Vapour Compresion Refrigeration V/S Vapour Absorption Refrigeration Systems
VAPOUR COMPRESSION REFRIGERATION:
VCR stands for Vapour Compression Refrigeration Cycle. It is the most widely used refrigeration system in which the working fluid is a vapor which readily evaporates and condenses (changes alternatively between the vapor and liquid phase) without leaving the refrigerating plant.
During evaporation, it absorbs heat from the cold body and this heat is used as its latent heat for converting it from liquid to vapour; whereas in condensing or cooling, it rejects heat to external bodies, thus, returning back to liquid form.
Components of Vapour Compression Refrigeration System:
1. Compressor:
The vapour at low pressure and low temperature enters the compressor from the evaporator where it is compressed to high pressure and high temperature. This high pressure and temperature vapour refrigerant is discharged into the condenser through the discharge valve.
2. Condenser:
The condenser or cooler consists of coils of pipe in which the high pressure and temperature vapour refrigerant are cooled and condensed. The refrigerant while passing through the condenser gives up its latent heat to the surroundings which is normally air or water.
3. Receiver:
The condensed liquid refrigerant from the condenser is stored in a vessel known as a receiver from where it is supplied to the evaporator through the expansion valve.
4. Expansion Valve:
It is also called a throttle valve. Its function is to allow the liquid refrigerant under high pressure and temperature to pass through it where it reduces its temperature and pressure.
5. Evaporator:
It also consists of coils of pipe in which liquid-vapour refrigerant at low pressure and temperature is evaporated and converted into vapour refrigerant at low pressure and temperature.
Working of Vapor Compression Refrigeration System:
The working of Vapor Compression Refrigeration System can be completed under 4 processes and are as follows.
Stage 1: Compression
The refrigerant (for example R-717 or R-22) enters the compressor at low temperature and low pressure. It is in a gaseous state. Here, compression takes place to raise the temperature and refrigerant pressure. The refrigerant leaves the compressor and enters the condenser. Since this process requires work, an electric motor may be used. Compressors themselves can be scroll, screw, centrifugal or reciprocating types.
Stage 2: Condensation
The condenser is essentially a heat exchanger. Heat is transferred from the refrigerant to the water which is flowing outside the condenser. This water goes to a cooling tower for cooling or is air-cooled. As the refrigerant flows through the condenser, it is in a constant pressure. To maintain this pressure, there are several pressure-controlling devices.
Stage 3: Throttling and Expansion
When the refrigerant enters the throttling valve, it expands and releases pressure. Consequently, the temperature drops at this stage. Because of these changes, the refrigerant leaves the throttle valve as a liquid vapor mixture, typically in proportions of around 75 % and 25 % respectively.
Throttling valves play two crucial roles in the vapor compression cycle:
- They maintain a pressure differential between low and high-pressure sides.
- They control the amount of liquid refrigerant entering the evaporator.
Stage 4: Evaporation
At this stage of the Vapor Compression Refrigeration Cycle, the refrigerant is at a lower temperature than its surroundings. Therefore, it evaporates and absorbs latent heat of vaporization. Heat extraction from the refrigerant happens at low pressure and temperature. Compressor suction effect helps maintain the low pressure.
Advantages of VCR:
The Advantages of Vapor Compression Refrigeration system are as follows.
1. The temperature at the evaporator section can be controlled by employing regulation of the expansion valve.
2. It exhibits a high Coefficient of Performance.
3. The initial cost is lower as compared to VAR because of a lesser number of components.
4. It produces a cooling effect quickly as compared to VAR cycle.
Disadvantages of VCR:
The disadvantages of Vapor Compression Refrigeration system are as follows.
1. Refrigerants can leak from the pipes/hose.
2. Refrigerants are not cheap, nor readily available.
3. Refrigerants can adversely affect the atmosphere.
4. The running cost of the system is high because vapour compression requires more power.
5. Load variation affects the working efficiency of VCR.
Vapour Compression Refrigeration System Applications:
The applications of Vapor Compression Refrigeration system are as follows.
1. It is used in domestic refrigeration for keeping the food.
2. It is helpful in food processing and cold storage.
3. It is useful in Industrial refrigeration for chemical processing, heating and cooling.
4. It is useful in Cryogenic refrigeration, Medical refrigeration, Transport refrigeration and Electronic cooling.
VAPOUR ABSORPTION REFRIGERATION:
The Vapor Absorption Refrigeration system comprises all the processes in the vapor compression refrigeration system like compression, condensation, expansion and evaporation. In the vapor absorption system the refrigerant used is ammonia with water as the absorber, or, water if used as a refrigerant with lithium bromide as the absorber. This cycle does not have a compressor. Instead, an absorber, a generator, a pump and a pressure reducing valve are used in order to replace the compressor of the VCR. The refrigerant produces a cooling effect at the evaporator and releases the heat to the atmosphere via the condenser.
Components and Working of Vapour Absorption Refrigeration System:
Refer to the above schematic diagram to understand the components and working of VAR.
1. Absorber — The absorber is used to absorb the refrigerant. The absorber should have a higher affinity for the refrigerant vapours at low temperature, and lower affinity for refrigerant at higher temperatures. For NH3-H2O pairs of refrigerant-absorber, a strong aqua ammonia (NH3-H20) solution is formed which is further fed to the generator through a pump. The formation of this solution releases heat which increases the solution’s temperature and hence, a cooling jacket for the absorber is required to maintain a low temperature for efficient absorption of the refrigerant.
2. Pump — The pump increases the pressure of the strong solution of ammonia and water to condenser pressure and pumps it from the absorber to the generator.
