The helical-rotor expander is one of numerous candidates for the direct ''total flow'' expansion of geothermal fluids. The potential exists for expansion efficiencies as high as 70 percent. Engine efficiencies will generally be below 70 percent because of an expansion-ratio limitation of 15 and a maximum pressure differential of 110 psi. Single-stage expansion of self-pumped geothermal fluids from reservoirs at 350 to 752 exp 0 F gives engine efficiencies in the range from 57.3 to 43.1 percent. Calculated rotor diameters range from 10.5 to 41.6 ft for a range of outputs of 5 to 40 MW. Two-stage expansion of fluid from the 572 exp 0 F reservoir gives an engine efficiency of 62.3 percent and a 21 percent reduction in the diameter of the larger rotor. Rotor diameters can be held to more reasonable sizes by using the expander in combination with a vapor turbine. (ERA citation 02:000513)
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