Sizing

The displaced volume of the sliding vane compressor can be calculated if certain geometric data are available. Unfortunately the vendor catalog data generally will not be very useful in establishing the geometry because frame designations are not directly related to any convenient measurement. The vendor's literature does give the expected capacity for air at various pressures, which include the displaced volume and the vol-

Figure 4-22. Pressure-volume diagram of the compression cycle of a sliding vane compressor.

Figure 4-22. Pressure-volume diagram of the compression cycle of a sliding vane compressor.

umetric efficiency. Geometric information might be available, such as may occur with a unit being considered for reuse where measurements can be made. The name plate, of course, would provide the rated conditions. The design ratio would be available and could be used to make a decision regarding the suitability for alternate service. With the bore, the rotor diameter, the cylinder length, and vane number and thickness, a calculation to determine the displaced capacity per revolution may be made. By applying an estimate of volumetric efficiency to the displacement value, the speed needed for a given output can be calculated. If the speed is within the allowable limits, and the pressure ratio is in the range of the original value, the compressor can be reapplied.

The following provides an estimate of Qr, the displaced volume per revolution.

e -- eccentricity, R - r R = radius of cylinder bore r = radius of rotor D = diameter of cylinder bore m = number of vanes s = vane thickness 1, = cylinder length

where

Some typical values of geometry are r/R = .88 and e = .12 R. The L/R ranges from 4.5 to 5.8, increasing with the size of the compressor. Volumetric efficiency ranges from approximately .90 at 10 psig to .85 at 30 psig for air service. Volumetric efficiency is slightly better for heavier gases and lower for the lighter gases. Typical maximum vane speed, calculated using the cylinder bore as the diameter, is 50 fps.

Power requirements and discharge temperatures are calculated using the same relationships as used with the other rotary compressors already discussed. The efficiency is .80 for air service and pressure in the 30 psig range. The mechanical losses are higher than the other rotaries. The mechanical loss is variable and dependent on gas, lubrication, and other factors. For an estimate, use .15 of the gas horsepower. This approximation should be close enough for an estimate.

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