MCUES SYSTEM (Maximum Comfortable and Ultimate Energy Saving System) Author: Engr. K.H., Kong is Mechanic Engineer. (IEM member, No: M21065) Bachelor s Degree with Honors with Distinction in Mechanical Engineering.
MCUES SYSTEM TABLE OF CONTENTS TITLE PAGE ABSTRACT 3 1) DEFINITION OF MCUES SYSTEM 4 2) MCUES AD-VENT UNIT 6 3) MCUES AND THE HEAT BALANCE EQUATION 8 4) CASE STUDY ON MCUES HEAT LOAD CALCULATION 9 5) EFFECT AND BENEFITS OF VAT SYSTEM 10 6) MCUES CONTROL SYSTEM 11 7) MCUES EXHAUST AIR SYSTEM 14 8) CASE STUDY ON ENERGY SAVING BY MCUES EXHAUST AIR SYSTEM 15 9) MCUES AIR FLUSHING CYCLE SYSTEM 18 10) MCUES PREDICTIVE AND PREVENTIVE MAINTENANCE 22 11) CASE STUDY AND COST EFFECT ANALYSIS ON MCUES SYSTEM 30 12) VALUE ENGINEERING ON MCUES SYSTEM 38 13) CONCLUSION 40 14) REFERENCES 40 Page 2 of 40
ABSTRACT In this book, a new HVAC design concept is introduce to the readers in enhancing the energy saving while providing a comfortable air-conditioning environment. Due to the increase of the price of energy, and the sources of energy is reducing, a more environmental friendly HVAC design system in introduced. In this book, MCUES system is categorized into 5 major items. There are: 1) MCUES System 2) MCUES Exhaust Air System 3) MCUES Air Flushing Cycle System 4) MCUES Predictive and Preventive Maintenance 5) Case study and cost analysis on MCUES system Chapter 1 to 6 describes the MCUES System design concept. MCUES system is more advance than VAV system where it is a combination of VAV (Variable Air Volume) and VAT (Variable Air Temperature) systems in achieving maximum comfortable and ultimate energy saving system. Chapter 1 introduces MCUES system, chapter 2 introduces Ad-Vent unit which is used in MCUES system. Chapter 3 introduces the MCUES system in technical and chapter 4 discuss about a case study on MCUES technically. Chapter 5 and chapter 6 describe about VAT system in MCUES system. Chapter 7 and chapter 8 describe about the MCUES Exhaust Air System where more than 10% of energy are saved compare to conventional system. Chapter 9 introduces the MCUES Air Flushing Cycle System which is applied on buildings that are not operating for 24 hours. An air flushing cycle is introduced to remove stored heat to reduce the initial cooling load of equipment. Chapter 10 discuss about the MCUES Predictive and Preventive Maintenance. This is a new concept of maintenance method, which is able to reduce the dependency of BMS (Building Management System) of equipment. Prediction on deterioration of equipment can be obtained to prevent the failure of a system. Chapter 11 and chapter 12 discuss case study and cost analysis on MCUES system and MCUES Exhaust Air System compare to conventional system. The equipment cost, materials cost, installation cost and commissioning are reduced. book. Chapter 13 and chapter 14 are the conclusion and reference respectively of this Page 3 of 40
1.0 DEFINITION OF MCUES SYSTEM MCUES is the abbreviation of Maximum Comfortable & Ultimate Energy Saving System. (MCUES is pronounced as Mac-Q-s ) The system is designed to achieve maximum comfortable condition according to individual needs wherever, whenever; and ultimate energy saving without wasting energy on unoccupied space and unnecessary period at anytime, anywhere. It is a new ACMV design concept which evolutes from VAV system with the creation of Variable Speed Drive (VSD) and MCUES Ad-Vent. 1.1 INTRODUCTION In constant-air-volume (CAV) systems, cooling or heating is accomplished by varying the supply air temperature and keeping the volume of the supply air constant. Variable-air volume (VAV) systems accomplish cooling or heating by keeping the air temperature constant and varying the volume of the air supply. MCUES system is the abbreviation of Maximum Comfortable and Ultimate Energy Saving system which is a more advance system than VAV system. It is a combination of VAV and VAT (Variable Air Temperature) systems to achieve energy saving while maintaining the internal air quality of conditioned space. VAT system is double energy saving compare to re-heating system where the supply air temperature is reduced and energy used for reheating is not required. 1.2 CONVENTIONAL VAV SYSTEM Proper design calls for a VAV terminal to be fully open when the zone under its control experiences maximum load. As the zone load increases, the action of the controller reduces flow. This, in turn, increase duct pressure, which is transmitted to the central air handler and produces a reduction in system airflow. This airflow reduction is the major cause for energy savings of VAV system compared to constantvolume systems. It is apparent that variable-speed fan control conserves the most energy. In the past, VAV systems have saved on fan energy but not without countervailing side effect. When 13 C supply air to interior zones is reduced to match internal heat gains, the volume may be reduced below minimum air change rate, leading to air quality complaints. The antidote involving reheat of the minimum supply air volume is also counterproductive for energy reasons. When the 13 C air is Page 4 of 40
supplied to perimeter zones, it must be reheated to room temperature in winter before it is heated further to over come envelope heat loss. The heat input from 13 C to room temperature is parasitic. One way to overcome this is to supply air that is thermostatically controlled for each solar zone. At the least, this implies separate air supply zoning for interior, south-facing perimeter, and other perimeter zones, which can be achieved by compartmented air-handling systems. Another solution that is gaining in popularity is to add a fan to each air terminal. The side-pocket fan in the air terminal would be sized to provide the minimum rate of air circulation. It would be started only when the cold air supply fell below this critical value. Reheating the 13 C air supply to perimeter zones would be avoided by ensuring that perimeter heat would not be activated until the cold air damper was completely shut. Since the addition of fans in the false ceiling adds unpopular maintenance for bearings and filters in occupied spaces, the same effect can be achieved with one central fan for each floor or major zone. The central fan would then recirculate air at a neutral temperature to double-duct type terminals connected jointly to the cold air supply and neutral air supply. However, it adds initial and operating cost on the central fan. Variable-air-volume systems are easy to control, are highly energy efficient and allow fairly good room control. A potential drawback includes the possibility of poor ventilation, particularly under low zone loads. They are suitable for offices, classrooms, and many other applications, and are currently the system of choice for most commercial and institutional buildings in spite of the fact that humidity control under widely varying latent loads is difficult. 