Control Engineering (Section A Only) Forth Year Course Version 1.0

Transcription

1 Department of Building Services Degree in Building Services D026 Dublin Institute of echnology Bolton Street Dublin 1 Control Engineering (Section A Only) Forth Year Course Version 1.0 Lecturer: Dr. John McGrory School of Control Systems and Electrical Engineering, Dublin Institute of echnology, Room 10, Kevin Street, Dublin 8. Phone: +353-(0) john.mcgrory@dit.ie Web Site: Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 1 of 113

2 Notes from the author. Each semester is 15 weeks in duration. his includes one week for revision and two weeks for exams. his equates to only 12 weeks teaching. Assuming this course is disseminated using four lectures per week, two for each section, this means that there are only twenty-four contact hours per section involved (not including laboratory time or your private study time for this subject). herefore, onus is on you from the beginning to perform to the best of your ability. he term luck refers to that which happens beyond a person's control. Passing the exam and handing in satisfactory laboratory reports is not a matter of luck it is a matter of effort and work. Remember woulda coulda shoulda are the last words of a fool. As a caveat to students, the contents of these notes should not be considered the complete course. Items raised during the lectures are just as important and revenant and you should note them for yourselves. hese notes are provided before the lecture takes place. his allows you read ahead and to make the best use of your contact time with the lecturer. Section B is contained in another document. Keep the notes separate, as both sections are to be attempted for the examination. In the diagram below you can see my office (Room KEG-010) location in Kevin Street. Beside my office in Room KEG-012 in Kevin Street is where the laboratory is located. So yes you have to get to Kevin Street for the laboratories once every three or four weeks. So let s get through it and work hard, Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 2 of 113

3 able of Contents NOES FROM HE AUHOR ABLE OF CONENS... 3 OBJECIVES OF HIS COURSE:... 5 AIM:... 5 OBJECIVE:... 5 COMPLEION IME... 5 SYLLABUS:... 5 MEHOD OF INSRUCION:... 5 ASSESSMEN PROCEDURES AND CRIERIA:... 5 CHAPER 1, ELEMENARY CONROL FUNCIONS... 6 OUSIDE AIR... 6 CONSAN MINIMUM OUSIDE AIR... 6 OA ECONOMY CYCLE EMPERAURE CONROL... 7 HE ECONOMY CYCLE SCHEDULE... 7 ENHALPY CONROL ROOM PRESSURE CONROL WIH DAMPER CONROL SRAIFICAION CHAPER 2, HEAING AND COOLING COILS CONROL OF HEAING COILS REHEA CONROL JUS AS A REMINDER "ENERGY EFFICIEN" HUMIDIFICAION HE WO SCHOOLS OF HOUGH ON HUMIDIFICAION: SENSIBLE HEAING, AND COOLING ALONG COOLING, OR HEAING COIL FROS PROECION AFER HEA COILS LIMIS OR OVERRIDE O CONROL HE SUPPLY AIR CONROL OF COOLING COILS CONROL OF CHILLED WAER (CW) COILS CONNECING CHILLED WAER IN COILS IN PARALLEL OR COUNER FLOW HE AIR WASHER WO SAGE EVAPORAIVE COOLING NON-ADIABAIC HUMIDIFICAION PROCESS SEAM HUMIDIFIERS (ELECRICAL) PRESSURE WAER HUMIDIFIERS (ELECRICAL) CHEMICAL DEHUMIDIFICAION CONROL CONROL OF ELECRIC HEAERS CONROL OF REFRIGERAION SYSEMS CAPACIY OF SPEED CONROL OF COMPRESSOR SAGED COMPRESSION SCROLL COMPRESSOR CYLINDER UPLOADING INLE VANE CONROL OPERAES AS PER FANS AIR COOLING CONDENSERS WAER COOLED CONDENSERS CONROL OF COOLING OWERS CHAPER 3, COMPLEE CONROL SYSEM HE SINGLE ZONE SYSEM WIH VARIABLE OA QUANIY FOR ECONOMY CYCLE Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 3 of 113

4 CONROL OF RELAIVE HUMIDIY (RH) VARIABLE AIR VOLUME (VAV) CONROL Room control Discharge temperature control Mixed Air emperature control OA Variable Flow Rate (VFR) Control Fan Control REHEA SYSEM Commercial application Industrial applications CHAPER 4, HEA RECOVERY CONROL SYSEM AIR-O-AIR HEA RECOVERY IN VENILAION SYSEMS CHAPER 5, PACKAGED PRESSURISAION AND FILLING SYSEM PACKAGED PRESSURISAION AND FILLING UNI CHAPER 6, BOILERS AND CHILLERS PUMPS CENRAL BOILER AND CHILLERS CIRCUIS Boiler control Common Header single boiler Frost protection Multiple boilers in sequence (Parallel connection) Boilers connected in Series Modular Boilers, Wall Hung type Based load Heating inertia CHAPER 7, SIZING VALVES FOR WAER SERVICE SIZING VALVES FOR WAER SERVICE Example PUMPS CIRCULAION SYSEM CHARACERISICS Example ACUAL PERFORMANCE HREE-POR VALVE WO-POR VALVES VALVE AUHORIY Example Example HREE-POR CONROL VALVES AND VALVE AUHORIY CAVIAION AND FLASHING Cavitation in liquids Flashing in liquids Avoiding cavitation CHAPER 8, FLOW CHARACERISICS FAS OPENING CHARACERISIC LINEAR CHARACERISIC Example REFERENCE GLOSSARY OF ERMS Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 4 of 113

5 Objectives of this course: Aim: he aim of the module is to achieve an appropriate knowledge and understanding of the principles of the subject matter and reach a level of appropriate academic competence in descriptive, analytical and computational elements and apply this knowledge in the optimisation of engineering solutions. Objective: On completion of the course the student will be able to: 1. Appropriately comprehend the principles involved. 2. Apply the principles in a logical and appropriate manner. 3. Where appropriate, measure and evaluate empirically the major issues. 4. Analyse and compute some of commonly encountered models in a manner, which leads towards a resolution of the situation presented. 5. Evaluate the often-competing potential solutions so as to formulate an optimum and appropriate solution. Completion ime Chapters 1 to 2 Chapters 3 to 8 Chapters 9 to 12 Chapter 12 to 15 xx Hours xx Hours xx Hours xx Hours Syllabus: 1. Introduction of Control heory 2. he principles of Operation of Electronic control equipment. 3. Direct Digital Control. 4. Building Management Systems 5. Communications Networks 6. Communications Systems Method of Instruction: Instruction is by lecture sessions. Assessment Procedures and Criteria: As per syllabus. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 5 of 113

6 Chapter 1, Elementary Control Functions Outside air Outside air (OA) is defined as air that is brought into the ventilation system from outside the building and, therefore, has not been previously circulated through the system [Cupton, 1989]. he use of OA within buildings varies from: Commercial buildings with 80% recirculated air (RA), and 20% OA, Laboratories with 100% OA. Some applications such as ventilation to toilet areas, special manufacturing processes may require 100% OA. Some areas such as electroplating shops and chemical laboratories may require to be kept at (-ve) pressure (partial vacuum) to prevent leaking of dangerous chemicals or substances in the air to other locations in the building. Constant minimum outside air. An application of the constant minimum outside air is a simple on/off control interlocked with the supply fan as shown in Figure 1.1. he damper in the RA duct is a simple balancing damper. OA may also need a balancing damper. Recirculation Manual Panel Filter emperature OA Motorised Damper MD Balance Manual ON/OFF Supply Fan Interlocked to damper motor Figure 1.1 Constant minimum outside air example. his example is a very crude way of ensuring the constant minimum OA, but should only be used where the OA requirement is fixed and the economy free cooling cycle is not required due to the size of the plant. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 6 of 113

7 OA economy cycle temperature control A typical ratio of OA to RA in a fixed or two positioned OA damper would be 20:80. A typical the supply air (SA) temperature would be 14 o C, and RA temperature of 24 o C. Hence the balance would be: Sensible Heat (OA) + Sensible Heat (RA) = Sensible Heat (SA) 0.2Ms CP + 0.8Ms CP = Ms CP OA RA SA = 14 OA = 14 OA OA = 26DegC So, when the outside OA temperature is above -26 C the supply temperature of 14 C cannot be provided and the temperature of 14 C has to be provided by mechanical cooling of the air. his is very wasteful in energy. It is therefore clear that while the OA temperature is at or below 14 C the temperature of 14 C can be provided without mechanical cooling hence the so called economy cycle. he normal arrangement is for three dampers (i.e. the OA, the Exhaust Air (EA) and the RA. he OA and RA operate in unison with the recirculation air in the appropriate position. he economy cycle schedule Based on the 20% OA and 80% RA with supply at 14 C and return at 24 C the following equation holds where X is the OA fraction, CP is the specific heat capacity, and Ms is the mass flow rate of the supply air. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 7 of 113

8 ( x) Ms CP + (1 x) Ms CP = Ms CP OA RA SA ( x) Ms CP + (1 x) Ms CP = Ms CP OA RA SA x + (1 x) = OA RA SA x + (1 x)24degc = 14DegC OA x + 24DegC x 24DegC = 14DegC OA x = 14DegC 24DegC + x 24DegC OA x = 10DegC + x 24DegC OA OA 10DegC + x 24DegC = x Using the above formula it is possible to calculate the OA temperature OA for various mixing ratios of OA. X OA able 1.1 Mixing ratio of OA Consider for example 50% OA and 50%RA 0.5 Ms CP Ms CP = Ms CP OA RA SA 0.5 Ms CP Ms CP = Ms CP OA RA SA = OA RA SA ( DegC) + (0.5 ( + 24 DegC) = 14DegC his is not a linear relationship but a curve relationship as shown graphically in Figure 1.2. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 8 of 113

9 In this region the OA makes some contribution to mechanical cooling MAX 100%OA Usual control modification at design OA MIN 20%OA Design Air emperature OA Supply Air emperature 14DegC Return Air emperature 24DegC Figure 1.2 Mixing Ratio to temperature As it is possible to supply the building without mechanical cooling while the OA remains below the design supply air temperature, the chillier can remain off. herefore an enabling stat is fitted in the OA duct to enable the chillers to operate only when the OA reaches the design supply air temperature typically 14 C in commercial buildings as shown in Figure 1.3. he SA temperature is sensed and transmits a signal to the controller. he controller is also supplied with the RA temperature (RA) from the building and the OA. While the OA is below the set point of the supply air the OA damper modulates opening incrementally to maintain the supply air temperature when OA>RA then return OA to the minimum position. Usually, if the OA is < the design winter temperature of the OA approximately -2 C in Dublin it also modulates to the min OA position. During all these operations the EA damper modulates as the exact position of the OA and the RA damper modulates but in the opposite position. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 9 of 113

