制冷技术(双语版)第五章、蒸汽及气体压缩循环

制冷技术(双语版)第五章、蒸汽及气体压缩循环内容摘要：

sorbent. This explains that the lithium bromide absorption system is strictly limited to evaporation temperatures above 0186。 C。 and the ammonia absorption system is mainly used for low temperatures below 0186。 C. 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 nonvolatile at the operating conditions so rectification is not necessary 在溴化锂吸收式制冷系统中，水是制冷剂，溴化锂是吸收剂。 这是溴化锂吸收式系统的低温要控制在 0186。 C以上；而氨水吸收式制冷循环可以在 0186。 C以下的原因。 在氨水吸收式系统中，水作为制冷剂会蒸发，所以要用精馏除去。 而在运行工况下，溴化锂（一种吸湿性盐）是几乎不挥发的，所以不需要精馏。 The refrigerants and absorbent in H2OLiBr system and NH3–H2O system 表 51, H2OLiBr系统和 NH3–H2O系统中的制冷剂和吸收剂 吸收式循环 制冷剂 吸收剂 H2O–LiBr 系统 H2O LiBr solution NH3–H2O 系统 NH3 H2O 2. Composition of mixtures Calculation of absorption refrigerators requires some knowledge of the thermodynamics of solutions and of how their properties depend on the position. 吸收式制冷机的计算要用到一 些溶液热力学和混合物成分特性的知识。 Composition of a mixture is expressed as the mass fraction  of one of the ponents. For example, in H2O–LiBr solution it contains mass Lm of LiBr and Wm of H2O, the mass fraction of LiBr is defined as: 混合物的组成体现为某一成分的质量分数 。 比如在 H2O–LiBr 系统中，溶液中包含 LiBr 的质量 Lm 和 H2O的质量 Wm ， LiBr 的质量分数定义为： WLLmm m (55) And the mass fraction of H2O is: H2O质量分数为： WLWmm m1 (56) Mixing two masses m1 and m2 of solutions having the mass fraction 1 and 2 , respectively, produces a solution having the intermediate mass fraction 3 . From the constancy of the mass of LiBr before and after: 将两个质量分别为 m1和 m2，质量分数分别为 1 和 2 的溶液会得到质量分数为中间值 3 的溶液。 由混合前后 LiBr 质量不变可得 ：   2211321  mmmm  (57) And: 321321   mm (58) 3. Vapor pressure of LiBrwater solution 3. LiBrwater溶液的蒸气压力 The vapor pressure of aqua lithium bromide solution is determined by its temperature and mass fraction. Their relationship is shown in . The abscissa is temperature in linear scale。 the ordinate on the lefthand is vapor pressure in logarithmic scale。 the ordinate on the righthand 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 0 , 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 righthand. 溴化锂水溶液的蒸气压力由它的温度和质量分数决定，它们的关系如图 53。 横坐标是以线性刻度表示的温度；左边的纵坐标是以对数刻度表示的 蒸气 压力；右边的纵坐标是以线性刻度表示的温度，显示了纯水的饱和温度，此时， LiBr 溶液的温度由横坐标给定，而纯水有和此温度下 LiBr溶液相同的蒸气压力。 在图表中同样可以看见纯水线，即相当于 0 的溶液，右边不管是横坐标还是纵坐标，在纯水线上的点都有相同的温度。 In , the accurate value of vapor pressure can be found from Table 52 from the saturated temperature of pure water on the ordinate on the righthand. 在图 53中根据右边纵坐标纯水的饱和温度可以从表 52得到 蒸气 压力的准确值。 For example, if a solution of LiBrH2O mass fraction  = is at 40℃ , from the lefthand scale the vapor pressure may be estimated between 8mbar and 9 mbar. From the righthand 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 ., the corresponding vapor pressure for 5176。 C is mbar. 比如，如果 40℃ 时， LiBr 溶液的质量分数为  ， 从左边的刻度可以估计出蒸气压力在8 mbar 和 9 mbar 之间。 从右边的刻度可以看出，与蒸气压力相的纯水的温度大概为 5℃。 从表 51中可以看到 5176。 C纯水对应的蒸气压力为 mbar。 , the vapor pressure of solutions of LiBr in water [6] 图 53, 溴化锂水溶液的蒸气压力 , the saturated vapor pressure table of pure water [7] 表 51, 水的饱和 蒸气 压力表 Temperature Co Saturated Pressure mbar Temperature Co Saturated Pressure mbar Temperature Co Saturated Pressure mbar Temperature Co Saturated Pressure mbar 1 11 21 31 2 12 22 32 3 13 23 33 4 14 24 34 5 15 25 35 6 16 26 36 7 17 27 37 8 18 28 38 9 19 29 39 10 20 30 40 50 4. Basic Lithium bromidewater absorption refrigeration system —水吸收式制冷基本循环 The diagram shown in is a basic lithiumbromide vapor absorption refrigeration system. A basic H2O–LiBr absorption refrigeration system consists of 8 main ponents. Apart from the evaporator, the condenser and the expansion valve which are found in a mechanical powered vapor pression refrigerator, other five ponents, namely, a pump, and absorber, a generator, a heat exchanger and a valve fulfill the function of ―thermal pressor‖: 图 54所示是溴化锂 —水吸收式制冷基本循环。 溴化锂 —水吸收式制冷基本循环包括 8个 主要部分，除了在机械驱动制冷循环中可以见到的蒸发器，冷凝器和膨胀阀，其余五个部分是完成热压缩的溶液泵，吸收器，发生器，换热器和一个阀 , a scheme of a basic absorption refrigeration system 图 54, 基本吸收式制冷系统体系 In the evaporator, the heat (Qe) from low temperature source (To) is transferred to the refrigerant which changes its state from liquid to vapor at low pressure from state 3 to state 4. The vapor is then absorbed by the absorbent LiBr in the absorber at low pressure and temperature. During the absorption, the vapor changes into liquid state 5. The absorption is an exothermic process. The heat (Qa) released during the absorption process is rejected to the surroundings. The weak solution is delivered by the pump to the generator (G) at high pressure at the state 7. The solution in the generator is heated to boil off the refrigerant (water) with heat (Qb) from the high temperature source so that the solution is concentrated. Solution in the generator bees strong and leaves the generator at state 8. The high pressure and temperature refrigerant vapor at state 1 leaves the generator and flows to the condenser where the refrigerant is cooling down and condensed. The condensing heat (QC) is rejected to the surrou。