Refrigeration System Control And Operation PDF

4th Class Edition 3 Part B Unit 9 Chapter 3 Refrigeration System Control and Operation Learning Outcome When you complete this cha...

4th Class Edition 3 Part B Unit 9 Chapter 3 Refrigeration System Control and Operation Learning Outcome When you complete this chapter you should be able to: Describe the purposes and operating principles of refrigeration system operational and safety controls. Learning Objectives Here is what you should be able to do when you complete each objective: 1. Describe refrigeration system controls. 2. List the safety shutdown devices specific to centrifugal compressor water chillers. 3. Describe typical refrigeration system safety shutdown devices. 4. Describe the construction and operation of refrigerant metering devices. 5. Describe the different methods used to control evaporator capacity. 6. Describe the different methods used to control the capacity of refrigeration compressors. 3-1  Refrigeration System Control and Operation Chapter 3 Chapter Introduction This chapter discusses controls used to help refrigeration plants operate safely and efficiently. Refrigeration systems operate under varying conditions. Though designed to operate at particular high-side and low-side temperatures, load changes and atmospheric conditions influence system operation. For example, in a cold storage facility, when a load of fresh goods are brought in to be frozen, the cooling load increases. In ice arenas, each time the ice is flooded, the cooling load increases. In HVAC systems, the cooling load increases during the day and decreases at night. All refrigeration systems carry greater load during hot summer months than in the dead of winter. Refrigeration systems therefore need controls to automate the response to load changes, which may vary from no-load to full-load. Refrigeration system controls can be classified into three types: Operating (system capacity) controls: These controls start or stop the compressor, or otherwise regulate its capacity, when the process conditions (temperature, pressure, or humidity) approach or deviate from their set points. Actuating or secondary controls: These controls either indirectly control the changes in operation called for by the primary controls, or they regulate the cycle during operation. Limiting and safety controls: This group of controls protects the system against operation beyond the limits for which it was designed. Many of the controls used in refrigerating systems are quite similar in design and operation to those used on boilers and in heating systems. Many large stand-alone refrigeration systems have human machine interface (HMI) panels, or have controls linked to the plant supervisory control and data acquisition (SCADA) systems. Every operator should review the manufacturer manuals and plant data for site-specific practices and procedures. 3-3 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 1 Describe refrigeration system controls. In HVAC systems, refrigeration system controls maintain the set points and safe operating conditions for the comfort and safety of the occupants. In cold storage and process plants, adherence to set point is also critical to maintain process conditions to prevent damage to goods or process equipment. Millions of production dollars can be lost if refrigeration systems do not perform according to design. As new technologies arise, controls become more complex and better able to efficiently and accurately control processes. Refrigeration plant controls are no exception. For example, controls can now sense the ice surface temperature of arenas for optimum hockey or figure skating conditions. They can also check buildings for occupancy, and adjust HVAC set points accordingly. Human Machine Interfaces (HMIs) An HMI is usually found mounted on a packaged refrigeration unit supplied by the manufacturer, or on a wall in the machine area close to the unit. These HMIs are designed to allow the operator to receive operational data and alarms on a screen or a series of screens as well as allowing him or her to make changes to set points or operational expectations. These allow the units to be “stand alone”, where the unit does not report to a central control system or hook up to the SCADA system, or can be interlinked to the master control system reporting to a control room. Figure 1 – Human Machine Interface for HVAC Chiller 3-4 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Figure 1 shows an HMI system status screen, which provides the operator with overall system operating conditions. HMIs may have several different control screens so that operators can: View and change operating set points Change system operating schedules View and change alarm set points View critical operating parameters View historical and current operating trends Print system status reports View, acknowledge, and reset alarms Figure 2 shows a series of screen shots from an ice arena control room operator interface. The operator can scroll through several screen to start and stop equipment, change set points, observe trends, set occupancy schedules, respond to alarms, check equipment runtimes, and review all pertinent operating parameters. 3-5 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 2 – Arena Ice-Making Equipment Control Screens 3-6 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Operating Controls Refrigeration plant cooling capacity must always equal the cooling load. If a refrigeration plant operates at full capacity when the load is low, the temperature of the refrigerated medium will drop below set point. If the refrigeration system operates at low capacity when the cooling load is high, then the temperature of the refrigerated medium will rise above set point. System capacity (kW or TR) is controlled by adjusting the mass of refrigerant circulated per unit time (kg of refrigerant circulated per hour). Because the compressor is the device that circulates refrigerant, system capacity control must influence the mass of refrigerant circulated by the compressor (the compressor capacity). This may be done directly or indirectly. A control system can directly adjust compressor capacity, or a control system can directly regulate evaporator capacity. In the second situation, the compressor responds to changing evaporator capacity; the compressor is therefore controlled indirectly. Compressor capacity, like the output of small boilers, can be adjusted by cycling it on and off (two-position control). Compressor capacity can also be regulated with multi-stage control (cylinder unloading) and full modulation (variable speed control). These control systems respond to: Temperature Pressure Humidity Temperature-Actuated Control (Thermostat) Electric controls can be used for starting and stopping compressors. When these are used, a thermostat with a bimetal or filled-system thermal sensing element is placed to detect the temperature of the refrigerated medium. Deviations from the set point operate electrical contacts that start or stop the compressor. Thermostats can be used as contact closures inputs used in a computerized control system to start or stop compressors. These thermostats “enable” or “disable” a compressor when its capacity cannot be controlled in other ways. A compressor running at minimum capacity may still circulate too much refrigerant. In this case, the thermostat would signal the control system to disable the compressor. When disabled, the compressor stops. When enabled, the compressor starts, and its capacity varies from low to high. Other thermostats may have electrical resistance elements or electronic temperature sensors. These can be used to produce variable resistance, variable voltage, variable current or digital signals which can be used with electrical or computerized control systems. Bimetal Thermostat A bimetal element can directly control the power supply to small compressor motors. Larger motors with a high current draw are indirectly controlled by a thermostat that actuates a magnetic motor starter (contactor). When the compressor is equipped with an unloader, a thermostat can control the operation of the unloader actuator. Filled System Remote Bulb Thermometer A filled system thermometer, when equipped with a switch, can also directly control the compressor motor power supply. If the temperature of the refrigerated medium increases above set point, the fluid within the thermometer expands, causing the bellows to expand. The bellows closes the thermostat switch, energizing the magnetic motor starter and starting the compressor. When the temperature of the refrigerated medium drops below set point, the opposite occurs. To keep the compressor from short-cycling, the thermostat has a differential setting. The temperature must drop a few degrees below the set point for the switch to open, and it must rise a few degrees above the set point for the switch to close. These are referred to as the cut-in and cut-out points. The cut-out point is always at a lower temperature than the cut-in. The difference in temperature between these two points is known as the temperature differential. 