College of Technological Studies Department of Power & Refrigeration Technology Course Contents Course Designation: Air Conditioning Control systems Course No. : 272 0463 Credit Hrs.: 3 Lecture Hrs.: 2 Lab Hrs.: 2 Lecturer: Eng. Raad Alsaleh Course Topics i. Background ii. Compressor Control System Reciprocating Compressors Control Capacity, safety and operation controls. Centrifugal Compressor Control Capacity, Surge, and safety control. iii. Air handling systems control Outside Air Control Minimum Outside Air, Economy Cycle Outside Air, Enthalpy Control, Static pressure Control, Air Stratification Heating Coil Control Preheat of outside or mixed air, Normal heating, Reheat cooling Coil Control Direct Expansion (DX) coils, Chilled water coils Humidity Control humidifying, Dehumidifying iv. Applications Single Zone, Multi zone and dual-duct, 3 deck Multi zone, Variable Air Volume Textbooks 1- Control Systems for Heating, Ventilating and Air Conditioning by Roger W. Haines 2- Handbook of Air Conditioning System Design, by Carrier Grading Policy 1. Attendance - 10 2. Homework - 10 3. Exam 1-15 4. Exam 2-15 5. Lab. - 20 6. Final - 30
I. Background A. The purpose of the control system: 1. Provide automatic operation. 2. Maintain the controlled conditions closer than could be achieved by manual operation. 3. Provide maximum efficiency and economy of operation. 4. Ensure safe operation. B. Control Fundamentals: 1. The desired temperature is. 2. the actual quantity which is being controlled. Examples:,,. 3. The measures the controlled variable and convey the value to the controller. Examples: Electric Pneumatic Mechanical 4. The compares the actual quantity (Controlled variable) with the desired quantity (set point). Examples: 5. The reacts to signals received from the controller to vary a flow Examples:
6. The is the medium manipulated by the controlled device. Examples: 7. The mathematical relationships of control systems are usually represented by. A is the symbol which is used to indicate a summing operation, and a is the symbol for multiplication. C. Components of Control Systems: Control systems are consisting of a loop that contain: 1. 2. 3. 4. The communication signals between control system components are: 1. 2. 3. D. Control Action: 1. Two-Position action: It is the simplest of automatic regulation. There is no intermediate position between the two extremes of full ON and full OFF.
2. Timed Two-Position action: A heater element is added to offset operation time lag. 3. Floating action: The control device will be turned off at any position if it reaches upper limit or lower limit. 4. Proportional action: Continuous action either toward OFF position or ON position, and it is of three types: a. Proportional. b. Proportional Plus Integral. c. Proportional plus derivate
II. Compressor Control System: A. General: The purpose of the compressor in the vapor compression cycle is to accept the low-pressure dry gas from the evaporator and raise its pressure to that of the condenser. Compressors may be of two types: 1. the general form is the piston type, or Reciprocating compressors, being adaptable in size, number of cylinders, speed and method of drive. Figure 3.1 Reciprocating compressor, (a) Suction stroke, (b) Discharge stroke
2. they impart energy to the gas by velocity or centrifugal force and then convert this force to pressure energy. The most common type is the Centrifugal compressors.
B. Reciprocating Compressors Controls: 1. Capacity Control: A refrigeration system will be designed to have a maximum duty to balance a calculated maximum load, and for much of its life may work at some lower load. Such variations require capacity reduction devices. Capacity reduction means mainly reducing the compressed by the compressor according to a set heat load. Methods of capacity control are: a) ON - OFF Control: This is generally used with residential air conditioners, where starting and stopping the compressor may be done directly by room thermostat. On a rise in room temperature the thermostat opens a Solenoid valve. The low-pressure switch closes and starts the compressor. When the Solenoid valve closes, the refrigerant is pumped out of the evaporator, opens the low-pressure switch, stopping the compressor. This type of capacity control is recommended only when the load on the system is moderately constant.
b) Multispeed Compressor Since the capacity of a compressor is proportional with its speed, multispeed motor may be used to regulate its capacity (Two-speed motor). This motor may be controlled by two stage thermostat and two speed magnetic starter. The added cost is the disadvantage of this control method.
c) Multiple Compressors: There are some applications where failure of refrigerant in equipment could result in financial loss beyond the equipment repair expense. In such cases it advisable to break the refrigeration load into multiple stages, each has its own compressor.
