Industrial Utility Efficiency    

The Benefits of Regular Air Compressor Cleaning

When some people think about compressed air, they imagine the big, loud, dirty, unreliable machine in the back corner of their facility. Many businesses around the world rely on compressed air, and an unreliable air compressor can mean stopping an entire facility, costing thousands of dollars in lost productivity and repair labor. Additionally, that loud machine in the back corner is also a major energy consumer. So much so that many industry professionals refer to it as the “fourth utility.”

One of the biggest causes of lost reliability and increased energy consumption in a compressor is varnish. Preventing, controlling and removing varnish from compressors may not be a topic of discussion over the water cooler, but a good plan that is continuously executed can result in thousands of dollars in savings.


What is Varnish?

Varnish is not a specific compound, but a term that broadly describes the by-products of lubricant degradation. When most people think of varnish, they typically think of the thick, black, sticky material that gums up control valves and causes their compressors to run hot. When left unchecked, this sticky material can begin to harden and become even more of a menace. Varnish also has a tendency to attract and hold onto harmful particulates, such as small pieces of wear metals, which can lead to wear and tear of lubricated components.

One of the biggest factors influencing lubricant degradation, leading to varnish creation, is heat. The commonly quoted Arrhenius rate rule specifies that for every 18°F (10°C) the lubricant operating temperature increases, the rate of oil oxidation will double. This means that a compressor running hot due to excessive varnish can fall into a trap of exacerbating the issue by reducing the rated life of fresh lubricants.

Some of the other common causes for varnish formation are:

  • Fluid Cavitation
  • Static Discharge
  • Coking on Hot Surfaces
  • Caustic Chemical Contamination
  • Reactive Intake Gases
  • Coolant Contamination
  • Hydrolysis
  • Grease Contamination
  • Chloride Contamination
  • Oil or Additive Incompatibilities

What Does Varnish Do To Your Air Compressor?

The side effects of varnish formation in your air compressor may not be immediately evident in day-to-day operations. Due to the process occurring slowly over time, it can be the invisible enemy of an equipment operator. Common symptoms of a varnish issue include an increase in the operating temperature of the equipment, increased energy consumption, and decreased oil life. The sticky film that coats every internal surface can also lead to increased wear and decreased reliability of your equipment. Some other common issues related to varnish include:

  • Deterioration of seal materials
  • Sticking of moving components, such as control valves
  • Catalytic degradation of lubricants
  • Increased bearing wear
  • Decreased heat transfer
  • Plugging of oil flow ports and strainers
  • Reduced filter efficiency and life


How Do You Prevent & Control Varnish?

Preventing and controlling varnish is not as easy as picking a high-quality lubricant. In fact, some synthetic base fluids such as polyalphaolefins (PAO) can actually have the opposite effect. While these base fluids have a greater resistance to oxidation, varnish can still form through the other mechanisms previously listed. Due to these high-purity base fluids having a lower natural solvency than synthetic esters and polyalkylene glycols (PAG), or less refined mineral oils, their ability to suspend and carry any varnish-like compounds is significantly reduced. In other words, when the varnish does form, they have a higher tendency to deposit it within the system.

Preventing and controlling varnish is a multi-step process that involves fully understanding not only your equipment, but also your oil. The first step is to perform regular oil analysis on the in-use lubricant. Oil analysis is an excellent tool for tracking lubricant degradation. While there are a number of lubricant properties to monitor, the three major watch points are the total acid number, metals counts and viscosity at 40°C. While laboratories will look at other lubricant properties, the previously listed three are the major watch points. Not changing a lubricant when the oil analysis indicates it is time is a major mistake that can quickly lead to severe varnish buildup. Over-utilization of the lubricant is another leading cause for varnish in equipment.

In addition to regular oil analysis, a qualified oil analysis laboratory can also perform tests that are specific to identifying and quantifying varnish within a lubricant. Each test has its own pros and cons; some will offer a lower cost or the ability to perform the test in the field in exchange for precision or quantification. Some of the additional tests that can be performed on the lubricant to help identify varnish in equipment are:

  • Ultra-Centrifuge Test
  • Colorimetric Analysis
  • Blotter Spot Test
  • Microscopic Particle Counting
  • Membrane Patch Colourimetry
  • Sediment & Oil Color Observation

Additionally, sources of cavitation and electrostatic discharge should also be investigated if a system is experiencing varnishing of the oil. New filter designs feature higher flow rates and smaller pore sizes that can cause static charges to build on the filter element. The discharges from this static buildup can lead to sparks with temperatures over 10,000°C (18,000°F), which can cause severe localized degradation of the oil. Similarly, the collapsing of air bubbles formed by cavitation can generate temperatures over 1000°C (1800°F), which can also lead to severe localized lubricant degradation.