10 Set Point = SA 14DegC OA Design Winter emperature Controller EA OA Exhaust Air Motorised Damper Outside Air Motorised Damper MD MD RA SA MD Recirculation Air Motorised Damper Figure 1.3 Enabling stat fitted in the OA duct At any given time the controller is receiving in data on the: OA RA SA If the OA > RA then Min OA Damper Min EA Damper Max RA Damper If the OA < Design Winter OA then Min OA Damper Min EA Damper Max RA Damper If the OA < Set point SA to building then Max OA Damper Max EA Damper Min RA Damper Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 10 of 113

11 If the OA > Design Winter < Set point SA to building then Modulate Modulate Modulate OA Damper EA Damper RA Damper o give the set point of the SA temperature, generally all OA dampers are interlocked with the supply operation to close when the fan is off as shown in Figure 1.4. he operation of the minimum OA damper setting can be either built into the software stops at a set angle, or use a separate minimum OA damper on/off with the modulating damper going then to fully closed. In order to operate with software setting the correct angle of the blade must be determined during commissioning by measurement. his might not always be that easy hence the alternative. ON/OFF Interlocked with supply fan Motorised Damper EA Modulating with controller Motorised Damper OA MD MD RA SA Figure 1.4 Interlocking damper to fan he control schedule described is to be used with a VAV system (VAV stands for Variable Air Volume or also known as VFR - Variable Flow Rate). VAV boxes provide constant or variable air depending on the temperature demands of the space. As the temperature rises the VAV damper opens to send a designed amount of airflow to the room. It supplies a constant (say 14 C) air temperature all year round. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 11 of 113

12 MAX 100%OA VFR VFR MIN MIN 20%OA Design Air emperature -2 DegC OA Supply Air emperature 14DegC Return Air emperature 24DegC Figure 1.5 VFR results overview However an economy cycle with a constant air volume (CAV) system can give problems with heating costs in winter. With a CAV system the supply temperature is varied at a constant VFR. With a CAV single zone system for example it may be required to supply air to the building above room temperature let alone above 14 C. In such cases the approach is to sense the room temperature (Room). If the Room < the room set-point, then heating is required, the dampers modulate to minimum. If the Room > the room set-point then cooling is required, the dampers modulate to the schedule previously described. Enthalpy Control Heat sources internal to the building such as people, lights, computers, copy machines, motors, printers and other equipment causes the temperature inside a structure to continuously increase. Heat soaked up by the building structure may also continue to heat the building long after the OA temperature has dropped. here are times when the OA temperature is lower than the temperature inside. Whenever the cooling system is calling for cooling and the temperature outside is cool enough it is economical to shut off the compressor and bring in cool outside air to satisfy the cooling needs of the building. Consider when the OA temperature is close to the desired SA temperature then the EA and OA dampers are fully opened, and the RA air damper is completely closed. Under these conditions, 100% OA enters the building and the chiller can be shut off, because there is no need for cooling the air. Such is the function of an air economiser system and night Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 12 of 113

13 purging. Strictly speaking the economiser mode should be used whenever the enthalpy difference between the OA and the SA is less than the enthalpy difference between the RA and the SA. It is assumed for this module that you are familiar with psychometric chart as shown in Figure 1.6!!!! Seven different components shown on the one diagram is a lot of detail to take in. Figure 1.6 Psychometric chart overview his is because the enthalpy is the energy and not just the temperature of the air as shown in Figure 1.7. he term enthalpy means, total heat. he enthalpy control measures both sensible and latent heat in the air and only allows outside air to be used for cooling if the air is both cool and dry enough to satisfy the space conditions. However, it is difficult to measure enthalpy so usually just the air temperatures are used if the humidity is not a large factor. he economiser mode is sometimes used at night to cool off a building mass in preparation for the next days cooling load. his is called night purging [Curtiss et al., 2002]. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 13 of 113

14 his refers Energy Figure 1.7 emperature or Energy Just refers to emperature here is one drawback to this type of control system. Even though the thermostat acknowledges that the outside air temperature is low enough to cool the building, the OA may be too humid to provide adequate comfort for the building occupants. he occupants will feel cool but clammy. he solution is an economiser that adds a second control which works in harmony with the outdoor thermostat and measures the OA humidity. Such a control is called an enthalpy control. If the indoor thermostat calls for cooling and the outside air enthalpy (total heat) is low enough then the economiser brings in this cooler and less humid air and uses it for cooling instead of operating the compressor. Using the outside air for cooling is less expensive than operating the compressor to provide cooling. So an enthalpy control is a control which checks to see if both the temperature (sensible heat) and the humidity (latent heat) are low enough to be used for cooling. his combination provides for the greatest comfort at the least cost. Not all economisers use enthalpy controls. Some just check the outside air temperature and do not check the outside air humidity. hose controls do not provide the same levels of comfort as enthalpy controlled economisers. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 14 of 113

15 Enthalpy or heat content is a description of thermodynamic potential of a system, which can be used to calculate the "useful" work. It is often contested that instead of sensing the OA and the RA the enthalpy of these air streams should be sensed instead. Figure 1.8 ypical controls & sensors for economiser system In Figure 1.8 the typical controls and sensors used in an economiser system. his is able to provide the minimum OA during occupied periods when it is warm outside, to use outdoor air for cooling when appropriate by means of a temperature based economiser cycle and to operate fans and dampers under all conditions. he numbering system used in the figure indicates the sequence of events as the air-handling system begins operation. 1. he fan control system turns on when the fan is turned in. his may be by a clock signal or a low temperature space condition. 2. he space temperature signal determines if the space is above or below the set-point. If above the economiser feature will be activated to control the outdoor and mixing dampers. If below, the OA damper is set to its minimum position. 3. he mixed air PI controller controls both sets of dampers (OA/RA and EA) to provide the desired mixing air temperature. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 15 of 113

16 4. he OA temperature rises above the cut-off point for economiser operation, the OA damper is returned to its minimum setting. 5. he supply fan is off the OA damper returns to its NC position and the RA damper returns to its NO position. 6. he supply fan is off the exhaust damper also returns to its NC position. So in the case where the OA temperature is >RA temperature, the RA would be taken in preference to OA. However taking RA which has a higher enthalpy would put a higher load on the plant and we actually ought to take the OA instead. So it is suggested the sensors in these locations should be enthalpy rather than dry bulb temperature. he difficultly is that enthalpy sensors and more expensive and also may need more maintenance than the dry bulb temperature sensors. So are they really required? In Ireland probably not, the RA from the building is unlikely to exceed 24DecC and 60% RH which has an enthalpy of 53kj/kg. he summer time design external condition in Dublin is typically 24.5 C and 10g/kg moisture content. his has a specific enthalpy of 50kj/kg and a Wet Bulb (WB) temperature of 18 C. Hence while the OA temperature is < 24 C its enthalpy will be almost < the RA enthalpy and it is very seldom that the OA enthalpy would be above the RA enthalpy, while the OA temperature is > RA temperature. Hence in the Irish conditions temperature sensing alone is sufficient. Also cost effective. he problem generally only arises in very humid coastal regions. he most humid cities on earth are generally located closer to the equator, near coastal regions. Cities in South and Southeast Asia seem to be among the most humid such as Kolkata, India, Bangkok and hailand. Room pressure control with damper control One effect of free cooling cycle is that pressure in the room may change. Room pressure can be controlled by cycling the dampers, but free cooling control is generally then not used. Usually with CAV systems room pressure is set up at commissioning in commercial applications. But in industrial applications where room pressure is critical then exclusive control of dampers by means of pressure sensors is generally adopted as shown in Figure 1.9. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 16 of 113

17 EA OA Exhaust Air Motorised Damper MD Outside Air Motorised Damper MD RA SA Recirculation Air Motorised Damper MD Set point of difference pressure input Designated damper controller Stabiliser Delays Reaction Controller Pressure Sensor Other room pressure or reference pressure Room pressure sensed Pressure Sensor Figure 1.9 Room pressure control with dampers Pressure in the space is generally controlled with reference to pressure in an adjacent space (+15Pa -15Pa) or with reference to outside pressure. If the room pressure is falling, the OA damper opens and the exhaust damper closes slightly, therefore more air is supplied than is removed from the space and the pressure rises in the room. Recirculation quantity will rise as the exhaust is restricted, which could reduce the effectiveness of the throttling of the exhaust. hrottling of the exhaust alone may not therefore reduce the extract from the room. Room pressure is corrected simply by increasing the supply fan. If the recirculation damper however is also throttled as the exhaust is throttled and the OA damper opened then the pressure is more effectively restored as shown in Figure 1.10 and as a air flow chart in Figure Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 17 of 113

18 Degree in Building Services (D026.4), Control Engineering (Section A) EA OA Outside Air Motorised Damper Exhaust Air Motorised Damper MD MD RA SA MD Set point of difference pressure input Recirculation Air Motorised Damper Room Figure 1.10 Room pressure control he capacity of the extract fan is more effectively reduced as both dampers are controlled. P P Q Q Figure 1.11 Supply-Exhaust fan curves Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 18 of 113

19 Stratification In larger mixing boxes with colder OA intakes can be a problem. Air stratification is the tendency of two or more airstreams to remain separated. his commonly occurs in central air handling units in commercial and industrial building as return and outside air are introduced into the mixing box of the air handling unit. Whether a significant temperature difference exists between the two air streams or not, these two air streams tend to remain separated due to the inherent momentum/velocity of each stream. he presence of air stratification creates many challenges in proper design and operation of air handling units. Among the most notable problems are: Freeze-Stat rips & Frozen Coils Poor emperature Control Accuracy Insufficient Fresh Air Distribution Poor Economizer Operation Uneven Velocity Profile he only way to positively address each of these issues related to air stratification is properly mixing the return and outside air streams. Exhaust Air Motorised Damper EA Outside Air Motorised Damper OA at a low temperature of 4DegC Supply Air in this branch is not at the mixed temperature MD MD RA at 24DegC SA Recirculation Air Motorised Damper MD Separation or Stratification of hot RA and cool OA. Incorrect Mixing Figure 1.12 Air stratification Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 19 of 113

20 A solution to the stratification problem is to force the air streams to mix with baffling as in Figure1.13, or stirring fans in the chamber as in Figure 1.14 or rearranging the RA inlet to force the opposed air streams to mix as in Figure Figure 1.13 Baffling he concept of baffling is based on introducing a folded sheet metal plate structure that forces a turbulence and mixing of air streams. Exhaust Air Motorised Damper EA Outside Air Motorised Damper OA at a low temperature of 4DegC Supply Air in this branch is now Mixed MD MD RA at 24DegC SA Recirculation Air Motorised MD Separation or Stratification of hot RA and cool OA. Incorrect Mixing Supply Air in this branch is now Mixed Figure 1.14 Stirring fans Stirring fans are mechanical propellers that are placed into the mixing box of the AHU. As they rotate they mix the air flow streams. his is a similar concept to overhead cooling/mixing fans which are attached to light fittings. Figure 1.14 is a crude representation of the propeller. Industrial implementations are more efficient. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 20 of 113

