专用术语中英文对照 Chapter 9 Refrigeration and Liquefaction 冷冻和液化 Refrigeration...

专用术语中英文对照 Chapter 9 Refrigeration and Liquefaction 冷冻和液化 Refrigeration 制冷，冷冻

Freezing water 冷冻水，（低于环境温度）（ -30 ~ 20℃ ）用于使温度下降至低于环境温度

Cooling water 冷却水，（高于环境温度）用于冷却热介质

Chemical Engineering Thermodynamics Zhengzhou University

Freezing water 冻水，冰镇水（蓝色）

Cold water 冷水（白色）

Hot water 热水（红色）


Freezing

冰点，结冰，摄氏零度 The temperature remained below freezing all day.

严寒的（非常冷 , 比 Chilly 更冷） I am freezing

冷冻（水）（温度低于环境温度） Freezing water is used to decrease temperature wh

ich is below the surrounding temperature.


Dehydration 脱水，干燥

Treatment 医疗处理，治疗

A standard of comparison 一个比较的标准 Throttle Valve 节流阀


Liquefaction Processes 液化过程

Linde liquefaction process 林德液化过程

Claude liquefaction process

克劳德液化过程


Chapter 9

Refrigeration and Liquefaction

By Dr. Zhenxi Jiang

School of Chemical Engineering


Chapter 9 Refrigeration and Liquefaction

Refrigeration is best known for its use in the air conditioning of buildings and in the treatment, transportation, and preservation of food and beverages. It also finds large-scale industrial application, for example, in the manufacture of ice and the dehydration of gases. Application in the petroleum industry include lubricating-oil purification, low temperature reactions, and separation of volatile hydrocarbons. A closely related process is gas liquefaction, which has important commercial applications.


The purpose of this chapter is to present a thermodynamic analysis of refrigeration and liquefaction processes.

The word refrigeration implies the maintenance of a temperature below that of the surroundings.


Refrigeration requires continuous absorption of heat at a low temperature level, usually accomplished by evaporation of a liquid in a steady-state flow process. The vapor formed may be returned to its original liquid state for re-evaporation in either of two ways. Most commonly, it is simply compressed and then condensed. Alternatively, it may be absorbed by a liquid of low volatility, from which it is subsequently evaporated at higher pressure.


The Carnot refrigerator can be considered as a standard of comparison.


9.1 The Carnot Refrigerator

In a continuous refrigeration process, the heat absorbed at a low temperature is continuously rejected to the surroundings at a higher temperature. Basically, a refrigeration cycle is a reversed heat-engine cycle.


9.1 The Carnot Refrigerator

Heat is transferred from a low temperature level to a higher one; according to the second law, this requires an external source of energy.

The measurement of the effectiveness of a refrigerator is its coefficient of performance ω. The definition of ω is

ω = QC / W = QC / (QH - QC ) = TC / (TH - TC )


Refrigeration cycleChemical Engineering Thermodynamics Zhengzhou University

Heat Engine cycle

9.2 THE VAPOR-COMPRESSION CYCLE Chemical Engineering Thermodynamics Zhengzhou University

Figure 9.1


The vapor-compression refrigeration cycle is represented in the Figure 9.1 above. Shown on the T-S diagram are the four steps of the process.

9.2 THE VAPOR-COMPRESSION CYCLE


In principle, this can be carried out in an expander from which work is obtained, but for practical reasons is accomplished by throttling through a partly open valve. The throttling process occurs at constant enthalpy.



The coefficient of performance is:

ω = (H2 - H1) / (H3 - H2)



For given values of TC and TH, the highest possible value of ω is attained for Carnot-cycle refrigeration. The lower values for the vapor-compression cycle result from irreversible expansion in a throttle valve and irreversible compression.


9.3 THE CHOICE OF REFRIGERANT

The efficiency of a Carnot heat engine is the working medium of the engine. Similarly, the coefficient of performance of a Carnot refrigerator is independent of the refrigerant.



