Blog

Rotary Screw Compressors: The Difference Between Oil-Injected and Oil-Free

Rotary screw compressors are commonly used in a wide range of commercial, industrial, and light manufacturing applications that range from 3hp to over 500hp. A conventional rotary screw compressor employs two intermeshing rotors in a stator housing that has an intake and a discharge port. Also referred to as helical screws, the rotors interlock precisely and spin at high speeds, in opposite directions.

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 “pocket” as 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 Rotary Screw Compressors

Rotary screw compressors are divided into two main categories: Oil-injected and oil-free.

Oil-Injected Compressors

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 lubricant 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 air flow 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 (oil carry-over) 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, the rotors do not come into contact to create friction. Thus, 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 lubricant will 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 rise the temperature above the limits recommended. To avoid this—as well as any potential leaks in the compression chamber during the discharge—compression 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.

If you’re in the market for a rotary screw compressor, please visit our products page or contact Compressors Unlimited today! Our experienced professionals can help you find the right compressor, spare parts, and/or accessories for your application.

SHARE THIS STORY:

Recent Posts

Remanufactured Commercial Compressor
When Is It Smarter to Remanufacture Your HVAC/R Compressor Instead of Buying New?
Remanufacturing a commercial HVAC/R compresor is not the same as repairing one. It’s not a patch job...
Carrier 5F Compressor
How to Identify the Right Replacement Compressor Using Your Existing Model Number 
The compressor model number is more than an ID tag; it’s a blueprint for system restoration. The model...
Maintenance For Commercial Compressors
Extending the Life of Your Carrier and Copeland Compressors: Maintenance Best Practices
Carrier and Copeland compressors are recognized throughout the HVAC/R industry for their reliability...
Carrier Commercial Compressor
Carrier 06D vs. 06E Compressors: Key Differences and Selection Guide
When it comes to reliable HVAC/R performance in commercial and industrial systems, the Carrier brand...
Remanufactured Copeland Compressor 3D
Top Reasons Copeland Compressors Are a Favorite for Remanufacturing and Replacement
For facility managers, service contractors, and system designers, choosing a remanufactured Copeland...
Oil Mangement Compressors
The Role of Oil Management in Enhancing Compressor Efficiency and Longevity
It’s about protecting your investment, reducing energy costs, and extending the life of one of the most...