制冷技术（英文版）.doc

Chapter 5. Vapor and Gas Refrigeration Cycles 5-1) Mechanical Powered Vapor Compression Refrigeration Cycle 5-2) Heat Powered Vapor Compression Refrigeration CycleVapor Absorption Refrigeration 5-3) Heat Powered vapor Compression Refrigeration CycleVapor Adsorption Refrigeration 5-4) Heat Powered vapor Compression Refrigeration Cycle Vapor Jet Refrigeration 5-5) Refrigeration Cycle by Gas Compression and Adiabatic Expansion 5-1) Mechanical Powered Vapor Compression Refrigeration Cycle(1) Introduction of mechanical powered vapor compression refrigeration cycle Hot heat reservoirHeat sink Refrigeration cycleCondenser Evaporator Compressor Metering DeviceFluid to be cooled(Refrigeration load) Cooling fluidHeat source Cold heat reservoirFig.2-1, The Model for Analysis of Refrigeration CycleThe mechanical vapor compression refrigeration is the most common refrigeration cycle. Its advantages, in comparison with other types of refrigeration systems, include the compact of the system; high coefficient of performance (COP); being reliable, safe and flexible in operation; relatively simple in maintenance; and low initial costs. (2) Basic vapor compression refrigeration cycle举个例子Here take the Refrigerant R134a as an example to show how to calculate the cycles cooling capacity and COP by assuming that the refrigerant leaves the evaporator at the temperature of -20°C and it is condensed at 40°C. For the case of evaporating temperature and condensing temperature, the thermal properties of R134a can be found from the diagram or table of the refrigerant R134a as below:Evaporating pressure Condensing pressure The specific enthalpies of R134a at these states are: (It is an isentropic process from point1 to point2.) Fig.5-1, Schematic and a log p-h diagrams for the basic vapor compression cycleThe process 1-2 is a reversible, adiabatic (isentropic) compression (5-1)The process 2-3 is an isobaric heat rejection process (5-2)The process 3-4 is an irreversible throttling process, The process 4-1 is an isobaric constant pressure heat admission process (5-3)The coefficient of performance of the cycle can be calculated as: (5-4)If the mass flow rate of the refrigerant R134a through this cycle is m=0.1kg/s, then the refrigeration capacity, the condensing load and the work of compression can be gotten as: 5-2) Heat Operated Vapor Compression Refrigeration Cycle (1)Vapor Absorption Refrigeration （还是蒸汽压缩式制冷，降温方法一样（p.53），区别只在于压缩方式）有热能可以利用的场合There is abundant thermal energy appeared in different forms in the world, such as solar thermal, geothermal, various wasted heats and biomass energy etc. These energies can be used to drive refrigeration and air-conditioning systems.3种热驱动的蒸汽压缩式制冷There are three kinds of vapor compression refrigeration cycles that can be driven by thermal energy. They are: 1，the absorption refrigeration cycle, 2，the adsorption refrigeration cycle and 3，vapor jet refrigeration cycle. These cycles share similar technologies that are used in the vapor compression refrigeration cycle, i.e., throttling evaporating and condensing. but they are driven by thermal energy. These refrigeration cycles will discussed in this chapter. (1) Principles of absorption refrigerationFig.5-2, essential components of the vapor absorption cycleThe mechanical compressor is replaced by a thermal compressor which consists of absorber, solution pump, generator (or boiler) and liquid valve. This group of components sucks vapor from the evaporator, and delivers high pressure vapor to the condenser, just as the mechanical compressor does but the vapor is actually absorbed by a liquid absorbent . Aqua ammonia and aqua lithium bromide solutions are commonly used in vapor absorption refrigeration systems.氨水吸收的蒸汽压缩式制冷系统The absorption of ammonia by water is an exothermic process. The strong solution formed in the absorber is pumped to the generator at higher pressure. In the generator, the strong solution is boiled by heating, and the vapor given off is rectified to nearly pure ammonia and delivered to the condenser. There is a heat exchanger interposed between the generator and absorber. The hot weak solution from the generator transfers the heat to the strong solution from the absorber. To maintain the difference in pressures between the generator and absorber, a valve is installed in the pipe 4, 5The refrigerants and absorbent in H2O-LiBr system and NH3H2O systemAbsorption cyclerefrigerantAbsorbentH2OLiBr systemH2OLiBr solutionNH3H2O systemNH3H2O溴化锂水吸收的蒸汽压缩式制冷系统In lithium bromide-water absorption refrigeration systems, water is the refrigerant and lithium bromide is the absorbent. This explains that the lithium bromide absorption system is strictly limited to evaporation temperatures above 0ºC; and the ammonia absorption system is mainly used for low temperatures below 0ºC. Water as a solvent in ammonia absorption system is present in the vapor so rectification is required to remove it, whereas LiBr (a hygroscopic salt) is almost non-volatile at the operating conditions so rectification is not necessary (2) Composition of mixtures Calculation