lesson 20

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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.
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However, it does have cost advantages for large installations operating at low temperatures.
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Refrigerant 502 has many advantages over R12 and R22 and is particularly suitable for hermetic(密封的) compressors, but is still an expensive gas and not readily available worldwide at present.
Refrigerant 22 is now the most common refrigerant for a wide range of temperature, including low temperature, 41oC being possible without negative evaporator pressures(负压).
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The compression refrigeration is the most commonly used type of marine refrigeration.
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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).
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Refrigerant 12 was the first of the halogenated hydrocarbon( 卤 化 烃 ) refrigerants to become widely available at sea.
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Drawing of compression refrigeration system
Compressor
Condenser
Liquid receiver Expansion valve
Evaporator or Cooling coils
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Dalian Ocean University
Apart from hermetic compressors, one end of the shaft has to project( 伸 出 ) through the crankcase in order to receive the motor drive, either direct coupled or belt.
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Then it flows to the compressor, where it is compressed to the condenser pressure(condensing pressure) and thereby becomes hot.
Lesson 20
Marine refrigeration(制冷)
Refrigerate, refrigerant
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Q3
常温高压液体 高温高压蒸汽
Q2
低温低压90%液体和 1cean University
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
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Oil lubrication is maintained between the rubbing faces.
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To allow capacity reduction cylinder unloading gear(气缸卸 荷 机 构 ) which allows suction valves to be held open has been introduced to multicylinder compressors.
The essential component of a typical seal is a carbon ring, stationary and sealed to the gland cover, with a flat surface against which is pressed the rotating face of a cast iron ring secured to the shaft.
卧式壳管式冷凝器
[Closed Shell and Tube Condenser]
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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.
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The sealing of this shaft presents a greater problem than the usual 'shaft seal' encountered on the other mechanical items.
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Dalian Ocean University
The most common refrigerants used on ship are now probably Refrigerant 22 and Refrigerant 12, Refrigerant 502 and Ammonia(氨).
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But because of its hazardous effect on ozonosphere(臭氧层), the using in refrigeration will be soon prohibited.
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Ammonia has never been popular refrigerant for marine use owing to its irritant(刺激的) and toxic(有毒的) properties in the event of a leak.
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The regulator or expansion valve has small orifice in order to effect the desired pressure reduction and ensure that the correct volume of liquid refrigerant flows from the high pressure side of the system through to the low pressure evaporator side.
The liquid refrigerant flows to the receiver and then to the expansion valve to start the circuit again.
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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.
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The most common compressor at sea today is the reciprocating compressor for use with Freon.
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Designs cover a power range of 1 to 2OO kW, the small machines being vertical in line and larger ones with cylinder in V or W formation.
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.