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Refrigeration Cycle


It is a well known fact that heat flows in the direction of decreasing temperature, i.e., from a high temperature region to a low temperature region.

But the reverse process (i.e. heat transfer from low to high temperature) cannot occur by itself (Claussius Definition of Second Law). This process requires a special device called Refrigerator.

Refrigerator and heat pump

Another device which transfers heat from low to high temperature is a Heat Pump. Heat pump and refrigerator cycles are very similar. The difference is in their objectives.

Reversed Carnot Cycle 

Carnot cycle is a totally reversible cycle which consists of two reversible isothermal processes and two isentropic processes.

 It has the maximum efficiency for a given temperature limit.

Since it is a reversible cycle, all four processes can be reversed. This will reverse the direction of heat and work interactions, therefore producing a refrigeration cycle.
The cycle consists of

1-2: Isothermal heat transfer from cold medium to refrigerant (Evaporator)

2-3: Isentropic (Reversible adiabatic) compression

3-4: Isothermal heat rejection (condenser)

4-1: Isentropic Expansion

Practical Difficulties of Carnot cycle

compression of two-phase mixture from 1-2

Expansion from 4-1 results in a very wet refrigerant, causing erosion of turbine blades.

Ideal vapour compression refrigeration cycle 

The impracticalities of the reversed Carnot Cycle can be eliminated by:

vaporising the refrigerant completely before it is compressed

replacing the turbine by a throttle valve

by implication; isothermal processes are replaced by constant pressure processes.

The cycle consists of:

1-2: Isentropic compression

2-3: Constant pressure heat rejection (Condenser)

3-4: Adiabatic expansion in a throttling device

4-1: Constant pressure heat absorption (Evaporator)

The throttling process 

Imagine a steady flow process in wich a restriction is introduced into a flow line or pipe. As a result a pressure drop occurs. The process is irreversible.

Applying SFEE: 

 Therefore, h2 = h1

Pressure-enthalpy chart 

The ideal vapour compression cycle consists of two constant pressure process and one constant enthalpy process. So in preliminary cycle calculations pressure-enthalpy diagrams are particularly useful.

Actual vapour-compression cycle 

Two main differences with ideal cycle:

Fluid frictions, causing pressure drop

Heat transfer to or from surroundings.

1-2: Irreversible and non-adiabatic compression of refrigerant. Heat transfer from surroundings to refrigerant è Entropy increases (S2>S1).

1-2': Heat transfer from refrigerant to surroundings è S2'<S1 (preferred)

2-3: Temperature (& pressure) drop due to fluid friction and heat transfer

3-4: pressure drops in the condenser because of fluid friction

4-5: temperature and pressure drop (as in 2-3)

5-6: Throttling process

6-7: The throttle valve and evaporator are usually located very close to each other, so pressure drop in connecting line is small.

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