ROTARY SCREW COMPRESSOR PRINCIPLES
Principles of the rotary screw compressor. Machines are often a combination of simplicity and complexity; this certainly holds true for the rotary screw air compressor. Simple in design, yet precisely engineered for high efficiency, rotary screw air compressors are the workaholics of the industrial world. They provide a constant supply of energy all day, every day, without interruption. And when sized properly, they are one of the most efficient forms of compressed air supply out there.
How does a rotary screw air compressor work?
The principle behind a twin rotary screw compressor was developed in the 1930s. Twin compressor elements include male and female parts that rotate in opposite directions. Air fills the space between the rotors, and as they spin, the volume between them and the surrounding casing decreases, squeezing or compressing the air into a smaller space; the length, screw pitch, and discharge port shape collectively determine the pressure ratio. Beyond that, there are no valves or other mechanical forces to cause imbalance, allowing a screw compressor to run at high speeds while combining high flow with small outside dimensions; packs a good punch for its size.
Oil-Free Screw Compressors vs. Oil-Injected Screw Compressors
Rotary screw air compressors are available in two main technologies: oil-free and oil-injected. Here's a breakdown:
Oil-free rotary screw compressors
External gears synchronize the position of the counter-rotating screw elements, and because the rotors do not contact and create friction, no lubrication is needed inside the compression chamber. As a result, the compressed air is free of oil. Precision engineering within the casing keeps pressure leaks (and drops) from the pressure side to the inlet to a minimum. And because the internal pressure ratio is limited by the difference in air temperature between the inlet and discharge ports, oil-free screw compressors are frequently built with multiple stages and interstage cooling to maximize pressure range. . The gearbox that drives the mechanism contains lubricants; oil-free refers to the compression chamber itself, and the supplied air is free of foreign contaminants beyond those inherently found in the air passing through the intake.
Oil-injected rotary screw compressors
In liquid-injected rotary screw air compressors, a liquid is injected into the compression chamber to accomplish the following: Cool and lubricate the moving parts of the compressor elements; to cool the air that is compressed in the chamber; and to help minimize leaks back into the chamber during discharge. Oil is the most widely used liquid due to its lubricating and sealing properties, although water and other polymers are also used. The oil is then separated and passes through a filter and cooler before going back into the process again. Compressed air can still be hot and often goes through a cooler, depending on the end use.
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