The water for frost formation comes from the articles placed in the refrigerator and the occasional air that gets into the gadget when it is opened and closed.
Refrigeration is the process of lowering the temperature and maintaining it in a given space. In mechanical refrigeration, constant cooling is achieved by the circulation of a refrigerant in a closed system, in which it evapourates to a gas and then condenses back into a liquid in an evapourates to a gas and then condenses back into a liquid in a continuous cycle.
The two main types of mechanical refrigeration systems used are the compression system (used in domestic units for large cold-storage applications and for most air conditioning) and the absorption system (now employed largely for heat-operated air-conditioning units).
In the compression systems four elements – compressor, condenser, expansion valve, and evapourates are involved in the refrigeration cycle. In the evapourator the refrigerant is vapourized and heat is absorbed from the material contents or the space being cooled. The vapour is next drawn into a compressor and elevated to high pressure, which raises its temperature. The resulting superheated, high-pressure, gas is then condensed to liquid in an air-or-water-cooled condenser. From the condenser the liquid flows through an expansion valve, in which its pressure and temperature are reduced to the conditions that are maintained in the evapourator. The cycle thus continues. The amount of water vapour in the atmosphere is termed as humidity. Air at a given temperature can contain only a certain amount of water vapour. This amount increases as the temperature rises and decreases as temperature falls. In air containing a given amount of water vapour, the temperature at which dew (water that condenses on cool objects from the water vapour in warm air) begins to form is known as the dew point. If the dew point is below the freezing temperature of water, frost is formed.
When the air surrounding the evapourator coil is cooled the moisture present in the air condenses and sticks on to the coil in the form of water droplets. These droplets are cooled in the subsequent cycle and results in frost.
The moisture present in the items placed is absorbed by the air and carried to the evapourated coil. The condensation takes place as explained earlier.