Vapour absorption refrigeration system
The vapour absorption system used heat energy instead of mechanical energy and hence compressor is not used. Instead it is replaced by an absorber, a heat exchanger and a generator. This system uses ammonia as refrigerant.
The basic principle of this system is that the ammonia vapour condenses when it is dissolved in water. Further heating of the ammonia liquid converts ammonia into vapour.
A simple vapour absorption system consists of the following parts.
Generator: The solution of aqua-ammonia received from the absorber is heated by some external sources such as electric heater or gas flame. Because of this heating the ammonia solution gets separated into ammonia vapour at high pressure and hot weak ammonia which mostly consists of water.
Condenser: Condenser converts the high pressure ammonia vapour received from the generator into high pressure ammonia liquid. This process is done by means of circulating cool water.
Expansion valve or throttle valve: The expansion that takes place in the expansion valve is throttling. The high pressure ammonia liquid is expanded to low pressure low temperature ammonia in the expansion valve.
Evaporator: Evaporator, also called cold chamber, is the actual freezing. The ammonia refrigerant passing through the evaporator absorbs the heat and evaporates.
Absorber: The function of the absorber is to absorb low pressure ammonia vapour from the evaporator and weak ammonia solution from the generator. The purpose of the absorber is to make this mixture into a string solution, which is pumped back to the generator.
Heat Exchanger: It is used to transfer the heat from strong ammonia solution to the weak solution.
Pump: It is used to circulate the strong ammonia through the heat exchanger. The pump increases the pressure of the solution.
The system works as follows:
The low-pressure ammonia vapour leaving the evaporator enters the absorber, where it is absorbed by cold water forming a strong solution of ammonia. This solution is known as aqua-ammonia. Due to absorption of ammonia, the pressure in the absorber reduces, which in turn, increases the temperature of the solution. The coolant removes the heat generated in the absorber. The strong solution from the absorber is pumped by a liquid pump to the generator through a heat exchanger. The pump increases pressure of the solution. The heat exchanger increases the temperature of the high pressure strong solution. The strong solution of ammonia is further heated in the generator with help of steam or a heating coil. The ammonia vapour is driven off or generated during this heating process. The hot, high-pressure ammonia vapour is then condensed in the condenser using water as a coolant. Meanwhile due to generation of vapour, weak solution remains in the generator. The weak solution at high pressure flows back to the absorber after passing through a pressure reducing valve. When the demand for refrigeration arises, the expansion valve in the circuit is opened. The ammonia in liquid passes through the expansion valve. The ammonia subsequently undergoes an expansion process and then enters the evaporator. In the evaporator, it absorbs heat and produces the refrigeration effect. The low-pressure ammonia vapour from the evaporator then re-enters the absorber, where it is again absorbed to form a strong solution and the cycle repeats itself. Thus completes the simple vapour absorption refrigerator cycle.
· Absence of moving parts so less noise.
· Exhaust system may be used as a source of heat energy.
· Load variation does not affect the performance of the system.
· Control is easy for absorption system.
· Cost of the system is less.
· COP of the system is low.
· Leakages are a problem.
· This system occupies more space.
· Costly pump is required.