Carnot Vapour compression Refrigeration cycle

(a) Schematic representation (b) T-s diagram

Processes: -

1-2: Isentropic compression from state 1 (wet vapour) to state 2 (saturated vapour)

2-3: Heat rejection (Q_H) in the condenser

3-4: Isentropic expansion from state 3 (saturated liquid)

4-1: Heat absorption ( Q_L) in the evaporator

The COP of the refrigerator,

Practical Vapour compression refrigeration cycle

(a) schematic diagram (b) T-s diagram

Application of the first law of thermodynamics to the control volume compressor, condenser, throttle and evaporator gives

(W_s)_compressor=h₂-h₁

Q_H=h₂-h₃

h₃=h₄

and Q_L=h₁-h₄

The COP of the refrigerator is given by,

In the ideal refrigeration cycle, the refrigerant leaves the evaporator as wet vapour.

In some cases the refrigerant leaves the evaporator as either saturated vapour or superheated vapour.

T-s diagram for a vapour compression refrigeration cycle when the refrigerant leaves the evaporator as (a) saturated vapour (b) superheated vapour

Gas refrigeration cycle

(a) Schematic diagram (b) T-s diagram

The simplest gas refrigeration cycle is the reversed Brayton cycle

Processes: -

1-2: isentropic compression for state 1 (atmospheric air) to state 2

2-3: energy exchange with the surrounding, air is cooled

3-4: isentropic expansion to state 4

Work obtained during the expansion process can be used to run the compressor

Work done on the compressor,

Work delivered by the expander,

The net work required= C_P (T₂-T₁-T₃+T₄)

The COP of this refrigeration system is given by,