Vapor Compression Refrigeration System | Basic, Working, Parts Of System, Advantages, and Disadvantages
Working Of Vapor Compression Refrigeration System:
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
Most of the modern refrigerators work on this cycle, in Its simplest form, there are four fundamental operations required to complete one cycle.
The low-pressure Vapour in the dry state is drawn from the evaporator during the suction stroke of the compressor. During compression Stroke, the pressure and temperature increase until vapor 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.
When the high-pressure refrigerant vapor enters the condenser heat flows from the condenser to the cooling medium thus allowing the vaporized refrigerant to return to the 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.
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).
The low-pressure refrigerant vapor 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 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:
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 vapor from the evaporator to suction inlet of the compressor.
The function of the compressor is to draw refrigerant vapor from the evaporator and to raise Its 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.
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:
6. Liquid Receiver
It acts as, a reservoir that 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.
An expansion valve is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controlling the superheat at the outlet of the evaporator.
Types of Expansion devices
- Thermostatic EV
- Capillary tube
- Hand operated EV
- Automatic or Constant Pressure EV
- Float expansion
Types of Vapor Compression Cycles :
We have already disused that vapor compression cycle essentially consists of compression, condensation, throttling and evaporation. Many scientists have focused their attention to increase the coefficient of performance of the cycle. Through there are many cycles, yet the following are important from the subject point of view:
1. Cycle with dry saturated vapor after compression,
2. Cycle with wet vapor after compression,
3. Cycle with superheated vapor after compression,
4. Cycle with superheated vapor before compression, and
5. Cycle with under cooling or sub cooling of refrigerant,
Advantages of Vapour Compression System :
- It has a smaller size for the given capacity of refrigeration.
- It has less running cost.
- It can be employed over a large range of temperatures.
- The coefficient of performance is quite high.
- Less time required to produce refrigerant effects.
Disadvantages of Vapour Compression System :
- The initial cost is high.
- The prevention of leakage of the refrigerant is the major problem in the vapor compression system.
- More wear and tear and noise due to Moving Parts
- Liquid droplets in suction line may damages.
Multi-Stage: In the case of a Refrigeration system very low-pressure refrigerant vapors (Evaporator pressure) are to be compressed to very high-pressure refrigerant vapors (Condenser pressure). As this pressure ratio is very high the size and work done required in case of a single-stage is more, thus COP decreases TO increase COP and reduce work done multi staging is needed.
Advantages of multi-staging in vapor compression system:
Advantages of multi-staging in vapor compression system are :
1. Work done per kg of refrigerant is reduces by using an intercooler
2. Volumetric efficiency of compressor increases
3. It reduces leakage of refrigerant
4. It gives uniform torque, therefore, the smaller flywheel may be used
5. Effective lubrication can be done
6. Cost of compressor reduces
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