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Boiling points are published with respect to the environmental pressure.
5) The vapor pressure of the liquid equals the ambient atmospheric pressure at the atmospheric boiling point.
6)The boiling point cannot be increased beyond the critical point.
At the vapor point a liquid boils into its vapor phase.
7) The boiling point decreases with decreasing pressure until the triple point is reached
8) Liquids may change to a vapor through the process of condensation.
V. Complete the sentences:
VI. Make up different kinds of questions to the text. Ask your partner about the boiling point. (Work in pairs or groups)
I group (Yes/No-questions)
Do all liquids have an infinite number of boiling points?
II group (Wh-questions)
What do all liquids have?
III group (Tag-question)
All liquids have an infinite number of boiling points, don’t they?
VII. Read and translate the text:
The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems.
The thermodynamics of the cycle can be analyzed on a diagram as shown in Figure 1. Figure 1
In this cycle, a circulating refrigerant such as Freon enters the compressor as a vapor. From point 1 to point 2, the vapor is compressed at constant entropy and exits the compressor superheated. From point 2 to point 3 and on to point 4, the superheated vapor travels through the condenser which first cools and removes the superheat and then condenses the vapor into a saturated liquid by removing additional heat at constant pressure and temperature. Between points 4 and 5, the saturated liquid refrigerant goes through the expansion valve (also called a throttle valve) where its pressure abruptly decreases. That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes). The adiabatic flash evaporation process is isenthalpic (i.e., occurs at constant enthalpy).
That results in a mixture of liquid and vapor at a lower temperature and pressure as shown at point 5. The cold liquid-vapor mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes. The evaporator operates at essentially constant pressure. The resulting refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.
The above discussion is based on the ideal vapor-compression refrigeration cycle, and does not take into account real-world effects like frictional pressure drop in the system, slight thermodynamic irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).
Vapor absorption cycle: In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapor compression cycle, it lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapor compression cycle). Nowadays, the vapor absorption cycle is used only where waste heat is available or where heat is derived from solar collectors.
The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which raises the pressure and a generator which, on heat addition, drives off the refrigerant vapor from the high-pressure liquid. Some work is required by the liquid pump but, for a given quantity of refrigerant, it is much smaller than needed by the compressor in the vapor compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent), and water (refrigerant) and lithium bromide (absorbent).
Gas cycle: When the working fluid is a gas that is compressed and expanded but doesn't change phase, the refrigeration cycle is called a gas cycle. Air is most often this working fluid. As there is no condensation and evaporation intended in a gas cycle, components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas-to-gas heat exchangers in gas cycles.
The gas cycle is less efficient than the vapor compression cycle because the gas cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. As such the working fluid does not receive and reject heat at constant temperature. In the gas cycle, the refrigeration effect is equal to the product of the specific heat of the gas and the rise in temperature of the gas in the low temperature side. Therefore, for the same cooling load, a gas refrigeration cycle will require a large mass flow rate and would be bulky.
Because of their lower efficiency and larger bulk, air cycle coolers are not often used nowadays in terrestrial cooling devices. The air cycle machine is very common, however, on gas turbine-powered 'jet' aircraft because compressed air is readily available from the engines' compressor sections. These jet aircrafts' cooling and ventilation units also serve the purpose of pressurizing the aircraft.
The Peltier effect uses electricity directly to pump heat; refrigerators using this effect are sometimes used for camping, or where noise is not acceptable. They are totally silent, but less energy-efficient than other methods.
Other alternatives to the vapor-compression cycle but not in current use include thermionic, vortex tube, magnetic cooling, Stirling cycle, acoustic cooling, pulse tube and water cycle systems.
vapor-compression cycle – парокомпрессионный цикл
compressor – компрессор
vapor – пар
to superheat – перегревать
condenser – конденсатор
liquid – жидкость
а expansion valve (a throttle valve) – регулирующий вентиль (для жидкого холодильного агента)
to decrease –уменьшаться
flash evaporation – мгновенное испарение
evaporator – испаритель
coil – спираль, змеевик
inlet – впуск, вход; входное отверстие
coefficient of performance – холодильный коэффициент (в компрессионной холодильной машине); тепловой коэффициент (в абсорбционной холодильной машине)
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