制冷技术(英文版)Ch5-090531.doc

1 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 reservoir Heat sink H T Refrigeration cycle Condenser Evaporator Compressor Metering Device Fluid to be cooled (Refrigeration load) Cooling fluid Heat source L T Cold heat reservoir W L Q H Q L Q H Q Fig.2-1, The Model for Analysis of Refrigeration Cycle 2 The 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 -20C and it is condensed at 40C. For the case of evaporating temperature and condensing temperature, Ctte20 1 Cttc40 3 the thermal properties of R134a can be found from the diagram or table of the refrigerant R134a as below: Evaporating pressure MPape1327 . 0 Condensing pressure MPapc0164 . 1 The specific enthalpies of R134a at these states are: kJ/kgh1 7 . 384 (It is an isentropic process from point1 to point2.)kJ/kgh2 3 . 427 kJ/kgh3 2 . 256 kJ/kghh 34 2 . 256 Fig.5-1, Schematic and a log p-h diagrams for the basic vapor compression cycle The process 1-2 is a reversible, adiabatic (isentropic) compression 3 (5-1)42.6kJ/kghhw 12in The process 2-3 is an isobaric heat rejection process (5-2)171.1kJ/kghhq 32C The process 3-4 is an irreversible throttling process, kJ/kghh 43 2 . 256 The process 4-1 is an isobaric constant pressure heat admission process (5-3)kgkJhhq 1e / 5 . 128 4 The coefficient of performance of the cycle can be calculated as: (5-4)016.3 /6.42 /5.128 12 41 kgkJ kgkJ hh hh w q COP in e 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: kW85.12 ee mqQ kW11.17 cc mqQ kW26. 4 inin mwW 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（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），区别只在于压缩方式） 有热能可以利用的场合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 refrigeration 4 Fig.5-2, essential components of the vapor absorption cycle The 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 NH3H2O system Absorption cyclerefrigerantAbsorbent H2OLiBr systemH2OLiBr solution NH3H2O systemNH3H2O 溴化锂水吸收的蒸汽压缩式制冷系统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 0C; and the ammonia absorption system is mainly used for low temperatures below 0C. 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 5 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 L m W m defined as: (5-5) WL L mm m (3) Vapor pressure of LiBr-water solution 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 0 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 6 In Fig.5-3, the accurate value of vapor pressure can be found from Table 5-2 from the saturated 6 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 5C is 8.72 mbar. Tab.5-1, the saturated vapor pressure table of pure water 7 Temperatur e C o Saturated Pressure mbar Temperatur e C o Saturated Pressure mbar Temperatur e C o Saturated Pressure mbar Temperatur e C o Saturated Pressure mbar 0.016.106 16.5711113.1272124.8773144.959 27.0601214.0262226.4483247.585 37.5801314.9782328.1043350.343 48.1351415.9872429.8513453.239 58.7251517.0552531.6923556.278 69.3531618.1842633.6313659.466 710.0201719.3802735.6733762.810 810.7281820.6432837.8223866.315 911.4811921.9782940.0833969.987 1012.2802023.3883042.4604073.835 50123.499 (4) Basic Lithium bromide-water absorption refrigeration system The 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 system Some 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. 7 Fig.5-5, a single-effect lithium bromide-water absorption refrigeration system 8 (5) Analysis for a basic absorption refrigeration system a) Circulation factor 循环倍率循环倍率 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) ws w For example, if and, the circulation factor is 7.05, 578 . 0 w 660 . 0 s b) Enthalpy of liquid and vapor The figure is based on the enthalpies of liquid water and solid anhydrous lithium bromide each being zero at 0. 8 Fig.5-6, specific enthalpy of solutions of LiBr in water c) Steady-flow analysis 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: 578. 0 w 660 . 0 s 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, ; ; kgkJh/2706 1 kgkJh/209 2 kgkJh/2510 4 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, ; ; kgkJhh/154 65 kgkJh/13 8 kgkJh/146 9 The heat transfer in the components can be calculated as follows: Condenser: (5-11)kgkJhhqcon/2497 21 9 Evaporator: (5-12)kgkJhhqe/2301 24 (6) Multiple-Effect （多效）（多效）and direct- or indirect-fired Absorption Chillers Absorption chillers can be direct- or indirect-fired and single- or multiple-effect. Direct-fired （直（直 燃）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）（双效）absorption cycle 10 Fig.5-8, a Triple-effect （三效）（三效）absorption chiller-heater (H.T. -High-temperature, I.T. -Intermediate-temperature, L.T. -Low-temperature) 缺点缺点 （p.98 第第2段）段） Some disadvantages