Presentation on theme: "Refrigeration Systems"— Presentation transcript:
1Refrigeration Systems Chapter 6Refrigeration SystemsRefrigeration • Mechanical Compression Refrigeration • Absorption Systems • Troubleshooting and Maintaining Refrigeration Systems • Refrigerant Regulations • Refrigerant Handling
2A refrigeration system controls the absorption and rejection of heat by refrigerant to move heat from inside a cooled space to outside the cooled space.Refrigeration is the process of moving heat from one area to another by use of a refrigerant in a closed system. A refrigeration system is a closed system that controls the pressure and temperature of a refrigerant to regulate the absorption and rejection of heat by the refrigerant. A refrigerant is a chemical that vaporizes (boils) at low temperatures. A refrigeration system moves heat from inside a cooled space and rejects it outside the cooled space. See Figure 6-1.
3In a mechanical compression refrigeration system, a compressor is used to produce the refrigeration effect.Mechanical compression refrigeration is a refrigeration process that produces a refrigeration effect using a compressor and a pressure control device. A mechanical compression refrigeration system consists of refrigerant inside the system, a compressor, condenser, metering device, evaporator, and accessories such as filter-dryers, sight glasses, accumulators, and liquid receivers. See Figure 6-2.
4Refrigerant vapor pressure charts list the saturation temperature and pressure of refrigerants. Most refrigerants have exact pressure and saturation temperature relationships. At any pressure, there is only one saturation temperature. A refrigerant that is vaporizing or condensing is at its saturation temperature. Saturated liquid is liquid at a certain pressure and temperature that vaporizes if the temperature increases. Saturated vapor is a vapor at a certain pressure and temperature that condenses if the temperature decreases. A saturated liquid is at its vaporization temperature and a saturated vapor is at its condensing temperature. For this reason, if the saturation temperature of a refrigerant is known, the pressures and temperatures inside the evaporator and condenser are known. Refrigerant vapor pressure charts list the temperature and pressure relationship of refrigerants and are used when testing refrigeration system performance. See Figure 6-3. For example, when refrigerant HCFC-22 is at 10°F, its saturation pressure is 32.8 psi.
5Refrigeration compressors include reciprocating, vane, centrifugal, and screw compressors. A compressor creates pressure in a mechanical compression refrigeration system. The compressor creates the low pressure that pulls the refrigerant from the evaporator, then compresses the refrigerant and forces it through the condenser. Compressors include reciprocating, vane, centrifugal, and screw compressors. See Figure 6-4.
6Refrigerant compressors are available in hermetic and semi-hermetic configurations. Compressors are available in different configurations based on the applications and size of the system. Compressors are available in hermetic and semi-hermetic configurations. See Figure 6-5. A hermetic compressor is a compressor that is completely sealed inside a welded case. The compressor motor is cooled by refrigerant passing around the motor windings. Hermetic compressors are usually not repairable and are replaced when defective. Hermetic compressors must be disposed of according to local and national hazardous waste regulations because they contain refrigerant oil. A semi-hermetic compressor is a sealed compressor that can be serviced through removable access plates. Semi-hermetic compressors are also known as serviceable compressors because the housing can be opened and the components can be serviced on the job site.
7An air-cooled condenser removes heat from high-pressure refrigerant vapor by air blown across the condenser coils.An air-cooled condenser is a condenser that uses air as the condensing medium. See Figure 6-6. Air-cooled condensers are the most common condenser used in refrigeration systems. Air is blown across finned condenser coils and the refrigerant vapor is cooled and condenses into a high-pressure liquid, giving up its heat to the air. All the vapor should condense by the time the refrigerant passes approximately two-thirds of the way through the condenser. In the final third of the condenser, additional heat is removed from the liquid, subcooling it.