of absorption refrigerators requires some knowledge of the thermodynamics of solutions （溶液热力学） and of how their properties depend on the composition.Composition of a mixture is expressed as the mass fraction of one of the components. For example, in H2OLiBr solution it contains mass of LiBr and of H2O, the mass fraction of LiBr is defined as: (5-5) (3) Vapor pressure of LiBr-water solution溴化锂水溶液的蒸气压The vapor pressure of aqua lithium bromide solution is determined by its temperature and mass fraction. Their relationship is shown in Fig.5-3. The abscissa is temperature in linear scale; the ordinate on the left-hand is vapor pressure in logarithmic scale; the ordinate on the right-hand is temperature in linear scale, shows the saturation temperature of pure water which has the same vapor pressure as a BrLi solution at the temperature given by the abscissa. The line of pure water is also shown in the figure, which is corresponding to a solution of, all the points on the line of pure water have the same values of temperature both on the abscissa and on the ordinate on the right-hand.Fig.5-3, the vapor pressure of solutions of LiBr in water 6In Fig.5-3, the accurate value of vapor pressure can be found from Table 5-2 from the saturated temperature of pure water on the ordinate on the right-hand.For example, if a solution of LiBr-H2O mass fraction = 0.578 is at 40, from the left-hand scale the vapor pressure may be estimated between 8mbar and 9 mbar. From the right-hand scale, the temperature reading of pure water for the same vapor pressure is very close to 5. From the table of pure water as shown in Tab.5-1., the corresponding vapor pressure for 5°C is 8.72 mbar. Tab.5-1, the saturated vapor pressure table of pure water 7TemperatureSaturatedPressureTemperatureSaturatedPressureTemperatureSaturatedPressureTemperatureSaturatedPressure0.016.10616.5711113.1272124.8773144.95927.0601214.0262226.4483247.58537.5801314.9782328.1043350.34348.1351415.9872429.8513453.23958.7251517.0552531.6923556.27869.3531618.1842633.6313659.466710.0201719.3802735.6733762.810810.7281820.6432837.8223866.315911.4811921.9782940.0833969.9871012.2802023.3883042.4604073.83550123.499(4) Basic Lithium bromide-water absorption refrigeration systemThe diagram shown in Fig.5-4 is a basic lithiumbromide vapor absorption refrigeration system. A basic H2OLiBr absorption refrigeration system consists of 8 main components. Apart from the evaporator, the condenser and the expansion valve which are found in a mechanical powered vapor compression refrigerator, other five components, namely, a pump, and absorber, a generator, a heat exchanger and a valve fulfill the function of “thermal compressor”: Fig.5-4, a scheme of a basic absorption refrigeration systemSome manufacturers construct the absorption refrigeration systems by placing the four major components (generator, absorber, condenser and evaporator) in a single shell divided into higher and lower pressure regions as shown in Fig.5-5.Fig.5-5, a single-effect lithium bromide-water absorption refrigeration system 8(5) Analysis for a basic absorption refrigeration systema) Circulation factor 循环倍率An important quantity in the calculation of an absorption system is the mass flow rate of the strong solution which is needed to absorb unit mass flow rate of vapor from the evaporator. This quantity is called the circulation factor . Hence: (5-10)For example, if and, the circulation factor is 7.05, b) Enthalpy of liquid and vapor液体和蒸气的焓The figure is based on the enthalpies of liquid water and solid anhydrous lithium bromide each being zero at 0. Fig.5-6, specific enthalpy of solutions of LiBr in waterfig.5-6，溴化锂水溶液的比焓c) Steady-flow analysis平稳流动分析Assume a lithium bromide system operating at the following conditions:Evaporation: 5 (pe=8.725 mbar), Condensation: 50 (pc=123.45 mbar), Generator: 110, Absorption: 40.Assuming equilibrium states leaving the generator and the evaporator, no pressure drops, and complete heat exchange, i.e. the strong solution leaves the exchanger at 40.The mass fractions of the strong and weak solutions are determined as: The circulation factor =7.05 by Eq.5-10.For the refrigeration cycle shown in Fig.5-4, in the processes from point 1 to point 4, the working substance is pure water or its vapor, therefore the data of enthalpies of superheated vapor and saturated vapor and liquid can be found from the steam tables,; ; In the processes from point 5 to point 10, the working substance is LiBr-H2O solution, therefore the enthalpies can be found from Fig.5-6,; ; The heat transfer in the components can be calculated as follows:Condenser: (5-11)Evaporator: (5-12) (6) Multiple-Effect （多效）and direct- or indirect-fired Absorption ChillersAbsorption chillers can be direct- or indirect-fired and single- or multiple-effect. Direct-fired （直燃）chillers contain a burner that runs on natural gas or other fuels to produce the heat required for the absorption process. Indirect-fired chillers use steam or hot water produced externally by a boiler or cogeneration system. Fig.5-7, a double-effect （双效）absorption cycle 10Fig.5-