The rotary screw compressor falls under the category of positive displacement compressors. These pumps work by applying increasing cavity in the suction side and a decreasing cavity on the discharge side of the hose. The gas trapped inside the positive displacement machine is normally of fixed volume and it is often displaced into the discharge manifold.
The two most common compressors used today include the reciprocating piston and the rotary screw. Unlike the reciprocating piston which is heavy and uses valves, the rotary screw compressor does not use valves and is much lighter in weight. Moreover, the rotary screw is pulsation free allowing the user to save in maintaining the design efficiency of the compressor over a longer operational time because the rotors never come in contact with each other.
The rotary screw compressor has two intermeshing helical rotors housed within the machine. The clearance between one rotor and the next and between the rotors and the housing is maintained at 005”. The male rotor is connected through a shaft extension by an engine or an electric motor. In case of oil injected machines, the male rotor drives the female rotor through a thin film of oil. Dry rotary motors on the other hand use a set of timing gears to maintain proper rotation.
The pressure and capacity that the machine can generate is determined by the length and the diameter of the rotors. This means that as you increase the length and diameter of the rotors, so will the capacity and pressure increase. The power applied on the male rotor moves it out of mesh together with the female rotor creating a void which in turn allows gas to be drawn via the inlet ports. As the rotor rotates and creates more interlobe space, the gas continues to expand until it fills the void created. Finally, when the male rotor enters the interlobe space, it compresses the gas conveying it to towards the discharge port. This gas is packaged in grooves and isolated by housing walls. The compression chamber created receives a lubricant which aids in lubrication, cooling and sealing.
As the rotation continues, the rotation causes the gas volume to reduce to the stated design pressure. The compressed gas mixed with the lubricant is finally pumped through the discharge port then it is channeled into a two phase separator whose work is to separate the gas from lubricant. The oil is filtered from the separator and cooled before it is pumped back to the compression chamber for reuse. The best type of oil that can be used in rotary compressors is the hydrocarbon synthetic of ISO 220, 150 or 100 viscosities selected based on the specific gravity of the gas. It is worth noting that the correct analysis of the gas is important during the initial stages because the gas used will dilute the viscosity of the oil.
In conclusion, there are many different types of rotary screw compressors in the market. Your choice should fit your budget and the specifications of your project.