8Water-cooled condensers transfer heat from refrigerant vapor to water. A water-cooled condenser is a condenser that uses water as the condensing medium. Heat is transferred from the refrigerant to the water. Water-cooled condensers are available in shell-and-coil, shell-and-tube, and tube-in-tube configurations. See Figure 6-7. In a shell-and-coil condenser, water flows through a coil and refrigerant vapor circulates in the shell surrounding the coil. In a shell-and-tube condenser, water flows through tubes and the refrigerant vapor circulates in the shell surrounding the tubes. In a tube-in-tube condenser, refrigerant flows through a small pipe inside a large pipe that contains water. The refrigerant enters as a vapor and leaves as a liquid. In all water-cooled condensers, cool water enters the condenser and hot water leaves the condenser.
9Evaporative condensers reject heat through the evaporation of water. An evaporative condenser is a condenser that uses the evaporation of water from the outside surface of the coils to remove heat from refrigerant. See Figure 6-8. In an evaporative condenser, water is sprayed down over the condenser coils as air flows upward. Heat from the condenser causes some of the sprayed water to evaporate off the coils, which draws heat from the condenser coils and causes the refrigerant vapor to condense. The airflow carries away the water vapor and the heat it contains. Makeup water is added to the condenser as required.
10A fin comb is used for condenser maintenance to straighten damaged or bent fins, which limit airflow and reduce condenser efficiency.Most air-cooled condenser problems result from dirt buildup on the coils or restricted airflow that reduces heat transfer. Air-cooled condensers must be kept clean. Fan operation and airflow levels should be checked regularly. Condenser coils can be cleaned with low-pressure compressed air (less than 15 psi), brushes, and chemical sprays. Fins that become bent should be straightened using a fin comb. See Figure 6-9. A fin comb is a device used to straighten the edges of the thin metal that extend from a condenser or evaporator coil. A fin comb has different teeth corresponding to a different number of fins per inch.Robinair Division, SPX Corporation
11A thermostatic expansion valve uses temperature readings at the evaporator outlet to control the rate of refrigerant flow into the evaporator.A thermostatic expansion valve (TXV) is a valve that uses temperature readings at the evaporator outlet to control the rate of refrigerant flow into the evaporator. A TXV controls the flow of refrigerant into an evaporator by maintaining a constant superheat setting. Superheat is sensible heat added to a substance after it has turned to vapor. The amount of superheat is the difference between the temperature of the refrigerant leaving the evaporator and the saturation temperature of the low-pressure side. For example, if an R-22 system has a low-pressure side reading of 33 psi, its saturation temperature is 10°F. If the temperature of the pipe at the outlet of the evaporator is 20°F, the superheat is 10°F. The superheat setting is the temperature difference between the point immediately after the TXV and the outlet of the evaporator. See Figure Toward the end of its passage through the evaporator, the refrigerant has completely evaporated into vapor and is superheated before leaving the evaporator. Superheating the vapor in the evaporator ensures that all liquid refrigerant is evaporated before leaving the evaporator.
12The opening and closing of a thermostatic expansion valve is controlled by the pressure in the remote bulb.A TXV is opened and closed by pressure exerted by a remote bulb acting on the top of the valve diaphragm. The pressure on the top of the diaphragm is created by the evaporation and condensation of refrigerant contained in a remote bulb. See Figure The remote bulb is attached directly to the outlet tubing of the evaporator. A capillary tube connects the remote bulb and the TXV. A capillary tube is a long, thin tube that resists fluid flow, which causes a pressure decrease. The temperature of the remote bulb is the same as the outlet tubing of the evaporator. A high temperature in the outlet tubing of the evaporator from increased superheat causes the pressure in the bulb to increase. Increased superheat causes the refrigerant to vaporize more vigorously and superheat the vapor earlier in the evaporator. This allows more time for superheating the vapor. The evaporator needs a greater flow of liquid refrigerant to replace what is being quickly vaporized. Each TXV is factory-set to maintain a specific superheat, usually between 8°F and 12°F.