d) Loading and Unloading Cylinders: Except in very small sizes, reciprocating compressors have multistage capacity control. This is generally achieved by loading and unloading cylinders under control of a suction pressure controller, by raising the suction valve off its seat. When the load increase: (Unloading stage) Suction pressure is high Bellow shrink and Bleed port arm go up closing bleed port Pressure in the piston room increase Piston pin moves down Suction valve operates No load When load decrease or in starting: (Loading stage) Suction pressure decrease Bellow expands moving bleed port arm down opening bleed port Pressure in piston room decrease Piston pin moves up Suction valve (on load)
e) Hot Gas Bypass Control: Hot gas bypass may also be used for capacity control, for system that operates at or below minimum temperature of compressor unloading. A constant pressure expansion valve is used to maintain the evaporator pressure and temperature at constant level, regardless of load.
2. Safety Controls Any gas control system must include such safety controls as high or low temperature limits and high or low pressure limits a) Oil safety switch All compressors except the smallest have mechanical lubrication and will fail if the oil pressure falls because of a pump fault or oil shortage A safety cut-out is required which will stop the compressor. This takes the form of a differential pressure switch with starting time delay. A pressure gauge on the pump discharge will indicate the total pressure at that point Any detection element for true oil pump pressure must sense both suction and pump outlet pressures and transduce the difference Oil safety cut-out have pipe connections to both sides of the oil pump and two internal bellows to measure the difference.
Since there will be no oil pressure at the moment of starting, a time delay must be fitted to allow the oil pressure to build up This timer may be thermal or mechanical or electric. Oil safety cut-out control indicates an unsafe condition and such controls are made with hand reset switch, and normally operate an alarm to warn of the malfunction. b) Refrigerant Pressure Controls Pressure controls are used for safety protection with compressor and condenser type systems.
High-pressure control actuates the system if high-side pressure exceeds the preset level When the switch in the pressure control opens, the compressor is stopped. This type of safety control is required to: Prevent the compressor motor from overloading. Prevent the system from rupture. Low-pressure control is used to stop the compressor motor whenever the low-side pressure falls below a preset level Too low suction pressure mast often occurs when there is a refrigerant leak in the system. Damage to the system might result if the unit to operate under this condition, drawing moisture and air.
High and low pressure switches are similar in appearance. The pressure range at which the switch bellows mechanism operates determines whether it is a high or a low pressure control. All pressure switches regardless of type have a common element which is the. PRESSURE OPERATED SWITCH
3. Operation Control a) Water Tower Controls A reverse-acting fan pressure control in a water-cooled air conditioning system. A water tower is often used to supply cooled water to the condenser. For the system to operate efficiently, the head(high-side) pressure of the compressor must maintained within certain limits. If the head pressure becomes too low, the evaporator does not function properly. Low head pressure could be the result of too cool water being supplied to the condenser. To solve this problem, a reverse-acting fan pressure control connected to the high side pressure line of the compressor is used. When pressure is below a set limit, the fan motor will stop.
b) Pump-Down circuit control: A low-pressure switch is used in conjunction with a thermostat and a solenoid valve to form the pump-down circuit. In this method of control, the thermostat does not stop the compressor but de-energizes the liquid line solenoid valve to stop the supply of refrigerant to the evaporator. The compressor continues to run and pumps down the evaporator until stopped by the low-pressure switch. When the thermostat again calls for cooling, it opens the solenoid valve to let the liquid refrigerant enters the evaporator and the compressor restart again by the low-pressure switch. This method is used to ensure that the evaporator is kept clear of liquid refrigerant when the plant, is off.
C. Centrifugal compressors Control: Centrifugal compressors are built for heavy duty continuous operation. Centrifugal compressors are of two types: a. Open - have a shaft which projects outside the compressors housing OPEN CENTRIFUGAL MACHINE
b. Hermetic - have the driver built into the unit, completely isolating the refrigerant space from the atmosphere Hermetic Centrifugal Machine
1. Capacity Control a. Hermetic Centrifugal Temperature control is obtained by means of variable inlet guide vanes at the suction inlet of the compressor. This control reduces capacity by varying the angle at which the suction gas is directed into the eye of the impeller. At low flow the change of inlet gas direction has little effect on capacity and the control operates primarily as a suction damper. The minimum partial load capacity of the machine is based upon the amount of gas leakage thru the fully closed capacity regulating vanes.
When the temperature changes, the thermostat signals the control to reposition the capacity regulating vanes which changes the capacity of the system, to maintain the desired temperature. When the vanes reach the closed position and the leaving temperature continues to decrease to a predetermined minimum, the low temperature cutout switch stops the compressor. b. Open Centrifugal Capacity control on an open centrifugal compressor may be obtained with a suction damper, variable inlet guide vanes or variable speed drive. Suction damper is controlled by a thermostat to reduce the capacity of the compressor by throttling the gas suction inlet. Variable inlet guide vanes control is identical to that discussed under Hermetic centrifugal. Variable speed drives may be controlled manually when the change in loading is gradual or when a suction damper is used for automatic
control. Automatic speed control is used with steam, gas turbine or gas engine drives. Automatic speed control provides very economical operation, and require less input than other methods of control. COMPARATIVE PERFORMANCE, CENTRIFUGAL COMPRESSOR CAPACITY CONTROL
1. Safety Control a. Surge Control Surge is a characteristic of centrifugal compressors which occurs at reduced capacities. This condition is a result of the breakdown in flow which occurs in the impeller. When this happen, the impeller can no longer maintain the condenser pressure, and there occurs a momentary reversal of flow and the compressor is unable to force the gas into the condenser. The following are some conditions which may cause compressor "surge": Air in System - increases condenser pressure. Low Refrigerant Charge - lowers cooler pressure. Dirty Condenser Tubes - increases condenser pressure. Low Condenser Water Flow - may increase condenser pressure. High Entering Condensing Water Temperature increase condenser pressure Faulty Float Valve Operation - decreases cooler pressure or increases condenser pressure. Division Plate By-pass - increases condenser pressure & decreases cooler pressure. Notice all tend to increase lift by either raising condenser pressure or lowering cooler pressure. Surge is solved by lowering the condenser pressure This allow the impeller to function normally again, and gas flow returns to its normal direction. The operation is stable until the condenser pressure builds up and surge occurs again. Surging can be detected primarily by the change in sound level of the machine.
Surge in a centrifugal machine does not occur at partial loads if the head or lift decreases sufficiently with the load. Lift is defined as the difference between chiller suction and discharge pressures. The figure below shows a typical lift versus load diagram at different positions of the inlet guide vanes: Line (B) when the loading is such that the total lift of the compressor reduces sufficiently as the loading also reduces. Line (A) when the lift remains almost constant or decreases slightly, it can be seen that line (B) does not enter surge until load is under minimum. Line (A) enters the surge region above minimum load. IIFT-IOAD DIAGRAM, HERMETIC CENTRIFUGAL
To control surge occurring at partial load, a hot bypass valve between the condenser and the evaporator is used to load the compressor artificially The valve may be either manual or automatic, and controlled in sequence with the automatic suction damper or speed of the compressor or position of the inlet guide vanes So that the valve starts to open just before there is an indication of surge HOT GAS BYPASS VALVE
b. Condenser high pressure cutout switch It stops the compressor when the condenser pressure becomes too high due to a condenser water stoppage, excessive condenser scaling or air in the system. c. Evaporator low pressure cutout switch It stops the compressor when the evaporator pressure becomes too low due to a chilled water stoppage, excessive cooler scaling, or insufficient refrigerant charge. d. Low oil pressure cutout switch stops the compressor when the oil pressure drops below the required minimum. e. Low chilled water temperature cutout switch stops the compressor when leaving chilled water temperature drops below the minimum allowable temperature f. Chilled water flow switch stops the compressor when chilled water ceases to flow, and prevents a start-up of the compressor motor until chilled water flow is established. TYPICAL SAFETY CONTROL SYSTEM. HERMETIC MACHINE TYPICAL SAFETY CONTROL SYSTEM, OPEN MOTOR DRIVEN MACHINE
III. Air Handling System Control: A. Outside Air Control: Before deciding on how to control the amount of outside air it is necessary to determine how much is required by the HVAC system and why. For example, Laboratories require 100% exhaust and makeup; so chemical labs require a negative pressure to prevent exfiltration.
Clean rooms require that a positive internal pressure be maintained to prevent infiltration from surrounding areas. When there are no special requirements, the minimum amount of outside air required is that needed to meet the code requirements for ventilation rates. Once the criteria have been determined, one of the following methods of control can be used 1. Minimum Outside Air The simplest method of outdoor air control is to open a minimum outside air damper whenever the supply fan is running.