associated with lithium bromide absorption refrigeration systems are: the risk of crystallization of solution at high concentrations; the corrosion in the system can occur. Lithium bromide, a highly corrosive brine, readily attacks ferrous metals such as steel. The corrosion process generates 10 hydrogen gas that reduces the internal vacuum inside the evaporator, which results in the unit operating poorly. In addition, the debris （碎粒）resulting from the corrosion fouls narrow openings in spray headers, heat exchangers, etc. (7) The Ammonia-Water Absorption System a) Principles of the ammonia-water absorption system 13 Fig.5-9, a typical ammonia absorption system 正压、低温正压、低温 主要的缺点主要的缺点： Probably the major disadvantage of the ammonia-water system is the fact that the absorbent (water) is also volatile, so that the vapor in the generator contains appreciable amounts of water vapor. If the water passes through the condenser and enters the evaporator, it will cause rising of evaporating temperature and reduction of the refrigerating effect. For this reason, analyzer and rectifier are used to remove the water vapor from the mixture before the condenser. As shown in Fig.5-9, the analyzer is essentially a distillation column that is attached to the top of the generator. b) Analysis of the ammonia-water absorption cycle 11 Fig.5-10, specific enthalpy of vapor in equilibrium with a solution of ammonia in water 1 The specific enthalpy of vapor in equilibrium with aqua ammonia solution is shown in Fig.5-10. It is based on 0 for liquid water at 0; 0 for liquid ammonia at -40 and the vapor kgkJ /C 0 kgkI /C 0 is in equilibrium with the solutions at bubble point. Fig.5-11, the h-diagram for the processes in Fig.5-9 For example, if the operating conditions of the ammonia-water system are give as: te= -20 C (pe = 1.9 bar), tc = 40 C (pc = 15.5bar), ta = 30 C (absorber temperature), tb = 170 C (temperature of weak solution leaving boiler), th = 40 C (temperature of weak solution leaving heat exchanger), then the heat ratio of the absorption system in an idea cycle can be obtained, which is about: . But 56 . 0 / be qq in practices, the COP or the heat ratio ranges from 0.3 to 0.4. 15 5-3) Heat Operated vapor Compression Refrigeration Cycle (2) Vapor Adsorption Refrigeration (1) Adsorption and desorption (吸附和解吸附吸附和解吸附) a) Adsorption Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a film of molecules or atoms (the adsorbate). It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution. The term sorption encompasses both processes, while desorption is the reverse process of adsorption. Adsorption is usually described through isotherms, that is, the amount of adsorbate on the adsorbent as a function of its pressure (if gas) at constant temperature. The quantity adsorbed is nearly always 12 normalized by the mass of the adsorbent to allow comparison of different materials. Fig.5-12, Adsorption isotherms of water vapor on zeolite-13X. b) Desorption Desorption is a phenomenon whereby a substance is released from or through a surface. The process is the opposite of sorption (that is, adsorption and absorption).As the temperature rises, so does the likelihood of desorption occurring. (2) The working pairs in adsorption refrigeration Tab.5-2, the main working substances in adsorption refrigeration AdsorbentsAdsorbate (refrigerant ) Activated carbon Activated carbon fiber Silica gel Zeolites Water Ammonia Ethanol Hydrogen Methanol (3) Principles of adsorption refrigeration Like the mechanical vapor compression refrigeration cycle and the absorption refrigeration cycle, the adsorption refrigeration cycle can accomplish the removal of heat through the evaporation of a refrigerant at a low pressure and the rejection of heat through the condensation of the refrigerant at a higher pressure. The pressure difference in the adsorption refrigeration system is created by adsorption and desorption of refrigerant vapor by adsorbent at low temperature and at high temperature respectively. A diagram of the cycle is shown in Fig.5-13. 13 Fig.5-13, the principle of the vapor adsorption refrigeration When the adsoprion system in operation, the adsorbent is alternatively being cooled and heated. Adsorption is a physical process by which a fluid molecule is fixed onto a solid matrix, typically a surface or a porous material. When the molecule is fixed, it releases some heat, therefore, the adsorption is an exothermic process. Fig.5-14 Heating and desorption + condensation 14 2 The working pairs in adsorption refrigeration Tab.5-2, the main working substances in adsorption refrigeration Adsorbents 吸附剂Adsorbate (refrigerant ) Act