Rotary Screw Compressors – How They Work
Among do-it-yourselfers and small shop owners who use pneumatic tools, chances are that their air compressors are the reciprocating type. Indeed, these are the most commonly used compressors and they do have a wide application in home shops, construction sites, and automotive garages.
Industrial users, however, tend to opt for rotary screw compressors. This is because the free air delivery (FAD) capability of these compressors can range from 10 CFM to as much as 2,000. A large factory where there are several workers doing their jobs with their pneumatic tools at the same time will probably need a rotary screw compressor. These are also seen in large construction sites mounted on trailers powering jackhammers and other tools with high consumption of compressed air.
Rotary screw compressors rely on two helical screws to compress air. These screws are called rotors since they rotate towards each other to perform their function. One of them is referred to as male, while the other as female. Their threads mesh with each other in tight tolerances. Their rotation creates a partial vacuum within their housing, which, in turn, draws free air from outside through the inlet valve.
Once inside the housing, the air moves within the flutes of the screws towards the other end. They are pushed by even more air being drawn inside by the partial vacuum. As they approach the opposite end of the screws, the threads become tighter thus compressing the air. An outlet port at this point allows the compressed air to escape into a tank for storage.
Rotary screw compressors can either be oil-free or oil-flooded. In an oil-free type, air is compressed solely by the action of the screws, resulting to lower discharge pressure. This is because the absence of an oil seal allows some of the air to escape through the threads of the screws. To compensate for this, a second set of screws may be installed for a multi-stage oil-free compressor. These compressors are often used in applications where clean oil-free air is required such as in drug, food, or semiconductor manufacture. They can compress air up to 150 PSI with an output of 2,000 CFM.
An oil-flooded system, on the other hand, has oil within the air compression area that seals and cools both the mechanical parts and the compressed air. This design, however, requires a gas-oil separator with an oil separator element. Compression can be as high as 200 PSI with an output volume of 1,500 CFM or even higher.
As with the reciprocating compressors, the oiled version tends to last longer because of the lubricating and cooling action of the oil. They are also more quiet, but require more maintenance.