21 MD MD MD Plan View of the system Figure 1.15 Rearranging the RA inlet By rearranging the location of the RA inlet to the mixing box it is possible to force a mixing of the air streams. his does cause some inefficiency in the ductwork and fans but may not be excessive when compared to installing a mixing fan. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 21 of 113

22 Chapter 2, Heating and cooling coils Control of Heating Coils he normal medium for supplying hot water (HW) heating coils is Low Pressure HW (LPHW) typically C. Although Heat Recovery Water at the about C is increasingly being used. Direct fired gas may also be used occasionally e.g. swimming pools, but control and efficiency needs to be checked first. Generally steam coils should be avoided. If steam is the source of heat availability it is better to use this to generate LPHW at 82 C for use in the coils. Cold Air + + Warmed Air Set at 82DegC Control Steam Supply S.V. S.V. Set at 90DegC HL S.. Condensate Return Figure 2.1 Heating Coils using steam Reheat Control Occasionally the heating requirements are divided into PREHEA and AFERHEA coils in the AHU. If adiabatic humidification is being provided then this separation is essential in countries where the OA temperature are not frequently < ODecC. (a) Adiabatic humidification occurs when water vapour is added to the atmosphere. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 22 of 113

23 (b) (c) Specific humidity will increase, along the wet bulb temperature line. Reduction in dry bulb temperature will happen as, the evaporated water will absorb heat. A humidifier will perform this function. Adiabatic dehumidification occurs similar to dehumidification, but on the opposite direction. Figure 2.2 Adiabatic moisture addition or removal Just as a reminder "ENERGY EFFICIEN" HUMIDIFICAION. here are often misconceptions that some humidification systems require significantly less energy to operate than others. While some energy savings between different types of humidifiers exist, large differences in overall facility energy consumption many times do not materialise. Often times, when two different humidification systems are compared, only the energy required to operate the humidifiers is considered. his is not the entire picture. Different humidification systems affect the entire HVAC system energy usage differently. he two schools of thought on humidification: One type of humidification that most people are probably aware of is isothermal humidification. With isothermal humidification, steam is injected into an air-handling unit or directly into the space that is to be humidified as shown in Figure 2.3. Water can be boiled with electric resistance, natural gas, or some other heat source to make steam. he other method of humidification is called adiabatic humidification. his method of humidification atomizes water into very small droplets. his can be done by forcing pressurized water through very tiny nozzles (high pressure humidification) or by vibrating a pan of water at very high frequencies (ultrasonic humidification). he droplets are then injected into the air- Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 23 of 113

24 handling unit or directly into the space that is to be humidified. he atomized water droplets are then evaporated into the air to increase the relative humidity. Figure 2.3 Steam humidification Many companies are advertising that the adiabatic humidification technologies are extraordinary energy savers. he fact is, when strictly comparing only the humidifier, an isothermal (boiling) unit will always use more energy than an adiabatic (evaporating) unit of equal capacity. But the energy implications are significantly more complex than this. he small droplets of water introduced by an adiabatic humidification system are not evaporated into the air for free. he water droplets absorb heat from the air to evaporate. We have all experienced heat absorption from evaporation. When we sweat, the moisture on our skin evaporates absorbing heat from our body to keep us cool, just as the water droplets absorb heat from the air during adiabatic humidification. Kg for Kg, evaporating water droplets requires nearly as much energy input as boiling. Adiabatic humidification can offer an advantage if humidification is required at the same time that cooling is needed. Believe it or not, some type of cooling is needed most of the year for many air-handling systems in some particular countries. his type of humidification will provide a free cooling effect along with the humidifying. However, during the heating season adiabatic humidification can result in increased space-heating energy. More heat may be required to maintain the desired space temperature. Isothermal humidification, on the other hand, does not significantly affect the temperature of the surrounding air. here may be several reasons to choose one type of humidification over the other, including energy usage. Every system should be considered case-by-case to determine what is right for your application. Remember, when analysing the energy consumption of a humidification system, it is important to look at the whole picture. Focusing just on the humidifiers can lead to some misleading results. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 24 of 113

25 Generally preheat coils are used with all OA plants. Where plants use recirculation however with a hold off stat set at 18 C (senses the return air from the building) preheat may not be required. he recirculation mixing chamber acts as a preheat coil. In this case only an after-heater is used. Where large quantities of OA are used, unless a preheat coil is used very cold air at <0 C is passing through the plant (and may damage it) until it reaches the heating coil. Freezing may occur in filters or in cooling coils themselves. It is common to place the last heating battery downstream of the cooling coil in order to allow some control of Relative Humidity (RH). he cooling battery can reduce the air temperature below the sensible cooling load, to remove sufficient moisture and then reheat to the t between the supply and return air required for the sensible cooling load as shown in Figure 2.4. A - + B C Wet Bulb B C A Moisture Content Dry Bulb t Supply-Return Figure 2.4 Cooling battery to reduce air temperature below sensible cooling load Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 25 of 113

26 Sensible heating, and cooling along cooling, or heating coil (a) Sensible cooling happens when the cooling or air, does not alter the specific humidity. However, the relative humidity will increase. Sensible cooling can only happen under condition that, the cooling coil's temperature, is not lower than the dew point temperature of entering air (b) Sensible heating is similar to sensible cooling (alteration in dry bulb temperature), but dry bulb temperature is increased in this case. he condition that has to be met is, there should be no water within the heating system, to avoid increase in specific humidity Figure 2.5 Steam humidification Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 26 of 113

27 Frost Protection Research has shown that water flowing in a coil at velocities > 0.7m/s will not freeze, once the air temperature is >-30 C. Hence the general principle of frost protection in heating systems is to use two sensors. An OA sensor which brings in the pumps at about +8 C if they are not already on and an immersion thermostat in the boiler return which brings on the boiler at about +4 C. Boiler Panel Filter emperature Set at 12DegC Or <= the design cooling supply air temperature to the building OA + + SA MD +4DegC Figure 2.6 Control of preheat coil on all OA plant Normal control of preheating coils is to raise the temperature after the preheat coil to a minimum low temperature (the minimum design supply temperature for cooling, for example). For instance the preheat coil always heats the air to 12 C, all year round, while the plant is on. his temperature clearly needs to be below the min supply temperature for cooling, or equal to the low limit settings of the supply air, as shown in Figure 2.6. he frost protection of the coil is provided by a thermostat which is located downstream of the coil. his is set to +4 C and locks out the supply fan if the air temperature is < +4 C at this point. his means the heating system has failed for whatever reason. Another option is to use the temperature sensor to open the valve to provide circulation but this is not effective if the pump is off at the time or is faulty. It is also common in OA plants to interlock with the ON/OFF control of the intake damper. A wind velocity of 8m/s which is common on Ireland will produce a velocity Pressure of 38Pa. his is sufficient to move significant quantities of air through the plant and duct, and may cause the coils to freeze when the plant is off as shown in Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 27 of 113

28 Figure 2.7. A solution other than just using isolation dampers is to use a Penthouse louver as shown in Figure 2.8. P 38Pa VFR, possible if wind only is driving force at 8m/s Q Figure 2.7 Wind velocity Figure 2.8 Penthouse louver Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 28 of 113

29 After heat coils All heating coils and chilled water (-) cooling coils may be controlled by: (i) A diverting valve (common) Figure 2.9 (ii) A mixing valve (accurate) Figure 2.10 (iii) A two port valve Figure 2.11 LPHW Return +4DegC LPHW Supply Panel Filter Diverting Valve OA + SA MD Figure 2.9 Diverting valve arrangement LPHW Return LPHW Supply Panel Filter Mixing Valve OA + SA MD Figure 2.10 Mixing valve arrangement Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 29 of 113

30 DPS LPHW Supply LPHW Return Panel Filter wo Port Valve OA + SA MD Figure 2.11 wo-port valve arrangement Each of these may have the usual possibility of: (i) ON/OFF control (ii) Proportional control (iii) Proportional and integral control (ii) and (iii) are referred to as modulating, and all of these methods of control have been discussed in section B of this course. Note: generally counter flow arrangements water flowing back and air flowing forward. In a single zone system the after heat coil is controlled from a room temperature sensor as shown in Figure Otherwise it is controlled from a plant temperature sensor as shown in Figure In a reheat system the reheat coil is controlled from a room temperature sensor. Room Sensor, Single Zone system Boiler Panel Filter OA + SA MD Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 30 of 113

31 Figure 2.12 wo-port valve arrangement Figure 2.12 Single zone room sensor to valve directly arrangement Boiler Panel Filter Plant Sensor for Multi-zone system alternative OA. Sensor on a schedule bases OA + SA MD Figure 2.13 Multi zone room sensor to valve directly arrangement In a reheat system the reheat coil is controlled from a room temperature sensor as shown in Figure Boiler Room Sensor Controls valve Panel Filter + OA SA + Room Sensor Controls valve MD + Room Sensor Controls valve + Figure 2.14 Preheat and reheat system control arrangement Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 31 of 113

32 Limits or override to control the supply air. It is sometimes necessary to put a limit on the minimum and maximum temperature which the plant delivers in response to controls located remotely from the plant, for example in a room or in a zone somewhere. Hence low limit (LL) and high limit (HL) may be set in the software to the temperature of the air delivered from the plant. A typical low level would be 12 C and a high level would be 50 C. If LPHW is at 82 C it would deliver air at up to 75 C. Which is far too high and would probably trip all the fire dampers? his could happen under fault conditions. A typical low level limit would be 12 C as the cold water flow temperature (CWF) of 5 C could produce air at 7 C which would probably cause condensation in the room. Hence supply temperature must be sensed even in plants in which coils are controlled from the room. Control of cooling coils hese are generally confined to the air handling unit. Very occasionally re-cooling coils are used, but these are certainly not at all as common as reheating coils. Occasionally when chemical dehumidifiers are use re-cooling may be used. Recooling deserves investigation in certain climates it may be a preferable approach to reheating. here is one argument for re-cooling cooling is using mainly electrical energy heating using fossil. Fossil is about 30% the cost of the electrical production, therefore it is better to use fossil directly. herefore would it make more sense to heat everything up first and then trim with the cooling rather than the other way around! Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 32 of 113

33 Control of Direct Expansion (DX) type evaporator coils. In a similar fashion to heating coils evaporator coils could be controlled using a basic two position control or more sophisticated control. So let s discuss the basic two position control aspect. Figure 2.15 Direct Expansion (DX) Refrigerant flowing through the coil tubes is controlled by an expansion device, usually a hermostatic Expansion Valve ("EV" or "XV"). he XV is mounted at the coil just ahead of the refrigerant distributor, and automatically feeds just enough refrigerant into the coil to be completely converted (boiled) from liquid to gas as shown in Figure he XV is controlled by a temperature sensing bulb mounted on the coil outlet (suction) connection. Proper operation of the XV depends on the bulb sensing the required amount of superheat in the refrigerant gas at the coil outlet (superheat = the number of degrees above the boiling point temperature of the refrigerant). Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 33 of 113