1.2.1 FCU-VAV system In FCU-VAV system, every room has a fan coil unit with minimum fresh air supply to each room. The ventilation in the room is governed by the blower fan of FCU. The cooling of each room is accomplished by the cooling coil with chilled water supply. Therefore, it is a good individual VAV system. However, the initial cost is expensive, the resources are not fully utilized, and the operating cost is high with constant air flow blower fan. 1.2.2 Re-heating VAV system In Re-heating VAV system, when the cooling load reduces until the air flow below the required ventilation for good air quality, the re-heat coil will be energized to heat up the supply air so that more air with higher temperature is supplied to the room. However, the initial cost is high due to heating system. The re-heating causes energy wastage, which causes higher operating cost. Permanent static loss across reheat coil causes higher fan capacity and higher operating cost. Furthermore, scheduled maintenance of reheat coil is required. Page 5 of 40
2.0 MCUES AD-VENT UNIT Ad-Vent Unit is a motorized adjustable ventilator applicable in Heating, Ventilating and Air-Conditioning (HVAC) system. An electrical motor or actuator is able to position or rotate the fins or blades to vary the opening area to control the air volume flow through the ventilator. The actuator is either to move the blade of the ventilator, or to adjust the air damper attached to a ventilator, or to control the air flow through air damper of a plenum box attached to a ventilator. Refer to figure 2.1. Incoming supply air enters Ad-Vent through the circular inlet. The circular inlet has a motorized controlled blade to adjust the supply air volume into the plenum box. The actuator is self-feedback controlled by setting the required room temperature through a remote control or wall-mounted controller. 2.1 COMPARISON BETWEEN AD-VENT UNIT AND VAV BOX Figure 2.1 Ad-Vent Unit. Figure 2.2 VAV Box. 2.1.1 Features of MCUES Ad-Vent Unit ceiling mounted consists of built-in self monitoring control system. To control air flowrate according to temperature through built-in temperature sensor controlled through wired controller or remote controller direct from users Noise reduction by noise absorption material in plenum box one time installation, which is final installation easy to be installed through flexible duct easy for maintenance, no access panel required. easy for replacement. Page 6 of 40
2.1.2 Features of Conventional VAV Box ducted type built-in air flow sensor or temperature sensor controlled through wired controller from users noise reduction by noise absorption material in plenum box first fix installation together with ducting inclusive of wiring work, required large ceiling space for installation to achieve uniform air flow transition duct for connection which is higher cost required access panel for maintenance and not easy for maintenance if obstructed by other services not easy for replacement. Flowrate vs Static Loss Coefficient 1100 1000 900 800 When Static Loss Coefficient of Ad-Vent is high, air flowrate is low, where cooling load reduces, and temperature in the room increases. When room occupation reduces, air flowrate reduces, IAQ still maintain for less occupation, energy saved. Flowrate (L/s) 700 600 500 400 300 Size 6 Size 7 Size 8 Size 10 Size 12 The upstream pressure to be maintained is 25Pa. 200 100 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2 Static Loss Coefficient Figure 2.3 The flowrate versus static loss co-efficient performance curve of varies sizes of Ad-Vent. Page 7 of 40
3.0 MCUES AND THE HEAT BALANCE EQUATION Basically, MCUES system evolutes from VAV system where it is a more advance system than VAV system in energy saving. It can achieve good air ventilation even though under low zone loads, without reheating the supply air which is energy wastage. Therefore, it saves more cost compare to VAV system while providing good air quality to zones. 3.1 HEAT BALANCE EQUATION Heat Load = Cooling load of refrigeration unit Q = mc. p T --- (1). where m = air mass flowrate, c p = specific heat of air T = temperature difference between incoming supply air and return air Note: This equation is valid when the latent heat is small. For VAV system using variable speed controller (VSD), when heat load increases, more cold air is supplied to the room and vise versa. When heat load reduces until certain limit where air flowrate is lower than required air change rate, it affects the air quality of zones. Therefore, a central circulation fan is required to overcome this situation. For MCUES system, instead of adding a central circulation fan to the system, which actually adding the initial and operating cost to the system, it achieves good air ventilation by reducing the temperature difference. It is achieved through the supply air temperature control system where a limit switch is added. The temperature limit switch is dominant to the return air temperature control system. During daytime when heat load is high, the temperature limit switch is set at low temperature. When heat load is low during night time, the temperature limit switch is set at high temperature. When supply air achieves the limit temperature, the chilled water supply or refrigerant will be cut off. Supplying higher air temperature to zones causes self-controlled Ad-Vent to throttle wider and allow more supply air flows into the zones to achieve the required room air temperature. Thus, maintaining the air quality of zones. The variation of temperature can be achieved by adding a timer to the limit switch control system where sequencing the variation of limit temperature. Thus, it saves cost on installing a central circulation fan and the noise treatment on the additional fan to the system. For rooms consist of large area of window facing east and west, a perimeter Ad-Vent might required to balance the solar heat gain. A combination of VAV and CAV system is able to counter for large fluctuation heat gain phenomena. CAV system provides the minimum cooling the all day while Ad-Vent is located near window to provide additional cooling to solar heat gain and top up to the CAV system. Page 8 of 40