3-7 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Thermostats often have adjustable differential settings. The range refers to the span of temperatures over which the compressor operates. The differential setting controls the size of the range. For example, suppose the range of thermostat “A” is from 8°C to 3°C, and that of thermostat “B” is from 15°C to 10°C. Though they operate over different ranges, in each case, the differential is 5 degrees. If the range of thermostat “C” was between 3°C and 10°C, its differential would be 7 degrees. Figure 3 shows the location of a filled system remote bulb thermostat in a compressor electrical control circuit. Figure 3 – Thermostatic Control of Compressor Cooling From Thermostat To Evaporator(s) Condenser Compressor Magnetic G H Starter (if required) Line When temperature-actuating devices control compressor operation, they are used in conjunction with refrigerant flow controls. These are typically solenoid valves that stop the flow of refrigerant when the compressor stops. Pressure-Actuated Control (Pressurestat) Since the suction pressure of the refrigeration compressor is directly related to the boiling temperature of the liquid refrigerant in the evaporator, a change in evaporator temperature is reflected by changes in the suction pressure. For example, when using the refrigerant R-717, if the boiling temperature of the refrigerant in the evaporator is -18°C, then the evaporator pressure will be about 309 kPag. If the temperature increases to -16°C, then the evaporator pressure will be about 328 kPag. A pressure-actuated control connected to the suction line of the compressor can be used to start and stop it. It is possible to maintain the evaporator temperature within close limits over varying load conditions using this system. This arrangement is shown in Figure 4. Figure 4 – Suction-Pressure Controlled Compressor 3-8 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 The pressure-actuated control used for this purpose consists of a switch actuated by a bellows or diaphragm that senses the suction pressure of the compressor. The control is similar to the thermostat, except it operates in the reverse manner. That is, it opens the compressor power switch when the suction pressure drops to the cut-off setting, and closes it when the pressure rises to a preset differential. A suction pressure-actuated control cannot be used when an automatic expansion valve is used as a metering device. Automatic expansion valves maintain a constant evaporator pressure. In this situation, the compressor would not cycle off. It also cannot be used with a capillary tube or orifice-type metering device, since these do not prevent liquid refrigerant flow from the high to the low sides of the system when the compressor shuts down. The pressures on both sides would equalize soon after the compressor stops. The rising evaporator pressure would then restart the compressor almost immediately. The frequent starts and stops would cause premature compressor failure, and excessive power consumption. Humidity-Actuated Control (Humidistat) In some air conditioning systems, refrigeration is used to lower the humidity of the air by condensing the moisture on the cold surfaces of the evaporator or dehumidifier. A humidistat can be used both for humidity measurement as well as starting and stopping refrigerant flow to an evaporator. A liquid solenoid valve, coupled with a metering device, would be used in conjunction with the humidistat. 3-9 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 2 List the safety shutdown devices specific to centrifugal compressor water chillers. Safety shutdown devices are used to protect equipment from damage due to mechanical failure or human error. Because of greater ODP and GHG awareness, there is greater focus today on keeping the refrigerant contained within the system by preventing leakage and accidental discharge of refrigerant. Failure of a refrigeration system component may cause a breach in the system and a catastrophic leak. Therefore, it is important that pressure-retaining components – including the compressor itself – are protected from failure. Centrifugal Chiller Safety Controls Centrifugal chillers are equipped with several safety devices that protect the system against abnormal conditions during startup and operation. Modern chiller systems have sophisticated controls and HMIs for monitoring safety controls and conditions prior to startup and during operation. The HMI can also be connected to a central computerized control system that operates the building HVAC system. Low Chilled Water Temperature Cut-Off Switch This safety device consists of a thermostat that senses the temperature of the chilled water leaving the chiller. If the water temperature is too low, the water may freeze in the evaporator. This will damage the evaporator tubing and tube sheet, due to the expansion that occurs when water freezes. If the temperature drops approximately 2.5 to 3°C below the set point of the control thermostat, it will shut down the compressor. When the water temperature rises approximately 5 to 6°C, the thermostat will permit the compressor to restart. Chilled Water Flow Switch This switch, installed in the chilled water outlet line, protects the chiller from freezing due to lack of water flow. It opens the compressor motor circuit when water flow drops below the safe minimum flow. It also prevents the compressor from starting if flow has not been established. Inlet Vane Closed Switch The inlet vane closed switch is a proximity switch. It is closed when the chiller capacity control vanes are in the closed position. This is a necessary precondition for centrifugal compressor start-up. With closed vanes, the compressor can be started under a no-load condition. If the vanes are open and the compressor is off, it cannot start. Low Oil Pressure Cut-Off This safety shutdown device prevents compressor operation when lube oil pressure is below a safe minimum value. This prevents compressor damage or destruction, and the possibility of a catastrophic refrigerant leak that may result. The low oil pressure cut-off normally requires a manual reset. This alerts the operator that a low oil pressure condition occurred, and may require more careful monitoring of the compressor oil pressure to determine the cause. Condenser High Pressure Cut-Off This is a high limit switch that shuts the compressor down when the condenser pressure reaches an excessively high value. This switch must be manually reset before the compressor can be re-started. Inadequate condenser cooling water or airflow, or condenser fouling, is generally the cause of a high condenser pressure trip. 3-10 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Refrigerant Low Temperature Cut-Off This safety switch is a low limit thermostat. It senses the temperature of the refrigerant in the chiller. If the temperature drops low enough to freeze water, the switch opens and shuts down the compressor. Motor Demand Limiter Most centrifugal chillers have a motor demand limiter, which limits the maximum current flow to the compressor drive motor. This control overrides the water temperature sensor if the motor load reaches the maximum amperage setting. One of the most common and useful purposes that this control serves is to limit the current flow when the machine is first started. Usually, the chilled water temperature is considerably higher than set point at the time of startup. Although the start temperature of chilled water loop may be high, the actual system load may be low. If no restriction is placed on the current flow, the motor will draw maximum current until the temperature of the water is brought down to the chilled water set point. This results in a very high and very costly electrical demand. Also, the water could reach its set point temperature very quickly under a low-load start. If the compressor does not automatically unload fast enough, the unit could trip on low chilled water temperature. The motor demand limiter can be adjusted to 40%, for example, and left there until the chilled water has been brought down to the required temperature. Increasing the demand limiter in gradual steps through 60%, 80% and 100% helps reduce the maximum electrical demand and the possibility of motor damage due to high current flow. The motor demand limiter sets the permissible opening of the compressor inlet vanes. The power consumed by the compressor motor is proportional to the mass of refrigerant moved per unit time. The inlet vanes restrict the flow of refrigerant, and thus the current draw of the compressor drive motor. Instead of demand limiters, some chillers use soft loading strategies on startup. These have the same effect on reducing electrical demand and preventing low temperature trips on startup. Soft loading is a PLC control strategy that brings the chilled water loop temperature from its start value to its set point in a controlled manner. Soft loading prevents the chiller from going to full capacity during the pull down period. Chillers that use variable speed drives