制冷技术(英文版)Ch5-090531.doc

Chapter 5. Vapor and Gas Refrigeration Cycles 5-1) Mechanical Powered Vapor Compression Refrigeration Cycle 5-2) Heat Powered Vapor Compression Refrigeration Cycle—Vapor Absorption Refrigeration 5-3) Heat Powered vapor Compression Refrigeration Cycle—Vapor 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 cycle’s 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, 5]The refrigerants and absorbent in H2O-LiBr system and NH3–H2O systemAbsorption cyclerefrigerantAbsorbentH2O–LiBr systemH2OLiBr solutionNH3–H2O 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 a。