4.0 CASE STUDY ON MCUES HEAT LOAD CALCULATION In this chapter, calculations are carried out to show that the temperature variations during peak heat load and low heat load and how the internal air quality is maintained at low heat load. Consider an example of a room having 1) area 7.5m x 4m and ceiling height 2.7m 2) occupancy load is 0.1 person/m 2, 3) direct lighting load is 21.5 W/m 2 (35% of total lighting load) 4) total room sensible load is 110 W/m 2 5) total room latent load is 0.66 W/m 2 (negligible) 6) supply air at 7.67 l/sm 2 (10 air change) 7) return air 5.7 l/sm 2 8) fresh air is 2 l/sm 2 9) exhaust air is 1.67 l/sm 2 10) Taking total room sensible load during night time is 40 W/m 2 From the heat balance equation, (1), =. Q m c p T During daytime, the designed heat load is 94.6 W/m 2, air flow per m 2 is 6.4 l/sm 2, therefore, the designed temperature raise is, 110 W/m 2 = 7.67 l/sm 2 x 1.1604 g/l x 1.005 J/g C x T T = 12.3 C Air supply at 13 C, and air return at 25.3 C during daytime at full load. The fresh air supply is 2 l/sm 2, which is 26% of total supply air 7.67 l/sm 2. While for night time, if the heat load is 40 W/m 2, for conventional VAV system, by reducing the air flow rate, thus, 40 W = m l/s x 1.1604 g/l x 1.005 J/g C x 12.3 C m = 2.8 l/sm 2, which is lower than the minimum air flow requirement, 3.1 l/sm 2. The air flow rate per m 2 is smaller than the minimum ventilation required for good air quality, which is 3.1 l/sm 2 (4 air change). Therefore, to achieve good air quality in a room, the supply air temperature is reduced, 40 W = m l/s x 1.1604 g/l x 1.005 J/g C x 10.3 C m = 3.3 l/sm 2, which is higher than the minimum air flow requirement, 3.1 l/m 2. Air supply at 15 C and air return at 25.3 C during night time. The temperature difference is 2 C between daytime and night time. Therefore, the variation of temperature can be achieved by adding a timer to the limit switch control system where sequencing the variation of limit temperature. The fresh air supply is 2 l/sm 2, which is 60.6% of total supply air 3.3 l/sm 2 during night time. The air flow reduces from 7.67 l/sm 2 to 3.3 l/sm 2, which is 57%. Thus, fan energy is saved as compare to CAV system while maintaining the minimum air flow requirement during night time. Page 9 of 40
5.0 EFFECT AND BENEFITS OF VAT SYSTEM By increasing the supply air temperature, the humidity will increase. Environment with higher humidity can reduce sickness and produce good effect such as: 1) Moisturizing skin and lips to prevent dry skin sickness. 2) Help to prevent dry eyes sickness. 3) Moisturizing throat and brochures which enhance air transfer through absorption. 4) Prevent accumulation of static charges. 5) Moisture absorbed in fabric during night time can help to cool the room faster through evaporation during day time, due to the high latent heat of water, which is high heat transfer rate to air flow rate. 6) Flushing effect. High humidity air has better performance in cleaning odors and dust which reduce the air contaminant and enhance air quality. 7) Occupants generate least heat and sweat during night time when sleeping. Thus, stuffiness will not happen with slightly higher humidity by increasing 2 C of supply air temperature. Page 10 of 40
6.0 MCUES CONTROL SYSTEM Figure 6.1 shows the typical control system on AHU. The blower fan is monitored by VSD to maintain the discharge air pressure of the ducting system. A discharge air pressure sensor feedbacks the signal to VSD so that the air pressure can be maintained. For temperature control, 2 temperature sensors are required. There are the return air temperature sensor and discharge air temperature sensor. Discharge air temperature sensor is dominant to return air temperature sensor. When the supply air temperature is lower than set temperature, the chilled water supply will stop although the return air temperature is high. The discharge air temperature is set by timer. Figure 6.1 - Control Schematic for MCUES Air Handling Unit. Figure 6.2 shows the control schematic for MCUES Ad-vent unit. Ad-vent is used to vary the air flow into the air-conditioned room. A temperature sensor will feedback the signal to Ad-vent. If the room temperature is lower than the set temperature, the air flow will be reduced and vise versa. The room temperature sensor is determined by the user. The suitable Ad-Vent size for the above design is size 10. It is suggested that before the customers enter to the room, a short flushing of supply air is recommended to clean the room with full throttle of Ad-Vent. Page 11 of 40
Figure 6.2 Control Schematic for MCUES Ad-vent unit. 6.1 VAT OPERATION METHOD To achieve good air quality with variations of supply air temperature can be achieve as follows as an example: 1) From 12 p.m. to 6 p.m., air is supplied at 13 C 2) From 6 p.m. to 12 a.m., air is supplied at 14 C 3) From 12 a.m. to 6 a.m., air is supplied at 15 C 4) From 6 a.m. to 12 p.m., air is supplied at 14 C 5) Repeat The supply air temperature limit switch acts as an AND gate to the return air temperature sensor. The control condition is described as follows: 1) If the return and supply air temperature higher than set temperature, the solenoid or modulating valve will open. 2) If the return and supply air temperature lower than set temperature, the valve will shut off. 3) If the return air temperature higher than set temperature while supply air temperature lower than set temperature, the valve will shut off. This phenomenon seldom happen because the return air of each room is set by individual ad-vent unit. It might happen if partial of MCUES system using CAV design. 4) If the minimal cooling still lower than above calculation, 3 C of temperature drop is considered with 7 steps of temperature variation sequence. Page 12 of 40
