lesson 20 Marine refrigeration(制冷) 轮机英语教学课件.pptVIP

Lesson 20 Marine refrigeration(制冷) The natural transfer of heat is from a hot body to a cold body, the function of a refrigeration plant is to act as a heat pump and reverse this process so that rooms can be maintained at low temperatures for the preservation(保存) of foodstuffs, or air can be cooled for the air conditioning. The compression refrigeration is the most commonly used type of marine refrigeration. The components required for a compression refrigeration system are a compressor, condenser, liquid receiver, expansion valve and evaporator or cooling coils(蒸发或冷却盘管). Drawing of compression refrigeration system The refrigerating agent, as so called refrigerant, flows from the expansion valve and through the evaporator coils where it absorbs heat from the space being cooled and becomes a gas or vapor. Then it flows to the compressor, where it is compressed to the condenser pressure(condensing pressure) and thereby becomes hot. In the condenser, the hot, high pressure refrigerant gas is cooled by the sea water and becomes liquid. The liquid refrigerant flows to the receiver and then to the expansion valve to start the circuit again. The air from the cooled space or air conditioning system is passed over the evaporator and boil off the liquid refrigerant, at same time cools the air itself. The boiling and condensation points of a refrigerant depend on the pressure exerted upon it. Because the expansion valve is just a little way open, the refrigerant is under low pressure in the evaporator and under high pressure in the condenser. Therefore it can evaporate at low temperature in the evaporator and condense at high temperature in the condenser. Thus it will be seen that heat that is transferred from the air to the evaporator is then pumped round the system until it reaches the condenser where it is transferred or rejected to the sea water. The most common refrigerants used on ship are now probably Refrigerant 22 and Refrigerant 12, Refrigerant 502 and Ammoni