can limit demand by varying compressor speed. Compressor Vibration Shutdown There is often a vibration switch on each motor and compressor bearing. If the vibration on any bearing reaches a certain set point, an alarm sounds to warn the operator of a vibration problem. If the vibration on that bearing increases further (to the shutdown set point), the compressor will stop. Suction Scrubber High Level Shutdown On chillers equipped with a suction scrubber, if the liquid level rises to a certain level, an alarm will sound to alert the operator of a high level in the scrubber. If the level continues to rise, a float switch will shut down the compressor to keep liquid from getting into the compressor. High Bearing Temperature Shutdown Each compressor and motor bearing is equipped with a temperature probe. If the temperature rises to a certain point (determined by the bearing manufacturer), a high bearing temperature alarm activates. This allows the operator to either take action to reduce the bearing temperature, or to prepare a standby chiller for service. If the temperature continues to rise, the compressor will shut down. 3-11 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Motor Winding Temperature Cut-Off Motor windings can fail if they get too hot. This may occur if the compressor and motor are overloaded. To protect the motor, chillers have overcurrent protection and locked rotor protection. In either case, if high motor current persists during the start or run period, the power to the compressor drive motor is shut off. Restart Inhibit Restart inhibit keeps compressors from short-cycling, or restarting in a short period of time. Motors sustain high currents when starting, which can damage motor windings. The restart inhibit feature allows the motor to cool off for 10 or more minutes before it can be restarted. Motor Overload Protection Motor overloading leads to overheating and eventual failure. One type of overload used with centrifugal chillers employs a current transformer with a resistor in the motor circuit. An increase in current flow causes a greater voltage drop across the resistor in the electrical circuit. This change in voltage is amplified by an electronic circuit to operate relays controlling the compressor inlet vanes. If vane control should fail to prevent overload, the relay operates a solenoid valve to force a pneumatic or hydraulic drive motor to close the vanes. This is achieved by applying oil or air pressure to one side of the piston and bleeding the other side. 3-12 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Objective 3 Describe typical refrigeration system safety shutdown devices. Limiting and Safety Controls Refrigeration systems must be equipped with safety devices to provide protection from damage that might occur due to equipment malfunction or operator error. High-Pressure Cut-Off During operation, the condenser pressure may become excessively high. This may be due to: a) Insufficient cooling of the high-pressure vapour in the condenser b) Lack of cooling airflow c) Atmospheric conditions Extremely warm temperatures High humidity Lack of wind d) Fouling of the condenser heat transfer surfaces e) Non-condensable gases accumulated in the system Refrigerant gas condenses under the normal temperature and pressure conditions found in the condenser. Under these same temperatures and pressures, other gases (such as air or products of lube oil decomposition) cannot condense. These gases accumulate at the top of condensers and liquid receivers. In the condenser, non-condensable gases occupy space, impede heat transfer, and raise condenser pressure. Refrigeration system piping and components are designed according to various ASME codes, to specific high- and low-side design pressures. To prevent the pressure from exceeding the design pressure, the CSA B52 Mechanical Refrigeration Code, in Clause 7.2, states that, with few exceptions: “Pressure-limiting devices shall be provided on all systems operating above atmospheric pressure.” A E It also states: “On systems equipped with a pressure-relief device, the setting of this device shall not be more than 90% of the system high-side design pressure… the pressure-limiting device shall stop the action of the pressure-imposing element.” CSA B52 also requires this device to be connected between the compressor and the first stop valve in the discharge line. The sensing line must not have any intervening valve, because closing it would render the pressure-limiting device inoperative. This pressure-limiting device is much like the high-pressure cut-off of a boiler. It has a set point and a non-adjustable differential. The high-pressure cut-off has a manual reset button, so that operator intervention is necessary. The operator must determine the cause of the high pressure and rectify the condition before restarting the compressor. When the compressor motor is controlled by a suction pressure control, the high-pressure cut-off is often combined in the same housing with the suction pressure control. One type is shown in Figure 5. 3-13 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 5 – Combined Low-Pressure Control and High-Pressure Safety Cut-Off Low Range Adjustment High Range Adjustment Low Side Connection High Side Connection Low-Pressure Cut-Off This pressure operated safety switch is used to protect the system against abnormally low suction pressure and temperature. This could occur if the pressure actuated operating control fails to stop the compressor at its cut-off pressure (a low suction pressure). It could also occur in a system with a temperature actuated motor control if the evaporator ices up excessively, preventing proper heat transfer. In this situation, the refrigerated space or medium would rise to above its temperature set point. However, the thermostat would run the compressor continuously. A low-pressure cut-off control operates on the same principle as the high-pressure safety cut-off control, and may also have a manual reset lever. When a low-pressure cut-off is used, it may be combined in the same housing with the high-pressure cut-off. This combination control is provided with a manual reset, so the operator is made aware of the cause of the trouble if the compressor shuts down before temperature conditions are satisfied. Low Limit Thermostat The low limit thermostat is used in systems where equipment damage could result if the temperature drops to below the minimum setting of the operating thermostat. It is important that chilled water is never allowed to freeze, since freezing could cause extensive damage to the evaporator. To prevent freezing, the sensing element of a low limit thermostat is immersed in the water at the coldest point of the chiller. The thermostat is set to open the control circuit of the compressor at a temperature several degrees above the freezing point. Low Oil Pressure Cut-Off Switch Failure of a forced lubrication system could cause extensive damage to a refrigeration compressor. Therefore, the compressor must have an oil pressure failure switch which shuts down the compressor when the oil pressure drops below the safe minimum limit for longer than a predetermined period. Such a device is shown in Figure 6. 3-14 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Figure 6 – Low Oil Pressure Cut-Off Switch Crankcase or Suction Pressure Power Supply In B Resistor A Timer Switch 220 Diff. Pressure 110 Switch Bimetal L Heater M Reset Button Oil Pump Starter Discharge Starter Holding Coil Pressure Overload Relays Normally Closed Refrigeration compressor crankcases and oil sumps are both under low-side pressure. The compressor bearings operate within the surrounding low-side pressure environment. Low side pressure varies during normal compressor operations. For example, many compressors pump down the low side before they cycle off. This reduces the low side pressure to just above atmospheric, which is well below normal. During the pump down cycle, the oil pump supply pressure falls considerably. When the temperature of the refrigerated space increases, a liquid line solenoid valve opens to admit refrigerant to the evaporator. The evaporator pressure rises, and the compressor starts. Therefore, due to the wide variations that occur in the low-side (and crankcase) pressure, sensing only the oil pump discharge pressure is an unreliable indicator of whether the bearings are receiving adequate lube oil. In fact, a normal single-point oil pressure-sensing switch would cause frequent nuisance shutdowns, compressor bearing failure, or both. The important parameter to measure is not the oil pump supply pressure; rather, it is the difference between the oil pump supply pressure and the crankcase pressure. This is the useful or “net” lube oil pressure. For example, consider a compressor with an oil supply pressure of 620 kPa and a crankcase pressure of 480 kPa. In this case, the useful (net) oil pressure is 620 – 480 = 140 kPa. This net pressure must be maintained to prevent compressor bearing damage. Refer to Figure 6. The low oil pressure cut-off control is a differential pressure switch with two opposed bellows. One bellows senses oil pump discharge pressure. The other bellows senses crankcase pressure. Variations in the relative positions of the bellows operates a differential pressure switch (A). This switch controls a small heater located near a bimetal strip. When hot, the bimetal bends, allowing a timer switch (B) to open. The timer switch is wired into the compressor motor run circuitry. When it opens, two things happen: 1. The compressor stops. 2. The current through the heater stops. After the bimetal cools, it cannot automatically restore the timer switch. The reset button must be depressed. This button moves the timer switch contacts together, and allows the bimetal to bend under the timer switch, holding the contacts closed. 3-15 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration When the compressor first starts, the net oil pressure is zero. During this start-up period, the oil pump continues to develop oil pressure. The cut-off switch timer allows the compressor enough time (usually from 30 to 120 seconds) to start and establish the correct oil pressure differential. If this pressure differential is not established within the required amount of time, the compressor motor shuts off. After the proper oil pressure differential increases to the require set point pressure, the differential pressure switch opens and de-energize the heater circuit before the bimetal strip opens the timer switch contacts. The compressor then continues to operate normally. During normal operation, if the differential oil pressure is greater than the suction pressure by the required amount, the timer heater circuit stays open and the heater stays cool. This keeps the timer switch closed, and the compressor continues to run. If the differential oil pressure drops too low, the bellows arrangement closes the differential pressure switch and energizes the heater circuit. After between 30 and 120 seconds, the bimetal becomes hot enough to bend and open the compressor motor circuit. This may occur if the sump oil level is too low, if the oil is foaming, or if the oil pump is not working properly. The switch cannot be manually reset until the heater and the bimetal strip cool off. Once the bimetal strip is cool, pressing the reset button causes the compressor to start. Therefore, the operator should investigate the cause of the low oil pressure condition, and correct the problem before resetting the cut-off. Flow Switch Flow switches are operated by the force exerted on a flexible vane immersed in a flowing fluid. They are used in chilled water lines, cooling water lines, and air ducts. Flow switches are often used as safety lockout switches if the fluid flow becomes insufficient or ceases. For example, in a chilled water system, the compressor must not run if a chilled water pump fails. Otherwise, the compressor may not be able to unload fast enough, and evaporator freeze-up may occur. Flow switches can also be used to close flow indicator circuits. High Oil Temperature Cut-Off If the oil temperature is too high, it cannot adequately lubricate the bearings. High oil temperature could indicate fouling or failure of the lube oil cooler, or failure of the cooling medium. If the oil temperature rises to above 80°C, the compressor shuts down immediately. This often requires a manual reset. The high oil temperature cut-off sensing element may be has a bimetallic strip or a thermistor. A thermistor changes electrical resistance quickly in response to temperature changes. Thermistor-type cut-offs respond faster than bimetal strips, so they are used when reaction speed is more critical. High Motor Temperature Cut-Off The high motor temperature cut-off stops the compressor if the drive motor windings get too hot. Hot windings lead to premature winding insulation failure. This may be due to high machinery room ambient temperature, overloading, damaged motor fan, excessive dirt buildup on the motor, or blocked cooling passages. Bimetal or thermistor temperature sensing elements may be used. Low Oil Temperature Start Inhibit and Cut-Off Switch When compressor lube oil is cool, it dissolves more refrigerant than when it is hot. Dissolved refrigerant dilutes the lube oil, which causes excessive bearing wear. As well, when the compressor starts, refrigerant in the oil vapourizes, forming bubbles. When the lube oil pump draws bubbles, it may lose its prime and starve the bearings of oil. When the lube oil temperature is below 60°C, a lube oil heater starts to raise the oil temperature. Below 35°C, the inhibit switch keeps the compressor from starting. 3-16 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Figure 7 shows a crankcase heater, installed near the base of a reciprocating compressor. The heater has a temperature set point adjustment dial. A low oil pressure cut-off can also be seen. Figure 7 – Crankcase Oil Heater 3-17 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 4 Describe the construction and operation of refrigerant metering devices. The refrigerating capacity of a system depends, in part, on the mass of refrigerant circulated per unit time. Consider two identical refrigeration systems. They have the same high and low side pressures, the size and make of compressor, the same evaporator and condenser, and use the same refrigerant. With all things equal, the system kW rating (or tonnes) depends on how much refrigerant circulated through the system over a period of time. The circulation rate depends on two things: the mass of refrigerant the compressor circulates, and the rate at which refrigerant enters the evaporator. Refrigeration system pressures and temperatures must remain constant while the system is in operation. For this to occur, the amount of refrigerant fed to the evaporator must always equal the amount of refrigerant circulated by the compressor. At low load, less refrigerant circulates. As cooling load increases, refrigerant flow must increase in proportion. The flow of refrigerant through the compressor is controlled with various compressor capacity control methods, including on-off cycling and unloading. The flow of refrigerant through the evaporator is regulated with a metering device. Without an automatic metering device, the evaporator may receive either an insufficient or excessive amount of refrigerant. In the first case, the evaporator pressure and temperature drops. But, because too little refrigerant is fed, the evaporator runs too dry, the vapour becomes excessively superheated, and the evaporator tonnage drops too low to adequately cool the refrigerated space. As well, excessively superheated low-pressure vapour may inadequately cool the compressor. If too much refrigerant enters the evaporator, the cooling load will not be able to vapourize all the refrigerant. Remaining liquid will enter the compressor suction line and destroy the compressor. In some older refrigerating systems, the flow of liquid refrigerant to the evaporator is manually controlled. In modern systems, the metering devices operate automatically, with the possible exception of very large systems where qualified operators are in attendance at all times. Metering devices include capillary tubes, fixed orifices, expansion valves, and float valves. There are six basic types of metering devices. These are: 1. Hand-operated expansion valve 2. Automatic (constant pressure) expansion valve (AEV) 3. Thermostatic expansion valve (TEV) 4. Low-pressure float valve 5. High-pressure float valve 6. Capillary tube Hand-Operated (Manual) Expansion Valve A manual expansion valve used in small capacity refrigerating systems is shown in Figure 8(a). Depending on the refrigerant, it may be made of brass or stainless steel. It is connected to the piping system by flared compression fittings, threading or welding. In order to withstand the severe requirements of throttling service, this type of valve is equipped with a valve stem tapered at the end and a matching tapered valve seat. The spindle has a fine thread which makes precise adjustment possible. The valve is adjusted by removing the cap and turning the spindle by means of a wrench or key. 3-18 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Figure 8(b) is another type of hand operated expansion valve suitable for larger flows. It has an index pointer for greater precision in adjustment. These manual expansion valves are seldom used in modern refrigeration systems. However, they are occasionally used in bypass lines around automatic control valves, or for defrosting systems. Figure 8 – Manual Expansion Valves Handwheel Valve Stem Packing Nut Index Packing Pointer (a) Valve Seat (b) Automatic Expansion (Constant Pressure) Valve The automatic expansion valve (AEV) is actually a pressure-regulating valve that maintains constant pressure in the evaporator whenever the compressor is running, regardless of load. In addition, the valve automatically shuts off the liquid flow when the compressor stops. A sketch of an automatic expansion valve is shown in Figure 9. Figure 9 – Schematic Diagram of Automatic Expansion Valve Adjustment Screw Spring Spring Bellows or Pressure Diaphragm Needle Evaporator and Seat Pressure Out In Strainer The valve stem is attached to a bellows or diaphragm in the upper part of the valve housing. A spring exerts a downward force on the diaphragm, which tends to open the valve. This force is counteracted by the evaporator pressure acting upward against the diaphragm, which tends to close the valve. The spring tension is adjusted so that during operation the two forces balance each other and valve is opened sufficiently to allow enough liquid to flow into the evaporator to maintain the desired pressure and, therefore, the desired temperature. 