Choosing the right rotary compressor oil

The operation of a compressor depends on picking the right rotary compressor oil, which is the lifeblood of the system.

By Kaishan Compressor USA May 28, 2024
Courtesy: Kaishan, New Products for Engineers Database

Any oil-flooded rotary screw air compressor has an oil lubrication system that seals the compression cycle, lubricates components and removes the heat of compression, lowering your system’s operating temperature.

Oil reduces friction and wear, controlling contamination and protecting your unit’s components from oxidation and corrosion.

It’s so central to the operation of your compressor that we consider rotary compressor oil the lifeblood of the system. It’s also there’s so much importance on selecting the right oil for compressors. While it’s true that oil-free units do not rely as much on oil, they still use oil to lubricate components and to lower the operating temperatures in the stage cooling jackets (keeping casting temperatures lower).

Importance of choosing the right oil

Kaishan worked with an expert global developer of industrial lubricants to formulate the rotary compressor oil we install in our compressors. Our KTL-8000 rotary compressor oil is a hydrologically stable synthetic with ester chemistry additives.

We chose a synthetic blend over a mineral-oil product to improve the thermal and oxidative stability, giving the oil much longer life. It has a higher viscosity index, enabling it to handle a wider range of temperatures than mineral oil. And it lessens the formation of sludge, which can negatively impact compressor performance. The longer life alone gives it significant cost advantages over mineral oil.

Our main oil is a blend of polyalphaolefin and polyol ester formulated with antioxidants, liquid additives that combine with the base oil to prevent oxidation. Oxidation produces acids that cause varnish or solids to form in the oil.

KTL-8000 rotary compressor oil has a low dissolved-water content, helping prevent corrosion in a compressor’s bearings. Its thermal stability means less oil carryover into your compressed air flow.

Maintenance and oil changes

Since rotary compressor oil plays such a crucial role in compressor operation, it’s essential to maintain fluid levels, change filters and replace oil regularly. Drain the old oil from the system when you do an oil change. Make sure to drain the oil cooler completely to ensure no residual oil is left behind. Leaving a small percentage of the old oil can significantly impact the life of the new oil. The good news is rotary screw air compressors have a separate drain point and atmospheric vent installed to make this process easy.

Oil sampling

Fluid sampling is critical to the life of an oil-flooded rotary screw air compressor. Sampling oil regularly will tell you whether it is being exposed to excessive heat or is taking in contaminants. It can also detect excessive bearing wear, allowing you to be proactive when you uncover a problem, even avoiding an unplanned shutdown.

That’s why we consider oil sampling the most important maintenance procedure for our compressors. And we require that, to maintain their warranty protection, our customers collect an oil sample and have it analyzed every 2,000 hours or every 1,000 hours for food-grade applications.

There are two critical readings you’ll see in your oil analysis:

  • Viscosity by ASTM D 445 measures the thickness of the fluid. We expect viscosity to increase as you use your compressor. That’s how the oil keeps possible contaminants in solution, preventing varnish or solids from “dropping out” of the fluid.

  • Total acid number indicates how much acid has formed. Once again, we expect it to increase as the oil ages.

Oil sampling extends the life of the lubricant and your equipment. It verifies that your rotary compressor oil will last as long as expected, and also measures the level of contamination, lubricant oxidation and additive depletion.

It’s especially critical if you expose your compressor to excessive heat or contaminants or if your air compressor does not run often enough—when it’s lightly loaded, condensate collects and causes rusting issues. In these situations, oil life can be decreased, so it is critical to follow the sample recommendations to avoid the risk of varnish.

It gives hard data to estimate how long your oil will remain effective in your application and how often you should change it. Make sure to change your oil before it is no longer effective.

In addition to the conditions of the rotary compressor oil, the analysis gives critical insight into the operational conditions of your compressor, indicating high levels of wear or corrosion. For example:

  • If the oil is becoming dirtier, there might be some concerns with the quality of the air you’re drawing into your compressor.

  • If the compressor starts running warmer or if we see the total acid number increase more quickly than expected, there might be issues with the temperature of the compressor’s environment or an indication that the cooling system needs service.

  • If metal fines are detected in the sample, it indicates future bearing failure; since the rotors never touch, you’ll only find metal in an oil sample if there is bearing failure.

  • If water is detected in the sample, it indicates the system is short-cycling and may not have time to exceed the pressure dew point temperature.

It’s an all-encompassing review of the lubricant and its integrity, and it can give you insight into the compressor’s operation and its environment.