E2.9 Refrigeration Cycle Experiment: Theory

One important application of the thermodynamic cycles is refrigeration. The transfer of heat from a higher temperature to a lower one is a natural phenomenon, but the unnatural reverse process happens in refrigeration cycles. The devices that produce refrigeration are called refrigerators and the working fluids are called refrigerants.

Fig. 2.1 Schematic diagram of refrigerator

A refrigerator is shown schematically in Figure 2.1. Here, QL is the magnitude of the heat removed from the refrigerated space at temperature TL, QH is the magnitude of the heat rejected to the warm space at temperature TH, and Win is the net work input to the refrigerator.

Refrigeration can be classified as:
1. Vapor cycle, and
2. Gas cycle

Vapor cycle refrigeration can further be classified as:
1. Vapor compression refrigeration
2. Gas absorption refrigeration

Fig. 2.2 Ideal vapor compression refrigeration cycle

An ideal vapour compression refrigeration cycle is defined as a refrigeration system that removes thermal energy from a low temperature region and transfers the heat to a high temperature region. The diagram in Figure 2.2 shows the schematic and the T-s graph for an ideal vapor compression refrigeration cycle.

For an ideal vapor compression refrigeration cycle, it consists of 4 main processes:

The refrigerant enters the compressor at state 1 as saturated vapour and is compressed isentropically to the condenser pressure. The temperature increases during this process to well above the temperature of the surrounding medium. The refrigerant then enters the condenser as superheated vapour at state 2 and leaves as saturated liquid at state 3 as a result of heat rejection. The temperature at this state is still above the surrounding temperature.
The saturated liquid at state 3 is throttled to the evaporator pressure by passing it through an expansion valve. The refrigerant enters the evaporator at state 4 as a low quality saturated mixture and is completely evaporated by absorbing heat from the refrigerated space. The refrigerator leaves the evaporator as saturated vapour and re-enters the compressor, completing the cycle.

However, in real life situations, the ideal cycle rarely occurs due to friction in the pipe and irreversibilities that occur in pipes and devices in the refrigeration cycle. The deviation from the ideal cycle is as shown below in Fig. 2.3.