《制冷技术(英文版)Ch3-090529》由会员分享，可在线阅读，更多相关《制冷技术(英文版)Ch3-090529（24页珍藏版）》请在金锄头文库上搜索。
1、Chapter 3. Methods of Decreasing Temperature ( I ) -Relative to Gas and Liquid Refrigerants3-1) Decreasing Temperature by Adiabatic ThrottlingJ-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, or a capillary tube rest
2、rictor, is implemented in a 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 J.P. Joule and W. Thomson (
3、Lord Kelvin), and is called Joule-Thomson process.For a stationary flow, the 1st law of thermodynamics can be written as (3-1)Where, - the heat absorbed by the flow during the process; Q = 0Porous plugW = 0(T)h,3=(T)S,32Fig.3-1, Adiabatic throttling Process through a porous plug - the work done by t
4、he flow during the process;- the mass flow rate of the flow; and are the specific enthalpy of the flow at the inlet and the outlet respectively.In an adiabatic throttle process as shown in Fig.3-1, since and , the adiabatic throttle process must have 高度不可逆的等焓过程 (3-2)(2) The Joule-Thomson effectThe c
5、hange of temperature with respect to a change in a Joule-Thomson process is the Joule-Thomson (Kelvin) coefficient:微分效应： (3-3)For an ideal gas, the enthapy is only the function of temperature; it does not vary with pressure, therefore and.The value of of real gases, may be larger, less, or equal zer
6、o, which is typically expressed in ，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 (), and thus If , the temperature of the gas is reduced, , which produces a cooling efect;, the
7、temperature of the gas is raised, , which produces a heating efect;, the temperature of the gas has no change, i.e., .积分效应： (3-4)(3) Adiabatic throttling in real gas regionFor all real gases, the value of will equal zero at some point, which is called the inversion point and Joule-Thomson inversion
8、temperature, which is the temperature where the coefficient changes sign (i.e., where the coefficient equals zero). The Joule-Thomson inversion temperature depends on the pressure of the gas before expansion.Fig.3-2 shows the isotherms（等温线） and the inversion locus （逆转轨迹）on the pressure-enthalpy diag
9、ram （压力-焓图）with pressure to a logarithmic scale.The isotherms at low pressure tend towards the vertical, indicating the approach to ideal gas behaviour. Adiabatic throttling at low pressure produces negligible change of temperature.The inversion locus comes down towards zero pressue at the maximum i
10、nversion temperature. At the left-hand side the inversion locus joins the saturated liquid line.Below the inversion locus the value of is everywhere positive; outside the locus it is everywhere negative, indicating a rise of temperature. The Joule-Thomson process, for example, is shown as the line A
11、-B-C in Fig.3-2. In the process from A to B, the temperature is increasing; and in the process from B to C, the temperature is decreasing. The reduction of temperature accompanying a given drop in pressure increases as the initial temperature falls.Fig.3-2, The Joule-Thomson Effect (ABC)on a logarit
12、hmic pressure and enthalpy diagramAB: ; BC: ; AC: Tab.3-1 gives the maximum inversion temperatureof some gases.Above this temperature no isenthalp （等焓线） shows the phenomenon of inversion, and adiabatic throttling is always accompanied by a rise of temperature.Tab.3-1 Maximum inversion temperatures (
13、最高转变温度) (K) of some gases1500794761621-625603250203-20540-45The Joule-Thomson Effect is applied in the Linde-Hampson liquefier as a standard process in the production of liquid oxygen, nitrogen and argon. However the simple Linde cycle cannot normally be used to liquify helium, hydrogen and neon, fo
14、r their Joule-Thomson inversion temperatures are lower than room temperature. Helium, hydrogen and neon can be liquefied by throttling when they are precooled by other method to the temperatures below their inversion temperatures.气体的Joule-Thomson效应很小：0.1-0.4The values of Joule-Thomson coefficient (
15、in ) are not very large, mostly at 0.1-0.4. Therfore the total temperature decrescing by the throttling is always not large too. The values of temperature decreasing of helium throttling from different high pressure to 1 bar are all less than 4 K.Adiabatic throttling in real gas region is widely used in cryogenic engineering, especially for the liquefaction of gases, such as air, nitrogen, and oxygen.Fig.3-3a shows t
