制冷技术(英文版)ch.doc

制冷技术(英文版)ch309052946 Chapter 3. Methods of Decreasing Temperature (I) ---------- R elative toG as and L iquid R efrigerants 3-1 ) Decreasing Temperature by Adiabatic Throttling — J-T Effect (1) Adiabatic throttle process : 绝热、节流降压、不做功、速度变化不大 When a hydraulic resistance part, such as throttle valve, a porous plug, ora capillary tube restrictor, is implemented ina gas stream, and the gas flows adiabatically through the resistance part without performing any external work or any velocity change at the system boundary, this process is called adiabatic throttle process. The process was first investigated by . Joule and W. Thomson (Lord Kelvin), and is called Joule-Thomson process. For a stationary flow, the 1 st law of thermodynamics can be written as WQhhm????)( 12 (3-1) Where, Q - the heat absorbed by the flow during the process; -1, Adiabatic throttling Process through a porous plug W - the work done by the flow during the process; ?m - the mass flow rate of the flow; 1h and 2h are the specific enthalpy of the flow at the inlet and the outlet respectively. Q=0P orous plug W=0 111,,hTP 222,,hTP 47 In an adiabatic throttle process as shown in -1, s ince0?Q and0?W , the adiabatic throttle process must have 高度不可逆的等焓过程 21hh?(3-2) (2) The Joule-Thomson effect The change of temperature with respect toa change ina Joule-Thomson process is the Joule-Thomson (Kelvin) coefficient : 微分效应: HTJP T)(?????(3-3) For an ideal gas, the enthapy h is only the function of temperature T ; it does not vary with pressure P , therefore 0)(?????HTJP T? and 12TT?. The value of HTJP T)(????? of real gases, may be larger, less, or equal zero, which is typically expressed in bar K/ , and depends on the specific real gas, as well as the temperature and pressure of the inlet gas. 是实际气体的物理性质 In an adiabatic throttle process, the gas pressure is reduced ( 12PP?), and thus If0)(?????HTJP T?, the temperature of the gas is reduced, 12TT?, which produces a cooling efect; 0)(?????HTJP T?, the temperature of the