How rotary screw compressors work

Group of DRM screw compressors
Thanks to their efficiency, reliability and durability, rotary screw air compressors have become extremely popular in a wide range of industries. To understand the secret of their success, it is important to understand the rotary screw compressor working principle.

As the name indicates, their design is based on a couple of helical screws, which are also called “rotors.” Essentially, these rotors are used to take the intake air and compress it.

Let’s take a look at how that happens:

The rotary screw compressor working principle is as brilliant as it is simple. The air compressor’s interlocking helical rotors are located in a housing. Usually, there are two of them, a male rotor and a female rotor. Each features a different shape and a different number of grooves or “teeth.” In the most common arrangement, one of them has four teeth and the other six.

Together, this heart of the air compressor is also referred to as the air-end. Using an intake valve, this housing, i.e. the space between the rotors, is filled with air. As the screws begin to turn, the volume of the air is reduced as it is pushed down. As a result, the pressure of that air increases.

While that is the basic compression principle for any type of rotary screw air compressor, there is a difference between oil-injected and oil-free compressors.

It is important to note that “oil-free” in this context only means that no oil is introduced in this process. The compressed air from this type of compressor may still contain traces from the oil present in the ambient air.

Oil-injected rotary screw compressor working principle

Let’s first look at the working principle of an oil-lubricated screw air compressor. Here, a motor drives the male rotor, which in turn drives the female rotor. The oil serves as a lubricant in this process. In addition to this main purpose, the oil also functions as a coolant, and it seals the compression chamber.

In this case, the air-end compresses a mixture of air and oil. This mixture then flows into a so-called separator tank. In order to keep the compressed air clean, this is where most of the oil is separated out. This separation process makes use of a centrifugal force, i.e. the air spins around the tank and the heavier oil particles drop out. In combination with an oil separator element, this gets rid of most of the oil in the air.

DRM air compressor ghost
The separated oil will either be reintroduced into the air-end or go through an oil cooler. This usually depends on its temperature, which is why the compressor also includes a thermostatic valve that routes the oil down the proper path.

The air, on the other hand, moves on to a cooler and from there to its application.

The compressor usually also has a minimum pressure valve. This doesn’t permit the air to leave the system until a minimum pressure has been reached that allows the compressor to lubricate itself. In addition, it features a couple of filters. There is an oil filter that removes the contaminants, like tiny particles or water, from the oil. And there is an air filter that keeps the intake air clean. Finally, the compressor includes a blow-down valve, which relieves the trapped pressure when the compressor is idling.

Oil-free rotary screw compressor working principle

In oil-free screw compressors, the timing between the two rotors is controlled by a set of gears. While the absence of oil ensures that the air is cleaner, it also means that the chamber cannot be sealed. As a result, a single-stage oil-free compressor cannot achieve a pressure that is as high as the pressure achieved by an oil-injected screw compressor. That is why many oil-free models are two-stage compressors.

In addition, because there is no oil that can function as a coolant, these compressors run hotter, which makes them less efficient.

While both types of compressors feature a similar design, they are not identical. For example, oil-free rotary screw air compressors usually have two air-ends with an intercooler between them. In addition, they typically include a lubricated gear box that contains the gears for these air-ends. To prevent the oil from finding its way into the air, there is usually an oil seal.

On the other hand, there is no need for a separator tank, an oil cooler, or the thermal valve. Apart from that, however, the components are the same.

As a result of their differences, oil-free and oil-injected rotary screw compressors are used for different applications. For example, an oil-free rotary screw compressor would be used whenever a very high air quality is required. In particular, this applies to applications in the pharmaceutical or food and beverage sectors.

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