Isenthalpic change, Joule-Thomson

There is another mode of gaseous expansion called the Joule-Thomson expansion, in which the change in gas volume occurs at constant enthalpy AHW = 0 without any change in energy. The vector of the isenthalpic expansion then stands perpendicular to the abscissa on the ordinate and points in the negative direction (exergy consumption) as is shown in Fig. 11.10(b). 

The Joule-Thomson coefficient p,j-r describes the extent and direction of the temperature change for an isenthalpic change of state (constant enthalpy h) ... 

E2.29(b) The Joule-Thomson coefficient fi is the ratio of temperature change to pressure change under conditions of isenthalpic expansion. So... 

The expansion phenomena (generation of low temperature by expansion) of compressible media is well described by the Joule-Thomson effect, which may be used as basic information on the applicability of a certain gas in the PGSS-process. If heat-transfer, changes in kinetic energy, and work-transfer for the flow process through an expansion valve are neglected, then the process may be considered to be isenthalpic [33]. The effect of change in temperature... 

Thus we conclude that in a Joule-Thomson throttling process the enthalpy is conserved. Therefore, the temperature of an ideal gas does not change as the heat capacity Cp and thus the enthalpy H do not depend on pressure. The change of temperature of a real gas during such an isenthalpic expansion is characterized by the Joule-Thomson coefficient... 

For an ideal gas, /jlj-j- is exactly zero, because enthalpy depends only on temperature (that is, at constant enthalpy, temperature is also constant). However, for real gases, the Joule-Thomson coefficient is not zero, and the gas will change temperature for the isenthalpic process. Remembering from the cyclic rule of partial derivatives that... 

Most practical refrigeration and liquefaction systems obtain a reduction in temperature with the aid of an expansion valve (Joule-Thomson valve), an expansion engine, or a combination of the two devices. In the case of the expansion valve, the flow within the valve is irreversible as well as non-isenthalpic. However, the inlet and outlet conditions have the same enthalpy. The change in temperature of a fluid obtained with an isenthalpic change in pressure is represented by the Joule-Thomson (IT) coefficient, defined as... 

We can determine the change in temperature that results as the pressure decreases in the isenthalpic throttling process if we know the derivative, dT/dP)h- We call this relation the Joule-Thomson coefficient, /x.jt-... 

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