3-19 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration When the temperature of the refrigerated space or medium drops below set point, a thermostat stops the compressor. The expansion valve stays open and allows liquid to enter the evaporator. This liquid continues to vapourize, causing the evaporator pressure to increase. The increased evaporator pressure acts on the underside of the valve diaphragm, overcomes the downward force of the spring, and closes the valve. This stops the flow of liquid refrigerant into the evaporator. When the temperature of the refrigerated space or medium rises above set point, the thermostat restarts the compressor. This causes the evaporator pressure to drop to where the pressure on the diaphragm becomes less than the downward spring force. The valve then opens to allow liquid refrigerant to enter the evaporator. AEVs are used on small refrigerating units, such as refrigerators and freezers. This is because these machines have relatively steady loads and small compressors that can start easily under load. Due to their inherent disadvantages, AEVs are not used in larger multi-evaporator systems, or systems with highly variable cooling loads. Consider a system with a single compressor and two evaporators, each fed with an AEV. The compressor is controlled by a thermostat located in the refrigerated space. The cooling load calls for the compressor to continually operate. This lowers the pressure in the coils, causing both AEVs to stay open. If the load in the refrigerated space is uneven, one evaporator may have sufficient load to vapourize all the refrigerant fed to it. The other may not have enough load, and could admit liquid refrigerant into the compressor suction line, damaging the compressor. The thermostatic expansion valve was designed to overcome this problem. Thermostatic Expansion Valve The thermostatic expansion valve (TEV) is the most widely used metering device. It is similar in construction to the automatic expansion valve, but features a thermal power element which consists of a bellows or a diaphragm chamber connected to a temperature-sensing bulb by means of a small internal diameter tube (capillary tube). The bulb is often charged with the same refrigerant used in the system. The refrigerant in the thermal bulb is in liquid form and the rest of the element is filled with refrigerant vapour. Figure 10 shows a cross sectional view of a diaphragm type TEV, and a TEV installed on an R-410A chiller. Figure 10 – Thermostatic Expansion Valve Diaphragm Capillary Tube External Equalizer Connection Inlet Outlet Balance Valve Bulb Spring 3-20 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 A simplified diagram of a TEV installed on the inlet of an evaporator is shown in Figure 11. The thermal bulb is strapped to the piping at the evaporator outlet. The bulb is sensitive to refrigerant temperature changes at the evaporator outlet. A change in outlet temperature changes the pressure of the refrigerant in the thermal power element. The TEV adjusts the amount of liquid admitted to the evaporator so that, under all load conditions, nearly the entire evaporator surface is used to transfer heat to the evaporating liquid refrigerant. It also ensures that no liquid leaves the evaporator with the vapour. To verify there is no liquid in the vapour leaving the evaporator, the vapour must be superheated. Typical TEVs maintain a constant superheat of around 6°C over the evaporator saturation temperature. This is shown in shown in Figure 11. At point B, all liquid has been evaporated, but the vapour temperature is still at the saturation temperature corresponding to the evaporator pressure. Between B and C, however, the heat absorbed by the vapour causes its temperature to rise above the saturation temperature, thus becoming superheated. Figure 11 – Schematic Diagram of Thermostatic Expansion Valve Bellows or Diaphragm Bulb Pressure Evaporation Pressure Needle A -7°C, 142 kPa and Seat In Spring Pressure Strainer Spring -7°C, 142 kPa Adjusting Screw -1°C, 197 kPa Remote Bulb -1°C, 142 kPa Superheat -7°C, 142 kPa C B Because the bulb has a fixed internal volume, the pressure inside the bulb increases as its temperature increases. As the bulb increases in temperature, its internal pressure exceeds the evaporator saturation pressure. The operation of the expansion valve is controlled by the interaction of three forces: 1. The force exerted by the thermal element, which tends to open the valve. 2. The forces exerted by the evaporator pressure, which tends to close the valve. 3. The force exerted by the spring, which also tends to close the valve. Consider an increase in cooling load. A load increase requires additional refrigerant circulation. If insufficient liquid refrigerant enters the evaporator, the refrigerant receives more superheat than normal. This raises its temperature above saturation temperature, and above the normal superheat set point. The pressure in the thermal element rises and increases the force on the diaphragm. This opens the valve, allowing more liquid refrigerant to enter the evaporator. Now, consider a decrease in cooling load. This requires less refrigerant circulation. If too much liquid enters the evaporator, the superheat is reduced or eliminated and the pressure in the thermal element drops. This pressure drop reduces the force on the diaphragm so that the valve closes partially and reduces the flow of liquid refrigerant. 3-21 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration The TEV shown in Figure 11 works quite well provided the pressure drop across the evaporator is small. In many coil type evaporators, however, pressure drop can be quite large, causing a reduction in temperature and pressure near the evaporator outlet. This reduction in temperature causes a pressure reduction in the thermal element, resulting in a reduced valve opening. To obtain a sufficient valve opening to supply enough liquid refrigerant to match the cooling load, the refrigerant superheat must increase further. This means more heat transfer surface area will be used for providing superheat than for evaporating liquid refrigerant. The net result will be reduced evaporator capacity. To compensate for a large evaporator pressure drop, a thermal expansion valve may be equipped with an equalizing line which connects the underside of the diaphragm with the outlet of the evaporator instead of the inlet. The reduction in the downward force of the thermal element is now compensated for by a reduction in upward force exerted by evaporator pressure. The valve will now admit enough liquid to maintain 5°C to 6°C of superheat at the outlet and provide for maximum evaporator capacity. A diagram of an evaporator equipped with a thermal expansion valve with an equalizing line is shown in Figure 12. Figure 12 – Thermostatic Expansion Valve with Equalizing Line 151 kPa Bulb Pressure 103 kPa 142 kPa, -7°C In Equalizing Strainer 118 kPa, -10°C Line 48 kPa Spring Pressure (saturation temperature) 151 kPa, -6°C 103 kPa, -6°C Super Heat 103 kPa, -12°C Low-Pressure Float Valve In a flooded evaporator, the low-pressure float valve is used to maintain a constant level of liquid refrigerant. Its name is derived from the fact that the float is installed in the low-side of the system. The float responds to the level of the liquid in the evaporator, and controls a valve that opens or closes to maintain the desired level, regardless of cooling load. The float may be installed directly in the evaporator or in the accumulator (Figure 13(a)). It may also be installed in an external float chamber, attached to the low-pressure receiver (accumulator) of a liquid overfeed system (Figure 13(b)). External float valves are common on large water chillers or on pasteurization coolers where rapid response to changes in product flow is necessary to ensure all product is properly chilled. A bypass line (Figure 13(b)), with a hand-operated expansion valve, is usually installed around the external float chamber on large capacity systems. This provides cooling if the float valve fails. Isolation valves on the liquid and vapour connections to the low-pressure receiver allow the float chamber to be isolated for repairs, without having to evacuate the refrigerant from the evaporator. In liquid overfeed systems, a liquid refrigerant pump provides forced circulation of refrigerant through the evaporator. If the pump fails, cooling can be maintained with a manual expansion valve that bypasses the pump. 3-22 