2. Economy Cycle Outside Air There many times when it is necessary to operate the cooling coil even when outdoor air temperatures are near or below the freezing mark, with outside and return dampers controlled by air temperature.
a. When the outside air at design winter temperature Outside air dampers are in minimum open position Return air dampers are in maximum open position b. When outside air temperature increases The mixed air thermostat (T1) gradually opens the outside air damper to maintain constant mixed air temperature. Return dampers modulate correspondingly c. When outside air temperature become moderate cooling. 100* outside air will be provided and used for d. As outside air temperature continues to increase Outdoor air high-limit thermostat(t2) cut the system back to minimum outside air, decreasing cooling load An interlock from the supply fan to close the outside air damper when the fan is off to keep out dirt and unwanted things from entering the system. 3. Enthalpy Control The outside air economy cycle control based on dry bulb temperatures is not always the most economical. That is, in very humid climates the total heat (Enthalpy) of the outside air may be greater than of the return air even though the dry bulb temperature is lower
Since the cooling coil must remove the total heat from the air to maintain the desired condition, it is more economical in this case to hold outside air to a minimum. 4. Static pressure Control For those spaces requiring a constant positive or negative pressure with respect to their surroundings, the outside and return air dampers will be controlled by static pressure controllers. The static pressure controller senses the difference in pressure between the controlled space and a reference location outdoors, and adjusts the dampers to maintain that pressure differential.
5. Air Stratification Stratification of return air and outside air streams in mixing plenums can be a serious problem. In a worst condition case, two air streams will not mix and remain separate for a long distance through filters and coils. If the outside air temperature is below freezing, the separate air stream can cause localized freezing in heating and cooling coils. Or if a good low temperature safety control is provided, the HVAC system will cycle OFF and ON.
The mixing plenum and its dampers should therefore be designed to promote good mixing of the air streams. Some of these methods are: a. Parallel Blade Dampers - to have the air streams meet head on b. Opposed Blade Dampers - the air streams enter opposite sides of the mixing plenum.
c. Dynamic mixing - propeller fan is added, set at right angles of the air streams, to provide mechanical mixing B. Heating Coil Control: Heating may be done as: Preheat of outside or mixed air Normal heating Reheat for humidity control 1. Preheat Preheating is used with large percentages of outside air to prevent freezing of downstream heating and cooling coils and to provide a usable mixed air temperature. a. The simplest control Is a two-position valve in the steam or hot water supply, with an outdoor thermostat which opens the valve whenever the outdoor temperature is below 35-40 o F.
b. Face and bypass dampers are added at the coil and controlled by means of a downstream thermostat (T2) to provide a usable mixture temperature. The difficulty here is stratification of the two air streams.
c. In many cases a sufficient distance for mixing is not available. The best solution in this case is to use hot water with recirculating pump. Now there can be full flow through the coil at all times with the temperature of the water varied to suit the requirements. No air is bypassed, so there are no mixing problems. Very accurate control of air
temperature is possible. Notice the opposed flow arrangement, with the hot water supply entering the air leaving side of the coil. When dealing with freezing air certain precautions are necessary. For hot water, it has been shown experimentally that water velocities of 2.5-3 ft./sec (0.9 m/s) in the coil tubes are sufficient to prevent freezing at outdoor temperatures down to -30 o F. 2. Normal Heating Normal heating refers to the coil in a single-zone, multi zone or dual-duct air system which handles all or a major portion of the system air at entering temperature of (45 o to 50 o F) or higher.
3. Reheat is used mainly with humidity control or individual zone control. In either case, control of steam or hot water supply valves is usually done by room thermostat.
C. cooling Coil Control: There are two types of cooling coils: Direct Expansion (DX) coils. Chilled water coils. 1. Direct Expansion coils: DX coils must by their nature use two-position control with its wide operating differential. This system is often used in small units. a. The room thermostat opens the solenoid valve, allowing refrigerant liquid to flow through the expansion valve to the coil. The expansion valve modulates to maintain a minimum refrigerant suction temperature. A low limit discharge thermostat (T2) keeps the supply air temperature from becoming too cold. b. Controllability can be improved by providing face and bypass dampers, but this may lead to lack of humidity control and coil icing at high bypass rates.
