Isoenthalpic expansion

The cooling process of the mixture can be represented on the enthalpy diagram of Fig. 6.13. Point A represents the starting condition after thermalization at 4.2K. The A-B line corresponds to the decrease of enthalpy in a J-T heat exchanger. In point B, we have a coexistence of liquid and vapour. The percentage of liquid decreases in the isoenthalpic expansion B-C which produces a decrease of p and T. The remaining vapour is condensed in the still exchanger (line C-D). 

The net effect of this exercise will be to save not 20 percent of the motive steam, but 10 percent. The 20 percent reduction in nozzle area is partially offset by the opening of the governor valve. The inefficient, irreversible, isoenthalpic expansion and pressure drop across the governor speed control valve are reduced. The efficient, reversible, isoen-tropic expansion and pressure drop across the nozzles are increased. 

Figure 20.1(a) An isoenthalpic expansion. The heat and speed of the steam remains constant. 

I like to call the isoenthalpic expansion a parasitic expansion because it wastes the potential ability of the steam to do work. On the other hand, we have the good isoentropic expansion that maximizes the ability of the expanded steam to do work. Figure 20.1(b) - Case 2, also illustrates this sort of good expansion. 

By trial and error procedure, determine the amount of liquid which flashes by an isoenthalpic (constant enthalpy) expansion to the critical flow pressure (or actual pressure if greater than critical) for the flashed vapor. 

Thus, the initial and final states of a Joule-Thomson expansion he on a curve of constant enthalpy (isoenthalp) and the Joule-Thomson process occurs at constant enthalpy. The Joule-Thomson coefficient, pJT, is defined as... 

Isoenthalpic An expansion of steam that does not convert heat to velocity. An irreversible process. 

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