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Figure 13 – Flooded Coil-Type Evaporator with Low-Pressure Float Valve Pressure Evaporator Vapour to Accumulator Relief Compressor Float Bypass Valve Baffle Flash Expansion Accumulator Liquid Chamber Valve from Liquid Level Receiver Vapour to compressor Float Control Liquid-Vapour Mixture Strainer External Pump Float Liquid from Chamber Receiver Oil Drain Pump Bypass (a) Pump Bypass (b) Expansion Valve Another method to control refrigerant level in large flooded evaporators uses a low-pressure float switch in combination with a solenoid valve. The float switch is mounted in a float housing, which is connected to the evaporator. The switch controls the operation of a solenoid valve in the liquid refrigerant line between the receiver and evaporator. This system is shown in Figure 14. Figure 14 – Water Chiller with Electric Level Control Suction Float Chilled Switch Manual Solenoid Control Water Valve Valve 3-23 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration High-Pressure Float Valve Like the low-pressure float valve, the high-pressure float valve is also a liquid level operated control valve. However, the high-pressure float is located on the high-side of the system, and operated by the liquid refrigerant level on that side. Figure 15 shows a cross-sectional view of this valve. Figure 15 – High-Pressure Float Valve Body Float Arm Head Pivots Valve Pin Valve Seat Outlet Float Vent Discharge Tube Inlet Ball Tube The float maintains a constant level in the float chamber. If the evaporation rate increases, the compressor pumps more vapour to the condenser. This causes more liquid refrigerant to flow into the float chamber. The level in the chamber then rises, and the float valve opens to allow more liquid to flow to the evaporator. Since the liquid refrigerant flows directly from the condenser into the valve housing, there is no provision in the system for storage of the refrigerant, other than in the evaporator. For this reason, the amount of refrigerant charge in a system with a high-pressure float valve is critical. Too much refrigerant in the evaporator may cause liquid to carry over with the vapour to the compressor. Insufficient liquid will starve the evaporator and reduce the system capacity. A high-pressure float valve installation is shown in Figure 16. An intermediate pressure-reducing valve is usually placed before the evaporator to reduce frosting of the line immediately after the float valve. Figure 16 – High-Pressure Float Valve Installation Weighted Valve Pin Evaporator Intermediate Valve (Pressure Reducing Valve) High Pressure Condenser Float Compressor 3-24 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Capillary Tube The capillary tube is the simplest of all metering devices. It consists of a fixed length of tubing with a very small inside diameter. Because of the high resistance resulting from its length and small bore, it creates a considerable pressure drop along its length. Therefore, it restricts the flow of liquid from the condenser to the evaporator and maintains the pressure difference between the high side and the low side. Because the bore of the tube is so small, capillary tubes are limited in application to small refrigeration units. Figure 17 shows the installation of a capillary tube in a low capacity system. Figure 17 – Refrigeration System with Capillary Tube Suction High Pressure Cooling Line Superheated Unit Vapour Low Pressure Superheated Hermetic Vapour Compressor Low Pressure Liquid/Vapour Solder Capillary Tube Screen High Pressure Condenser Liquid (Courtesy of Borg-Warner) The solder connection in Figure 17 permits heat exchange between the cool vapour leaving the evaporator, and the warm refrigerant liquid entering the evaporator. This improves system efficiency by using cool low side refrigerant vapour to sub cool the liquid refrigerant. When sub-cooled, refrigerant produces less flash gas in the evaporator. This increases the net refrigerating effect of the system. Since the capillary tube is not very sensitive to load changes, and because of its small capacity, it is used only on small refrigerating equipment with fairly constant loads, such as domestic refrigerators, freezers, and air conditioners. 3-25 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 5 Describe the different methods used to control evaporator capacity. Refrigeration systems are designed to meet maximum cooling loads. Most of the time, the cooling load is not at the maximum. For this reason, refrigeration systems must be able to modulate their cooling capacity to meet lower cooling loads effectively and efficiently. Otherwise, the refrigeration system must be cycled on and off, which is inefficient, costly, and hard on the machinery. On-off control is only a reasonable capacity control solution for small household or commercial appliances. To efficiently control the capacity of a refrigeration system that operates continuously, the evaporator capacity (evaporator tonnage) must be controlled. Use of evaporator capacity control affects the amount of vapour produced. Therefore, evaporator capacity control requires simultaneous compressor capacity control. Evaporator Capacity Control Evaporator capacity can be controlled by using sectional evaporators or evaporator dampers. Sectional Evaporators A sectional evaporator is divided into two or more sections (Figure 18), each with a refrigerant flow control valve. Sections of the evaporator can be shut off as the cooling load decreases. The evaporator in Figure 18 has two sections, “A” and “B”. When the load decreases, a solenoid valve installed in the liquid refrigerant line closes to make section “A” inoperative. The area of each section is proportional to the reduction in cooling load. Figure 18 – Two-Section Evaporator Solenoid Evaporator A Valve Refrigerant Flow Control Valve B Liquid from Receiver Vapour to Compressor 3-26 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Evaporator Dampers An evaporator may be equipped with a face damper to vary the quantity of air passing over the evaporator coils (Figure 19). These face dampers are controlled by a positioner that opens and closes according to a control signal. The positioner is designed so it will only close completely for maintenance or in the event of a fire alarm shutdown. One problem with using only face dampers is that the airflow varies with cooling load. In HVAC applications, the reduction in airflow caused when the dampers close reduces the ventilation air to unacceptable levels. Figure 19 – Face Damper Control Air Duct Evaporator Air Flow Face Damper Figure 20 shows another type with both face and bypass dampers. The dampers are connected to the same damper drive, so that the bypass damper opens as the face damper closes. The cooling capacity is controlled by varying the amount of air passing through the evaporator coils. Regardless of the damper position, the quantity of air passing through the duct remains constant. Figure 20 – Face and Bypass Damper Capacity Control Air Duct Bypass Damper Air Flow Face Damper Multispeed blowers and dampers are often used in combination to provide a better balance between the amount of air supplied by the fan and the amount required for cooling. 3-27 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 6 Describe the different methods used to control the capacity of refrigeration compressors. Compressor Capacity Control Refrigeration compressors are most often driven by constant speed electric motors. Since the capacity of the compressor is designed for maximum calculated cooling load, and often exceeds the actual load, compressor capacity must be regulated. Various methods used to control compressor capacity are: a) Intermittent operation: Two-position start-stop control b) Continuous operation with reduced output: Cylinder unloading Cylinder bypass Hot gas bypass Compressor speed control Suction throttling Variable inlet guide vanes Variable position slide valves Intermittent Operation The compressor is stopped when the desired low temperature of the substance to be cooled is reached, and it is started up again when the temperature rises to a certain level. This method is only used on small systems with fairly constant loads as the most power is consumed when starting compressors and if used on large compressors the demand charges for the repetitive starting would be too costly. Continuous Operation with Reduced Output Reciprocating Compressor Capacity Control In larger systems, especially when operated on light loads, frequent starts and stops would put undesirable stresses on the motor and switchgear, cause power fluctuations, and create a very high and costly electrical demand. Therefore, in larger systems, the compressor operates continuously, but at reduced capacity. The capacity of reciprocating compressors is reduced by: Cylinder unloading Cylinder bypass Hot gas bypass Variable speed drive 3-28 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Cylinder Unloading Cylinder unloading is a method of capacity reduction that deactivates one or more cylinders in sequence, as the cooling load