Compressor is very critical for any refrigeration or air conditioning system. There are multiple variety of compressors like reciprocating, screw, centrifugal, etc., and are available in wide range of capacities and sizes. In this article we will learn about rotary screw compressor which is popular and commonly used in many commercial and industrial air conditioning applications.
All about rotary screw compressors
Rotary screw compressors was designed during mid-1950s and are widely used in most of the industrial and commercial applications. They are known for their efficiency, durability, smooth and noise-free operation.
Screw compressors are very rugged and are capable of supplying high volume compressed air continuously i.e., 24/7 and 365 days. They are internally cooled and contains very few moving parts that makes them operate continuously for long periods without getting damaged or heating up. For this reason, they are widely used in industries like food processing, gas pipelines, etc., were uninterrupted supply of air is required continuously.
Screw compressors are noise-free and rugged compared to other compressors and are little expensive compared to other compressors. But, their durability and low maintenance justifies their cost. However, going for a re-manufactured screw compressor will be viable.
Operation of rotary screw compressor
Rotary screw compressor operates on the principle of positive displacement but instead of cylinder and piston it contains two rotors (screws with lobes), one male and other female. The two rotors rotates in a proper alignment such that the air is sucked in and is compressed between the two rotating screws and then the compressed air is discharged at the other end. A well designed screw compressors will not have a metallic contact between the two screws.
Types of rotary screw compressor
Rotary screw compressors are of two types, oil-injected screw compressor and oil-free screw compressor.
- Oil-injected screw compressors
- These are also called as wet compressors.
- In oil injected screw compressors, a thin film of oil is used between the two screws to compress the air.
- The compressed air is then filtered to remove oil contaminants.
- However, the air still contains a very minor quantity of oil called ‘oil-carryover’ in compressed air no matter how much it is filtered.
- Because of oil-carryover, oil-injected compressors are generally not used in food processing industries and some high technology industries.
- It is suitable for factories and industries where metallic equipment are used as the minor oil residue in the compressed air prevents metallic equipment from rusting.
- Oil-free screw compressors
- These are also called as dry compressors.
- As there is no oil film to make two rotors rotate together, a timing gear is used to rotate the two screws in alignment to compress air.
- Though it is oil free, some quantity of oil is required for lubrication of the parts. For this purpose a clean oil or pressurized water is used (lubricated parts are however located far from the compressing space).
- The dry compressors in which pressurized water is used for air compression are called water-injected compressors.
- Air from oil-free or water-injected compressor is 100% oil-free and so it is suitable for industries where very clean and quality air without any contaminants is required.