With so many different routes that can lead to varnish formation inside of equipment, it is inevitable that an operator will have to deal with the effects of varnish. Luckily, new technology on the market has made the restoration process to remove this harmful material from the equipment easier and safer.


How Can You Clean Varnish?

Even the highest quality lubricants cannot withstand thermal degradation caused by the high, localized temperatures of some degradation mechanisms. One common method to eliminate varnish from the lubricant is through off-line filtration. Technologies such as electrostatic separators, cellulose media and balanced charge agglomeration has proven this is a viable technique. However, what about the varnish that has adhered to the internals of the compressor? This is where a top-treat or run-in cleaner can help. Not only do these cleaners allow varnish removal without the need for expensive off-line filtration techniques, but they also make these off-line techniques more effective by removing the varnish from internal surfaces and allowing it to be carried to the separation equipment.

Typically, these cleaners are either a fully formulated lubricant or a concentrate that is poured in with the existing lubricant. Fully formulated cleaners are designed to replace the lubricant every two to four oil changes to help remove any light varnish from internal surfaces. These cleaners typically have a service life of approximately 2000 hours and can be useful for maintenance teams that cannot service a machine twice in one week. The downside to fully formulated cleaners is they are not as effective on heavy varnish buildup that is sometimes found in compressors.

Concentrated cleaners, such as Isel 5031, are typically added at a 10 percent concentration to the existing lubricant and run in the compressor for a short period time. These cleaners have the ability to quickly dissolve light varnish and cut through heavy varnish coatings. The downside to these cleaners is they cannot be used for an extended period of time and must be removed from the compressor within one to two weeks of adding to the sump.

Not all cleaners are the same, however. Some of them contain harmful chemicals that require specialized handling and disposal. Others are formulated utilizing volatile components that can vaporize and end up in the discharge gas stream, while re-depositing any varnish they had solubilized. An ideal cleaner is not only non-toxic and non-hazardous, but also non-volatile, such as Isel 5031. This will prevent it from evaporating into the gas stream and re-depositing the varnish it cleaned up, while also ensuring it can be easily disposed of with standard used oil.


Test Cases

By understanding how varnish is formed and what factors influence it, operators can be more prepared on how to effectively eliminate it. A recent study performed by Isel showed that cleaning a compressor with heavy varnish buildup can result in an average energy savings of 3 to 5 percent, while also reducing operating temperatures by approximately 5°F. Not only will the reduced operating temperature help extend the life of future oil changes, but the reduced energy consumption can quickly pay off the cost of the cleaning.

The table “Isel Compressor Cleaning Test Cases” lists four compressors that were experiencing varnishing issues and were cleaned utilizing a concentrated cleaner. Cleaning the compressors resulted in better cooling with the removal of the insulating layer of varnish, and lower energy consumption. When averaged, the four compressors consumed $1600 less worth of electrical energy alone – when calculated for 2600 hours of run time per year at $0.12/kWhr. In facilities with longer operational times or multiple compressors, the cost savings from electrical energy consumption alone become a significant factor. This proves that cleaning your equipment of any varnish buildup has an immediate return of value for the facility.

Isel Compressor Cleaning Test Cases


Case #1

Case #2

Case #3

Case #4


Brand A

Brand A

Brand B

Brand B






Hours On Compressor





Initial Viscosity (cSt)





Initial TAN (mgKOH/g)





Initial Power Draw (K.watts)





Initial Oil Temperature (°F)





Post-Cleaning Viscosity (cSt)





Viscosity % Change





Post-Cleaning TAN (mgKOH/g)





TAN % Change





Post-Clean Power Draw





Power Draw % Change





Post-Cleaning Oil Temperature (°F)





Oil Temperature % Change





Cost Savings per Year*





*Based on 10 hours per day, 5 days per week, 52 weeks per year, at $0.12/kWhr


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