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.
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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