6.2 PSYCHROMETRIC CHART Figure 6.3 Psychrometric Chart for conventional and MCUES system. (A off coil air, B return plenum air, SA Supply Air, RA Return Air, MA Mixed Air, and FA Fresh Air) The black line in figure 6.3 shows the psychrometric cycle for conventional ACMV system. While the blue line represents the MCUES system, where the inlet air temperature is reduced from 13 C to 15 C. The humidity still maintain at comfortable stage. The hatched region shows the reduction of cooling load by reducing the supply air temperature. The reduce of air flow rate further saves energy on fan and cooling load, while maintaining the internal air quality (IAQ). Page 13 of 40
7.0 MCUES EXHAUST AIR SYSTEM 7.1 INTRODUCTION For conventional toilet exhaust system, ducted fan ventilated system is used. While the return air system is free return system where using ceiling plenum as return plenum. Large lighting heat load are collected by return air. For MCUES exhaust air system, the toilet exhaust system is using ceiling plenum as exhaust plenum while return air system using insulated air ducted return system. By using MCUES exhaust air system, heat generated by lighting in ceiling plenum can be removed through toilet free exhaust air system and thus reducing cooling load by return air. Furthermore, the initial cost of MCUES exhaust air system can be minimized by combining the smoke extract system and toilet exhaust system through dual-fan smoke extract riser system or through VSD control of smoke extract fan. Moreover, it can achieve better smoke extract performance through distributed exhaust grilles during fire. For hotels or hospitals designs, where the toilet exhaust air plays a main role in ventilation system. The toilet exhaust air can significantly reduce the cooling load by removing the ceiling plenum heat generated by lighting. The cooling load can be further reduced by insulated ducted return air system, insulated ceiling and foamed soffit of slab. The warm toilet exhaust air can be used as good sources for regeneration of desiccants. 7.2 SUPPLEMENTARY SUPPORTING DESIGN To achieve MCUES exhaust air system, the ceiling air tightness is important. This can be achieved by using plaster board ceiling. However, fire mode dependant motorized damper or pressure relief damper with louvers must be installed to perform smoke extraction during fire. Pressure relief damper is a device where during normal operating period, where the pressure of the ceiling plenum and air-conditioned space below ceiling is within a small fluctuative difference. During fire mode, when the smoke extract system is energized, the ceiling plenum pressure reduces, the pressure relief damper is flipped open to allow for smoke extraction. Smoke extract louvers are distributed within a zone. Ceiling fixtures such as lighting are air tight type. Page 14 of 40
8.0 CASE STUDY ON ENERGY SAVING BY MCUES EXHAUST AIR SYSTEM Calculations are carried out to compare the conventional ducted exhaust and free exhaust air system to show that energy is saved with free exhaust air system. It is because free exhaust air carried more heat at higher temperature and discharge out from the building. Therefore, it reduces the heat carried by the ducted return air where less cooling of return air is required. Consider a zone with ceiling plenum as shown in figure 8.1 below: Figure 8.1 Heat Balance in Ceiling Plenum. 8.1 HEAT BALANCE EQUATION Lighting heat generated = heat transfer through slab,q1 + ceiling,q2 + exhaust air,q3 + heat transfer to supply and return air duct. Q = Q1 + Q2 + Q3 + Q T 1 = RS 1 + R C. + mc p D ( TC TR ) + QD Consider 1 C temperature raise in 50mm fiber glass insulated supply and return air duct and the four-face duct surface area is 50% of the floor area, the heat transfers to duct is 1 C x 0.69 W/ Cm 2 x 0.5 = 0.345 W/m 2 40 W/m 2 = (2.23 + 0.65 + 1.17) W/ Cm 2 x (T C 23 C) + 0.345 W/m 2 (T C 23 C) = 9.8 C T C = 32.8 C Page 15 of 40
Heat removed by exhaust air, Q3 = 1.17 W/ Cm 2 x 9.8 C = 11.5 W/m 2 / 40 W/m 2 = 28.75% of ceiling plenum lighting load Exhaust air is able to remove 28.75% lighting load from ceiling plenum, which saves energy in the interior area of building and during evening. In conventional free return air system, 25% fresh air, where exhaust air is 70% of 25% fresh air and exfiltration is 30% of 25% fresh air. The lighting load distribution in general, the supply air collected 63.6% (35% direct lighting load and 44% of 65% indirect load through ceiling and floor); free return air collected 35.4% (54.5% of 65% ceiling plenum), and the bare ducted exhaust air cooling is roughly 1% (1.5% of 65% ceiling plenum lighting load) with 1 C temperature raise. By using MCUES exhaust air system, the insulated ducted return air load is negligible, supply air collected 81.3% (35% direct lighting load and 71.25% of 65% indirect lighting load), while exhaust air collected 18.7% (28.75% of 65% indirect ceiling plenum lighting load). In conclusion, MCUES exhaust air system removes 18.7%-1% = 17.7% of total lighting load and energy is saved. For a design with 65% lighting load of total heat load, the total cooling load reduces 11.5% (17.7% of 65% lighting load). If 25mm thickness polyurethane foam is sprayed on the soffit of slab, the overall conductivity of floor slab becomes 0.68 W/ Cm 2. The ceiling temperature, Tc raise to 38.9 C. The heat removed by exhaust air is 46.5% of ceiling plenum lighting load, 30.2% of total lighting load (46.5% of 65% indirect ceiling plenum lighting load). Total cooling load reduces 19.6% (30.2% of 65% lighting load). Anyhow, in actual situation, small portion of the heat will be absorbed by other services sharing the same ceiling plenum such as hydraulic, chilled water pipes and etcetera. Furthermore, the lighting load will reduce depending on the diversity factors because not all the lights are switched on all the time. 8.2 ADVANTAGES AND DISADVANTAGES OF MCUES EXHAUST AIR SYSTEM 8.2.1 Advantages of MCUES Exhaust Air System 1) Saves energy. 2) Longer life span of filter. 8.2.2 Disadvantages of MCUES exhaust air system 1) Require insulated ducted return air system. 2) Require air tight ceiling. 3) Require air tight ceiling fixtures. 4) Require smoke extract for smoke extract system. 5) Require ceiling insulation. Page 16 of 40
