Compression Cycle

Straight-lobe compressors, or blowers, as they are commonly called are low-pressure machines. The feature unique to these compressors is that the machines do not compress the gas internally as do most of the other rotaries. The straight-lobe compressor uses two rotors that inter-mesh as they rotate (see Figure 1-7). The rotors are timed by a set of gears. The lobe shape is either an involute or cyclodial form. A rotor may have either two or three lobes. As the rotors turn and pass the inlet port, a volume of gas is trapped and carried between the lobes and the outer

Pressure Ratio

Figure 4-21. Efficiency improvement with variable volume ratio screw compressors [4].

Pressure Ratio

Figure 4-21. Efficiency improvement with variable volume ratio screw compressors [4].

Slippage is a function of the rotor diameter and differential pressure for a given gas. Slippage is determined by test. For the test, the differential pressure is imposed on the blower and the speed gradually increased until the point is reached where the output just matches the slip leakage. This point is detected by watching for the machine to just begin to give a positive output. The speed at which this occurs is called the slip speed. A slip speed is determined for each of several pressure differentials.

Output volume is calculated from displaced volume multiplied by the difference between a desired speed and the slip speed, rather than using the concept of volumetric efficiency as in the other positive displacement compressors. Output volume is expressed by

where

Qi = delivered volume Qr = displaced volume per revolution N = rated speed = slip speed

While volumetric efficiency is not commonly used, the following relationship is an approximation for comparison purposes.

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