They are commonly used to replace piston compressors where large volumes of high-pressure air are needed, either for large industrial applications or to operate high-power air tools such as jackhammers and impact wrenches. For smaller rotor sizes the inherent leakage in the rotors becomes much more significant, leading to this type of mechanism being unsuitable for small air compressors.
An industrial rotary screw compressor has a 100% duty cycle. It can run 24/7 without a break, and in fact it usually works better and lasts longer when it’s used that way. A piston compressor normally works better when it can take a break – it likes a intermittent duty cycle. However, the rotary can go all out, all day without stopping – it doesn’t like starting and stopping constantly.
The basic principle of a screw compressor is as the male and female rotors are rotating in opposite direction they draw air in between them. As the air progresses along the rotors the air is compressed as the volume space between the rotors decreases, hence creating compressed air that is displace to the outlet. The speed of the rotors is optimised at a certain level to minimise mechanical loses (due to heat at very high speed) and volumetric losses (air losses due to very low speed).
How does its works ?
The screw compressor can be visualized as a development of the gear pump. For gas pumping, the rotor shapes are modified to give maximum swept volume and no clearance volume where the rotors
mesh together, and the pitch of the helix is such that the inlet and outlet ports can be arranged at the ends instead of at the side. The solid portions of the screws slide over the gas ports to separate one
stroke from the next, so that no extra inlet or outlet valves are needed.
The more usual form has twin meshing rotors on parallel shafts. As these turn, the space between two grooves comes opposite the inlet port, and gas enters. On further rotation, this pocket of gas is cut off from the inlet port and moved down the barrels. A meshing lobe of the male rotor then compresses the pocket, and the gas is finally released at the opposite end, when the exhaust port is uncovered by the movement of the rotors. Sealing between the working parts is usually assisted by the injection of oil along the length of the barrels. This extra oil must be separated from the discharge gas, and is then cooled and filtered before returning to the lubrication circuit.
The other form has a single grooved rotor, with rotating star tooth seal vanes to confine the pockets of gas as they move along the rotor slots .Gas sealing at these surfaces is effected by injecting a small amount of the liquid refrigerant. This obviates the need for the oil lubrication and cooling circuit, with its pumps, and leaves the compressor and the circuit oil-free.
Screw compressors have no clearance volume, and may work at high compression ratios without loss of ‘volumetric efficiency’. In all screw compressors, the gas volume will have been reduced to a
pre-set proportion of the inlet volume by the time the outlet port is uncovered, and this is termed the built-in pressure ratio. At this point, the gas within the screws is opened to condenser pressure
and gas will flow inwards or outwards through the discharge port if the pressures are not equal.
The absorbed power of the screw compressor will be at its optimum only when the working pressure ratio is the same as that of the built-in one. This loss of efficiency is acceptable since the machine has no valves and no working parts other than the screws and sealing vanes.
The other form has a single grooved rotor, with rotating star tooth seal vanes to confine the pockets of gas as they move along the rotor slots .Gas sealing at these surfaces is effected by injecting a small amount of the liquid refrigerant. This obviates the need for the oil lubrication and cooling circuit, with its pumps, and leaves the compressor and the circuit oil-free.
Screw compressors have no clearance volume, and may work at high compression ratios without loss of ‘volumetric efficiency’. In all screw compressors, the gas volume will have been reduced to a
pre-set proportion of the inlet volume by the time the outlet port is uncovered, and this is termed the built-in pressure ratio. At this point, the gas within the screws is opened to condenser pressure
and gas will flow inwards or outwards through the discharge port if the pressures are not equal.
The absorbed power of the screw compressor will be at its optimum only when the working pressure ratio is the same as that of the built-in one. This loss of efficiency is acceptable since the machine has no valves and no working parts other than the screws and sealing vanes.
Capacity reduction of the twin-screw compressor is effected by a sliding block covering part of the barrel wall, which permits gas to pass back to the suction, so varying the working stroke. Variation
down to 10% of maximum is usual.
The oil separation, cooling and filtering for a twin-screw compressor adds to the complexity of an otherwise simple machine. Some commercial screw compressors are available which have the oilhandling circuit built into the assembly, with a small loss of overall efficiency.
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