In VCR cycle, the main cost of the refrigeration unit is compressor and the work input in the compressor is also very high because of compressing the refrigerant in vapour phase.
Thus, to reduce the cost of the system, compressor can be replaced by the pump which also increases the pressure of the refrigerant. But the main drawback is that pump can compress the refrigerant in liquid phase & after compressing the liquid in pump it will be in liquid form which is to be converted in vapour form at condenser pressure which required a lot amount of heat.
Thus, a new refrigeration system comes into picture in which
The compressor which is used in VCRS is replaced with:
- Absorption to convert refrigerant vapour into liquid state.
- Pump to compress it from evaporator pressure to compressor pressure.
- Generator to convert compressed liquid refrigerant into vapour form.
Important Points of VARS
(i) Vars system is operated on low grade energy because a lot of heat energy going to used in the generator as compare to pump work.
(ii) The Cop of VARS system is having lower value and generally varies from:
(COP)VARS ⇒0.3 – 0.5
(iii) VARS system is generally preferable in remote locations or where the cost of electricity is high.
(iv) Waste heat can be effectively utilized in VARS.
(v) Heat rejection occurs in condenser and absorber.
(vi) heat absorption is occurs in evaporator and generator.
The most commonly used absorber refrigerant pair is:
(1) Ammonia -water VARS
In this Ammonia is used as a refrigerant and water is used as a absorber.
After absorbing the heat from the evaporator, refrigerant is passed through the absorber where it rejects heat and convert into liquid form. Ammonia has special property that it’s absorptivity in water decreases as increase in temperature. So at absorber, ammonia mixes with the water and get absorbed. Now this liquid form of ammonia water mixture is compressed to condenser pressure by pump. After compressing it is passes through a heat exchanger in which it absorbed the heat and ammonia present in the water get separated. But there is possibility that some water particles remain suspended in the ammonia. Thus, to remove the water particles completely from the ammonia vapour, Analyser and rectifier assembly is used. Here water is removed in two stages and the complete elimination of water particles is taken place in rectifier.
And then it is passed through condenser where it rejects heat and expanded in throttling device to evaporator pressure and passes through the space where low temperature is to be maintained.
COP of VARS system:
The work required to operate VARS system is much less than that to required to operate a compression system.
Figure: COP of VARS system:
Let us assume QE and QG are the heat absorbed by the refrigerants across evaporator and generator. Whereas Qc and Qa are the heat rejected by the refrigerant across condenser and absorber. To is the atmospheric temperature.
Thus, from first law of thermodynamics:
GAS REFRIGERATION CYCLE
Air cycle refrigeration systems belong to the general class of gas cycle refrigeration systems, in which a gas is used as the working fluid. The gas does not undergo any phase change during the cycle, consequently all the internal heat transfer processes are sensible heat transfer processes. Gas cycle refrigeration systems find applications in aircraft cabin cooling and also in liquefaction of various gases.
AIR STANDARD CYCLE
Air cycle refrigeration system analysis is considerably simplified if one makes the following assumptions:
(i) The working fluid is a fixed mass of air that behaves as an ideal gas.
(ii) The cycle is assumed to be a close loop cycle with all inlet and exhaust processes of open loop cycles being replaced by heat transfer processes to or from the environment.
(iii) All the processes within the cycle are reversible, i.e. the cycle is internally reversible.
(iv) The specific heat of air remains constant throughout the cycle.
BELL COLEMAN OR REVERSED BRAYTON CYCLE
This cycle can be thought of as a modification of reversed carnot cycle, as the two isothermal processes of carnot cycle are replaced by two isobaric heat transfer processes.
This cycle is also called as Joule or Bell-Coleman or Reversed Brayton cycle.
The actual reverse Brayton cycle differs from the ideal cycle due to:
(i) Non-isentropic compression and expansion processes.
(ii) Pressure drops in cold and hot heat exchangers.
Isentropic efficiency of compressor is defined as the ratio of ideal work required to that of actual work.
Whereas, isentropic efficiency of turbine is defined as ratio of actual work to that of ideal work.
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