Nevertheless, such characteristics as its toxicity, flammability, cost, corrosion properties, and vapor pressure in relation to temperature are of greater importance in the choice of refrigerant.



So that air cannot leak into the refrigeration system, the vapor pressure of the refrigerant at the evaporator temperature should be greater than atmospheric pressure.



On the other hand, the vapor pressure at the condenser temperature should not be unduly high, because of the initial cost and operating expense of high-pressure equipment.



The requirements limit the choice of refrigerant to relatively few fluids. Ammonia, methyl chloride, carbon dioxide, propane and other hydrocarbons can serve as refrigerants


9.4 ABSORPTION REFRIGERATION

In vapor-compression refrigeration the work of compression is usually supplied by an electric motor. But the source of the electric energy for the motor is probably a heat engine (central power plant) used to drive a generator.



Thus the work for refrigeration comes ultimately from heat at a high temperature level. This suggests that the direct use of heat as the energy source for refrigeration. The absorption-refrigeration machine is based on this idea.


9.5 THE HEAT PUMPChemical Engineering Thermodynamics Zhengzhou University

9.5 THE HEAT PUMP A heat pump is a device which applies external

work to extract an amount of heat QC from a cold reservoir and delivers heat QH to a hot reservoir. A heat pump is subject to the same limitations from the 2th law of thermodynamics as any other heat engine and therefore a maximum efficiency can be calculated from the Carnot cycle. Heat Pumps are usually characterized by a coefficient of performance which is the number of units of energy delivered to the hot reservoir per unit work input.


http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatpump.html#c3

One Equation for Three Cycles:• Heat Engine

• Refrigeration

• Heat Pump


9.6 LIQUEFACTION PROCESSES

Liquefied gases are used for a variety of purposes. For example, liquid propane in cylinders serves as a domestic fuel, liquid oxygen is carried in rockets, natural gas is liquefied for ocean transportation, and liquid nitrogen provides low-temperature refrigeration. Gas mixtures are liquefied for separation into their component species by distillation.



liquefaction results when a gas is cooled to a temperature in the two-phase region. This may be accomplished in several ways:

1. by heat exchange at constant pressure.

2. by an expansion process from which work is obtained.

3. by a throttling process.



The first method requires a heat sink at a temperature lower than that to which the gas is cooled, and is most commonly used to pre-cool a gas prior to its liquefaction by the other two methods. An external refrigerator is required for a gas temperature below that of the surroundings.



The three methods are illustrated in Figure 9.5. The constant-pressure process (1) approaches the two-phase region (and liquefaction) most closely for a given drop in temperature. The throttling process (3) does not result in liquefaction unless the initial state is at a low enough temperature and a high enough pressure for the constant-enthalpy process to cut into the two-phase region.



Liquefaction by isentropic expansion along process (2) occurs from lower pressure (for given temperature) than by throttling. For example, continuation of process (2) from initial state A ultimately results in liquefaction.



The Linde liquefaction process, which depends solely on throttling expansion, is shown in Figure 9.6. After compression, the gas is pre-cooled to ambient temperature. It may be even further cooled by refrigeration. The lower the temperature of the gas entering the throttle valve, the greater the fraction of gas that is liquefied.



A more efficient liquefaction process would replace the throttle valve by an expander, but operating such a device into the two-phase region is impractical. However, the Claude process, shown in Figure 9.7, is based in part on this idea.



Gas at an intermediate temperature is extracted from the heat-exchange system and passed through an expander from which it exhausts as a saturated or slightly superheated vapor. The remaining gas is further cooled and throttled through a valve to produce liquefaction as in the Linde process.



The un-liquefied portion, which is saturated vapor, mixes with the expander exhaust and returns for recycle through the heat exchanger system.

The Linde process is a limiting case of the Claude process, obtained when none of the high-pressure gas stream is sent to an expander.


Example Problems

Example 9.3


This the end of the lecture

Thanks !


专用术语中英文对照 Chapter 9 Refrigeration and Liquefaction 冷冻和液化 Refrigeration...

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