13An automatic expansion valve controls the temperature of the refrigerant by controlling the pressure in the evaporator.An automatic expansion valve is a valve that is opened and closed by the pressure in the line ahead of the valve. Automatic expansion valves maintain a constant pressure in the evaporator to control the flow and temperature of refrigerant. See Figure As the evaporator pressure falls, the valve opens to allow a greater flow of refrigerant to replace the refrigerant that is evaporated. As evaporator pressure rises, the valve closes to restrict flow of refrigerant. The opening and closing of the valve occurs as evaporator pressure is applied to a diaphragm through an internal port. Automatic expansion valves are used on refrigeration systems that have constant cooling loads. Automatic expansion valves cannot compensate for changes in the cooling load or condensing medium temperature.
14As refrigerant is forced through the capillary tube, it loses pressure until it is at the desired evaporator pressure.A capillary tube is a long, thin tube that resists fluid flow, which causes a pressure decrease. A capillary tube is used as a metering device on small refrigeration systems, such as refrigerators or window-mounted air conditioners. Refrigerant liquid is forced through the capillary tube by pressure from the compressor. See Figure The high pressure is reduced as the liquid is forced through the small diameter and long length of the capillary tube. At the end of the tube, the pressure of the refrigerant is reduced to the desired evaporating point in the evaporator.
15An evaporator vaporizes low-pressure refrigerant liquid into a low-pressure vapor. An evaporator is the component in which low-pressure refrigerant liquid vaporizes into a low-pressure vapor. The most common evaporator is the air-cooled evaporator. See Figure An air-cooled evaporator uses a fan to circulate air from the cooled space across the evaporator coils. Heat in the air from the cooled space flows across the cool evaporator coils, vaporizing the refrigerant and cooling the air. The low-pressure refrigerant vapor is sent to the compressor, carrying away the heat that caused the refrigerant to vaporize. The constant flow of refrigerant through the evaporator keeps it cool. Air-cooled evaporators are usually classified as dry evaporators. By the time the refrigerant flows to the end of the evaporator, all the liquid has vaporized and the refrigerant is dry (contains no liquid refrigerant). Most evaporator problems result from dirt on the coils or restricted airflow across the coils. Evaporator coils must be kept clean. Fan operation and airflow levels should be checked regularly. Coils can be cleaned with low-pressure compressed air and straightened using a fin comb.
16A hot-gas defrost uses hot gas from the compressor to melt frost on the evaporator. Hot-gas defrost is evaporator defrosting using hot gas from the compressor. Hot-gas defrost is used in large refrigeration systems. See Figure When the timer starts the defrost cycle, a solenoid valve is opened in an additional line running between the compressor discharge and the evaporator. The hot refrigerant vapor flows from the compressor to the evaporator, bypassing the condenser and metering device. The hot refrigerant vapor from the compressor flows to the evaporator, melting the frost from the inside. The hot refrigerant vapor gives up its heat to the cold evaporator and condenses. The liquid refrigerant flows to a reevaporator, where it is evaporated into vapor by warm air blown across the reevaporator coil that is located outside the cooled space. During defrost, the defrost heater is energized to speed the defrost and melt the ice in the drain pan so the water can drain quickly. After defrosting, the system resumes normal operation. The defrost heater, reevaporator fan, and bypass solenoid valves are de-energized, allowing the system to resume cooling.
17An evaporator pressure regulating valve allows two evaporators running from the same compressor to maintain different temperatures.An evaporator pressure regulator is a valve that allows two evaporators running from the same compressor to maintain different temperatures. An evaporator pressure regulating valve is used when a cold freezer and a relatively warm cooler are located side-by- side and operate from the same compressor. See Figure The two different temperatures are maintained by an evaporator pressure regulating valve (hold-back valve) on the suction line of the warmer evaporator. The evaporator pressure regulating valve holds back the refrigerant and maintains a higher pressure in the warmer evaporator. This creates the warmer temperature because the saturation pressure has been raised. The compressor draws a low pressure directly from the freezer evaporator to maintain its cold temperature.