34 EV Refrigerant Load SA Room Refrigerant Return Return emperature Figure 2.16 Cooling coil control EV measures the refrigerant to the coil in order to maintain a specific temperature (generally 1 C). his ensures that only refrigerant in a gaseous state enters the compressor. he control of the most basic form is via an ON/OFF solenoid valve and the control arrangement is shown in Figure In this basic system the LLS is a low level temperature sensor is simply an on/off solenoid valve as shown in Figure Otherwise the process can be accomplished with temperature sensors and a minimum limitation set at the supply air. Electrical Coil LLS Room temperature sensor Spring Figure 2.17 On/off solenoid valve Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 34 of 113

35 Controllability is limited and is best used at <85kw load (small load) and where load is stable (industrial load or internal load the building). Improved control is obtained, but at great expense, with multistage coils, initially two stages as shown in Figure EV ON/OFF EV Refrigerant Load ON/OFF wo Stage hermostat Refrigerant Return Figure 2.18 wo-stage cooling coils Just to put an image to the equipment being used Figure 2.19 shows thermostats which have one, two, three and four stages. Alternatively a temperature sensor set at two different limits can be used in sequence. Figure 2.18 wo-stage cooling coils Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 35 of 113

36 Room temperature sensed = Valve No = Valve No. 2 In this case the lower limit can be override the output signal if the supply goes below say 12 C he alternative is to use a modulating Back Pressure Valve (BPV). he back pressure valve shown in Figure 2.19 is for fixed pressure settings but motorised versions of this valve are available which allow the modulating of the back pressure setting. BACK PRESSURE / RELIEF REGULAOR Figure 2.19 Back Pressure Regulator Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 36 of 113

37 When installed in the control system the valve works by raising the evaporator pressure and hence temperature, therefore reducing the heat transfer as shown in Figure 2.20 and Figure As the room temperature falls, the BPV throttles thereby increasing the refrigerant temperature and reducing the heat output of the evaporator. EV Refrigerant Load SA Room Refrigerant Return Return emperature Modulating Back Pressure Valve Figure 2.20 Back Pressure Regulator location P P-Across BPV h Figure 2.21 Back Pressure Regulator P-h diagram Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 37 of 113

38 Generally a small portion of the fluid entering the evaporator is in vapour form. By using Hot Gas Bypass Pressure valve (HGBP) this portion can be artificially increased as shown in Figure Hot gas bypass used for capacity control is provided to maintain a constant evaporator pressure when the system load decreases. If the compressor does not have an unloading mechanism (reduced volume flow capability), the hot gas bypass allows the evaporator temperature to remain constant. his also provides a constant volume flow, at the appropriate evaporating temperature, to the compressor. When the system load decreases (less cooling or heating required) without a hot gas bypass valve or capacity control, the evaporating temperature will be reduced, since the compressor is still pumping the same volume (m^3/min). As cooling can only be provided by evaporating refrigerant, this reduces the coil capacity. Generally on/off solenoids are used on situations in which a series of parallel evaporators are in operation. he advantage is that the compressor can be kept running because of the constant volume flow. Figure 2.22 Hot gas bypass valve Keys to successful implementation: (Note reference to OIL refers to refrigerant/oil). (1) Position the HGBP valve above the discharge line, near the compressor. If the system includes pump-down, provide a means to shut off refrigerant flow. (2) Pitch the line upstream of the HGBP valve to drain oil back into the discharge line. (3) Pitch the line downstream of the HGBP valve toward the evaporator, away from the valve. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 38 of 113

39 (4) If the HGBP line includes a riser, regardless of height, provide a drain leg of the same diameter as the riser. Add an oil-return line 25 mm from the bottom of the trap; use tubing that is 6 mm and at least 3000 mm long. Precharge the trap with oil. (5) Divert hot gas to each active distributor at the expected operating points for hot gas bypass. (6) If the HGBP line feeds multiple distributors, provide a check valve for each distributor. (7) Insulate the entire length of the HGBP line. Control of chilled water (CW) coils Chilled water cooling coils are controlled as per LPHW heating coils by: (1) wo port valves (2) hree-port diverting (3) hree-port mixing he most common form of control is 3 port diverting although 2 port is becoming more common to save energy of the pump. 3 port mixing is rarely used on CW coils (more uncommon that with heating coils). As with heating coils it gives more accurate control of the air-off-temperature (i.e. the air coming off the cooling unit). However, it has the disadvantage of eliminating latent cooling at port load which may be a disadvantage in many situations where RH control is important. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 39 of 113

40 Connecting chilled water in coils in Parallel or Counter Flow Does the direction of the flow into the coil matter? In Figure 2.32 the two types of flow are discussed. emp Counter Flow emp Parallel Flow 27 Air 27 Air 11 Water Water Distance Distance Figure 2.23 Counter flow or Parallel flow Log mean temperature difference (LMD) For Counter Flow (27 11) (13 6) = (27 11) ln (13 6) 9 = 0.82 = For Parallel Flow (27 6) (13 11) = (27 6) ln (13 11) 19 = 2.35 = 8.08 he Heat ransfer in the parallel arrangement is the counter flow, therefore this is very significant = 100 = 74% of that in the in Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 40 of 113

41 It has been shown that a change from parallel to counter flow can result in the CW temperature increasing by 3 C without any decrease in the heat transfer as shown in Figure emp Counter Flow emp Parallel Flow 27 Air 27 Air 14 Water Water Distance Figure 2.24 Distance-emperature Flow Distance Hence more efficient coils can be produced and energy saved by increasing the coefficient of performance (COP) (i.e. temperature of the chilled water). Where Q is the useful heat supplied by the condenser and W is the work consumed by the compressor. (Note: COP has no units, therefore in this equation, heat and work must be expressed in the same units). Note that counter flow has an impact with cooling coils as they are deep 6-14 rows, but not so much with heating coils which have 2-4 rows. he Air Washer Most basic the desert cooler as shown in Figure 2.25 Eliminator A B Evaporation Rate 100 Mw 100 Figure 2.25 Desert cooler Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 41 of 113

42 Generally no thermostatic control other than fan and pump interlocked and manual on/off control of the circuit. B B A Figure 2.26 Psychometric diagram for desert cooler As the efficiency of the process is almost constant at all conditions the OFF condition depends on the ON condition as shown in Figure Generally used for relief-cooling, what ever cooling can be obtained is taken. Generally used where cooling is required all year (for example North-Africa). In theory it might be thought that control could be exercised by putting a modulating control on the pump, but as the pump circulates many multiples of the evaporator, there would be little impact on the rescaling off condition. Speed control of the fan is an option but this is often ruled out based on cost. Figure 2.27 shows an operation diagram of the desert cooler. Figure 2.27 Operational diagram for desert cooler Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 42 of 113

43 wo stage Evaporative Cooling With two stage operation you can switch from single to two stages and this gives some control and better efficiency flexibility. Another option is to add a heating coil to the process, which initially seems incorrect. However as the degree of evaporation depends on the incoming condition wet bulb temperature the heating coil can control this and therefore control the degree of evaporation by controlling the coil condition Heating Eliminator A + B C Room No Heating coil C C A B B Figure 2.28 Pre-heating before desert cooler Hence more or less cooling can be obtained. his is a more effective way then trying to control the circulation pump. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 43 of 113

44 Non-adiabatic humidification process In thermodynamics, an adiabatic process is a thermodynamic process in which no heat is transferred to or from the working fluid. he term "adiabatic" literally means impassable, corresponding to an absence of heat transfer. Non-adiabatic humidification is where heat does transfer to the passing medium as part of the humidification. he system shown in Figure 2.29 details the heating or cooling of the process air to obtain the desired supply air state. Heating Eliminator Space Humidity Sensor H E L + S Room Heat Removed Heat Added E-summer LPHW CW L S Controller E-winter Figure 2.29 Non-adiabatic humidification Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 44 of 113

45 Steam Humidifiers (electrical) High Level RH Relative Humidity Sensor RH Room Heater Room or more often the return air sensed Figure 2.30 Steam humidification High level sensor as shown in Figure 2.30 is required in the duct as at high levels of humidity condensation can occur in the duct. Water treatment may be necessary with some systems to avoid the nozzles clogging as the nozzle aperture is quite fine as shown in Figure 2.31, for example areas where there is high calcium content. Figure 2.31 Steam humidification Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 45 of 113

46 Pressure Water Humidifiers (electrical) Compressed air Water Relative Humidity Sensor RH Room Figure 2.32 Steam humidification An alternative to the steam humidifier is to use a combination of compressed air and water as shown in Figure hese combine in the nozzle, increases the liquids velocity and when passed through the fine aperture a very fine mist is produced. his system avoids the addition of heat to the air as is the case with the steam. But some people consider the lack of the steam heating to be a cost saving, but in reality the cost of running the compressor can be comparable and so no real saving will be achieved. Chemical Dehumidification In some environments a combination of high temperature and high humidity defies a remedy by convectional air-conditional air-conditioning, which is biased towards temperature rather than humidity. Dehumidification is merely a by-product of bringing the temperature down below the due point of the air causing condensation. A desiccant is a hygroscopic material whether it is liquid or solid, which can extract moisture from humid air, gas, and liquids. Hygroscopy is the ability of a substance to attract water molecules from the surrounding environment through absorption. Liquid desiccants work by absorption where moisture is taken up by chemical action. Solid desiccants have a large internal area capable of absorbing significant quantities of water by capillary action. Examples of efficient desiccants are: Silica Gel Activated Alumina Lithium salts riethylene glycol Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 46 of 113

47 his is generally used where low relative humidity areas are encountered, typically less than 40% are required. o achieve very low humidification using refrigerants requires very low evaporating conditions, hence the coefficient of performance (COP) falls dramatically and the capital cost is high. he chemical dehumidification process reduces the moisture content of the air by absorption, but increases the sensible temperature as shown in Figure 2.3. he vapour in the air is absorbed as water in the chemical. Hence it gives it latent heat and the dry bulb (DB) temperature rises as a result. his method of dehumidification requires a heating stage in the process. his is to dry or regenerate the desiccant material and requires a temperature range of C. One option is to supply the heat by means of evacuated tube solar collectors, backed up by natural gas when insolation is inadequate (Insolation is a measure of solar energy received on a given surface area in a given time. It is commonly expressed in kilowatt-hours per square meter per day (kwh/m²/day)). Alternatively waste heat, for example from a combined heat and power (CHP) unit may be exploited. As an alternative to air-conditioning, desiccant dehumidification can be used in conjunction with evaporative cooling. After being dried by the revolving desiccant wheel the air passes through a heat exchange such as a thermal wheel for cooling as shown in Figure If necessary, further cooling may be achieved by an evaporative cooler before the air is supplied to the building as discussed earlier. A B Wet Bulb A B Moisture Content Dry Bulb Figure 2.33 Dehumidification using desiccant material Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 47 of 113

