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Vapor Compression Refrigeration System | Basic, Working, Parts Of System

Vapor Compression Refrigeration System | Basic, Working, Parts Of System


Vapor-compression refrigeration, in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air-conditioning of buildings and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems.

Working Of Vapor Compression Refrigeration System:

Figure shows the basic components of a vapour compression refrigeration system.
As shown in the figure the basic system consists of an evaporator, compressor, condenser and an expansion valve. The refrigeration effect is obtained in the cold region as heat is extracted by the vaporization of refrigerant in the evaporator. The refrigerant vapour from the evaporator is compressed in the compressor to a high pressure at which its saturation temperature is greater than the ambient or any other heat sink. Hence when the high pressure, high temperature refrigerant flows through the condenser, condensation of the vapour into liquid takes place by heat rejection to the heat sink. To complete the cycle, the high pressure liquid is made to flow through an expansion valve. In the expansion valve the pressure and temperature of the refrigerant decrease. This low pressure and low temperature refrigerant vapour evaporates in the evaporator taking heat from the cold region. It should be observed that the system operates on a closed cycle. The system requires input in the form of mechanical work. It extracts heat from a cold space and rejects heat to a high temperature heat sink.

Vapor Compression Cycle -Working Diagram
Vapor Compression Cycle -Working Diagram

A refrigeration system can also be used as a heat pump, in which the useful output is
the high temperature heat rejected at the condenser. Alternatively, a refrigeration system can be used for providing cooling in summer and heating in winter. Such systems have been built and are available now

Vapor Compression Thermodynamic Cycle

PV And TS Diagram For Vapor Compression System
PV And TS Diagram For Vapor Compression System

Most of the modern refrigerators work on this cycle, in Its simplest form there are four fundamental operations required to complete one cycle.

(a) Compression
(b) Condensation 
(c) Expansion 
(d) Vaporization 

a) Compression 

The low pressure Vapour in dry state is drawn from the evaporator during the Suction stroke of the compressor. During compression Stroke the pressure and temperature increase until vapour temperature is greater than the temperature of condenser cooling medium (air or water) 

At point 1 in the diagram, the circulating refrigerant enters the compressor as a saturated vapor. From point 1 to point 2, the vapor is isentropically compressed (i.e., compressed at constant entropy) and exits the compressor as a superheated vapor.

b) Condensation 

When the high pressure refrigerant vapour enters the condenser heat flows from condenser to cooling medium thus allowing the vaporized refrigerant to return to liquid State.

From point 2 to point 3, the vapor travels through part of the condenser which removes the superheat by cooling the vapor. Between point 3 and point 4, the vapor travels through the remainder of the condenser and is condensed into a saturated liquid. The condensation process occurs at essentially constant pressure.

 c) Expansion 

After condenser the liquid refrigerant is stored in the liquid receiver until needed. From the receiver it passes through an expansion valve where the pressure is reduced sufficiently to allow the vaporization of liquid at a low temperature of about -10°C. 

Between points 4 and 5, the saturated liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure. That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes).

d) Vaporization 

The low pressure refrigerant vapour after expansion in the expansion valve enters the evaporator or refrigerated space where a considerable amount of heat IS absorbed by it and refrigeration is furnished.

Between points 5 and 1, the cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator where it is totally vaporized by the warm air (from the space being refrigerated) that a fan circulates across the coil or tubes in the evaporator.

The resulting refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

Main Parts Of Vapor Compression Refrigeration Cycles:

1. Evaporator
 Its function is to provide a heat transfer surface through which heat can pass from the refrigerated space into the vaporizing refrigerant.

This is generally a Fin & Tube (Hair-Pin type) heat exchanger, similar to Air-Cooled Condensers.

 2. Suction Line
It carries the low pressure vapour from the evaporator to suction in let of the compressor.

 3. Compressor
The function of the compressor is to draw refrigerant vapour from the evaporator and to raise It temperature and pressure to such a print to that it may be easily condensed with normally available condensing media. It also maintains a continuous flow of the refrigerant through the system.

Compression Ratio = Absolute Discharge Pressure / Absolute Suction Pressure

The Capacity of a Compressor is determined by its Mass Flow rate (Lb/Min) and not by Volume Flow(CFM).

The most common compressors used in chillers are reciprocating, rotary screw, centrifugal, and scroll compressors. Each application prefers one or another due to size, noise, efficiency and pressure issues.

 4. Discharge Line
It conveys the high pressure and high temperature refrigerant from the compressor to the condenser.

 5. Condenser
The function of the condenser is to provide a heat transfer surface through which heat passes from the refrigerant to the condensing medium which is either water or air.

Types of Condensers:
• Air-Cooled
• Water-Cooled

 6. Liquid Receiver
It acts as, a reservoir which stores the liquid refrigerant coming from the condenser and supplies it to the evaporator according to the requirement.

7. Liquid Line
It carries the liquid refrigerant from the receiver and conveys it to the expansion valve.

8. Expansion valve
Function Of This valve is to Supply a proper amount of refrigerant to the evaporator after reducing its pressure considerably so that the refrigerant may take sufficient amount of heat from the refrigerating space during evaporation

The Metering Device converts the High Pressure and High Temperature Liquid from Condenser to Low Pressure and Low Temperature Liquid-Vapor mixture, which will be fed to the Evaporator.

Commonly used types of Metering Device:

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