dictates. Cylinder unloaders work by keeping the intake valves of one or more cylinders in the open position, preventing the compression of the vapour drawn in during the suction stroke. Figure 21 shows a cylinder unloader and the controlling solenoid valve. Figure 21 – Cylinder Unloader and Solenoid Valve Solenoid Valve Connection to Discharge Side To Unloader of Compressor Wires to Electrical System Unloader Piston From Solenoid Valve Cylinder Head Valve Plate Suction Valve Cylinder 3-29 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration As the compressor suction pressure falls to a preset value, a pressure switch, sensing low pressure on the suction side, energizes a solenoid valve. The valve opens and admits refrigerant at condenser pressure to the unloader piston. This pressure moves the unloader piston downward to depress the suction valves, holding them in the open position. When this occurs, refrigerant vapour is drawn into the cylinder during the suction stroke and is discharged back to the suction line during compression. When the suction pressure rises to a certain value, the pressure switch de-energizes the solenoid valve, causing the unloader piston to return to its normal position, allowing the suction valves to once again operate. A similar unloader is required on an intermittently driven (on-off) compressor, so the compressor can start in an unloaded condition until it reaches operating speed. This condition reduces the starting current of the electric motor. Capacity control of multi-cylinder continuously operating compressors may be accomplished by unloading one or more cylinders. The cylinders load and unload in sequence, in response to changes in load. Cylinder Bypass Another method of controlling the capacity of reciprocating compressors is by bypassing the discharge from one or more cylinders back to the suction side of the compressor. This method is called cylinder bypass. Figure 22 shows a two-cylinder compressor that can bypass one cylinder. When the suction pressure at the compressor drops to a preset value, a pressure switch energizes a normally-closed solenoid valve. This permits refrigerant to discharge from cylinder “B” to flow back into the compressor suction line. The discharge from cylinder A is allowed to pass to the condenser. A check valve in the line connecting the two cylinder discharge lines prevents the discharge from cylinder “A” bypassing to the suction. In this situation, the compressor capacity is reduced to 50%. When the suction pressure increases to the preset cut-off pressure, the pressure switch de-energizes the solenoid. The bypass line closes, returning the compressor to full capacity operation. Figure 22 – Cylinder Bypass De-superheating Expansion Valve Mixing “T” Liquid Refrigerant Cylinders Vapour from Evaporator A B Compressed Vapour to Condenser S Solenoid Valve 3-30 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Hot Gas Bypass Figure 23 shows a simple hot gas bypass capacity control. When a reduction of compressor capacity is required, a normally closed solenoid valve located in the bypass line is energized by the pressure or temperature at the compressor inlet. When energized, some hot high-pressure gas is allowed to go directly into the suction line. Figure 23 – Hot Gas Bypass Thermal Bulb Vapour from Evaporator De-superheating Cylinders Expansion Valve Solenoid Valve S S A B Liquid Refrigerant Mixing “T” Solenoid Valve Compressed Vapour to Condenser This type of capacity control has several disadvantages. Because compression occurs normally, there is little or no reduction in power consumption when the bypass line is open. As well, compressor overheating can occur. The simple hot gas bypass is used only on small compressors. It is often used in conjunction with other types of capacity control when it is necessary to provide capacity control down to 0% loading or to unload a compressor before starting. It is also used with some centrifugal compressors. Centrifugal Compressor Capacity Control Speed Control Centrifugal compressor capacity varies according to its rotational speed. There are several ways of controlling this speed. Large compressors may be driven with steam turbines. In this case, a temperature-sensing device, located in the chilled water outlet, controls the speed of the compressor by regulating the steam flow to the turbine. If the compressor is driven by an electric motor, a variable frequency drive can be employed to vary the speed. Finally, hydraulic couplings installed between a constant-speed driver and the compressor can also vary compressor speed. Suction Throttling Suction throttling is accomplished by a butterfly damper installed at the inlet to a centrifugal compressor. Although the damper can be easily adapted to automatic control with the use of a piston type positioner, its application is not economical because the input power to the compressor does not decrease by the same amount as the capacity. Suction throttling has been replaced by variable inlet guide vanes. Variable Inlet Guide Vanes Variable inlet guide vanes are commonly used on centrifugal compressors for capacity control (see Figure 24). The vanes are linked so that they operate together. They are operated by a rack and gear arrangement attached to a piston. An increase in air or oil pressure moves the piston and rack against the force of a spring. This opens the inlet vanes. When the air or oil pressure to the piston decreases, the spring returns the vanes to the closed position. A vane position indicator shows the position of the blades. A vane switch acts as a safety device to prevent the compressor from being started unless the vanes are closed (fully unloaded). 3-31 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration The inlet guide vanes can occupy an infinite number of positions. Therefore, inlet vane control can finely adjust centrifugal compressor capacity. As well, inlet vane dampers control compressor capacity very efficiently, compared to suction throttling. As the vanes move towards the closed position to reduce compressor capacity, they swirl the refrigerant vapour the same direction that the compressor impeller turns. This results in a reduction in the compressor power consumption. So, as compressor capacity is reduced, compressor power consumption is also reduced. Figure 24 – Centrifugal Compressor with Diffuser Vanes Adjustable Diffuser Vanes Impeller Adjustable Inlet Vanes (Courtesy of Worthington Corporation) Figure 25 show centrifugal compressor inlet vanes removed from a centrifugal chiller undergoing maintenance. On the left, the inlet vanes are in the fully unloaded position. The photo on the right shows the inlet valves in the fully loaded position. Figure 25 – Centrifugal Compressor Inlet Vanes Screw Compressor Capacity Control Variable Position Slide Valve Constant speed screw compressors often use variable position slide valves to adjust their capacity. Slide valves also permit either unloaded or low-load compressor starts. Screw compressors with variable position slide valves are far more economical than compressors with start-stop capacity control. 3-32 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 A slide valve is a variable opening in the screw compressor housing. The slide valve may be positioned in steps, depending on the required cooling capacity. These steps provide 50, 75, and 100 percent capacity. Solenoid valves operate in sequence, applying hydraulic pressure (lube oil) to a piston that moves the slide valve into the correct position. Some compressors have infinitely variable, stepless capacity control, ranging from 10 to 100 percent capacity. A slide valve position indicator mounted on the compressor shows the operator how much load is on the compressor. Slide valve position switches can also be used to indicate the compressor capacity on a central computerized control display. Figure 26 shows how the slide valve works. In the top diagram, the compressor is fully loaded. The slide valve is against the fixed end. Refrigerant is drawn in on the right-hand side of the screw, and compressed over its entire length. In the bottom diagram, the compressor is partially unloaded. Compression begins at the leading edge of the slide valve. In a low load position, compression begins at a point further into the compressor housing. Therefore, only a portion of the screw is involved in gas compression. The part of the suction gas that is not compressed is diverted back the suction. Figure 26 – Screw Compressor Capacity Control Using Slide Valve Variable Speed Drive Variable speed drives, including variable frequency drives (VFDs), have been successfully used for capacity control of screw-type air compressors, and are being increasingly applied to refrigeration screw compressors. Screw compressors are categorized as positive displacement machines. Therefore, their capacity (kg/h) changes with rotational speed. In the case of an HVAC chiller, a temperature controller monitors the chilled water supply temperature. On startup, when the load is low but the chilled water temperature is high, built-in control algorithms prevent the VFD from operating the compressor at full capacity. This keeps the chilled water temperature from overshooting the temperature set point. As well, this limits the start-up current. During regular operation, the temperature control algorithm adjusts the capacity of the compressor via a control output signal to the VFD. If the chilled water supply temperature is too low, the compressor speed decreases. If the temperature is too high, the compressor speed increases. 