48 RH Air Conditioning units Supply EA air to the building Room Desiccant Wheel Interlocked with Fan Return Air Air Inlet Heat for regeneration Exhaust Air Figure 2.34 Dehumidification using rotating desiccant material he process needs to cool the air therefore after drying. As the absorbed moisture must be removed from the chemical so that the process can begin again a reprocessing cycle is used. Control he room thermostat controls the cooling coil while the room RH stat controls the regenerated air heater, the approach being that drying the supply air falls off as the air is not regenerated. he desiccant wheel is usually interlocked with the fan such that it rotated only when the fan is on. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 48 of 113

49 Control of Electric Heaters hese are often used for convenience in application which may have a short intermittent running time, for example ventilation of a meeting in use < 1000 hours/year, in which the capital cost of a LPHW coil and connections to boilers is not warranted. Special control is required, because if insufficient air passes the coil it could burn out the coil element. Hence (i) (ii) Heater is interlocked with fan. Differential Pressure Switch (DPS) is used to confirm airflow is adequate before the power is allowed to the heater element. he electrical diagram is shown in Figure Contactor ~ Interlock High Limit + Room DPS Figure 2.35 Electric Heaters arrangement Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 49 of 113

50 Star or Delta configuration Star Delta Fan interlock Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 50 of 113

51 Figure 2.36 Electric heaters electrical arrangement he configuration of the heater elements can be either star or delta. he star needs an additional neutral to complete the circuit as shown. Various forms of control for the electrical elements are possible including: (i) Staging the heater (possible across phases not recommended if large). (ii) Solid state controller, involving alteration of the voltage chopping the cycle, frequency generated. (iii) ransformer type, reduces the voltage, hence current and wattage. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 51 of 113

52 Control of Refrigeration systems. Capacity of speed control of compressor. his has now become very important with the development of compressor technology. Methods of controlling compressors are: Staging, where multiple compressors are used. Cylinder uploading Speed selection (scroll compressors) Inlet vanes (centrifugal) Staged compression Since compression generates heat, the compressed gas is to be cooled between stages making the compression less adiabatic and more isothermal. he interstage coolers cause condensation meaning water separators with drain valves are present. he compressor flywheel may drive a cooling fan. Scroll Compressor. he flow rate capacity for scroll compressors can be controlled by varying the speed at which the scroll rotated. Use the following hyperlink to see it in operation Figure 2.37 Scroll Compressor Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 52 of 113

53 Cylinder uploading. Cylinder uploading is a method of holding the discharge valve open on one or two of the cylinders of a multi-cylinder compressor. he discharge stroke therefore does not build up the pressure on these cylinders and hence capacity is reduced. Figure 2.38 Cylinder uploading Figure 2.38 illustrates the pistons of a car engine, but can also be used to demonstrate the operation of a cylinder compressor. he main difference is that no fuel is added so no need for a spark plug, and that the main crankshaft (C) is turned by an electric motor (not the combustion of the fuel). (E) Exhaust camshaft, (I) Intake camshaft, (S) Spark plug which does not exist in the compressor approach, (V) Valves an inlet and outlet, (P) Piston, (R) Connecting rod, (C) Crankshaft, (W) Water jacket for coolant flow because when a gas is compressed it heats up in accordance with the Combined Gas Law. Whenever we are dealing with problems that have initial and final conditions. he link below will also show a moving image of the system. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 53 of 113

54 Cshaft.gif Figure 2.39 multi-cylinder arrangement Inlet vane control operates as per fans. Inlet guide vanes are more efficient than butterfly valves by pre-swirling the air to help minimize power consumption by allowing lower turndown. Figure 2.40 illustrates a standard inlet vane arrangement. he internals are sealed to prevent contamination from ambient conditions. he stainless steel vanes virtually eliminate corrosion. Figure 2.40 Inlet vane arrangement Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 54 of 113

55 All refrigeration compressors operate between two pressure limits or settings or the high pressure and low pressure sensors. When considering control keep this in mind! For the condenser to maintain its pressure it must received gas to be condensed at the same rate as it has the ability to condense the gas. HP LP Figure 2.41 Compressor HP and LP As the Variable Flow Rate (VFR) of the refrigerant falls the speed of the compressor is reduced to match the VFR of the refrigerant being generated. he capacity of the condenser must also be turned back to prevent the condenser pressure falling as volume of gas produced from the compressor falls. Likewise if the load on the evaporator falls then the volume of gas produced is reduced the compressor and condenser must now reduce in capacity to match the volume of gas being generated as illustrated in Figure his is what the controls of the system are designed to do. Condenser EV Evaporator Compressor Speed Control Return emperature Room Figure 2.42 Compressor to cooling coil arrangement Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 55 of 113

56 Air cooling condensers Generally speaking the condenser fans start on/off with the compressor on/off. An illustration of the arrangement is shown in Figure 2.43 where the coils carry the refrigerant medium. he fins attached to the coils are used to maximise the contact of the air passing over the coils in order to cool the keep the refrigerant medium in the plates. he cool air entering the coil is heated up by the warm refrigerant medium and this warmer air leaves the system. Coils Conduction fins Warmed air after passing over plates o evaporator: Condensed Vapour Liquid Cool air Vapour from compressor Figure 2.43 Air cooled condenser arrangement Heat is rejected to the ambient air the temperature of the ambient air varies from about -2 C to +25 C in Dublin. Hence a given condenser designed to reject its full load at 25 C (say refrigerant at 45 C) would have far greater capacity at say +5 C (t = 40k) than at t = 20k design. So the capacity to reject heat of the condenser must be reduced to match the ambient conditions and load available. his is referred to Lead Pressure control. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 56 of 113

57 In its simplest form the fans are simply switched on in turn. In a more sophisticated form the speed of the fans is controlled using a variable speed drive as shown in Figure Coils Conduction fins Warmed air after passing over plates o evaporator: Condensed Vapour Liquid Cool air P Vapour from compressor Rotation, Increases air flow over the fins and coils Figure 2.44 Forced air cooled condenser arrangement he basic from of control is to hold a particular set point pressure and reduce the air flow rate over the coil as the condenser pressure would start to fall. his form of control however does not give any energy benefits. his is possible as with falling ambient conditions it should be possible to maintain the design t (20k) at lower condenser pressures. Hence if the ambient is 15 DecC then a condenser temperature of 35DecC should be possible to reject the same load. his would increase the coefficient of performance (COP) of the refrigerant process. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 57 of 113

58 Coils Conduction fins Warmed air after passing over plates o evaporator: Condensed Vapour Liquid Cool air OA temperature P Vapour from compressor Controller Rotation, Increases air flow over the fins and coils Figure 2.45 Forced air cooled condenser arrangement with controller he OA temperature sensor senses the OA condition and the controller reads a suitable set point for the pressure Required 9 pressure in Bar OA emperature Figure 2.46 Change in pressure to OA temperature Hence the system can be controlled in this way. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 58 of 113

59 Water cooled condensers An alternative to the air cooled condenser is the water cooled condenser. In this example the condenser is enclosed in a pocket of circulating water. Water enters the condenser coils at say 27 C and leaves the condenser at 33 C. he water at the higher temperature is delivered to the spray bars of the cooling tower and atomises the water. As the atomised water in the cooling tower falls the cool air comes in and cools down the droplets, thus cooling the water collecting at the bottom. he volume of air passing through the tower can be controlled by the fan speed. An alternative approach is to use a 3-port diverting valve arrangement which acts by maintaining the water temperature above the set point of 27DecC. Warm Air leaving Fan to force air through tower Spray Bar Indirect cooling, where the cooling is performed away from the condenser Cool OA entering tower Water droplets Cool OA entering tower Cooled water circulates around the cooling fins Cooled Water Say, 27DegC entering 3-port valve o evaporator: Condensed Vapour Liquid Warm water leaving the condenser Say, 33DegC leaving Vapour from compressor Figure 2.47 Water cooled condenser Control of cooling towers. he degree of control necessary depends on the annual load pattern. owers designed to operate only in the summer months may have sufficient control with fan speed alone. Summer towers are usually drained in the winter time. Where towers are designed to be operated in the winter then additional control and freeze protection may be required. As the minimum speed is limited by the necessity to reject heat from the motor the min speed is usually limited to 20%. Also when the fan is off some heat may be rejected. Hence the necessity to provide additional control such as the diverting circuit as this allows a full range of possible control. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 59 of 113

60 Frost immersion heaters in additional to trace heating may be required. Glycol (Ethylene Glycol is an alcohol based chemical compound widely used as automotive antifreeze) is not used in towers as it is a recirculation system and is not an evaporating systems. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 60 of 113

61 Chapter 3, Complete Control System he single zone system with variable OA quantity for economy cycle. A variable OA system for a single zone is shown in Figure 3.1 EA Exhaust Air Motorised Damper Outside Air Motorised Damper OA MD MD Heating Cooling RA + - SA MD Recirculation Air Motorised Damper Duct sensor Limiter Room 3 Output Sequence Controller Set point for emperature Room Set point for emperature Delivery Figure 3.1 Single zone system with variable OA he normal arrangements for operating this type of system are: If the Room emperature < Room Set point emperature then, (i) Stop cooling (ii) Decrease OA (iii) Increase Heating If the Room emperature > Room Set point emperature then, (iv) Stop heating (v) Increase OA (vi) Increase cooling Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 61 of 113

62 Note the set point of the room temperature can be varied manually at the controller similar to that shown in Figure 3.2. his can also be done automatically in response to an OA temperature sensor. his may help to save energy. Figure 3.2 hermostat for room Set Point of Room OA emperature Figure 3.3 Room Set-point to OA emperature he duct temperature sensor as shown in Figure 3.1 is a limiter and typically it prevents the supply air from going below 12 C, even though the load on the room may be heavy. A point to note is the sequence of the response. If Room emperature > Room Set point emperature then the sequence is: (i) Cooling coil valve opens (ii) Chilled water admitted (iii) Air is cooled (iv) Colder supply air will eventually reduce the Room emperature and the valve will close. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 62 of 113

63 his process will take time. In the meantime the room air temperature will have risen still further. he time delay depends on (i) (ii) (iii) he length of the ducting involved he location of the emperature room sensor he thermal storage features of the room. Remember the concept of PID in the lecture notes for section B. his means that the control could utilise the proportional, integral and derivative parts of the PID controller. In large spaces this delay can lead to a wide controlled band in the room. One way for shortening this band is to use the supply air temperature sensor as the controller (duct temperature as apposed to the room temperature). But to use the room temperature to reset the set point of duct temperature. Hence as follows: Room set point temperature = 23 C Duct set point temperature = 14 C Room temperature = 25 C Duct temperature = 14 C So with the duct set point temperature reset to 12 C, the cooling valve would open quickly to satisfy the duct temperature. he process is quicker because the control loop is very short and independent of the length of duct or absorption in the space. herefore in effect what happens is that the room condition is not allowed to get as far out of control as it could. Control of Relative Humidity (RH) his area is broadly divided into two applications. (i) Commercial: minimum RH 30%, maximum 65% (ii) Industrial, narrow limits of say 40% to 3%. In the case of the commercial application low and high limit humidity sensors are used usually in the return duct. he high limit humidity sensor is also an input to a sequence controller and overrides control of the cooler coil by the air temperature sensor using a selector relay. For most of the time there is no input signal form the RH sensor, therefore the selection relay simply puts the temperature sensor input as the output. When the RH rises above 65% the RH sensor now controls the cooling coil and dehumidifying the air. his removed moisture from the air but over cools the room. he reheat coil now activates to restore room temperature. So for this to occur the heating coil must be placed after the cooling coil. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 63 of 113