8.3 PSYCHROMETRIC CHART Figure 8.2 Psychrometric chart for conventional ACMV system and MCUES Exhaust Air system. (A off coil air, B return plenum air, SA Supply Air, RA Return Air, EA Exhaust Air, MA Mixed Air, FA Fresh Air) The black line in figure 8.2 shows the psychrometric chart for the conventional ACMV system and the blue line represents the MCUES Exhaust Air System. For black line, from MA to A, is the total cooling load (from on coil to off coil). The hatched region shows the reduction of cooling load where heat is removed by exhaust air. The exhaust air temperature is the same as the return air temperature. The on coil air temperature, B reduced and shifted to C, thus less cooling is required Note that the return air portion, from RA to B, is reduced by roughly 20% of the total cooling load. Page 17 of 40
9.0 MCUES AIR FLUSHING SYSTEM MCUES Air Flushing System is a new ACMV design system which to save energy and provide good internal air quality to residents within an air-conditioned building. It is most suitable for buildings that are not 24 hours operated such as office and commercial buildings. For office buildings, general operating hours are 10 hours from 8 a.m. to 6 p.m. For commercial building such as shopping mall, general operating hours are 12 hours from 10 a.m. to 10 p.m. 9.1 CONVENTIONAL ACMV DESIGN SYSTEM For all buildings, there is storage of heat in building structures. A large portion of radiant heat such as solar heat, lights and etcetera do not become an instantaneous load on the cooling equipment, because it must be strike on a solid surface and be absorbed before becoming a load on equipment. Therefore, there is a lag time for the radiant heat becomes load on equipment as shown in figure 9.1. The portion of radiant heat being stored depends on the ratio of the resistance to heat flow into the air film. However, as this process of absorbing heat continues, the material becomes warmer and less capable of storing more heat and will leak to the surroundings. Therefore, cooling load is required to cool the leakage. The interaction between instantaneous heat gain and cooling load of equipment is shown in figure 9.1. When instantaneous heat gain reduces drastically, the cooling load continues to remove the stored heat in building structures until heat balance achieved and the cycle repeats. Figure 9.1 Interaction between actual cooling load and solar heat gain through glass for average construction, with 24 hours operating equipment. Page 18 of 40
Figure 9.2 shows the interaction between 12-hour operating equipment actual cooling load and solar heat gain through glass for average construction. The cooling load is higher during daytime as compare to 24-hour operating equipment. Furthermore, the pull down load when begin energizing the equipment is required, to pull down the remaining stored heat in the structure building. In general, the cooling load for 24 hours operating and 12 hours operating cycles are the same, which to remove all the solar heat gain during daytime. Figure 9.2 Interaction between actual cooling load and solar heat gain through glass for average construction, with 12 hours operating equipment. 9.2 MCUES AIR FLUSHING SYSTEM For MCUES Air Flushing System, the 12 hours operating cycle is used as shown in figure 9.3. From time 5 to 6 a.m., outside air which at the lowest temperature among the day, is drawn into the building. The low temperature outside air is used to remove the stored heat in the structure building. Also, outside air is used to flush away the contaminated air in the building and bring in 100% fresh air to the building. The fresh air is drawn in with only fan operating. The pull down load is reduced by flushing the air-conditioned space with outside air. Therefore, the starting load of air-conditioning system is lower and the fan speed is lower if VAV system is used. Thus, reduce cooling load and achieve energy saving with MCUES air flushing system. Detail study on the heat removed by air flushing is required for the actual energy saving. Page 19 of 40
Figure 9.3 Interaction between actual cooling load and solar heat gain through glass for average construction, with 12 hours operating equipment for MCUES air flushing cycle. On top of that, further energy saving can be achieved by reducing the fresh air intake. For an office building design features as follow: A) Occupancy = 11.6 m 2 / person B) Fresh air = 11.8 L / s person C) Therefore, fresh air required = 1 L / s m 2 D) Supply air = 8.6 L / s m 2 E) Percentage of fresh air over supply air = 11.6% For an office space with 2.7 m height, the 100% fresh air contained in the building after flushing is equivalent to 45 minutes of 11.6% fresh air supply. By terminating fresh air supply for 45 minutes during peak outside air temperature (between 1 to 2 p.m.) can achieve reduction of cooling load. Thus, more energy is saved. Furthermore, for the first and the last hour of cooling (7 to 8 a.m. and 6 to 7 p.m.), fresh air can be terminated due to low occupancy load. It further saves energy. For a simplified rough estimation on energy saving, for a 12-hour operating system, consider 3 hours are without outside air supply, which is 25% of the operating period. For conventional ACMV system with 12 hours operating equipment, the total cooling of supply air is 103.2 L hr / s m 2, (8.6 L/s m 2 x 12 hrs), 12 L hr / s m 2 (11.6%) of the load is used to cool outside air. With the 3 hours reduction, the amount of cooled outside air is 9 L hr / s m 2 (75% of outside air). Therefore the energy saved is roughly 3%. Page 20 of 40
9.3 PSYCHROMETRIC CHART FOR MCUES AIR FLUSHING SYSTEM Figure 9.4 shows the psychrometric chart for conventional ACMV system and MCUES air flushing system. The black line shows the conventional ACMV operating cycle. When fresh air is not supplying to the system, the blue line is the actual cooling load required. The hatched region in figure 9.4 represents the reduction of cooling load without outside air supply. Figure 9.4 Psychrometric chart for conventional ACMV system and MCUES Air Flushing Cycle system. (A off coil air, B on coil air, SA Supply Air, RA Return Air, MA Mixed Air, FA Fresh Air) Page 21 of 40
