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What is the difference between an oil-free screw compressor and an oil-injected screw compressor?

  • 2021-07-21

What is the difference between an oil-free screw compressor and an oil-injected screw compressor?


Screw air compressors are commonly used in a variety of commercial, industrial and light manufacturing applications, with power ranging from 10hp to more than 500hp. The conventional screw compressor uses two mutually meshing rotors in a stator housing with an air inlet and an air outlet.


One of the rotors pulls the air in through the intake port, which is equipped with an air inlet filter that prevents airborne particles to enter the compressor. Then, both rotors push the air toward the discharge port.

Because the distance between the rotors decreases gradually, from the intake port toward the discharge port, the air is captured in a pocketas it moves down the axial length of the screws and forced (compressed) into a smaller chamber. In a continuous process, the compressed air is discharged from the opposite end of the intake, as new air is drawn in.


Types of screw compressors

Screw compressors are mainly divided into two categories: oil-injected air compressor and oil-free air compressor.

Oil-Injected Compressor

In these compressors, oil is injected into the compression chamber to lubricate and keep all the elements within permissible operating temperature limits. Although oil is the preferred choice because of its lubricating and sealing properties, synthetic lubricants are also used, particularly in applications that involve higher temperatures.

 

Using oil or other lubricants as a medium of heat transfer to remove some of the waste heat resulted from the compression process will reduce the amount of vapor in the compressed air or gas. This will minimize leaks from returns into the compression chamber during the discharge. Some compressor models also include a fan on the drive motor shaft that provides airflow in order to cool the lubricant as well as the other elements of the compressor.

 

To remove the lubricant from the compressed air or gas, most oil-injected compressors use a centrifugal separator. However, a small amount of lubricant or oil  often passes through the separator, together with the compressed air or gas. The oil or lubricant is filtered and cooled before being re-injected into the compression chamber. In applications that require compressed air or gas at lower temperatures, the air or gas can be run through an additional aftercooler.  

 

 

Oil-Free Compressors

In oil-free rotary screw compressors,they do not require lubrication within the compression area. Because the compressed air or gas is completely oil-free, these compressors are suitable for applications that cannot tolerate contamination of the compressed air or gas with oil or other lubricants.

However, the bearings must be adequately lubricated. The lubricantWaterwill protect bearing surfaces against corrosion, reduce friction, inhibit wear, and ensure efficient and reliable operation.

Because oil-free compressors tend to heat up fast, compressing air or gas in just one step can raise the temperature above the limits recommended. To avoid thisas well as any potential leaks in the compression chamber during the dischargecompression is done in two stages.

 

Each stage compresses the air or gas gradually, by several bars; the air or gas reaches the desired pressure level at the end of the second stage. To further remove the waste heat, most designs are built with an intercooler between the two stages and an aftercooler after the second stage.


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