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Rotary Screw Compressors

Rotary Screw Compressors: Understanding the Basics

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Rotary screw compressors are the workhorses behind a majority of manufacturers worldwide. If you see a big building, and they make stuff there, there’s a good chance there is a rotary screw air compressor powering their manufacturing process.

There is a good reason for this. 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 an intermittent duty cycle. However, the rotary can go all out, all day without stopping – it doesn’t like starting and stopping constantly.

Another reason is that when sized correctly, rotary screws can be some of the most energy-efficient compressors on the market. The keys are correct sizing, proper air system design, and intelligent compressor control. You can put the most efficient compressor in the world into an air system, but if the system and control scheme are poorly designed, the compressor won’t be efficient.

How they compress air?

A typical rotary screw air compressor has two interlocking helical rotors contained in a housing. Air comes in through a valve, typically called the inlet valve and is taken into the space between the rotors. As the screws turn, they reduce the volume of the air, thus increasing the pressure.

There are rotary screw air compressors with just one screw, as well. However, they’re not very popular when it comes to compressing air. You’ll see them more in refrigeration applications. Their principle of operation is beyond the scope of this blog, but if you’re interested, you can read more here. For the rest of this blog post, it can be assumed that we are talking about compressors with more than one screw.

The assembly comprising the rotors and the housing they’re in is called an “air end” or airend. This is the terminology for all rotary compressors, whether they are rotary vane, scroll, screw, or lobe. The part responsible for compressing the air is referred to as the airend.

Oil vs. Non-oil rotary compressors: What’s the difference?

Some rotary screw compressors use oil while others do not, but all compressors need to filter out the oil present in the ambient air. In compressors that use oil, the motor drives the male rotor, which in turn drives the female rotor. The oil forms a film between the two rotors and also serves as a sealant and coolant for the compression chamber.

In an oil-free compressor, no oil is used to drive the compression process. The two rotaries in an oil-free model are controlled by gears. Without oil to serve as a chamber sealant, compressors of this type are not as capable of reaching high levels of pressure. These oil-free compressors are less efficient as they are also liable to run hotter due to the lack of cooling oil.

Due to these limitations, oil-free rotary screw compressors are mostly confined to special types of use. Though rare, there are certain oil-free models that employ water instead of oil as a coolant.

The air end serves another function besides the compression of air, as this is where oil is compressed within the air. After the air end-stage is complete, the newly compressed air passes into the sump — also known as the separator tank — where oil is extracted from the air. The spinning motion effectively shakes the oil particulates from the compressed air so that the latter can be pure once it reaches its endpoint.

The process of oil separation is assisted with baffles. Once the air has passed through the separator tank, rarely more than three-parts per million (3 ppm) of the oil remains. Afterwards, the air passes through a cooler and onward to the endpoint, whether that happens to be a pneumatic tool or an air-powered machine.

Depending on the temperature of the now-separated oil, a thermostatic valve treats the oil appropriately. The purpose here is to prevent the oil from becoming either hot or cold. If the oil gets hot, it will fry and wear down the internal machinery. If the oil is cold, there will not be sufficient temperature to separate it from all the water extracted from the air during the compression stage.

Air is not allowed into the system until it has enough pressure to be self-lubricated. If the oil contains too much water, the air end will not function properly.

In a rotary screw air compressor with a stationary blade, the driving shaft has an eccentrically mounted roller inside the pump chamber. Within this chamber, the blade divides the inlet and outlet valves. The blade itself is sandwiched by the roller surface and interior body of the air compressor.

As the roller moves, the blade goes up and down to the rotary motion. As such, the compressor consists of three parts that move — the blade, the roller and the shaft. Each of these moving parts is lubricated. In the cylinder, vapours of low temperature and pressure are compressed to high temperature and pressure. This is all made possible by the motion of the roller.

All Air Compressors provides service and repairs of rotary screw compressors for your business. Get in touch today or visit All Air Compressors for more information about our services!