64 With modern controllers this selector relay function is built into the sequence controller and does not appear as a separate relay. Methods of implementing this with the PLC or controller are also discussed in Section B under the parts relating to ladder logic and sequence diagrams. As the moisture for outside air in Dublin is 10g/kg and the moisture content for room air at 23 C is 11.5g/kg. he chances of having an RH above 65% is very rare unless the VFR is very low as discussed earlier. Addition of up to 1.5g/kg is possible. Also at a room temperature of 23DecC the minimum room moisture for 30% RH the moisture content is 5.2g/kg and at -1 C the moisture is 3.4g/kg, requiring an addition of 1.8g/kg is required. Now, if the ASHRAE standard of 25% RH is required then the minimum moisture in the room is 4.4g/kg and only an addition of 1.0g/kg is required to bring the system within limits. Except in high occupancy rooms humidity control in commercial buildings is not often provided. A scheduling of the room temperature from 20 C in winter to 24 C in summer is often a better approach. High limit humidity is provided by a steam humidifier generally as shown in Figure 3.4. Humidifier placed in the discharge duct at the fan exit. Clouds of steam in air stream Steam humidifier using electrical element Low Limit in return air controls the steam humidifier RH Low Limit in return air duct Figure 3.4 Steam Humidity When sizing a humidifier do not size it to add enough steam for the design room condition, but just to add enough for the minimum condition at the design OA winter condition. Saving from moisture at design air condition of 3.4g/kg to about 5.2g/kg. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 64 of 113

65 Variable Air Volume (VAV) control Exhaust Air Motorised Damper EA Outside Air Motorised Damper OA Controller 3 Controller 1 Set point MD MD 3 1 RA + + SA MD Heating Cooling Recirculation Air Motorised Damper MD Controller 2 2 Room Figure 3.5 Control of the VAV system Room control emperature sensor 2 senses the room temperature and controller C2 controls the damper actuator. he set point of the room temperature and the high and low VFR is set at the controller C2. Discharge temperature control his would typically be controlled to 12 C set point. he chiller (-) and heater (+) are adjusted in sequence to do this. A common modification is to fit a discriminator relay. his receives an input from all the room temperature sensors and gives as an output the highest of the inputs. If no room is at the maximum setting say 25 C then the set point of 1 at C1 is adjusted upwards. his has the result: (a) Saving energy (b) (c) Avoiding low turndown, i.e. turnover quantities of air in the room. Possible disadvantage of using more fan energy as the VFR is greater than it would otherwise be. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 65 of 113

66 Mixed Air emperature control emperature sensor 3 senses the condition of the mixed air prior to cooling or heating, and C3 controls the 3 dampers to give the required condition. he set point of C3 is equal to C1 as this is the minimum temperature required to be supplied, this gives free cooling up to typically 12 C OA temperature. he reset line from the discriminator relay can also be used here to raise the set point if possible. OA Variable Flow Rate (VFR) Control 0-10Vdc signal measures VFR Set-point of VFR Controller Controls Min Limit MD ransducer Grid Sensor, also known as a Wilson Flow Grid Figure 3.6 Grid Sensor he grid sensor measures static and velocity pressure across the width of the duct as shown in Figure 3.6. From this by calibration the VFR can be inferred. he transducer converts this VFR to a 0-10Vdc signal. he controller has the set-point of the VFR set. his is the calculated required minimum VFR for effective ventilation. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 66 of 113

67 he arrangement acts as a minimum limit on the OA damper. his will allow the VFR at the damper to go above this set point but not below. Hence the damper acts to mix the quantities to get the correct free cooling but only when it does not compromise the ventilation. Fan Control he speed of the return air fan is controlled to be a certain % of the supply fan. he measurement of air can be completed in many ways using a turbine (Figure 3.7), varying resistance (Figure 3.8) and pressure tube as shown in Figure 3.6 etc. Controller 2/3 1/3 Figure 3.7 urbine Sensor here are various measurements of air flow: urbine/vane Pressure tubes, etc. Figure 3.8 Resistance Sensor Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 67 of 113

68 Control of the fan speed and air volume can be performed in a number of different ways (i.e. variable speed drives or adjustable vanes etc). In Figure 3.9 a comparison of the different types of ways which are used to vary the volume of air through a fan. he ideal relationship where Power proportional to Q is shown as a dashed line. (a) is where the fan speed is constant and the ductwork is changed in some way to achieve the change in volume. (b) is where the duct work is fixed and the speed of the motor is the only thing that changes. Figure 3.9 Constant Speed Fan or Variable Speed Fan A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustablefrequency drives (AFD), variable-speed drives (VSD), AC drives, micro-drives or inverter drives. Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives. Figure 3.10 shows an image of the VSD and also shows the AC incoming wave being sliced up into pulses. It is important to note that the motor acts on the principle of magnetism and it is not essential for the incoming wave to be a sine AC. It can be + and pulses, with the amplitude and frequency of the wave being used to generate the equivalent magnetism affect in the motor. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 68 of 113

69 Figure 3.10 Variable Speed Drive Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 69 of 113

70 Reheat system Exhaust Air Motorised Damper EA Outside Air Motorised Damper OA Controller 2 MD MD 3 s RA + + SA MD Heating Cooling Recirculation Air Motorised Damper + R Room Reset line (highest of inputs chosen) Controller 1 Rx Discriminator Controller R2 R1 Figure 3.11 Reheat System Room conditions Inputs he reheat system is a lot simpler than the VAV control. here are two main options as shown in Figure (i) Systems use primarily for room sensible temperature control, usually in commercial applications. (ii) System used primarily for room RH accurate control (industrial application) Commercial application Sequence controller controls the supply temperature in the normal way. ypically supply temperature would be 14 C. he supply temperature can be reset upwards when no room is on a full load. he test reference year (RY) was developed as a replacement for energy analyses. It produces a set of data which measures the temperature for every hour in a year and produces a cumulative % in relation to the temperature. he dry bulb temperature for Dublin is given in Figure 3.12 (a). he dry bulb temperature is shown under the Bin column ranging from -8 to 26Deg C, the number of hours is given under the Frequency column which sums up to 8760 Hours (24hr/day by 365 days of the year 24x365=8760Hrs). his shows for how many hours the temperature was reached. Similar RY data is available for the Wet Bulb in a given year is also given in Figure 3.12(b). Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 70 of 113

71 Note from test reference year (RY) data shown below that a supply temperature it design of 14DecC gives a cumulative % of 87.8%, while the 12 C gives 68.4% on a RY bases. Hence if 14 C is chosen as a reheat supply temperature it is not necessary that energy inefficient. Also with a load analyser fitted the energy waste can be reduced still further. Hence the combination of the sequence controller and the discriminative/load analyser gives a more energy efficient system. (a) (b) Figure 3.12 Dry Bulb and Web Bulb est Reference Year Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 71 of 113

72 Industrial applications With this arrangement the sensor should be fitted downstream of the cooling coil and set to control the off moisture content at a constant level. his is done by setting the dry bulb sensor so called dew point sensor. his is based on the idea that the off coil condition is 95% saturation. Hence a dry bulb sensor can control the supply moisture content in this way. If the RH in the space is too high the duct sensor set point can be reset downwards. If RH in the space is too low then humidification is introduced. 95% Figure 3.12 Dew point sensor Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 72 of 113

73 Chapter 4, Heat Recovery Control System Air-to-Air Heat Recovery in Ventilation Systems A heat recovery unit, also known as a recuperator, transfers heat (some units also moisture) from the exhaust air stream over to the supply air stream, thus reducing the heat loss due to ventilation, and reducing the need to condition the cold supply air. Conversely, in hot and humid outdoor conditions, a heat recovery unit can keep heat (some units also moisture) outside, thus reducing air conditioning costs. Figure 4.1Air-to-Air Heat Recovery However the control of the air-to-air heat recovery unit as shown in Figure 4.1 is difficult because there is no way to limit the air flow and therefore the heat. An alternative arrangement is to add a damper as shown in Figure 4.2, where it acts as a bypass. It may not be accurate but is capable of allowing an element of control. MD Figure 4.2 Air-to-Air Heat Recovery with bypass his is only needed to divert the incoming stream, not the exhaust. A good linear type control depends in good damper authority sizing. he tendency with the system is that as the damper opens slightly most of the air tends to go through the damper. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 73 of 113

74 100% Fully Open Damper position 20% 0% Open, Fully closed Flow thro the damper 80% 100 Figure 4.3 Damper position to flow rate Figure 4.3 illustrates a scenario where 20% opening of the damper would result in an 80% flow through the damper. A better control of the incoming and exhaust air would be using a face and bypass arrangement, similar to that shown in Figure 4.4. Figure 4.4 Face and Bypass arrangement Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 74 of 113

75 Chapter 5, Packaged Pressurisation and Filling System Packaged Pressurisation and Filling unit Before describing further the common heating systems, it is important to discuss the controls used to ensure the water systems continue to have adequate water reserves. he system illustrated in Figure 5.1 is used only to fill and pressurise the water in the system. hese systems are available off the shelf as a package plant. Some contain methods to dose the water with different types of chemicals and detergents. he mains water enters the storage tank through the ball valve. he pump pressurises the water but is controlled via the Low/High pressure sensors and the Operational Pressure sensor. here is a bypass loop also which is shown as an alternative to quickly fill the system is the pump is damaged or inoperable due to maintenance. In some cases there is a level sensor in the water storage tank to indicate if low or high water levels are present. he purpose of the expansion vessel is to maintain a constant pressure in the system. Low Pressure sensor Diaphragm ype Expansion Vessel High Pressure sensor Mains water supply PL PH POP Pump (perhaps multistage) But not a circulating pump LSV System to be filled Anti Gravity Loop Figure 5.1 Pressurisation system ypical settings would be: PL set at 1 bar POP set at 2 bar PH set at 4 bar SV set at 5 bar Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 75 of 113

76 Chapter 6, Boilers and Chillers Pumps he flow of water around a heating system is maintained using pumps. he pumps can be arranged in series or in parallel. Both arrangements have different impacts on the flow and total head as shown in Figure 6.1. Pumps in Series Pumps in Parallel Figure 6.1 Boiler and shunt pump Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 76 of 113