3. Generator — The generator receives the high pressure solution of refrigerant. This solution is heated in the generator using a low-grade energy source, such as heat energy. This heat energy could be obtained from a large heat reservoir (such as a furnace), or solar energy may be used. This increase in temperature lowers the absorber’s affinity to the refrigerant. Since the boiling point of NH3 is lower than that of water, high temperature- high pressure refrigerant vapours are released from the generator into the further cycle.
4. Pressure Reducing Valve — The weak aqua ammonia solution remaining in the generator after refrigerant vapours are released is fed back to the absorber via a Pressure Reducing valve which lowers its temperature and pressure.
5. Condenser- Condenser is used to convert the ammonia vapour back into liquid phase. This condenser can be either water-cooled or air-cooled. The liquid refrigerant has a high temperature and pressure.
6. Expansion Valve- It reduces the temperature and pressure of the liquid refrigerant obtained at the condenser.
Practical Vapour Absorption Refrigeration:
The replacement of the compressor by the simple arrangement as shown in the above figure is not very economical in practice. In order to make improvements certain additional auxiliary items are provided in the system. They include an analyzer, a rectifier, and two heat exchangers. The practical absorption cycle was developed after incorporating these all auxiliary items.
Analyzer: The ammonia vapours leaving the generator may contain some moisture, and therefore it should be made free from any trace of water vapour before passing on to the condenser and then to the expansion valve, otherwise the water vapour is likely to get freeze at some small valve passage and may choke the flow. The analyzer is an open type of cooler and is mounted on the top of the generator. Both the strong aqua-ammonia solution from the absorber and the condensate removed in the rectifier are introduced from the top of the generator and they flow downwards.
Rectifier: It is a closed type of cooler and is actually a miniature condenser where any traces of water vapour left in the ammonia vapour are removed by condensation. The condensed water is drained back to the generator through the analyzer.
Heat Exchangers: Two heat exchangers are provided to internally exchange heat from the higher temperature fluid to the lower temperature fluid so that one is cooled and the other is heated. One heat exchanger is provided between liquid receiver and evaporator so that the liquid is sub-cooled and vapour is heated up. Another heat exchanger is located between generator and absorber so that the strong aqua-ammonia solution is heated up before going on to the generator and weak aqua-ammonia solution is cooled before entering the absorber.
Single-Effect and Double-Effect VAR:
In vapour absorption refrigeration system, the temperature of the heat source plays an important role. The heat given by the source may be used in one single stage or in more than one stage. Accordingly the system is called a single stage or single effect cycle of operation and if the heat is supplied in two stages then the system is called two stage or double effect cycle of operation.
It is found that when the temperature of the heat source is up to 105°C, the heat is utilized in a single stage and the cycle is a single effect cycle. When the temperature of a heat source is greater than 105°C, then generally the heat is utilized in two stages and the system becomes a double-effect cycle.
1. Single Effect VAR:
A simple and practical absorption system using ammonia as refrigerant and water as absorbent described previously is an example of a single-effect cycle system for vapour absorption refrigeration system.
2. Double Effect VAR:
The schematic sketch of the double-effect VAR Cycle with Water and Lithium-Bromide (Li-Br) as the refrigerant- absorber pair is shown below. Here, water is the refrigerant and Lithium-Bromide is the absorber. Vapors of refrigerants are generated in two stages as shown. For this purpose two heat exchangers are used.
A double effect system consists of two heat exchangers and two generators (high and low pressure). It is an absorption system with improved efficiency which employs a second generator which uses water vapour from the first generator to provide its heat supply.
The evaporator pressure is maintained very low, so refrigerant water evaporates quickly after absorbing the heat, and the water vapors are absorbed by a strong Li-Br solution in the absorber. This absorption makes the Li-Br solution weak.
This weak solution is then pumped to the first, high pressure generator where it is heated up. While going towards the generator, it passes through a heat exchanger where it absorbs heat from the strong, high temperature Li-Br solution coming from the high pressure generator.
Heating up of the weak Li-Br solution in the first generator leads to evaporation of some portion of water, thereby making the Li-Br solution strong. This strong Li-Br solution is now throttled to lower pressure to that of the second generator causing the solution to develop flash vapour which is liquefied at the condenser and the remaining strong Li-Br solution is passed back to absorber via another heat exchanger. The strong Li-Br solution in the second, lower pressure generator is also heated, thereby releasing more water vapor to the condenser.
The condensate from the condenser is sent to the evaporator thus completing the cycle.
Advantages of VAR:
- The whole system will operate over a huge temperature range.
- Running cost of the cycle is low because liquid compression requires lesser power.
- Load variation does not affect performance of the cycle.
- No need of supplying any high-grade energy source for the running of the refrigerator.
- Refrigerants used are cheap and readily available.
Disadvantages of VAR:
- Efficiency of the absorption system is low as compared to VCR.
- Time required to produce a cooling effect is more than time taken by the VCR system.
- If kerosene/oil/gas flame is used for heating in the generator, it will emit a bad smell.
- Initial cost of the Vapor Absorption Refrigeration System will be higher than Vapour Compression Refrigeration System because of additional components required.
- Lithium bromide which will be used in VARS can be corrosive in nature which may affect the overall life of the system.
Applications of VAR:
1. Used for Air Conditioning in Recreational Vehicles, campers and caravans.
2. Used for waste heat recovery industries where a large amount of heat is generated.
3. Used in Absorption chillers for Conditioning.
4. Used in Domestic Electrolux Refrigerators.
COMPARISON BETWEEN VAPOUR COMPRESSION & VAPOUR ABSORPTION REFRIGERATION SYSTEMS:
