Vapor compression refrigeration system consists of the following important components: 1. Compressor 2. Condenser 3. Receiver 4. Expansion Device 5. Evaporator.
Vapor compression refrigeration system is the most popular and universally used system for the production of low temperature. In this system a working fluid known as refrigerant (such as Freon-12, Freon-22, NH3, SO2, CO2, etc.) is used. It condenses at high temperature and pressure close to atmospheric condition. It evaporates at low temperature and pressure close to the system which is to be cooled.
The refrigerant is circulated throughout the closed system, alternately condensing and evaporating. During its evaporation, the refrigerant absorbs its latent heat from the system to be cooled.
During its condensation, it rejects heat to the surrounding and releases its latent heat to the circulating water or air, i.e., to the atmosphere. The vapor compression refrigeration system is nowadays used for all purposes. It is generally used for all industrial purposes, from a small domestic refrigerator to large air conditioning plants.
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Figure 6.2 illustrates schematically the parts which make up the compression system. In this process, a refrigerant is used which can be alternately vaporized and liquefied. The heat energy required to change the liquid refrigerant into gas is obtained from air or water surrounding evaporator coil. The vapor formed in the evaporator is drawn through the suction line into the compressor.
While passing through the compressor, the heat-laden gas is raised from low to high pressure, thereby raising its temperature. It is then forced from the compressor through the discharge line into the condenser where the heat is removed from the vapor by water or air. The release of heat from vapor in condenser causes it to liquefy and then flow into the receiver.
The high-pressure liquid refrigerant is again returned to be admitted through the expansion valve to the evaporator, and the vaporization takes place at low pressure. The expansion valve is adjusted to control the flow of refrigerant into the evaporator at a rate which is sufficient to maintain the desired temperature.
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In the evaporator, the pressure is reduced and the boiling point is lowered. Vaporization takes place at low pressure and temperature; heat is extracted from the system to be cooled and is picked up by the refrigerant. The process is repeated continually as long as the compressor is functioning.
Component # 1. Compressor:
A mechanical device which is necessary to keep the refrigerant circulating to undertake the refrigeration cycle is known as compressor.
Vapor formed in evaporator at low temperature and pressure is sucked by the compressor and compresses it at higher temperature and pressure and supplies to the condenser where heat is rejected to the surrounding through water or air, according to the type of condenser (either water-cooled or air-cooled). Generally, compressor is driven by an electrical motor. The electrical energy supplied is the power input to refrigeration system.
Thus, the main functions of a compressor are as follows:
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(a) It removes the refrigerant vapor from evaporator, so that the desired pressure and temperature can be maintained.
(b) It increases the pressure and temperature of refrigerant vapor by the process of compression.
Compressors are classified into three major types:
(a) Reciprocating compressor
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(i) Open-type compressor
(ii) Hermetic sealed compressor
(b) Rotary-type compressor
(c) Centrifugal compressor
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(a) Reciprocating Compressor:
The compressor in which vapor refrigerant is compressed by the reciprocating (back and forth) motion of the piston in a cylinder is known as reciprocating compressor. It is suitable with the refrigerant which requires a comparatively small displacement at comparatively high pressure (such as R-12, R-22, NH3, etc.). Reciprocating compressors are available in sizes as small as 0.5- 150 kW.
Small size compressors are used for small size refrigerating units such as domestic refrigerator, window air conditioners, and up to 150 kW for large-capacity installations such as cold storage, NH3 plant, and central air conditioning system.
(i) Open-Type Compressor:
The open-type compressor is flexible in the sense that by varying the speed of the compressor, different capacities can be maintained. It can be operated by any type of prime mover/system such as electrical motors, petrol/diesel engine, etc. The motor and compressor both are separate units and are coupled with a common shaft for power transmission. The motor can be easily replaced in case of a motor burn out.
Figures 6.3(a) and 6.3(b) show a single-stage single-acting reciprocating open-type compressor in the simplest form.
(ii) Hermetic Sealed Compressor:
This type of compressor is known as a sealed unit shown in Fig. 6.4. The compressor and electric motor both are coupled and sealed in a metallic dome. In this type of compressor there is no need of shaft seal. The compressor and motor have a common shaft and are assembled in a single body. All joints are completely welded, such that there is no leakage of refrigerants. In this case, the noise and vibrations are completely eliminated. This is used for low- capacity unit, such as domestic fridges, window air conditioner, etc.
(b) Rotary Compressor:
The rotary compressors are suitable for pumping refrigerant vapors having moderate or low condensing pressure (such as R-21 and R-114).
(c) Centrifugal Compressor:
A centrifugal compressor increases the pressure of the refrigerant vapor by centrifugal force. This type of compressor is generally suitable for handling refrigerant vapor that requires large displacement and operates at low condensing pressure (such as R-11, R-113, etc.).