77 Central boiler and Chillers Circuits Boiler control. Some pointers to note: (1) Distinguish between high heat inertia boilers and low heat inertia boilers. (2) Distinguish between the fuels used for firing. (3) Distinguish between single boilers and multiple boilers in a sequence. (4) Heat dissipation stats. Cast iron sectional boilers have a very high heat capacity in metal and water content. Small natural gas fired wall hung boilers have a copper heat exchanger and have a very low heat capacity. Fire tube steel boilers have a less cast iron, but still have a massively high inertia. Shunt pumps are required if: (1) Low return waste temp (2) Oil fuel with sulphur content he shunt raises the return water temperature to prevent sulphuric acid from attacking the rear of the boiler. Shunt arrangement is sized to raise the return water temperature by diverting the water around the boiler as shown in Figure 6.2. Boiler Shunt Figure 6.2 shunt pump Boiler and Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 77 of 113

78 Common Header single boiler S.V. Boiler LSV Package pressurisation and filling unit Supply when needed Figure 6.3 Single Boiler and multiple loads Primary pump interlocked with the boiler. his primary pump is sized on the bases of the total simultaneous circuit demand +5%. his would typically result in Boiler _ kw = kg / s (82 71) 4.2 he specific heat capacity for water is kj/kg 4.2 kj/kg and the 82 and 71 are the ring main temperatures in C. More information is available on Frost protection Immersion thermostat in the boiler return is usually set at 4 C and brings on the burner. Air thermostat outside set at 4-8 C at a set OA temperature to protect the building and system from frost damage when the boiler is off. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 78 of 113

79 Multiple boilers in sequence (Parallel connection) S.V. Boiler #1 Boiler #2 Boiler Sequence Controller Figure 6.3 Boilers in Parallel he arrangement is shown in shown in Figure 6.3. Return water temperature is sensed. When the temperature of the return water falls below a set level the second boiler is brought on. ypically set at 71 C if the flow is set at 82 C. If the return temperature is 76 C then it is possible to meet demand with one boiler running. All boilers have their own operation and high level thermostat which maintains the outlet water temperature by controlling the burner. Operating thermostat is set typically at 82 C and the high level thermostat at about 90 C. P B1 B1+B2 In some cases boilers may have on/off two port valves to control the flow of water thro the idle boiler. Care is needed as this will change the pressures as the boiler is the main resistance in the primary circuit. (Remember it is the boiler in parallel not the pump) Q Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 79 of 113

80 Boilers connected in Series Normal direction of flow Manual Bypass valve S.V. Boiler #1 Boiler #2 Boiler Sequence Controller Figure 6.4 Boilers in Series he return water thermostat is set at 71 C and the flow at 82 C. In this arrangement as shown in Figure 6.4 boilers may operate by a sequence controller in the normal way. Alternatively the boiler thermostat can be set differently with Boiler 2 being set at 82 C and Boiler 1 being set at 76 C. his latter method is not favoured as the boiler settings can be easily changed at the dial and the boiler cannot be used in isolation, except with the bypass as shown. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 80 of 113

81 Modular Boilers, Wall Hung type Boiler Sequence Controller Figure 6.5 Modular Boilers Normally used with wall hung gas boilers. Modules fire in sequence in response to load demand. Low inertia system with good energy efficiency. his arrangement also has the advantage that further modules can be added if demand increases in the future. Be careful of the changes in system pressure as a result of the circuits being run in parallel. Advantages: Low inertia Flexibility Small space required Disadvantage Very little heat stored therefore not suitable for high based load applications say hospitals, it is more suited to an office application. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 81 of 113

82 Based load Load Load Base Load Base Load ime High Base Load ime Low Base Load Figure 6.6 Base load he base load refers to the on going minimum load experienced by the system. For example buildings such as hospitals and swimming pools have continuously long periods where the heat in the system needs to be maintained at a high level. he heating of pool water is ongoing 24hr/7 and the heating of the hospital space is 24hr/7. his causes the base load to be high. However office buildings are heated up by the sun during daylight and most of the workers would work 8-7 so the building heating load is not maintained throughout the day. Heating inertia Modern office buildings have insulation fitted to the inside of the building as shown in Figure 6.7 Case III. his means that the office space is the only heating load on the system not the heating of the building itself. However this means that the building will also lose it heat just as quick. his is termed heating Inertia. If the heating is placed on the outside of the building then the walls will have to be heated up before the space really heats up. his takes time and energy. he building will also take longer to cool as the heat has to return to the building from the walls. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 82 of 113

83 Figure 6.7 Heating Inertia Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 83 of 113

84 Chapter 7, Sizing valves for water service Sizing valves for water service In order to size a valve for a water application, the following must be known: he volumetric flow rate through the valve. he differential pressure across the valve. he control valve can be sized to operate at a certain differential pressure by using a graph relating flow rate, pressure drop, and valve flow coefficients. Alternatively, the flow coefficient may be calculated using a formula. Once determined, the flow coefficient is used to select the correct sized valve from the manufacturer's technical data. For liquid flow generally, the formula for K v is shown in Equation 7.1. Equation 7.1 Where: K v = Flow of liquid that will create a pressure drop of 1 bar (m 3 / h bar) = Flow rate (m 3 /h) G = Relative density/specific gravity of the liquid (dimensionless). Note: Relative density is a ratio of the mass of a liquid to the mass of an equal volume of water at 4 C P = Pressure drop across the valve (bar) Sometimes, the volumetric flow rate needs to be determined, using the valve flow coefficient and differential pressure. Rearranging Equation 7.1 gives: For water, G = 1, consequently the equation for water may be simplified to that shown in Equation 7.2. Equation 7.2 Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 84 of 113

85 Example 1 10 m 3 /h of water is pumped around a circuit; determine the pressure drop across a valve with a Kv of 16 by using Equation 7.2: Where: = 10 m 3 /h K v = 16 Equation 7.2 Alternatively, for this example the chart shown in Figure 7.1, may be used. (Note: a more comprehensive water K v chart is shown in Figure 7.2): 1. Enter the chart on the left hand side at 10 m 3 /h. 2. Project a line horizontally to the right until it intersects the K v = 16 (estimated). 3. Project a line vertically downwards and read the pressure drop from the 'X' axis (approximately 40 kpa or 0.4 bar). Note: Before sizing valves for liquid systems, it is necessary to be aware of the characteristics of the system and its constituent apparatus such as pumps. Figure 7.1 Extract from the water Kv chart Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 85 of 113

86 Figure 7.2 Water Kv chart Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 86 of 113

87 Pumps Unlike steam systems, liquid systems require a pump to circulate the liquid. Centrifugal pumps are often used, which have a characteristic curve similar to the one shown in Figure 7.3. Note that as the flow rate increases, the pump discharge pressure falls. Figure 7.3 ypical pump performance curve Circulation system characteristics It is important not only to consider the size of a water control valve, but also the system in which the water circulates; this can have a bearing on which type and size of valve is used, and where it should be positioned within the circuit. As water is circulated through a system, it will incur frictional losses. hese frictional losses may be expressed as pressure loss, and will increase in proportion to the square of the velocity. he flow rate can be calculated through a pipe of constant bore at any other pressure loss by using Equation 7.3, where 1 and 2 must be in the same units, and P 1 and P 2 must be in the same units. 1, 2, P 1 and P 2 are defined below. Where: 1 = Flow rate at pressure loss P 1 Equation = Flow rate at pressure loss P 2 Example 2 It is observed that the flow rate ( 1 ) through a certain sized pipe is 2500 m 3 /h when the pressure loss (P 1 ) is 4 bar. Determine the pressure loss (P 2 ) if the flow rate ( 2 ) were 3500 m 3 /h, using Equation 7.3. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 87 of 113

88 It can be seen that as more liquid is pumped through the same size pipe, the flow rate will increase. On this basis, a system characteristic curve, like the one shown in Figure 6.3.4, can be created using Equation 7.3, where the flowrate increases in accordance to the square law. Figure 7.4 ypical system curve Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 88 of 113

89 Actual performance It can be observed from the pump and system characteristics that as the flow rate and friction increase, the pump provide less pressure. A situation is eventually reached where the pump pressure equals the friction around the circuit, and the flow rate can increase no further. If the pump curve and the system characteristic curve are plotted on the same chart - Figure 7.5, the point at which the pump curve and the system characteristic curve intersect will be the actual performance of the pump/circuit combination. Figure 7.5 ypical system performance curve hree-port valve A three-port valve can be considered as a constant flow rate valve, because, whether it is used to mix or divert, the total flow through the valve remains constant. In applications where such valves are employed, the water circuit will naturally split into two separate loops, constant flow rate and variable flow rate. he simple system shown in Figure 7.6 depicts a mixing valve maintaining a constant flow rate of water through the 'load' circuit. In a heating system, the load circuit refers to the circuit containing the heat emitters, such as radiators in a building. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 89 of 113

90 Figure 7.6 Mixing valve (constant flow rate, variable temperature) he amount of heat emitted from the radiators depends on the temperature of the water flowing through the load circuit, which in turn, depends upon how much water flows into the mixing valve from the boiler, and how much is returned to the mixing valve via the balancing line. It is necessary to fit a balance valve in the balance line. he balance valve is set to maintain the same resistance to flow in the variable flow rate part of the piping network, as illustrated in Figures 7.6 and 7.7. his helps to maintain smooth regulation by the valve as it changes position. In practice, the mixing valve is sometimes designed not to shut port A completely; this ensures that a minimum flow rate will pass through the boiler at all times under the influence of the pump. Alternatively, the boiler may employ a primary circuit, which is also pumped to allow a constant flow of water through the boiler, preventing the boiler from overheating. he simple system shown in Figure 7.7 shows a diverting valve maintaining a constant flow rate of water through the constant flow rate loop. In this system, the load circuit receives a varying flow rate of water depending on the valve position. he temperature of water in the load circuit will be constant, as it receives water from the boiler circuit whatever the valve position. he amount of heat available to Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 90 of 113

91 the radiators depends on the amount of water flowing through the load circuit, which in turn, depends on the degree of opening of the diverting valve. Figure 7.7 Diverting valve (constant temperature in load circuit with variable flow) he effect of not fitting and setting a balance valve can be seen in Figure 7.8. his shows the pump curve and system curve changing with valve position. he two system curves illustrate the difference in pump pressure required between the load circuit P 1 and the bypass circuit P 2, as a result of the lower resistance offered by the balancing circuit, if no balance valve is fitted. If the circuit is not correctly balanced then short-circuiting and starvation of any other sub-circuits (not shown) can result, and the load circuit may be deprived of water. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 91 of 113

