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.
Therefore, h2 = h1
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.