18Accessories are used for maintaining and controlling the flow of refrigerant in a refrigeration system.Accessories are used for maintaining and controlling the flow of refrigerant in a refrigeration system. Accessories include filter-dryers, sight glasses, accumulators, and liquid receivers. See Figure 6-17.
19Pressure switches control refrigeration system temperature through changes in system pressure. A pressure switch is a switch operated by the amount of pressure acting on a diaphragm, bellows, or electronic element. Pressure switches control refrigeration system temperature through changes in system pressure. A system not controlled by a thermostat has its temperature controlled by a pressure switch connected to the low-pressure side of the system. See Figure 6-18.Ranco Inc.
20Ammonia systems operate at high temperatures and pressures and must have special controls and fittings to control the release of ammonia gas.An ammonia system is a refrigeration system that uses ammonia as the refrigerant. See Figure Ammonia systems are used in applications that require the removal of large quantities of heat, such as dairies and ice making plants. Ammonia is capable of moving much more heat per pound of liquid vaporized than other refrigerants. Unfortunately, ammonia can cause death and serious injury if inhaled and can also damage food products. Therefore, many components in ammonia systems are used to contain and control the release of ammonia.
21The direction of refrigerant flow in a heat pump is controlled by a reversing valve. A heat pump is a mechanical compression refrigeration system that can reverse the flow of refrigerant, switching between heating and cooling modes. The heating and cooling is accomplished by operating the heat pump as a refrigeration system to cool the space, then reversing the refrigerant flow to provide heat. See Figure Reversing the refrigerant flow is accomplished by using a reversing valve. A reversing valve is a four-way valve that reverses the flow of refrigerant in a heat pump. The reversing valve consists of a spool and a cylinder that has four refrigerant line connections. A solenoid shifts the spool to cover or uncover ports allowing refrigerant to flow in the desired direction.
22Heat pumps may use air or water as the heat source. Heat pumps are classified as air-to-air or water-to-air. See Figure In an air-to-air heat pump, air circulates across the indoor and outdoor coils. In a water-to-air heat pump, air circulates across the indoor coil and the outdoor coil is immersed in water. Heat pumps can move heat from one part of a building to another or from outside to inside a building. Heat pumps can be designed as split systems with one coil inside and one coil outside, or as packaged units with both coils in one enclosure.
23Chillers use chilled water to cool large building spaces. A chilled water system (chiller) is a refrigeration system that cools water that is used to cool air. See Figure Chillers are used to cool large buildings such as hotels and stadiums. Chillers are classified as secondary refrigeration systems because the chilled water does the actual cooling of the building. Water is cooled to about 45°F in the chiller evaporator and pumped to the building where it cools the air.
24A cooling tower cools water from a condenser by the evaporation of water as it cascades through the tower.A cooling tower uses evaporation and airflow to cool water. A cooling tower cools water from the condenser by evaporating some of the water as it cascades through the tower. See Figure Cooling towers are usually located outside a facility. A fan forces air upward through the tower as warm condenser water is sprayed down and drips through rows of tiles. Some of the water evaporates as it falls, removing heat by evaporation and cooling the water left behind. The warm water vapor is carried away by the air rising through the tower. Moisture eliminators (louvers or baffles) remove water droplets before the air leaves the tower. The cool water falls to the bottom of the tower for reuse in the condenser. Makeup water is added to replace the water lost to evaporation. All cooling towers require water treatment to prevent algae growth. Cooling towers must be kept clean and fans must be inspected regularly due to the wet conditions in which they operate.
