The central principle to a Rotary Screw Compressor is the male (helical lobes) and female (helical grooves) rotors spinning opposite directions. The rotary screw uses these two rotors to push air through the compressor, which creates pressure. Compression is accomplished through the rotors meshing within the housed section. Simultaneously, the air inlet port sits at the top of the housed section (towards the drive shaft end of the machine) to draw in air, and the discharge port sits at the opposite bottom end to release the final pressurized air.
In the oil or water injected designs, fluid is introduced to remove the heat of compression, seal internal clearances, and prevent rotor-to-rotor contact. In oil-free designs, timing gears are attached to the rotor shafts to prevent rotor-to-rotor contact. These multiple stages improve the efficiency of the compressor and enable it to achieve higher pressures.
Although they belong to the same class, rotary screw compressors are more complex than piston compressors. As such, the speed of the screw rotors optimizes at a certain level, minimizing the mechanical losses (due to heat at very high speed) and volumetric losses (air losses due to very low speed) during compression.
The rotary screw compressor contains twin elements that rotate in opposite directions - known as male and female parts. Air fills the gap between these rotors. As the rotation commences, the space between the rotors and the surrounding housing gets smaller. The air compresses into this smaller space, which increases the pressure - thus creating pressurised air.
Other elements that contribute to the pressure, including the length, pitch of the screw, and the discharge port's form, collectively determine the pressure ratio.
With the rotary screw model, there are no additional valves or mechanical forces that can cause unbalance to the compression process. So, the compressor can comfortably operate at high speeds while maintaining a high flow rate even while packed into small exterior dimensions.
In layman's terms, the screw compressor derives its name from its source of power. As air penetrates a sealed chamber, it becomes trapped and squeezed between the two opposed rotating screws. These screws inter-mesh the trapped air, and the volume is compressed, with the final product being the compressed air.
For those seeking a more detailed technical description, the rotary screw compressor uses a rotary-type positive-displacement mechanism to produce compressed air. The gas compression process of a rotary screw is a continuous sweeping motion, resulting in minimal pulsation, which means screw compressors are significantly quieter and produce less vibration even in large sizes.
To describe how the screw air compressor compresses air, let's begin with the rotary screw compressor element; this element consists of two rotors, each shaped to 'fit' into the other with 4 to 6 teeth. Simply put, the air is pulled into the screw element, and as it progresses, the volume space between the rotor teeth decreases in the direction from the suction to the outlet, and the air becomes compressed.
The air circuit works by air being drawn through the filter and an open inlet valve, making its way inside the compressor element to which it then becomes compressed. Following on, a mix of compressed air and oil makes its way into the air receiver/oil separator via a check valve. This air is then employed through an outlet valve via a minimum pressure valve and the air cooler. A minimum pressure valve keeps the pressure in the separator tank above a minimum value during loaded operation, required for lubrication. An integrated check valve prevents the compressed air from travelling downstream, and the value stops the air from being vented during the uploading operation. Finally, as the compressor stops, the check and inlet values close, preventing the compressed air (and if applicable oil) of being vented into the air filter.
In the case of an oil circuit, most of the oil is removed from the air/oil mixture through centrifugal action; then, any leftover oil is removed by the oil separator if necessary. This oil collects in the lower section of the air receiver/oil separator, the 'oil tank'. The oil system contains a thermostatic bypass valve; the job of this value is to shut off the supply of oil to the oil cooler if the oil temperature gets below a set point. The air pressure then forces the oil from the air receiver/oil separator through the oil filter and oil stop valve into the compressor element. The bypass valve will start opening the supply from the cooler when the oil temperature reaches the set point. The value will open the supply from the cooler when the oil temperature has increased to the set point. At approx. 15 ˚C above the set temperature point, all of the collected oil flows through the oil cooler, and the oil stop valve prevents the compressor element from flooding with oil in the situation where the the compressor stops.
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