c. A different approach adds a variable back pressure valve in the refrigerant suction line, controlled by room thermostat. As the room thermostat decreases, the valve is throttled, increasing the suction temperature at the coil and decreasing the coil capacity. d. Hot gas bypass may also be used for capacity control. There are limitations on the percentage of total refrigeration flow which may be bypassed, and on pressure drops in the piping system.
e. Two-Stage direct expansion will often provide adequate capacity control. The stages should be made by rows of the coil rather than by sectioning the coil. In sectioning the active section may ice up forcing most of the air flow through the inactive section and reducing the coil capacity. In multi-row coil the first stage should be first row in the direction of air flow, and second stage the rest of the rows A two stage thermostat is used
2. Chilled Water Coils: Chilled water coils are controlled by Three-way valve (Two-position or Modulating). Recirculating pump. Recirculating pump is used in two cases: For extremely accurate temperature control. To avoid coil freezing. Parallel and Counter flow: Consider the cooling coil with air flowing through it, decreasing in temperature from 80 o to 55 o F, and with water flowing parallel or counter to the air, and increasing in temperature from 42 o to 50 o F. The heat transfer from air to water in the coil is a function of Tube wall Air and water films inside the tube wall. Total internal tube surface area. MED (Mean Equivalent Temperature Difference). where: CTD = greatest temperature difference between water and air. LTD = least temperature difference between water and air. MED = mean equivalent temperature difference. ln = natural log (log to base). If we calculate the MED for each of the two flow arrangements, we get: 1. Parallel flow: GTD = 80-42 = 38 LTD = 55-52 = 3 MED = 38 3 ln 38 3 = 13.3
2. Counter flow: GTD = 80-52 = 28 LTD = 55-42 = 13 MED = 28 13 ln 28 13 = 18.7 This is an increase of nearly 1/3 in the heat transfer capacity. This should allow water temperature of 47 o F in and 57 o F out, and still get the design condition (55 o F leaving air), which is impossible with parallel flow. The higher water temperature, increases chiller efficiency and capacity. D. Humidity Control: Sometimes it may be necessary to raise or lower the humidity of the supply air in order to maintain selected humidity conditions in the air conditioned space.
1. Air Washer Humidifier: Often used for its sensible cooling capability, it is then known as an evaporative loader. The cooling is accomplished by using the sensible heat of the air to evaporate water. Thus the air passing through the washer changes conditions along a constant wet bulb line, with the final state being dependent on the initial state and the saturation efficiency of the washer. The is no control of humidity.
2. Steam Humidifier: Steam humidifiers are often used because of their simplicity. A piping manifold with small orifices is provided in the air duct or plenum. The steam supply valve is controlled by space or duct humidistat. A duct high-limit humidistat must be provided to avoid condensation in the duct.
3. Chemical Dehumidifier: Chemical dehumidifiers use a chemical adsorbent. One form of dehumidifiers uses a wheel containing silica gel, which revolves first through the conditioned air stream, absorbing moisture, and then through a regenerative air stream of heated outside air, which dries the gel. In the process, a great deal of heat is transferred to the conditioned air, and a cooling is necessary. Space humidistat control the heating coil in the regenerative (drying) air stream, and room thermostat control the space temperature.
4. Dehumidifying by Refrigeration: Low temperature cooling coil is used to reduce humidity to low values. Special DX coils with wide fin spacing must be used.
A provision must be made for defrosting by: a. Hot gas b. Electric heater c. Warm air This approach has some limitations: a. Inefficient at low humidifies. b. Intermittent shutdown for defrosting. c. Reheat is necessary. Since space humidity is largely a function of coil temperature, fairly good control can be achieved through a humidistat. A room thermostat controls the cooling coil when the humidistat is satisfied. V. Applications: A. Single Zone: A space thermostat controls heating and cooling directly. If used, a humidifier is controlled by the humidistat. B. multi Zone and dual duct: The mixing dampers controlled by the zone space thermostat for each zone. If used, humidifiers are usually controlled by a return air humidistat.
C. Three-deck multi Zone system: The zone dampers operate with sequenced damper motor either with: 1. Mix hot supply air with bypass air when the cold deck damper is closed. 2. Mix cold supply air with bypass air when the hot deck damper is closed.
D. Variable air volume system: Motorized dampers in each zone supply duct. A related zone space thermostat controls each damper by a (flow sensor controller), that is reset by the thermostat. If used, humidifiers are controlled by a return air humidistat.