10.0 MCUES PREDICTIVE AND PREVENTIVE MAINTENANCE MCUES Predictive and Preventive Maintenance (PPM) is a new methodology that able to predict and perform preventive maintenance on equipment with less dependency on BMS. In conventional Building Management System (BMS), a lot of sensors are used to check and monitor on the equipment status, such as filters and fans. The sensors are costly, unreliable, required high maintenance and commissioning. PPM is able to reduce the dependency on BMS. However, PPM is only applicable to pressure dependant VAV system, such as MCUES system, which using variable speed controller (VSD) to achieve constant supply pressure. By varying the frequency through VSD, the air flow rate can be varied. Therefore, from the frequency status shown by VSD, it can be used to predict the potential failure or weakness of the system. It is similar to checking the pulsation on human body to predict the potential sickness on a patient. Initial simulation of the system is required as a base to predict precisely and to prevent failures of system. 10.1 FAN PERFORMANCE CURVE FORWARD CURVE FAN 3P, VARIABLE SPEED DRIVE (VSD) OPERATED MEDIUM FREQUENCY HIGH FREQUENCY POWER (W) FREQUENCY (Hz) STATIC PRESSURE (Pa) LOW FREQUENCY FLOW RATE, Q Figure 10.1 Fan performance curve for a 3 phase forward curve fan which operated by variable speed drive (VSD). Page 22 of 40
When VSD is used to monitor the blower fan of AHU, the characteristic of the fan is shown in figure 10.1. For a fixed fabricated fan, the density of air and airconditioning system (no changes in ductwork or damper position) are held constant, from the graph shown in figure 10.1, it is noted that the fan laws: A) Q varies with RPM or Hz. B) Static Pressure varies with RPM 2 or Hz 2 C) Power varies with RPM 3 or Hz 3 10.2 CONSTANT PRESSURE WITH VARIOUS FLOW RATE For a constant system static loss (no changes in ductwork or damper position) and to maintain constant system supply pressure, the air flow rate is varied by frequency as shown in figure 10.2. In VAV system, when cooling load increases, the air flow rate increases by increasing the rotation of fan, while maintaining the supply pressure. The air flow rate increases from Q1 to Q2 by increasing the frequency from F1 to F2 and the power consumption increases from P1 to P2. Note that the power consumption is proportioned to Q 3, by reducing the flow rate in VAV system, the energy saved is significant. FORWARD CURVE FAN 3P, VARIABLE SPEED DRIVE (VSD) OPERATED HIGH FREQUENCY MEDIUM FREQUENCY F2 SP1 LOW FREQUENCY F1 P2 POWER (W) FREQUENCY (Hz) STATIC PRESSURE (Pa) P1 FLOW RATE, Q Q1 Q2 Figure 10.2 Supply air pressure is kept constant. By varying fan rotation, the air flow rate is changed. Page 23 of 40
10.3 CONSTANT FLOWRATE WITH VARIOUS STATIC PRESSURE In the other hand, if the air flow rate is kept constant but the system static loss increase, VSD will increase the fan speed to achieve the same required flow rate. The phenomenon is shown in figure 10.3. When the system static increases, from SP 1 to SP 2, the fan frequency increased from F1 to F2 and power consumption increases from P1 to P2. FORWARD CURVE FAN 3P, VARIABLE SPEED DRIVE (VSD) OPERATED HIGH FREQUENCY SP2 MEDIUM FREQUENCY F2 SP1 LOW FREQUENCY F1 P2 POWER (W) FREQUENCY (Hz) STATIC PRESSURE (Pa) P1 Q1 FLOW RATE, Q Figure 10.3 Supply air flow rate is kept constant. By varying fan rotation, the static pressure is changed. In short, from the expression above, we can conclude that if the static pressure is held constant, the air flow rate increases with the increase of frequency, which is VAV system. If the air flow rate is held constant, the static pressure increases with the increase of frequency, which forms the basis of Predictive and Preventive Maintenance (PPM) methodology. Page 24 of 40
10.4 PRELIMINARY UNDERSTANDING OF MCUES PPM P1 A 1 2 P2 B 3 4 P3 Figure 10.4 Relationship of fan curve and pressure sensor at upstream and downstream of sensor. (P1 pressure at upstream, P2 pressure sensor, P3 pressure at downstream, A damper A, B damper B) In this chapter, we study the variation of fan curve related to the fluctuation of pressure along the ducting by maintaining the pressure at location P2. The Bernoulli equation relates to the above simple air system is as follow: 2 P1 K A Q = P2 eq. 10.1 2 P2 K B Q = P3 eq. 10.2 where K A and K B are the static loss coefficient across damper A and B respectively. 10.4.1 Variation of Downstream Pressure FORWARD CURVE FAN 3P, VARIABLE SPEED DRIVE (VSD) OPERATED HIGH FREQUENCY MEDIUM FREQUENCY SP1 LOW FREQUENCY C FC B FA A POWER (W) FREQUENCY (Hz) STATIC PRESSURE (Pa) D FLOW RATE, Q Q2 Q1 Figure 10.5 Changes of fan curve to the variation of downstream pressure. Page 25 of 40
First, we look at the effect of downstream pressure to the changes of fan curve. As shown in figure 10.5 above, when damper B is at position 4, the point A shows the condition, where fan is running at frequency F A at flow rate Q1. From eq. 10.2, if damper B moves from location 4 to 3, the value K B will increase and the pressure at P2 will increase. In figure 10.5, the fan curve shift from point A to B, where the static pressure increase and the air flow reduces from Q1 to Q2. To maintain the pressure at P2, the VSD received signal from pressure sensor and reduce the fan frequency from F A to F C, where from point B to point C as shown in figure 10.5. Therefore, energy is saved. When the heat gain increases, damper B will open from position 3 back to 4. Again, the static loss coefficient K B reduces, and the pressure at P2 reduces, as shown in point D in figure 10.5. Thus the air flow rate increases from Q2 to Q1. To maintain the pressure at P2, the frequency increases from F C to F A and back to the original condition. 10.4.2 Variation of Upstream Pressure FORWARD CURVE FAN 3P, VARIABLE SPEED DRIVE (VSD) OPERATED HIGH FREQUENCY MEDIUM FREQUENCY SP1' SP1 LOW FREQUENCY B FA A C FC D POWER (W) FREQUENCY (Hz) STATIC PRESSURE (Pa) Q2 Q1 Q3 FLOW RATE, Q Figure 10.6 - Changes of fan curve to the variation of upstream pressure. Page 26 of 40
