Mechanical refrigeration principle is based on the phenomenon that any liquid boil and evaporate into gas at lower temperature when its pressure is decreased (i.e. as less as liquid pressure, as less as boiling or evaporation temperature. This why water boils at temperature less than 100 °C at mountain tops where atmospheric pressure is less than 1 bar). Mechanical refrigeration cycle (Fig.1.5) is a mechanical system designed to simulate above mentioned phenomenon.
Fig.1.5 Mechanical refrigeration cycle
Cycle is charged with refrigerant (refrigerants are fluids with low temperature evaporation at pressures near atmospheric pressure). Cycle is mainly composed of four elements;
Evaporator; is a long metal pipe (copper or aluminum) with one inlet and one outlet, arranged in rows, low temperature liquid flows inside pipe, heat is transferred from refrigerated space to evaporator through pipe walls, and accordingly liquid evaporates and sucked by compressor suction.
Compressor; driven by an electric motor, its function is compressing refrigerant gas to a pressure that can be cooled and condensate by air at ambient temperature (if condenser is air cooled).
Condenser; has similar construction to evaporator (rows arranged piping), but its function is too cool & condensate hot gas coming from compressor (change it into liquid).
Expansion valve; its function is to reduce pressure of liquid coming from condenser and as a result of pressure reduction, temperature will be reduced. Reduced temperature liquid will flow into evaporator, and cycle will be repeated in a continuous manner.
Refrigeration condenser is either to be air cooled (Fig.1.6, static or forced by a fan) or water cooled (Fig.1.7).
Fig.1.6 Air cooled condenser
Fig.1.7 Water cooled condenser
Selecting of condenser type depends on several factors such as refrigeration capacity, capital cost, energy cost, maintenance cost, water availability, space availability, etc. Simply; air cooled condenser reduces equipment capital cost, more suitable for small capacities, lower maintenance cost and higher running energy costs. Water cooled condenser is either cooled by water from cheap source like a river or by using of a cooling tower (common selection, Fig.1.8 shows a diagrammatic sketch for cooling tower), which cools water and return it by a pump to cool condenser again, this allow reusing of water with small make-up to balance for evaporated water, cooling tower requires dry climate to function properly.
Fig.1.8 Cooling Tower
Fig.1.9 Evaporative condenser
Another type of condenser water cooling is “evaporative condenser” in which water temperature is decreased in a similar way to cooling tower, but with condenser coil exists inside the tower (Fig.1.9 shows a diagrammatic sketch for evaporative condenser).
Water cooled condenser as explained has higher capital cost, requires additional space for cooling tower, has little higher maintenance cost and has noticeable lower running energy cost which is the main advantage, more suitable for large capacity refrigeration plants, such as large capacity cold stores, large super markets refrigeration system (central system for refrigerated displays), etc.