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Refrigeration is generally understood to mean a lowering of the temperature from ambient temperature down to the standard boiling point of ethylene (ca. 170 K); low-temperature technology is from 170 K down to the melting point of nitrogen (63 K); and cryogenics is below 120 K. In a food factory refrigeration equipment is needed for cooling, chilling and freezing.
Field of application
In many food manufacturing processes cooling is an important process step. Often mechanical cooling (using refrigeration equipment) is applied. Furthermore, many food products are stored and distributed either chilled or frozen.
Description of techniques, methods and equipment
The main components of a mechanical refrigeration plant are the evaporator, the compressor, the condenser and the throttling valve. The refrigerant circulates through these four parts of the refrigeration plant, changing state from liquid to gas, and back to liquid again. In the evaporator, heat is absorbed from the surrounding. This causes part of the refrigerant to vaporise. Where ammonia is used as the refrigerant, the common temperatures of evaporation are – 20 °C to – 25 °C, which corresponds to a pressure of 100 to 200 kPa. Refrigerant vapour goes from the evaporator to the compressor where the pressure is raised to about 1000kPa, which corresponds to a temperature of about 25 °C. The pressurised vapour then goes into the condenser, where the vapour is condensed. The heat absorbed by the refrigerant in the evaporator is released in the condenser. The condenser is cooled by water or air. The resultant liquid refrigerant then goes to the expansion (throttling) valve, where the pressure and temperature are reduced to restart the refrigeration cycle. The important components of the refrigerants are: ammonia (NH3) and halogen refrigerants (chlorofluoro-carbons (CFCs) and partially halogenated CFCs (HCFCs)). Ammonia has
excellent heat transfer properties and is not miscible with oil, but it is toxic and flammable.Halogen refrigerants are non-toxic, non-flammable and have good heat transfer properties. The interaction of halogen refrigerants with ozone in the atmosphere has resulted in a ban on CFCs. HCFCs are less environmentally harmful, but have to be phased out before the year 2015. Some new ozone-friendly HCFCs are currently being developed.
Condensers of refrigeration equipment can be divided into three types:
a) Air-cooled condensers
In the air-cooled condenser, the refrigerant goes through fin elements, around which the cooling air circulates.
b) Water-cooled condensers
The most common type of water-cooled condenser is the tube condenser. It operates by circulating cooling water inside the tubes. This condenses the refrigerant on the external tube surface. The water-cooled condenser is cooled according to the water once-through-system or with water which circulates over a cooling tower.
c) Evaporative condensers
The evaporation condenser is a combination of an air-cooled condenser and a cooling tower. Water evaporates at the condenser surface.