THERMODYNAMICS TUTORIAL 5 HEAT PUMPS （热力学教程5热泵）.pdf

THERMODYNAMICS TUTORIAL 5 HEAT PUMPS AND REFRIGERATION On completion of this tutorial you should be able to do the following. •Discuss the merits of different refrigerants. •Use thermodynamic tables for common refrigerants. •Define a reversed heat engine. •Define a refrigerator and heat pump. •Define the coefficient of performance for a refrigerator and heat pump. •Explain the vapour compression cycle. •Explain modifications to the basic cycle. •Sketch cycles on a pressure - enthalpy diagram. •Sketch cycles on a temperature - entropy diagram. •Solve problems involving isentropic efficiency. •Explain the cycle of a reciprocating compressor. •Define the volumetric efficiency of a reciprocating compressor. •Solve problems involving reciprocating compressors in refrigeration. •Explain the ammonia vapour absorption cycle. 1. INTRODUCTION It is possible to lower the temperature of a body by use of the thermo-electric affect (reversed thermo-couple or Peltier effect). This has yet to be developed as a serious refrigeration method so refrigerators still rely on a fluid or refrigerant which is used in a reversed heat engine cycle as follows. Figure 1 Heat is absorbed into a fluid (this is usually an evaporator) lowering the temperature of the surroundings. The fluid is then compressed and this raises the temperature and pressure. At the higher temperature the fluid is cooled to normal temperature (this is usually a condenser). The fluid then experiences a drop in pressure which makes it go cold (this is usually a throttle valve) and able to absorb heat at a cold temperature. The cycle is then repeated. Various fluids or refrige