Introduction

Refrigeration is necessary for the cooling process in full air-conditioning as well as process cooling and freezing.

The refrigeration cycle takes in energy at a relatively low temperature and discards it at some relatively higher temperature. For example in the domestic 'fridge the evaporator takes in heat from the body of the fridge or the 'ice box' and discards heat at the back of the fridge through a series of coiled pipework called the condenser.

Basis of Mechanical Refrigeration

The principle of mechanical refrigeration is that a liquid is made to 'boil' at a low temperature.

When a liquid boils a significant amount of energy is required to make the molecules vibrate enough to break free of the surface and change state into vapour.

If ethyl alcohol is poured onto a sheet of glass the alcohol will evaporate quickly at room temperature and the heat required for this vapourisation is taken from the glass thus cooling the glass, as shown below.

Also, if petrol is poured onto your skin (not recommended) after a while the skin feels cooler because the petrol has evaporated. This principle is adopted for mechanical refrigeration. A highly volatile liquid is passed through a heat exchanger where it boils when in contact with warmer air or water. Heat is extracted from the air or water thus cooling it down.

This heat exchanger is called an EVAPORATOR since the volatile liquid is evaporated into the vapour state from the liquid state as shown below.

In the case of water it takes about 2500 kJ of energy per kg of water at 100^oC to change water into steam.

This is also known as the latent heat of vapourisation.

For a continuous cycle where the volatile liquid doesn’t need to be replaced, the liquid can be condensed back to a liquid and then re-boiled in the evaporator. The CONDENSER is another heat exchanger where the vapourised gas is turned back into liquid.

If we call the volatile liquid the REFRIGERANT and ethyl alcohol is used as in the previous example given, then the vapourised alcohol needs to come in contact with something cool to condense it back to the liquid state. If mains water at 10^oC was used as shown below a simple condenser can be made.

If the evaporator shown above is put in a system with the condenser then a continuous refrigeration cycle is the result as shown below. A pump or compressor can be used to circulate the refrigerant.