Vapor-compression refrigeration

Vapor-compression refrigeration is one of the many refrigeration cycles available for use. It has been and is the most widely used method for air-conditioning of large public buildings, private residences, hotels, hospitals, theaters, restaurants and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems.

Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. A device that performs this function may also be called a heat pump.

The vapor-compression refrigeration cycle consists of condensing coil, expansion valve, evaporator coil, and compressor.

In the vapor-compression refrigeration cycle, heat is transferred from a lower temperature source to a higher temperature heat sink. Heat naturally flows in the opposite direction, and due to the second law of thermodynamics work is required to move heat from cold to hot. A food refrigerator or freezer works in much the same way; it moves heat out of the interior into the room in which it stands.

This most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile the compressor is usually driven by a belt connected to a pulley on the engine's crankshaft, with both using electric motors for air circulation. Since evaporation occurs when heat is absorbed, and condensation occurs when heat is released, air conditioners are designed to use a compressor to cause pressure changes between two compartments, and actively pump a refrigerant around. A refrigerant is pumped into the low pressure compartment (the evaporator coil), where, despite the low temperature, the low pressure causes the refrigerant to evaporate into a vapor, taking heat with it. In the other compartment (the condenser), the refrigerant vapour is compressed and forced through another heat exchange coil, condensing into a liquid, rejecting the heat previously absorbed from the cooled space. The heat exchanger in the condenser section (the heat sink mentioned above) is often cooled by a fan blowing outside air through it, or in some cases, such as marine applications, by other means such as water.

Figure 2

 

The vapor-compression refrigeration system uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Figure 2 depicts a typical, single-stage vapor-compression system. All such systems have four components: a compressor, a condenser, an expansion valve (also called a throttle valve), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at temperature and pressure at which it can be condensed with typically available cooling water or cooling air. That hot vapor is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air (whichever may be the case).

The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated.

The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser.

To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor.

Note: Saturated vapors and saturated liquids are vapors and liquids at their saturation temperature and saturation pressure. A superheated vapor is at a temperature higher than the saturation temperature corresponding to its pressure.

The most common compressors used in chillers are reciprocating, rotary screw, centrifugal, and scroll compressors. Each application prefers one or another due to size, noise, efficiency and pressure issues.

Reciprocating compressors are piston-style, positive displacement compressors.

Rotary screw compressors are also positive displacement compressors. Two meshing screw-rotors rotate in opposite directions, trapping refrigerant vapor, and reducing the volume of the refrigerant along the rotors to the discharge point.

Centrifugal compressors are dynamic compressors. These compressors raise the pressure of the refrigerant by imparting velocity or dynamic energy, using a rotating impeller, and converting it to pressure energy.

Scroll compressors are also positive displacement compressors. The refrigerant is compressed when one spiral orbits around a second stationary spiral, creating smaller and smaller pockets and higher pressures. By the time the refrigerant is discharged, it is fully pressurized.

 

ACTIVE VOCABULARY

vapor-compression refrigeration – паровое компрессионное (парокомпрессионное) охлаждение

oil refinery – нефтеочистительный завод

to utilize – использовать, расходовать, употреблять

a device – устройство, приспособление

to perform – исполнять, выполнять; делать, совершать

a heat pump – тепловой насос

а expansion valve (a throttle valve) – регулирующий вентиль (для жидкого холодильного агента)

condensing coil – змеевиковый конденсатор

evaporator coil – испарительный змеевик; змеевик испарителя

refrigerator – рефрижератор, холодильник, холодильная установка

freezer – рефрижератор, холодильник, холодильная установка

interior – внутренняя часть

to drive (drove, driven) – приводить (к какому-л. состоянию)

evaporation – испарение

condensation – сжижение; конденсирование, конденсация

a compartment – отделение

a liquid – жидкость

heat exchanger – теплообменник

fan – вентилятор

medium – среда

a saturated vapor – насыщенный пар a superheated vapor – перегретый пар

to reject – отвергать, отклонять; отказываться

to undergo (underwent; undergone) – испытывать, переносить

adiabatic – адиабатический, адиабатный

to lower – снижать, уменьшать

to transfer – переносить, перемещать

to complete – завершать, заканчивать

refrigeration cycle – холодильный цикл

reciprocating – возвратно-поступательный

rotary screw – ротационный винтовой

centrifugal – центробежный

piston – поршень

displacement – объём чего-л.

to rotate – вращаться

velocity – скорость; быстрота

impeller – рабочее колесо

to convert – преобразовывать; превращать

a spiral – спираль,

to orbit – вращаться (по орбите)

 

V. Find the Russian equivalents for the English ones:

 

air-conditioning, to trap, large-scale warehouse, an abrupt reduction, condensing coil, to impart, a lower temperature source, discharge point, an impeller, saturation pressure, stationary spiral, heat exchange coil, piston-style, a saturated liquid, scroll compressor

 

VI. Find the English equivalents for the Russian ones:

 

Холодильный цикл, тепловой насос, энергия давления, испарительный змеевик, теплоотвод, воздушный карман, электродвигатель, температура насыщения, динамический компрессор, термодинамическое состояние, насыщенный пар, динамическая энергия, желательный, объёмный компрессор, объем охладителя

 

VII. Translate the following sentences into Russian:

 

1) Сhemical processing plants and natural gas processing plants often utilize large vapor-compression refrigeration systems.

2) Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere.

3) In an automobile the compressor is usually driven by a belt connected to a pulley on the engine's crankshaft, with both using electric motors for air circulation.

4) Air conditioners are designed to use a compressor to cause pressure changes between two compartments, and actively pump a refrigerant around.

5) In the condenser the refrigerant vapour is compressed and forced through another heat exchange coil, condensing into a liquid.

6) Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor and is compressed to a higher pressure.

7) The hot vapor is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes.