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Vapour Absorption Refrigeration Systems
Vapour Absorption Refrigeration Systems (VARS) belong to the class of vapour cycles similar to vapour compression refrigeration systems. However, unlike vapour compression refrigeration systems, the required input to absorption systems is in the form of heat. Hence these systems are also called as heat operated or thermal energy driven systems. Since conventional absorption systems use liquids for absorption of refrigerant, these are also sometimes called as wet absorption systems. Similar to vapour compression refrigeration systems, vapour absorption refrigeration systems have also been commercialized and are widely used in various refrigeration and air conditioning applications. Since these systems run on low-grade thermal energy, they are preferred when low-grade energy such as waste heat or solar energy is available. Since conventional absorption systems use natural refrigerants such as water or ammonia they are environment friendly.
Lithium Bromide
Vapour absorption refrigeration systems using water-lithium bromide pair are extensively used in large capacity air conditioning systems. In these systems water is used as refrigerant and a solution of lithium bromide in water is used as absorbent. Since water is used as refrigerant, using these systems it is not possible to provide refrigeration at sub-zero temperatures. Hence it is used only in applications requiring refrigeration at temperatures above 0 o C. Hence these systems are used for air conditioning applications. The analysis of this system is relatively easy as the vapour generated in the generator is almost pure refrigerant (water), unlike ammonia-water systems where both ammonia and water vapour are generated in the generator.
Heat sources for water-lithium bromide system
Water-lithium bromide systems can be driven using a wide variety of heat sources. Large capacity systems are usually driven by steam or hot water. Small capacity systems are usually driven directly by oil or gas. A typical single effect system requires a heat source at a temperature of about 120 o C to produce chilled water at 7 o C when the condenser operates at about 46 o C and the absorber operates at about 40 o C. The COPs obtained aor in the range of 0.6 to 0.8 for single effect systems while it can be as high as 1.2 to 1.4 for multi-effect systems.
Water-Lithium Bromide System Process
Single stage systems operate under two pressures – one corresponding to the condenser-generator (high pressure side) and the other corresponding to evaporator- absorber. Single stage systems can be either:
- Twin drum type, or
- Single drum type
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| A commercial, twin-drum type, water-lithium bromide system |
As shown in the figure, the cooling water (which acts as heat sink) flows first to absorber, extracts heat from absorber and then flows to the condenser for condenser heat extraction. This is known as series arrangement. This arrangement is advantageous as the required cooling water flow rate will be small and also by sending the cooling water first to the absorber, the condenser can be operated at a higher pressure to prevent crystallization. It is also possible to have cooling water flowing parallelly to condenser and absorber, however, the cooling water requirement in this case will be high. A refrigerant pump circulates liquid water in evaporator and the water is sprayed onto evaporator tubes for good heat and mass transfer. Heater tubes (steam or hot water or hot oil) are immersed in the strong solution pool of generator for vapour generation. Pressure drops between evaporator and absorber and between generator and condenser are minimized, large sized vapour lines are eliminated and air leakages can also be reduced due to less number of joints.
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| A commercial, single-drum type, water-lithium bromide system |
Capacity control means capacity reduction depending upon load as thecapacity will be maximum without any control. Normally under both full as well aspart loads the outlet temperature of chilled water is maintained at a near constantvalue. The refrigeration capacity is then regulated by either:
- Regulating the flow rate of weak solution pumped to the generator through the solution pump
- Reducing the generator temperature by throttling the supply steam, or by reducing the flow rate of hot water
- Increasing the condenser temperature by bypassing some of the cooling water supplied to the condenser
a) Are used in large air conditioning systems
b) Are used in large frozen food storage applications
c) Operate under vacuum
c) All of the above
ANS. C
2. For a required refrigeration capacity, the solution heat exchanger used in water- lithium bromide systems.
a) Reduces the required heat input to generator
b) Reduces the heat rejection rate at absorber
c) Reduces heat rejection rate at condenser
d) Reduces the required heat source temperature
ANS. A and B
3. In water-lithium bromide systems
a) Crystallization of solution is likely to occur in absorber
b) Crystallization of solution is likely to occur in solution heat exchanger
c) Crystallization is likely to occur when generator temperature falls
d) Crystallization is likely to occur when condenser pressure falls
ANS. A and D
4. In commercial water-lithium bromide systems
a) Crystallization is avoided by regulating cooling water flow rate to condenserb) Crystallization is avoided by adding additivesc) An air purging system is used to maintain vacuumd) All of the above
ANS. D
5. Commercial multi-effect absorption systems
a) Yield higher COPsb) Yield higher refrigeration temperaturesc) Require lower heat source temperaturesd) Require higher heat source temperatures
ANS. A and D
6. In water-lithium bromide systems
a) The required heat source temperature should be higher than minimum heat generation temperature
b) The required heat source temperature decreases as cooling water temperature increases
c) The required heat source temperature is higher for air cooled condensers, compared to water cooled condensers
d) All of the above
ANS. A and C
7. In commercial water-lithium bromide systems, the system capacity is regulated by
a) Controlling the weak solution flow rate to generator
b) Controlling the flow rate of chilled water to evaporator
c) Controlling the temperature of heating fluid to generator
d) All of the above
ANS. A and C