Now, we look at the effect of upstream pressure to the changes of fan curve. As shown in figure 10.6, when damper A is at position 2, the point A shows the condition, where fan is running at frequency F A at flow rate Q1. From eq. 10.1, if damper A moves from location 2 to 1, the value K A will increase and the pressure at P1 increase and P2 decrease. In figure 10.6, the fan curve shift from point A to B, where the static pressure increase and the air flow reduces from Q1 to Q2. To maintain the pressure at P2, the VSD received signal from pressure sensor and increase the fan frequency from F A to F C, where from point B to point C as shown in figure 10.6. When air flow increase from Q2 to Q1, the static pressure P2 is maintained as indicated by eq. 10.2. When damper A open from position 1 back to 2. Again, the static loss coefficient K A reduces, and the pressure at P2 increases, as shown in point D in figure 10.6. Thus the air flow rate increases from Q1 to Q3. To maintain the pressure at P2, the frequency reduces from F C to F A and back to the original condition point A. The air flow reduces from Q3 to Q1 and the static pressure P2 is maintained as shown by eq. 10.2. 10.4.3 Pressure Variation of AHU P1 FILTER FAN T P2 Q P3 AD-VENT Figure 10.7 Air Handling Unit and the ducting system. (P1 static pressure at the return plenum, P2 discharge pressure, and P3 static pressure at the outlet grilles of duct work system. T temperature sensor and Q air flow sensor) Figure 10.7 above is a simplified AHU, which shows the actual situation of figure 10.4 where the damper A is represented by the static loss at upstream such as filter, cooling coil and etcetera. Damper B is represented by the static variation along ductwork and terminal unit. P2 is the pressure sensor where the discharge pressure is held constant. When filter is getting clogged, the upstream static loss increases, the discharge pressure (P2) is maintained. Therefore, fan speed needs to be increased to achieve the same performance. When filter is replaced, the fan speed reduces back to the original condition. For AHU downstream static pressure, when heat gain reduces, Ad-vent will close up, static loss increases. Thus, fan speed reduces to reduce the air flow to the terminal units where P2 is kept constant. When heat gain increases, Ad-vent open and fan speed increases to increase the air flow to terminal units. Page 27 of 40
10.5 MCUES PREDICTIVE AND PREVENTIVE MAINTENANCE METHODOLOGY FORWARD CURVE FAN 3P, VARIABLE SPEED DRIVE (VSD) OPERATED HIGH FREQUENCY SP2 MEDIUM FREQUENCY E F FF SP1' SP1 LOW FREQUENCY C A FB FD B D P F POWER (W) FREQUENCY (Hz) STATIC PRESSURE (Pa) F A P A P B Q1 Q2 FLOW RATE, Q Figure 10.8 The relationship between fan curve and MCUES PPM When an ACMV system is ready for operation, a simulation of the system is required as the base of the MCUES PPM. First simulation is done by putting in a new filter, then, by increasing the flow rate and recording the data of frequency and power, we can produce a fan performance graph as shown in figure 10.8. For example, the data will produce line A-B. Where at minimum air flow, Q1, the fan runs at lowest frequency, and at maximum air flow. Q2, the fun runs at medium frequency. Second simulation is repeated with a dirty filter and the line E-F will be obtained. Where usually, SP2 = SP1 + 200Pa (static loss of filter). Where the fan runs from medium frequency to high frequency curve with minimum and maximum air flow rate from Q1 to Q2 respectively. Page 28 of 40
During design stage, the static calculations always include the static loss of filter, therefore, in many system such as CAV system, fan is always runs at high frequency (50Hz), which causes energy wastage. Furthermore, adjusting the air damper to high static loss to achieve the initial air flow requirement will cause high static loss across the duct work. When filter clogged up, the air flow in the system to terminal units reduces and causes insufficient air ventilation to the system. Usually, static loss consideration across filter can be 20% to 30% of the total static loss calculation. Therefore, when filter clogged up, it will significant reduce the air flow in the system. After the simulation is done and the fan performance curve is produced, the MCUES system begins operation by maintaining the discharge pressure at SP1 as shown in figure 10.8. At maximum flow rate, Q2, with the fan running at frequency F B shown in the display screen of VSD. When the filter is getting clog, to maintain the discharge pressure, the fan rotation will increase from time to time, from F B to F D as indicated by VSD and slowly to F F at the maximum flow rate, Q2. When F F is achieved at the same flow rate, which means the filter needs to be replaced. Or at any flow rate, if the line E-F is achieved, the filter needs to be replaced. Line SP1 and SP2 show that the system static pressure increases to maintain the discharge pressure due to clogging of filter. As mentioned, from time to time, by noticing the sequence of frequency change from line A-B to line E-F, we can predict the life span of filter and preventive maintenance can be done to prevent system failure. In certain situation that if filter has been replaced, but the system performance offset higher from the medium frequency fan curve, (dashed blue line), or the system running on line C-D with new filter, this means there are others sickness occurred on the system, such as bio-film build up on cooling coil, deterioration of bearing or other parts of the system. All the sickness can be developed from time to time by studying the changes of frequency of the system. Or by simulation of sickness to produce the relation of frequency to air flow volume. Sickness such as deterioration of fan, bearing can be predicted and prevention can be done before system failure. Which is similar to the difference of human pulsation represents different kind of sickness. Further study on the power consumption can be done to find out the costenergy relationship of the system. For example, at certain stage, the clogged filter can be continued to use with running at higher frequency. However, system running at higher frequency required more power consumption, which causes more expenses. At certain point where is the breakeven between the filter cost and power consumption cost, the filter needs to be replaced. Applying MCUES Predictive and Preventive Maintenance methodology, the pressure differential across filters and AHU can be saved and reduces the initial and maintenance cost of the system. Furthermore, many sickness of the system can be predicted from MCUES Predictive and Preventive Maintenance. Page 29 of 40
