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ORIGINAL: KnotBand
my only comment is about terminoligy.....the title says positive displacement and yet, you talk about rotary type compressors....
positive displacement usually refers to something of the piston type.....like your engine, or your shop compressor, or your home AC compressor......they rotary technology (screw and scroll type compressors) is more efficient, more reliable, and more versitile than positive displacement equipment and usually more expensive. Screw compressors are used a lot on large chillers for unloading and on large industrial air compressors for efficiency since compressed air is the most expensive utility...
just thought I would point that out....but as a disclaimer, I don't know anything about superchargers and this might be an industry standard term eventhough it would not be consistant with normal industrial terminology.
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This is incorrect as positive displacement means there is a certain amount of air displaced in a movement. In the case of a piston, it's the volume that is taken up by one piston stroke, thus moving or compressing that air. The roots and twin screw are positive displacement because they always move the same amount of air in one rotation, just like a piston moves a certain amount of air in a stroke.