25Absorption systems use a generator and absorber in place of the compressor to raise system pressure. An absorption system is a refrigeration system that uses the absorption of refrigerant by another chemical to facilitate heat transfer. Absorption systems have a generator and absorber in place of the compressor to raise system pressure. Absorption systems are used in small refrigerators in mobile homes and in industrial settings where large quantities of surplus heat are available to fuel the generator. A solution of absorbant and refrigerant is heated in the generator. The heat source can be steam, exhaust from diesel engines, electricity, or an oil- or gas-fired furnace built into the generator. See Figure The absorbant is a liquid capable of absorbing and transporting the refrigerant. For example, water absorbs and transports ammonia. Water is the absorbent and ammonia is the refrigerant. The refrigerant is vaporized as a high-pressure vapor in the generator and condenses in the condenser. The refrigerant flows through the metering device to the evaporator. The refrigerant leaves the evaporator as a low-pressure refrigerant vapor and flows into the absorber, where it mixes with the absorbant from the generator. The solution of absorbant and refrigerant is pumped back to the generator, where the cycle begins again.
26Gauge manifolds are used to take pressure readings, add or remove refrigerant, and remove air from a system before it is filled with refrigerant.System pressure readings are taken using a gauge manifold. A gauge manifold is a set of valves and pressure gauges used to determine refrigerant system pressures and add or remove refrigerant. Refrigerant gauge manifolds include three- and four-hose manifolds. See Figure The left-hand gauge and valve are blue and are used to take low-pressure side pressure and vacuum readings. This gauge is referred to as a compound gauge because it can register positive pressure and vacuum readings. The right-hand gauge and valve are red and are used to take high-pressure side readings. The hoses attached to each side of the manifold are color-coded to match the low-pressure side and high-pressure side gauges. Only the red and blue hoses are used when taking pressure readings. The third hose, which is usually yellow, is left attached to the manifold, as is the fourth hose on a four-hose gauge manifold. These hoses are used to evacuate and charge systems. Evacuation is the removal of all air from a system before charging it with refrigerant.
27Service valves are front-seated for isolating parts of the system, mid-seated for adding or removing refrigerant or taking system pressures, and back-seated during normal operation.The blue and red hoses are attached to the service port outlets of system service valves. A service valve is a three-way manually operated valve used to charge or remove refrigerant or monitor system pressure. Service valves are usually located on or near the compressor, though they can be located at other points in large systems. Service valves are placed in the front-seated, mid-seated, or back-seated position. See Figure 6-26.
28Refrigeration system pressure readings are taken by connecting the blue hose to the low-pressure side service valve and the red hose to the high-pressure side service valve with both gauge manifold valves front-seated.System pressure readings are taken by connecting the blue hose to the low-pressure side service valve and the red hose to the high-pressure side service valve. The third hose remains connected to the gauge manifold. If a four-hose manifold is used, the fourth hose also remains connected to the manifold. Both gauge manifold valves remain front-seated so no refrigerant enters the third or fourth hose. See Figure The service valves are opened slightly to purge air from the hoses at the manifold connection, allowing a small amount of refrigerant to escape. This removes any air from the hoses, which can lead to false readings. No air should be introduced into a refrigeration system.
29A head pressure controller prevents the condenser pressure from falling too low and starving the evaporator for refrigerant.A head pressure controller is a refrigeration system component that prevents the high-pressure side pressure from falling too low. In cold conditions, the condenser can work too efficiently and lower the refrigerant temperature and pressure so much that not enough refrigerant is forced into the evaporator. This occurs when there is not enough pressure forcing refrigerant through the metering device. The evaporator becomes starved for refrigerant and the cooled space temperature rises because there is not enough vaporizing refrigerant to absorb heat in the evaporator. This situation is prevented by using a head pressure controller that is connected to the high-pressure side tubing. See Figure 6-28.