92 Figure 7.8 Effect of not fitting a balance valve wo-port Valves When a two-port valve is used on a water system, as the valve closes, flow will decrease and the pressure upstream of the valve will increase. Changes in pump head will occur as the control valve throttles towards a closed position. he effects are illustrated in Figure 7.9. A fall in flow rate not only increases the pump pressure but may also increase the power consumed by the pump. he change in pump pressure may be used as a signal to operate two or more pumps of varying duties, or to provide a signal to variable speed pump drive(s). his enables pumping rates to be matched to demand, saving pumping power costs. wo port control valves are used to control water flow to a process, for example, for steam boiler level control, or to maintain the water level in a feed tank. hey may also be used on heat exchange processes, however, when the two-port valve is closed, the flow of water in the section of pipe preceding the control valve is stopped, creating a 'dead-leg'. he water in the dead-leg may lose temperature to the environment. When the control valve is opened again, the cooler water will enter the heat exchange coils, and disturb the process temperature. o avoid this situation, the control system may include an arrangement to maintain a minimum Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 92 of 113

93 flow via a small bore pipe and adjustable globe valve, which bypass the control valve and load circuit. wo-port valves are used successfully on large heating circuits, where a multitude of valves are incorporated into the overall system. On large systems it is highly unlikely that all the two-port valves are closed at the same time, resulting in an inherent 'self-balancing' characteristic. hese types of systems also tend to use +variable speed pumps that alter their flow characteristics relative to the system load requirements; this assists the self-balancing operation. Figure 7.9 Effect of two-port valve on pump head and pressure Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 93 of 113

94 When selecting a two-port control valve for an application: If a hugely undersized two-port control valve were installed in a system, the pump would use a large amount of energy simply to pass sufficient water through the valve. Assuming sufficient water could be forced through the valve, control would be accurate because even small increments of valve movement would result in changes in flow rate. his means that the entire travel of the valve might be utilised to achieve control. If a hugely oversized two-port control valve were installed in the same system, the energy required from the pump would be reduced, with little pressure drop across the valve in the fully open position. However, the initial valve travel from fully open towards the closed position would have little effect on the flow rate to the process. When the point was reached where control was achieved, the large valve orifice would mean that very small increments of valve travel would have a large effect on flow rate. his could result in erratic control with poor stability and accuracy. A compromise is required, which balances the good control achieved with a small valve against the reduced energy loss from a large valve. he choice of valve will influence the size of pump, and the capital and running costs. It is good practice to consider these parameters, as they will have a bearing on the overall lifetime cost of the system. hese balances can be realised by calculating the 'valve authority' relative to the system in which it is installed. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 94 of 113

95 Valve authority Valve authority may be determined using Equation 7.4. DP1 DP2 Where: Equation 7.4 N = Valve authority P 1 = Pressure drop across a fully open control valve P 2 = Pressure drop across the remainder of the circuit P 1 + P 2 = Pressure drop across the whole circuit Valve authority expresses the ratio between pressure drop across the control valve compared to the total pressure drop across the whole circuit. he value of N should be near to 0.5 (but not greater than), and certainly not lower than 0.2. his will ensure that each increment of valve movement will have an effect on the flow rate without excessively increasing the cost of pumping power. Example A circuit has a total pressure drop ( P 1 + P 2 ) of 125 kpa, which includes the control valve. a) If the control valve must have a valve authority (N) of 0.4, what pressure drop is used to size the valve? b) If the circuit/system flow rate ( ) is 3.61 l/s, what is the required valve K v? Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 95 of 113

96 Part a) Determine the P Equation 7.4 Consequently, a valve P of 50 kpa is used to size the valve, leaving 75 kpa (125 kpa - 50 kpa) for the remainder of the circuit. Part b) Determine the required K V Where: = 3.61 l/s (13m 3 /h) P = 50 kpa (0.5 bar) Equation 7.2 Alternatively, the water K V chart (Figure 7.2) may be used. Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 96 of 113

97 Example A heating coil has a load of 100kW. he coil is supplied with hot water from a boiler at 82 C and which returns to the boiler at 71 C. he coil is controlled by a diverting valve and it has a water pressure drop of 30kPa at the design load. he control valve is required to have an authority of 0.5. Using the data on the attached sheet select a suitable valve size for this application. V [l/min] Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 97 of 113

98 Q = m c where Q is the heat energy put into or taken out of the substance, m is the mass of the substance, c is the specific heat capacity, and is the temperature differential. So Q = 100kW m =? c = at 75 C = = 11 C Rearranging this calculation so we can find m: m = 2.168kg/s Which is m^3/s or 2.22 l/s Pv 0.5 = Pv + 30kPa Pv = 30kPa he flow rate is m^3/s x3600(second to minutes to hours) = 7.992m^3/hr Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 98 of 113

99 V [l/min] Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 99 of 113

100 hree-port control valves and valve authority hree-port control valves are used in either mixing or diverting applications. When selecting a valve for a diverting application: A hugely undersized three-port control valve will incur high pumping costs, and small increments of movement will have an effect on the quantity of liquid directed through each of the discharge ports. A hugely oversized valve will reduce the pumping costs, but valve movement at the beginning, and end, of the valve travel will have minimal effect on the distribution of the liquid. his could result in inaccurate control with large sudden changes in load. An unnecessarily oversized valve will also be more expensive than one adequately sized. he same logic can be applied to mixing applications. Again, the valve authority will provide a compromise between these two extremes. With three-port valves, valve authority is always calculated using P 2 in relation to the circuit with the variable flow rate. Figure 7.10 shows this schematically. Figure 7.10 Valve authority diagrams showing three-port valves Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 100 of 113

101 Note: Because mixing and diverting applications use three-port valves in a 'balanced' circuit, the pressure drop expected over a three-port valve is usually significantly less than with a two-port valve. As a rough guide: A three-port valve will be 'line sized' when based on water travelling at recommended velocities (ypically ranging from 1 m/s at DN25 to 2 m/s at DN150). 10 kpa may be regarded as typical pressure drop across a three-port control valve. Aim for valve authority (N) to be between 0.2 and 0.5, the closer to 0.5 the better. Cavitation and flashing Other symptoms sometimes associated with water flowing through two-port valves are due to 'cavitation' and 'flashing'. Cavitation in liquids Cavitation can occur in valves controlling the flow of liquid if the pressure drop and hence the velocity of the flow is sufficient to cause the local pressure after the valve seat to drop below the vapour pressure of the liquid. his causes vapour bubbles to form. Pressure may then recover further downstream causing vapour bubbles to rapidly collapse. As the bubbles collapse very high local pressures are generated which, if adjacent to metal surfaces can cause damage to the valve trim, the valve body or downstream pipe work. his damage typically has a very rough, porous or sponge-like appearance which is easily recognised. Other effects which may be noticed include noise, vibration and accelerated corrosion due to the repeated removal of protective oxide layers. Cavitation will tend to occur in control valves: On high pressure drop applications, due to the high velocity in the valve seat area causing a local reduction in pressure. Where the downstream pressure is not much higher than the vapour pressure of the liquid. his means that cavitation is more likely with hot liquids and/or low downstream pressure. Cavitation damage is likely to be more severe with larger valves sizes due to the increased power in the flow. Figure 7.11 shows the damage to an impeller due to the Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 101 of 113

102 Figure 7.11 Damage to an impeller due solely to cavitation Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 102 of 113

103 Flashing in liquids Flashing is a similar symptom to cavitation, but occurs when the valve outlet pressure is lower than the vapour pressure condition. Under these conditions, the pressure does not recover in the valve body, and the vapour will continue to flow into the connecting pipe. he vapour pressure will eventually recover in the pipe and the collapsing vapour will cause noise similar to that experienced with cavitation. Flashing will reduce the capacity of the valve due to the throttling effect of the vapour having a larger volume than the water. Figure 7.12 illustrates typical pressure profiles through valves due to the phenomenon of cavitation and flashing. Figure 7.12 Cavitation and flashing through a water control valve Avoiding cavitation It is not always possible to ensure that the pressure drop across a valve and the temperature of the water is such that cavitation will not occur. Under these circumstances, one possible solution is to install a valve with a valve plug and seat especially designed to overcome the problem. Such a set of internals would be classified as an 'anti-cavitation' trim. he anti-cavitation trim consists of the standard equal percentage valve plug operating inside a valve seat fitted with a perforated cage. Normal flow direction is used. he pressure drop is split between the characterised plug and the cage which limits the pressure drop in each stage and hence the lowest pressures occur. he multiple flow paths in the perforated cage also increase turbulence and reduce the pressure recovery in the valve. hese effects both act to prevent cavitation occurring in case of minor cavitation, or to reduce the intensity of Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 103 of 113

104 cavitation in slightly more severe conditions. A typical characterised plug and cage are shown in Figure Figure 7.13 A typical two-port valve anti-cavitation trim he pressure drop is split between the orifice pass area and the cage. In many applications the pressure does not drop below the vapour pressure of the liquid and cavitation is avoided. Figure 7.14 shows how the situation is improved. Fig Cavitation is alleviated by anti-cavitation valve trim Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 104 of 113

105 Figure 7.15 A typical two-port valve with anti-cavitation trim Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 105 of 113

106 Chapter 8, Flow characteristics All control valves have an inherent flow characteristic that defines the relationship between 'valve opening' and flow rate under constant pressure conditions. Please note that 'valve opening' in this context refers to the relative position of the valve plug to its closed position against the valve seat. It does not refer to the orifice pass area. he orifice pass area is sometimes called the 'valve throat' and is the narrowest point between the valve plug and seat through which the fluid passes at any time. For any valve, however it is characterised, the relationship between flow rate and orifice pass area is always directly proportional. Valves of any size or inherent flow characteristic which are subjected to the same volumetric flow rate and differential pressure will have exactly the same orifice pass area. However, different valve characteristics will give different 'valve openings' for the same pass area. Comparing linear and equal percentage valves, a linear valve might have a 25% valve opening for a certain pressure drop and flow rate, whilst an equal percentage valve might have a 65% valve opening for exactly the same conditions. he orifice pass areas will be the same. he physical shape of the plug and seat arrangement, sometimes referred to as the valve 'trim', causes the difference in valve opening between these valves. ypical trim shapes for spindle operated globe valves are compared in Figure 8.1. Figure 8.1 he shape of the trim determines the valve characteristic Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 106 of 113

107 In this utorial, the term 'valve lift' is used to define valve opening, whether the valve is a globe valve (up and down movement of the plug relative to the seat) or a rotary valve (lateral movement of the plug relative to the seat). Rotary valves (for example, ball and butterfly) each have a basic characteristic curve, but altering the details of the ball or butterfly plug may modify this. he inherent flow characteristics of typical globe valves and rotary valves are compared in Figure 8.2. Globe valves may be fitted with plugs of differing shapes, each of which has its own inherent flow/opening characteristic. he three main types available are usually designated: Fast opening. Linear. Equal percentage. Examples of these and their inherent characteristics are shown in Figures 8.2. Figure 8.2 Inherent flow characteristics of typical globe valves and rotary valves Dr.JMcGrory & Dr.BCostello, DI, cont_engineering_dt026_4_building_year_4_section_a_v1.0 Page 107 of 113