The impeller of the centrifugal compressor has specially designed blades running radially from the center to the outer periphery of the impeller. The impeller of the compressor is coupled with the shaft of motor. When this rotates, the refrigerant vapor is pushed out from the center of the impeller to its outer periphery by centrifugal force.
Component # 2. Condenser:
A condenser is a heat exchanger in which the two fluids at different temperatures exchange heats. The condenser consists of copper or steel coil in which high-pressure and high-temperature refrigerant vapor is condensed. The refrigerant vapor rejects heat at high temperature and pressure to another fluid, either water (in case of water-cooled) or atmospheric air (in case of air-cooled) condenser.
The refrigerant vapor gives off its latent heat to either water or atmospheric air. Consequently, the vapor refrigerant condenses and gets recirculated in the refrigeration cycle. The heat which is rejected in the condenser might have been picked up while evaporating the refrigerant in evaporator.
The refrigerant vapor discharged from the compressor is sometimes in the state of superheated vapor, and hence, the condenser de-superheats the high-temperature and high-pressure refrigerant vapor first and then condenses it and also sub-cools the liquid to enhance the performance of the cycle.
There are three types of condenser:
(a) Air-cooled condenser
(b) Water-cooled condenser
(c) Evaporative condenser
(a) Air-Cooled Condenser:
An air-cooled condenser is cooled by the ambient air (Fig. 6.5), the temperature of which is more than the water temperature. Hence, the condensing temperature will be higher in an air-cooled condenser than that in a water-cooled condenser.
There are two types of air-cooled condensers:
(i) Natural Air Circulation Condenser/Natural Convection:
The movement of air over the surface of condenser tubes is by natural means. As air comes in contact with the condenser tubes, it picks up heat from the refrigerant and thus the temperature of air increases. The hot air moves up due to low density and the cool air comes from below and rises up.
This process is continued. Since the air movement is less, hence the heat removal is also less. The air-cooled condenser is incapable of removing heat rapidly and thus it requires a large surface area of condenser for heat transfer. The air- cooled condenser is suitable for small-capacity plants such as domestic fridge, window air conditioner.
(ii) Forced Air Circulation Condenser:
In this type of condenser, a fan or blower is employed for the circulation of air over the condenser coil. The condenser coil is of finned type. This increases the heat transfer surface which increases the heat removal rate.
(b) Water-Cooled Condenser:
A water-cooled condenser uses water as a cooling medium to transfer heat at a higher rate. The vapor refrigerant transfers heat to the cold circulating water (Fig. 6.6).
There are three types of water-cooled condensers:
(a) Tube in tube
(b) Shell and coil (Fig. 6.7)
(c) Shell and tube
Component # 3. Receiver:
The condensed liquid refrigerant from the condenser is stored in a vessel known as receiver from where it is supplied to the evaporator through expansion device.
Component # 4. Expansion Device:
The pressure of the liquid refrigerant from the condenser has to be reduced so that the liquid refrigerant can vaporize at the desired low pressure of evaporator. An expansion valve which is also known as throttle valve is a device to reduce the pressure and temperature of the liquid refrigerant before it is supplied to the evaporator where refrigerant gets evaporated.
The valve actually creates a pressure difference between condenser pressure and evaporator pressure. The liquid refrigerant from the condenser is passed through the valve where it reduces the pressure and temperature equivalent to evaporator pressure. Sometimes, this valve is also known as refrigerant control valve. The function of the valve is to allow high- pressure and high-temperature liquid refrigerant to pass through evaporator.
Thus, the main functions of an expansion device are as follows:
(a) It must reduce the pressure of the liquid refrigerant.
(b) It must regulate the flow of refrigerant to the evaporator.
During expansion, some of the refrigerants get evaporated in the valve itself and most of the liquids get evaporated in the evaporator.
There are three main types of expansion devices:
(a) Capillary tube
(b) Thermostatic expansion valve
(c) Automatic expansion valve
(a) Capillary Tube:
A small diameter tube of certain length is called capillary tube. Its inside diameter used in refrigeration system is about 0.5-2.28 mm. Longer the capillary tube and/or smaller the inside diameter of the tube, greater is the pressure drop it can create in the refrigerant flow (Fig. 6.8). A small diameter tube can offer the same restrictive effect as the small orifice produces. The capillary tube is quite a simple device and it is not so costly. This is used in small hermetic unit such as domestic refrigerator, fridges, and window air conditioners.
For small capacity plant, capillary tube is used as an expansion device. In operation, the liquid refrigerant coming out from condenser enters the capillary tube. Due to frictional resistance offered by a small diameter tube, the pressure drops down to evaporator pressure.
(b) Thermostatic Expansion Valve:
Thermostatic expansion valve is one of the most important components of refrigerating plant. The function of the valve is to reduce the condenser pressure down to evaporator pressure and also to regulate the flow rate of liquid refrigerant through evaporator.
