制冷与空调专业英语第2版 林慧珠 主编 4-3新.pptVIP

Unit Four Lesson 13 Thermodynamic Principles of Refrigeration Lesson 14 Mechanical Refrigeration Principles Lesson 15 Evaporation Lesson 16 Condensation Lesson 15 Evaporation Text The first step in the refrigeration cycle is the evaporation of the liquid refrigerant. It is during this phase that actual work of refrigeration is performed. The equipment which forms a part of the complete system and in which evaporation takes place is called the evaporator or the cooling coil. Text: To effect a decrease in temperature, heat must be removed from a body. The heat thus removed is not lost, but is absorbed by a surrounding body whose heat content increases in proportion to the amount absorbed. The rule that heat lost equals heat gained is thus established. Text: The cooling coil or the evaporator of a refrigeration system is designed to affect a heat flow from the area or the substance to be cooled to a fluid medium (refrigerant) forced through the coil.1 Text: The medium employed should induce such a flow of heat by one of the following two methods, or a combination of both:1) temperature differential, where by the refrigerant is lower in temperature than the area or the substance to be cooled;2) evaporation of the refrigerant, whereby the latent heat of vaporization is utilized to remove heat from the area or the substance to be cooled. 2 Text: 1. Temperature Differential If temperature reduction is effected by means of the first method alone, the heat thus transferred is that of sensible heat only. An example of this method of heat exchange is the case of a copper coil submerged in a body of warm substance with one end of the coil connected to a pump delivering cold water and the other end connected to a drain. Heat flow would then be induced from the warmer substance to the cold water flowing through the coil, resulting in a decrease