The rotary screw belongs to the positive displacement c […]
The rotary screw belongs to the positive displacement compressor family. Positive displacement pumps create flow by applying an expanding cavity on the suction side and a decreasing cavity on the discharge side. Gas that is trapped inside the positive displacement machine is a fixed volume which is then compressed or displaced into the discharge manifold.
The two most commonly used compressors today are the rotary screw (helical rotor) and the reciprocating piston. In comparison of the two, the rotary screw does not use valves, is lighter in weight than the reciprocating piston, is pulsation free making foundation requirements less extreme and maintains its design efficiency over operational time as the rotors never come in contact with each other. The screw compressor was originally designed in the mid 1950's and eventually developed to operate between the reciprocating piston and centrifugal machine capabilities for commercial, industrial and gas type applications.
The Rotary screw compressor is composed of two intermeshing helical rotors contained in a housing. Clearance between the rotors and between the housing and the rotors is typically.003" to.005". The male or drive rotor is connected through a shaft extension by an electric motor or engine. In the case of an oil injected machine, the female rotor is driven by the male rotor through a thin film of oil. A dry rotary screw compressor employs a set of timing gears to achieve proper rotation.
The diameter and length of the rotors regulate the final pressure and capacity the machine can produce. As the rotor diameter increases, so does the air pumps capacity; As the length of the rotor increases, so does the pumps final pressure.
As power is applied to the male rotor it begins to move out of mesh with the female rotor creating a void allowing gas to be drawn in through the inlet port. As the rotor maintains past the inlet port the intermesh space continues to expand until the gas completely fills the interlobe space. When the male rotor enters the interlobe space it begins to convey and compress the gas towards the discharge port. As the rotors turn the gas filled grooves are isolated by the housing walls, creating a compression chamber, where lubricant is then injected to provide cooling, sealing and lubrication.
Continued rotation causes the gas volume to reduce to the stated design pressure. The compressed gas and lubricant is finally sent through the discharge port, then into a two phase separator where the oil and gas are divided. The oil is filtered by a 10 micron automotive type spin on filter and then cooled via air or water before being re-injected into the compression chamber. The oil type used in these machines is a hydrocarbon synthetic of ISO 100, 150 or 220 viscosities and is selected based upon specific gravity of the gas. Proper gas analysis is critical in oil selection as during initial start up, gas will dilute the viscosity of the oil. In the case of an air compressor the gas is then directed to an air cooled after-cooler where up to 70% of the ingested water vapor is condensed out of the gas stream before entering the supply manifold.
The compression porting is located and cut to attain the application pressure ratio. To achieve the greatest efficiency, it's central the corresponding geometry match the application pressure requirements. Some rotary screw compressor designs employ a variable discharge valve that continuously seek maximum efficiency by opening and closing depending on system pressure conditions. When the compressor senses a decreased system air demand (rising pressure) the discharge valve allows air to circulate back to the inlet without being compressed to meet system demand. The net effect is a shorter length rotor resulting in variable displacement operation allowing power requirements to drop.
The displacement of the screw compressor is a function of the interlobe volume and speed. The interlobe volume is a function of rotor profile, length and diameter. The interlobe volume can be expressed by the equation;
Qr = d3 (L/d) / C
Where: Qr = Displacement/Revolution d = rotor diameter C = typical profile constant
Qd = Qr x N
Where: Qd = discharge volume N = compressor speed Qi = Qd x Ev
Where Qi = actual inlet volume Ev = volumetric efficiency
Volumetric Efficiency is a function of rotor slip which is the internal leakage created by gas expansion back to the low pressure side therefore reducing the potential volume capacity of the compressor.
The screw compressor discharge temperature can be evaluated assuming adiabatic compression, assumes no energy (heat) is transferred to or from the gas during the compression and all supplied work is added to the internal energy of the gas resulting in increase of temperature and pressure.
Discharge temperature can be calculated by taking the adiabatic temperature rise and dividing by the adiabatic efficiency, and then multiplying by the temperature rise efficiency to account for cooling. To find the final discharge temperature, add the inlet temperature to the temperature rise.
t2 = t1 + T1 (rpk-1/k-1) / na x nt
Where: rp = pressure ratio na = adiabatic efficiency nt = temperature rise efficiency
