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） Chemical Engineering Thermodynamics Zhengzhou University

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 which is below the surrounding temperature. Chemical Engineering Thermodynamics Zhengzhou University

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 Chemical Engineering Thermodynamics Zhengzhou University

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 Chemical Engineering Thermodynamics Zhengzhou University

Chapter 9 Refrigeration and Liquefaction By Dr. Zhenxi Jiang School of Chemical Engineering Chemical Engineering Thermodynamics Zhengzhou University

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. Chemical Engineering Thermodynamics Zhengzhou University

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. Chemical Engineering Thermodynamics Zhengzhou University

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. Chemical Engineering Thermodynamics Zhengzhou University

The Carnot refrigerator can be considered as a standard of comparison. Chemical Engineering Thermodynamics Zhengzhou University

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. Chemical Engineering Thermodynamics Zhengzhou University

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 = Q C / W = Q C / (Q H - Q C ) = T C / (T H - T C ) Chemical Engineering Thermodynamics Zhengzhou University

Refrigeration cycle Chemical Engineering Thermodynamics Zhengzhou University Heat Engine cycle

9.2 THE VAPOR-COMPRESSION CYCLE Chemical Engineering Thermodynamics Zhengzhou University Figure 9.1

9.2 THE VAPOR-COMPRESSION CYCLE Chemical Engineering Thermodynamics Zhengzhou University Figure 9.1

Chemical Engineering Thermodynamics Zhengzhou University 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

Chemical Engineering Thermodynamics Zhengzhou University 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. 9.2 THE VAPOR-COMPRESSION CYCLE

Chemical Engineering Thermodynamics Zhengzhou University The coefficient of performance is: ω = (H 2 - H 1 ) / (H 3 - H 2 ) 9.2 THE VAPOR-COMPRESSION CYCLE

Chemical Engineering Thermodynamics Zhengzhou University For given values of T C and T H, 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. 9.2 THE VAPOR-COMPRESSION CYCLE

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. Chemical Engineering Thermodynamics Zhengzhou University

9.3 THE CHOICE OF 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. Chemical Engineering Thermodynamics Zhengzhou University

9.3 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. Chemical Engineering Thermodynamics Zhengzhou University

9.3 THE CHOICE OF REFRIGERANT 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. Chemical Engineering Thermodynamics Zhengzhou University

9.3 THE CHOICE OF REFRIGERANT The requirements limit the choice of refrigerant to relatively few fluids. Ammonia, methyl chloride, carbon dioxide, propane and other hydrocarbons can serve as refrigerants Chemical Engineering Thermodynamics Zhengzhou University

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. Chemical Engineering Thermodynamics Zhengzhou University

9.4 ABSORPTION REFRIGERATION 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. Chemical Engineering Thermodynamics Zhengzhou University

9.4 ABSORPTION REFRIGERATION Chemical Engineering Thermodynamics Zhengzhou University

9.4 ABSORPTION REFRIGERATION Chemical Engineering Thermodynamics Zhengzhou University

9.5 THE HEAT PUMP Chemical 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 Q C from a cold reservoir and delivers heat Q H 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.2th law of thermodynamics Carnot cyclecoefficient of performance Chemical Engineering Thermodynamics Zhengzhou University

One Equation for Three Cycles: Heat Engine Refrigeration Heat Pump Chemical Engineering Thermodynamics Zhengzhou University

One Equation for Three Cycles: Heat Engine Refrigeration Heat Pump Chemical Engineering Thermodynamics Zhengzhou University

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. Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES 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. Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES 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. Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES 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. Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES 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. Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES 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. Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES 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. Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES 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. Chemical Engineering Thermodynamics Zhengzhou University

9.6 LIQUEFACTION PROCESSES 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. Chemical Engineering Thermodynamics Zhengzhou University

Example Problems Example 9.3 Chemical Engineering Thermodynamics Zhengzhou University

This the end of the lecture Thanks ! Chemical Engineering Thermodynamics Zhengzhou University