Aux 2 Unit 8 Refrigeration General overview Q1 Answer

(a) Explain EACH of the following refrigeration terms:
(i) superheated;
(ii) saturation temperature;
(iii) sub-cooled.
(b) Sketch a basic vapour compression refrigeration system, showing the refrigerants condition as explained in part (a).

(a) Refrigeration Terms

Refers to refrigerant vapour that has been heated above its saturation temperature at a given pressure.
Example: After leaving the evaporator, vapour may be heated further to ensure no liquid droplets return to the compressor.
Purpose: Prevents compressor damage and improves efficiency.

The temperature at which a liquid refrigerant boils or condenses at a specific pressure.
At this point, liquid and vapour co-exist in equilibrium.
Example: In the evaporator, the refrigerant absorbs heat at its saturation temperature while changing phase.

Refers to liquid refrigerant cooled below its saturation temperature at a given pressure.
Example: After leaving the condenser, refrigerant is sub-cooled to avoid flash gas formation before the expansion valve.
Purpose: Improves efficiency and ensures proper expansion valve operation.

The four main components:

Compressor
- Draws in low-pressure superheated vapour from the evaporator.
- Compresses it to high-pressure, high-temperature superheated vapour.
Condenser
- Vapour rejects heat to cooling water/air.
- First cools to saturation temperature, condenses to liquid, then further cooled as sub-cooled liquid.
Expansion Valve (TXV or throttling device)
- Reduces pressure suddenly.
- Sub-cooled liquid partially flashes into vapour and becomes a low-temperature mixture at saturation temperature.
Evaporator
- The refrigerant absorbs heat from the cooled space.
- At constant pressure, liquid evaporates at its saturation temperature, producing low-pressure vapour.
- Vapour leaving is made slightly superheated before returning to the compressor.