11.0 CASE STUDY AND COST EFFECT ANALYSIS ON MCUES SYSTEM Under this session, 2 systems are studied. There are: 1) Case Study and Cost Effect Analysis on MCUES System. 2) Case Study and Cost Effect Analysis on MCUES Exhaust Air System. The case study in based on the ACMV design of a zone of a hospital consists of floor area 40m x 19m, where MCUES system replaces the FCU-VAV systems. The zone of hospital consists of 10 single rooms and 1 suite room, 1 manager room, 1 staff meeting room, 1 staff station, 1 staff office, 1 interview room, 1 supervised children play area, 1 equipment room, 1 store room, 1 pantry, 1 dirty utility, 1 clean utility, 1 staff W.C. and corridor. The ceiling of all the rooms are plasterboard, only corridor is using 2 x 4 perforated metal sheet ceiling. Ceiling is insulated with fiber glass insulation. The ACMV system within the zone consists of supply air system (SAD), fresh air supply system (PAD), exhaust air system (E/D), free return air system, smoke extract system, chilled water system, and condensate drain system. There are 2 risers which are duct riser and pipe riser. The PAD, E/D and smoke extract systems are from duct riser, while chilled water piping is from pipe riser. 11.1 CASE STUDY AND COST EFFECT ANALYSIS ON MCUES SYSTEM The design criteria are as follows: 1) Total Supply air to zone is 4545 L/s 2) Total Fresh air to zone is 1150 L/s (25% fresh air) 3) Total Return air from zone is 3395 L/s 4) Total Exhaust air from zone is 805 L/s 5) Total Exfiltration from zone is 345 L/s The detail design criterion of the rooms is described in chapter 4.0 (Case Study on MCUES Heat Load Calculation). 11.1.1 The Conventional ACMV Design System The conventional ACMV Design System for the above hospital using FCU- VAV system. A constant fresh air supply is distributed to each FCU. Cooling is achieved by chilled water supply to each FCU, then cold supply air is distributed to their destinies through insulated supply duct. Each FCU consists of BMS status monitoring. Then, air is circulated back to each FCU through free return air system. For rooms, bare ducted return air system is used. The walls between rooms and corridor are full height wall. The detail design is shown in figure 11.1 below. Page 30 of 40
Figure 11.1 Conventional ACMV Design System for a zone of a hospital. Page 31 of 40
11.1.2 The MCUES System MCUES system for the above hospital is using a common special AHU system with BMS control. A constant fresh air supply is delivered to AHU from fresh air riser. Cooling is achieved by chilled water supply to AHU, then cold supply air is distributed to Ad-Vent unit at strategic locations through insulated supply air duct. The air is returned through free return system. The walls between rooms and corridor are ceiling height wall. The return air is then mixed with fresh air in the return plenum of AHU. During full operation, the minimum fresh air in the supply air is 25%. The fresh air percentage will increase when the return air reduces due to low heating load. Figure 11.2 shows the detail design of MCUES system. 11.1.3 Advantages and Disadvantages of MCUES System 11.1.3.1 Advantages of MCUES System Less FCUs, filters, chilled water piping, silencers for each room. Less replacement parts and less and easier for maintenance. Less labour cost, manpower and fast installation. Reduce installation and T&C time. Less cabling and switchboard. Less BMS. Reduce noise generated by the individual FCU blower fan. More flexibility for architect room design. Save energy, money and time. Reduce initial installation cost. Low operating cost. Create comfortable working environment and increase productivity. Routine maintenance can be performed without disturbing the system operation through special but common design of AHU. Flushing of rooms when air humidity is high, cleaning away odors and air contaminants. For MCUES system in a Hospital, all supply air is treated, distilled, and filtered by installing UV lights in the AHU or return air duct. So that clean air is supplied into the room all the time. While for conventional ACMV design system, bacteria, viruses and germs released from patient is accumulated in the room and stayed in the filter and FCU, where the germs might infect other patients. One way air flow direction from inside to outside to prevent germs going in to the bed and also to prevent germs bringing in by visitors. 11.1.3.2 Disadvantages of MCUES System Bigger FCU / AHU Bigger Ducting If the AHU fan broke down, the whole zone is affected. Page 32 of 40
Figure 11.2 MCUES ACMV Design System for a zone of a hospital. Page 33 of 40
11.1.4 Cost and Time Effect Analysis of MCUES System Comparison between conventional FCU-VAV system and MCUES system is shown below: Item Description Conventional MCUES Difference % reduce 1 Estimated pricing RM 129,000 RM 110,000 RM 19,000 15% 2 Estimated installation 5 men 30 days 5 men 18 days 12 days 40% period 3 Estimated T&C period 4 men 5 days 4 men 2 days 3 days 60% MCUES system reduces the initial installation cost regardless of the operating cost. It also reduces the time for installation, testing and commissioning. 11.2 CASE STUDY AND COST EFFECT ANALYSIS ON MCUES EXHAUST AIR SYSTEM Using the same example from chapter 11.1, the design criteria are as follows: 1) Total Supply air to zone is 4545 L/s 2) Total Fresh air to zone is 1150 L/s (25% fresh air) 3) Total Return air from zone is 3395 L/s 4) Total Exhaust air from zone is 805 L/s 5) Total Infiltration from zone is 345 L/s The detail design criterion of the MCUES exhaust air system is described in chapter 8.0 (Case Study on Energy Saving by MCUES Exhaust Air System). The zone of hospital consists of 10 single rooms and 1 suite room, 1 manager room, 1 staff meeting room, 1 staff station, 1 staff office, 1 interview room, 1 supervised children play area, 1 equipment room, 1 store room, 1 pantry, 1 dirty utility, 1 clean utility, 1 staff W.C. and corridor. The ceiling of all the rooms and corridor are plasterboard. Ceiling is insulated with fiber glass insulation. From MCUES system in chapter 11.1.2 (The MCUES System), we replace the free return air system to insulated ducted system with the free exhaust air system through smoke extract system. The smoke extract explosion proof is drive by Variable Speed Drive (VSD). At normal condition, the fan runs at low frequency while at high frequency during fire. During fire, smoke is extract through smoke extract air grille, which shown as SEG. Figure 11.3 shows the detail design of MCUES Exhaust Air System. It is modified from figure 11.2 MCUES ACMV Design System. Page 34 of 40