Rotary Screw Versus Rotary Vane Air Compressors

Rotary screw air compressors are electrically powered

, positive displacement compressors. They are available in lubricant-injected and lubricant-free makes.

Lubricant-injected rotary screw compressors are made of two intermeshing rotors; one rotor has helically formed lobes spanning its length and the complimenting rotor has helical grooves or flutes. They are known as the male and female rotors, respectively.

When air enters through the inlet port, it fills the spaces formed between the male and female rotor. Air becomes trapped between the lobes as the rotating movement pushes the air downward along the length of rotors. During this movement, the space between the rotors decreases, leaving the air with less space to occupy, and thus increasing the pressure. Compression occurs until the inter-lobe spaces until they meet a discharge port to release the compressed air.

During compression, in the lubricant-injected rotary screw compressor, lubricant is introduced into the compression chamber. A lubricant can be oil, a hydrocarbon product, cleaner or a synthetic such as diesters, polyglycols, polyalphaolefines, polyol esters and silicon-based lubricants. Cleaners and synthetic lubricants are most commonly and support a wider temperatures range. Lubricants are needed to lubricate the rotors and beatings, remove heat from compression, and seal the clearances between rotors.

Removal of the lubricant from the compressed air occurs when the mixture is passed into the sump/separator. Most of the liquid is separated through directional and velocity changes. Using a coalescing filter, remaining aerosols in the compressed air are removed.

Lubricant-free rotary screw compressors function in the same way, only varying at the point where lubricant enters the compression chamber. The two types of lubricant-free rotary screw compressors are the dry-type and the water-injected type. The dry-type does not allow for the intermeshing rotors to touch; the rotors are held in place by external timing gears. It does not inject any fluid into the compression chamber. Heat is removed with an intercooler between compression stages and an aftercooler. The water-injected type uses water instead of lubricant in the compression chamber and functions as a seal and removes heat. Moisture is removed from the compressed air through a moisture separation device.

Rotary vane air compressors are electrically powered and use centrifugal force to compress air. It is composed of a barrel and vanes. Air enters the system through an inlet valve. The vanes slide in and out of the walls of the barrel to create pressure, allowing for an air-tight seal to form. As the shaft of the barrel rotates the vane-housing, the compartments the vanes slide into become smaller and compress the air. The compressed air leaves through an outlet valve.

Both types of air compressors are easy to maintain. Rotary screw air compressors are more commonly found in industrial settings. They maintain constant pressure, have a low oil carry-over, and do not waste energy. Rotary vane air compressors have a long life expectancy and are ideally suited for moderate pressure applications. They tend to be used for specialty gas requirements, hobby trade, vacuum generation, pumping and other areas outside of industrial air supply.

by: Bill Fitzell

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