制冷与空调专业英语.pptVIP

Unit Four Lesson 13 Thermodynamic Principles of Refrigeration Lesson 14 Mechanical Refrigeration Principles Lesson 15 Evaporation Lesson 16 Condensation Lesson 13 Thermodynamic Principles of Refrigeration Lesson 13 Lesson 13 Text The principle of operation of refrigeration machines is based on the second law of thermodynamics. A certain amount of work has to be done to transfer heat from a cold body to a hot body. Text: The second law of thermodynamics is mathematically expressed as Qk =Q0+L, J; (Qk = Q0+AL, kcal), Where Qk is the quantity of heat transferred to the hot body; Q0is the quantity of heat given out by the cold body; L is the work done to transfer the heat; A=1/427 is the heat equivalent of work. Text: Machines that gather heat from a cold body and give it to a hot body are called refrigeration machines. Heat is transferred form a body at a lower temperature to a body at a higher temperature by a working substance (fluid) circulating in the refrigeration machine.1 Text: Refrigeration machinery in which mechanical energy is utilized is called compression-type refrigeration machine; refrigeration machines in which heat energy is utilized are subdivided into the absorption and vapor-jet type machines. Text: The working substance used in cooling may be refrigerants or air. In the first case the cooling effect is achieved as a result of a change in the state of the working substance-boiling of a liquid are called vapor-compressing machines. In the second case cooling is achieved as a result of transfer of heat from the cooled product to cold air. Refrigeration machines working on this principle are called air refrigerators. Text: Figure13-1 shows the working principle of a refrigeration machine. The working substance extracts a quantity of heat Q0 from the cold body (surrounding medi