Sizing

Sizing for the straight-lobe compressor is normally done using catalog data. Rotor lengths range from approximately one to two times the rotor diameter. Individual frame sizes within a given vendor's line may exceed these limits. Maximum tip speeds are in the 125 fps range with some units approaching 140 fps. The following relationship will permit size approximation if no catalog is available. It should be remembered that this is a rough size and that it may not be a standard with any of the vendors. It is helpful, however, to know at least one dimension from the vendor's line to make an estimate more meaningful.

A sizing constant of 1.2 can be used to make a reasonable approximation of many commercial sizes. The constant, c, varied from 1.11 to 1.27 for a number of the frames investigated. With the displaced size approximated, the delivered volume can be calculated. Use Equation 4.10 and an assumed volumetric efficiency of .90. This is arbitrary, as the actual volumetric efficiency varies from .95 to .75 or lower for the higher differential pressure applications. Once a slip speed has been determined, Equation 4.9 can be used to complete the calculation. The tip speed should stay near 125 fps.

Discharge temperature and horsepower can be calculated using the expressions given in the rotary screw compressor section, Equations 4.6 and 4.7. Efficiencies are a function of the differential pressure. For differential pressures of 5 psi and less, the efficiency is .80 or higher. It falls off fast with increased pressure rise, dropping to .60 in the 10 to 12 psi differential range. Mechanical losses are near .07 of the gas horsepower in the smaller sizes and .05 to .06 in the larger sizes. Larger is considered

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