30The EPA has established regulations under Section 608 of the Clean Air Act to regulate the handling of ozone-depleting substances.Section 608 of the CAA promotes minimizing emissions and maximizing recycling of ozone-depleting substances. See Figure Under Section 608 of the CAA, the EPA has established regulations that do the following:• Require service practices that maximize recycling of ozone-depleting substances such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) during the servicing and disposal of air conditioning and refrigeration equipment.• Set certification requirements for recycling and recovery equipment, technicians, and reclaimers.• Restrict the sale of refrigerants only to certified technicians.• Require persons or technicians servicing or disposing of air conditioning and refrigeration equipment to prove to the EPA that recycling and recovery equipment being used is in compliance with EPA rules.See complete list on pages 223–224.
31A recovery unit is used to recover refrigerant for reuse. Refrigerant must be recovered when removed from a system. Refrigerant recovery is the process of removing refrigerant from a system and capturing it in a recovery cylinder, with no cleaning of the refrigerant. Recovery is accomplished using a recovery unit. See Figure Refrigerant that is lightly contaminated can be recycled by passing it through a filter in a recovery machine that is capable of recycling refrigerant.
32A leak detector is a device used to detect refrigerant leaks in air conditioning or refrigeration systems.Refrigerant releases during the normal operation of air conditioning and refrigeration equipment are permitted, such as from mechanical purging or leaks. However, the EPA does require the repair of leaks above a specific size in large equipment. See Figure 6-31.
33A vacuum pump removes all air from a refrigeration system. Evacuation of a refrigeration system involves removing all air from inside the system. Air enters the tubing and components when a system is opened for repairs or after new construction. All the air and moisture contained in a system must be evacuated from the system before charging the system with refrigerant. This is accomplished with a vacuum pump that creates a deep low pressure that draws all the air from inside the system. See Figure Before beginning evacuation, the vacuum pump oil must be changed according to manufacturer’s instructions to prevent the formation of sludge in the pump. Only vacuum pump oil should be used and the oil level must be checked before operating the pump.
34The vacuum pump is connected to the system service valves using a gauge manifold and hoses. The vacuum pump is connected to the system service valves using a gauge manifold and hoses. See Figure As the air is removed by the vacuum pump, the pressure inside the system falls below atmospheric pressure and any water in the system vaporizes and is removed. The vaporization point of water is controlled by the pressure exerted on it. Water vaporizes (boils) at 212°F at sea level, but at higher elevations, the pressure is lower because there is less air pressing on the surface of the water. As pressure is reduced, water vaporizes at a lower temperature. In a complete vacuum, water vaporizes at a temperature above –90°F. See Appendix.
35Electronic leak detectors are extremely sensitive and indicate the general location of a leak. Leaks in an empty system are found using a standing pressure test. See Figure Refrigerant vapor is added to raise the system pressure to 10 psi. Dry nitrogen gas is then added to the system to raise the pressure to about 150 psi. If the pressure does not fall after a few hours, the system does not have a leak. If the pressure falls, an electronic leak detector can be used to locate the general location of the leak.
36The refrigerant container is placed upside down to charge with liquid and right side up to charge with vapor.A system is charged with refrigerant after it has been completely evacuated. Liquid refrigerant is usually charged into the high-pressure side when a system is completely empty. Smaller systems and some refrigerants require that refrigerant be added through the low-pressure side. A special fitting is attached to the red charging hose or the refrigerant container. This fitting contains a small opening that causes the liquid refrigerant to flash into vapor so that only vapor is added to the system, even though it is being charged from the liquid side of the refrigerant container. The system is not operated during liquid charging and the refrigerant must be weighed into the system. See Figure The system is operated when the correct amount of refrigerant has been added or when the refrigerant stops flowing because pressure in the system equals pressure in the refrigerant container. If additional refrigerant is required to fill the system, it is charged into the low-pressure side as a vapor while the system is operating. An operating system that is low on refrigerant is also charged at the low-pressure side as a vapor. The low pressure of the system’s low-pressure side allows the high-pressure refrigerant from the container to flow into the system. Charging is complete when the correct amount is added by weight or when the bubbles disappear from the sight glass and system pressures and temperatures are within normal operating specifications.