What exactly is a Screw Compressor?
A rotary-screw compressor is an air compressor that uses a rotary-type positive-displacement mechanism to replace piston compressors in applications requiring large volumes of high-pressure air, such as large industrial applications or the operation of high-power air tools.
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What are twin screw compressors?
During the 1930s, when a rotating compressor with a high flow rate and stable flow under varying pressure conditions was required, the principle for a twin-screw rotating displacement compressor was developed. The male and female rotors of the twin-screw element rotate in opposite directions as the volume between them and the housing decreases.
Each screw element has a fixed, built-in pressure ratio that is determined by its length, screw pitch, and discharge port shape. The build-in pressure ratio must be adapted to the required working pressure in order to achieve maximum efficiency. The screw compressor is typically devoid of valves and devoid of mechanical forces that cause unbalance. This means it can operate at high shaft speeds while maintaining a high flow rate and small exterior dimensions. The bearings must overcome an axial acting force that is dependent on the pressure difference between the inlet and outlet.
What Is the Operation of a Rotary Screw Air Compressor?
The concept of a twin rotary screw compressor was invented in the 1930s. The compressors’ twin elements include male and female parts that rotate in opposite directions. The space between the rotors is filled with air, and as they rotate, the volume between them and the surrounding housing decreases, squeezing or compressing the air into a smaller space; the pressure ratio is determined by the length, pitch of the screw, and shape of the discharge port.
Aside from that, there are no valves or other mechanical forces that can cause unbalance, allowing a screw compressor to operate at high speeds while combining a high flow rate with small exterior dimensions – it packs a powerful punch for its size.
Oil-Free Screw Compressors vs. Oil-Injected Screw Compressors
Rotary screw air compressors are classified into two types: oil-free and oil-injected. Here’s how it works:
Oil-free screw compressors
The first twin screw compressors used a symmetric rotor profile and no cooling liquid inside the compression chamber. These compressors were known as oil-free or dry screw compressors. Asymmetric screw profiles in modern, high-speed, oil-free screw compressors result in significantly improved energy efficiency due to reduced internal leakage. External gears are commonly used to synchronize the position of opposing rotors. There is no need for lubrication inside the compression chamber because the rotors do not come into contact with each other or the compressor housing.
As a result, the compressed air is completely free of oil. To minimize leakage from the pressure side to the inlet, the rotors and housing are manufactured with extreme precision. The limiting temperature difference between the inlet and discharge limits the build-in pressure ratio. To achieve higher pressures, oil-free screw compressors are frequently built with multiple stages and interstage cooling.
Oil-Injected Rotary Screw Compressors
A liquid is injected into the compression chamber in liquid-injected rotary screw air compressors to cool and lubricate the compressor elements’ moving parts, to cool the air being compressed in the chamber, and to help minimize leaks from returning into the chamber during discharge. Because of its lubricating and sealing properties, oil is the most commonly used liquid, though water and other polymers are also used. The oil is then separated and goes through a filter and cooler before being reintroduced into the process. Depending on the end-use, the compressed air can still be hot and is frequently run through a cooler.
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sources: thecompressedairblog.com and ashair.co.nz