3-33 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter Summary This chapter discussed controls used to help refrigeration plants operate safely and efficiently, under varying conditions. Though designed to operate at particular high-side and low-side temperatures, load changes and atmospheric conditions influence system operation. Refrigeration systems need capacity controls to automate the response to load changes. These controls include HMIs, metering devices, evaporator capacity controls, and compressor capacity controls. These automatically adjust the refrigeration plant operation so that the cooling load matches the plant cooling capacity at all times. The control system manages complex system interactions in real time, so manual intervention is not continually required while the equipment is in operation. Occasionally, operating condition deviate from design parameters. These deviations may cause product spoilage or system damage. These are costly, and may even be dangerous. Therefore, refrigeration plants are equipped with safety limits that prevent damage due to excessively high system pressures or excessively low temperatures. These include low water temperature cut-offs, discharge pressure cut-offs, low oil pressure cut-offs, and flow switches, to name a few. Operators must be aware of the function of all operating and safety limit controls in their plants. When a control fails, or operates outside of normal parameters, an operator must be able to control elements of the system manually. With this knowledge, operators can respond effectively when adverse conditions arise in their plants. 3-34 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Knowledge Exercises – Chapter 3 Name: _____________________________ Date: _______________________________ Instructor: __________________________ Course: _____________________________ Objective 1 1. What happens if the refrigeration plant capacity does not match the cooling load? 2. What are the units used to rate refrigeration plant capacity? 3. Explain how thermostats may control refrigeration system compressor operation. 4. When equipped with temperature-actuated controls, a refrigeration compressor may short-cycle. How is this prevented? 5. A temperature limit control cuts-in at -5°C, and cuts out at -8°C. What is the differential setting? What is the range? 3-35 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter 3 (Cont.) 6. Make a simple diagram of a refrigeration compressor controlled by a cooling thermostat. 7. Explain the operation of a compressor controlled by suction pressure. Objective 2 8. What is the purpose of the low chilled water temperature cut-off switch? 9. What feature prevents centrifugal compressors from starting at full-load? 3-36 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Chapter 3 (Cont.) 10. What are the conditions that may cause an HVAC chiller to trip on low chilled water temperature when first started? What can on operator do to prevent such a trip? Objective 3 11. What conditions can lead to excessively high condenser pressure? 12. What are non-condensable gases? How are they a problem in a refrigeration system? 13. An ammonia system has a high-side design pressure of 1500 kPa. It is equipped with a high-side safety valve set to 1500 kPa. What is the maximum setting for the pressure-limiting device required under CSA B52 code? 3-37 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter 3 (Cont.) 14. The oil supplied by an ammonia compressor lube oil pump is at 430 kPa. The compressor crankcase pressure is 250 kPa. What is the net lube oil pressure? 15. Explain why refrigeration compressor low oil pressure cut-off switches sense differential pressure. 16. How does a low oil pressure cut-off switch protect a refrigeration compressor if the oil pump fails when the compressor is running? Objective 4 17. When comparing identical refrigeration systems, what determines refrigerating system capacity? 18. What happens when a hand-operated expansion valve flows more refrigerant than the evaporator needs to meet the cooling load? 3-38 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Chapter 3 (Cont.) 19. What happens when a hand-operated expansion valve flows less refrigerant than the evaporator needs to meet the cooling load? 20. Draw a simple diagram that shows how automatic expansion valves work. 21. Why are thermostatic expansion valves called “constant superheat” valves? 3-39 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter 3 (Cont.) 22. Why do some TEVs use external equalizer piping connections? 23. What type of evaporator would use a low-pressure float valve? Objective 5 24. Why is it important to control the capacity of an evaporator? 25. Why are face dampers not used in HVAC applications to control evaporator capacity? 3-40 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Chapter 3 (Cont.) 26. Why are face and bypass dampers better for evaporator capacity control than face dampers? Objective 6 27. List the common methods of controlling the capacity of refrigeration compressors in continuous operation. 28. How does cylinder unloading reduce the capacity of a refrigeration compressor? 3-41 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter 3 (Cont.) 29. With the aid of a sketch, describe the operation of a cylinder bypass system for refrigeration compressor capacity control. 30. What are the disadvantages of hot gas bypass capacity control? 3-42 4th Class Edition 3 Part B  Refrigeration System Control and Operation Chapter 3 Chapter 3 (Cont.) 31. With the aid of a simple sketch, explain the operation of a screw compressor slide valve. 3-43 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter Glossary Term Definition AEV See Automatic Expansion Valve (AEV). Automatic Expansion A pressure-actuated flow control device that regulates the supply of liquid Valve (AEV) refrigerant into an evaporator. Catastrophic Leak A major uncontrolled release of refrigerant that presents serious danger to employees, building occupants, the public, or the environment. Centrifugal Chiller A packaged refrigeration plant that uses a centrifugal compressor to lower the temperature of water. Chiller An evaporator designed to cool water or brine. Cylinder Bypass A method of controlling reciprocating compressor capacity that uses a bypass piping arrangement to equalize the suction and discharge pressures of cylinders sequentially in response to the load requirements. Cylinder Unloading A method of controlling reciprocating compressor capacity that effectively shuts off cylinders in sequence in response to the load requirements. Demand Limiter A device or control strategy used to prevent excessive current draw during the initial startup of a piece of equipment. Expansion Valve A device in a refrigeration system that regulates the flow of liquid refrigerant into the evaporator. High-Pressure Float A flow control device that regulates the flow of liquid refrigerant into an Valve evaporator by responding to the accumulations of liquid in the high-pressure side of the system. Hot Gas Bypass A piping system used to control compressor capacity, which directs the flow of hot, pressurized refrigerant gas back to the suction side of the compressor. Low-Pressure Float A flow control device that regulates the flow of liquid refrigerant into a Valve flooded evaporator, and is actuated by the level of refrigerant liquid in the evaporator. Low-Pressure A vessel installed in liquid overfeed refrigeration systems that provides a Receiver reservoir of low temperature liquid for circulation through the evaporators, and intercepts liquid returning from the evaporators to prevent slugging the compressor with liquid. Also called an accumulator. Manual Expansion An expansion valve operated by hand. Valve Non-Condensable Gas A gas that does not change state under the pressure and temperature conditions within a refrigeration or steam system. Pull Down The act of reducing the temperature of a refrigerated medium, on initial cooling plant startup. Pump Down A procedure for removing refrigerant liquid from a component of a refrigeration system, often for prolonged shutdown, maintenance, or repairs. Short-Cycling The detrimental repetitive starting and stopping of equipment within a short time period. TEV See Thermostatic Expansion Valve (TEV). Thermostatic A metering device that regulates refrigerant flow into an evaporator, in Expansion Valve response to changes in evaporator pressure and refrigerant gas superheat. (TEV) Unloader A mechanism for reducing the capacity of air and refrigeration compressors during operation or startup. 3-44 4th Class Edition 3 Part B