Figure 6.9 shows a complete assembly of the expansion valve and evaporator. The device is incorporated in between receiver and the evaporator. If the receiver is not used in small capacity plant, the expansion valve is placed in between condenser and evaporator directly.
As regards, the constructional details, the expansion valve consists of a valve controlled by spring, a metallic diaphragm and an adjusting screw. It has a feeler bulb which is mounted with the suction line near the outlet of the evaporator coil. The other end of the feeler bulb is connected with the expansion valve through a small capillary tube.
The feeler bulb is filled inside with the same refrigerant as used in refrigeration plant. Since, the feeler bulb is installed on the exit of evaporator; it will have the same temperature as refrigerant. Any change in temperature of refrigerant will cause a change in pressure in the feeler bulb. In normal condition, there is no unbalanced force acting on diaphragm.
If load increases on evaporator that means more evaporation rate of refrigerant, the temperature of feeler bulb increases and thus pressure is increased and it will exert a pressure on diaphragm there by the opening of the valve and more liquid will enter to the evaporator. If evaporation takes place at the slower rate in the evaporator, the valve will be lifted up and it will close the opening and flow of liquid refrigerant is restricted.
(c) Automatic Expansion Valve:
The automatic expansion valve is an alternative arrangement of thermodynamic expansion valve. The valve is shown in Fig. 6.10. The function of automatic expansion valve is similar to thermostatic expansion valve.
The automatic expansion valve controls the liquid refrigerant supply to evaporator by maintaining a desired pressure. It is very much suitable for dry type of evaporator. In evaporator, the liquid refrigerant enters form one side of evaporator coil and the refrigerant vapor is sucked from the other end of the coil by the compressor. Thus, the evaporator coils are always dry. It is also called DX coil.
The condensate from the receiver enters the expansion valve and then to the evaporator. The liquid refrigerant pressure is decreased while passing through the valve which is controlled by the metallic bellow. Thus, the mass flow rate of refrigerant through the evaporator is automatically controlled by automatic expansion valve.
Component # 5. Evaporator:
Evaporator is a heat exchanger in which a liquid refrigerant is evaporated at low pressure and temperature. The process of heat removal from the system to be cooled is applied in evaporator. An evaporator consists of copper or steel tube in which liquid refrigerant evaporates at low pressure and temperature.
The liquid refrigerant is evaporated inside evaporator coil in order to remove heat from air, water, and brine. In the evaporator, the liquid refrigerant absorbs its latent heat of vaporization from the fluid (air, water, or brine) which is to be cooled (Fig. 6.11).
i. Direct Expansion System:
The system in which a liquid refrigerant gets evaporated inside the evaporator cooling coil and the fluid to be cooled is passed over the surface of cooling coil is known as direct expansion system (Fig. 6.12).
ii. Indirect System:
In a large air conditioning system, water or brine is chilled in an evaporator and the chilled fluid is circulated through a copper or steel tube. The air is passed over the chilled copper and steel tube. Such a system is called indirect system.
Below are mentioned different types of evaporators:
(a) Air-cooled evaporator/dry expansion evaporator (DX-coil)
(b) Liquid cooling evaporator:
(i) Shell and coil
(ii) Wire and tube
(c) Plate-type evaporator
i. Shell and Coil Evaporator:
The shell and coil evaporator as shown Fig. 6.13 is a dry expansion evaporator (DX coil). The liquid refrigerant goes into the evaporator coil and vapor comes out; the coil is always dry. The cooling coil is a long metallic tube having good thermal conductivity is placed in a perfect sealed shell.
Both the ends of the coil are passing through the corner of the shell and attached with it to prevent leakages. The liquid refrigerant coming out from condenser through expansion valve is passed through coiled tube and gets evaporated and the vapor thus formed in evaporator is sucked by the compressor.
Water is continuously supplied through the shell and chilled water comes out at the other end of the shell. The heat of water is picked up by cooling coil. Thus cooling effect is produced in evaporator. This type of evaporator is suitable for water cooler or usually used in small capacity unit up to a capacity of 10 TR.
ii. Wire and Tube Evaporator:
The wire and tube evaporator as shown in Fig. 6.14 consists of a coiled tube over which metallic wires are arranged parallel to each other and welded or brazed with the coiled tube surface. In this entire surface of coil is in contact with the refrigerant inside. The wires act as fins to enhance heat transfer rate.
The liquid refrigerant is passed through the coil and gets evaporated. This can be used for small capacity plants.
(c) Plate-Type Evaporator:
A most common type of plate evaporator is shown in Fig. 6.15. In this type of evaporator, the tubes made in coil form are either welded on brazed one side of the plate. The combination of coiled tube and plate forms the evaporator. The plate evaporators are used in domestic fridges and freezers etc.
Sometimes, the coiled tubes are placed in between two parallel plates and the tubes are embedded in grooves made in both the plates. The tubes carrying refrigerant has very good thermal contact with the plate. Such plate evaporators are used in plate freezers.