Flushing out air compressor lubricants

Picking the right lubricant for an air compressor can reduce downtime and prolong the machine’s life. Five lubricant base stocks are highlighted.

By Kyle Treble April 14, 2022
Courtesy: Sullair, LLC

Picking out a compressor lubricant can feel a bit like venturing down the cereal aisle. There are so many options, how do you even begin to choose? Air compressors need a lubricant to ensure maximum uptime and performance, but it can be difficult to pick the right one with so many options on the market today. Many questions may come to mind such as: Why do compressors need lubricants? Which one do I choose? What are the different base stocks? Does the base stock even matter? What about that pesky varnish?

Before deciding on a new compressor lubricant, consider these questions and more to ensure you are making the right decision and keeping the company’s and compressor’s best interests in mind.

Why do rotary screw air compressors need lubricants?

To understand why air compressors need lubricants, it’s important to understand the primary functions of lubricants in rotary screw air compressors. A compressor lubricant is to an air compressor what blood is to the human body. Lubricants are the lifeline of an air compressor. The heart pumps and circulates blood to keep your body functioning the same as an air compressor circulates compressor lubricant to keep the machine functioning.

The three primary functions of compressor lubricants are:

  1. Lubricate the bearings. The lubricant needs to keep the bearings lubricated.
  2. Seal the rotors. Since the rotors do not touch, the lubricant creates the necessary air tight seal with its film.
  3. Remove heat. The lubricant needs to remove the heat of compression. This heat is not caused by friction, but by the physics of the air compression itself.

Four other important functions of compressor lubricants include:

  1. Varnish-free operation

Varnish is the leading cause of air end failure. Varnish leads to higher temperatures and inefficiencies.

  1. Low oil carryover. More top-off lubricant per year costs money.
  2. High flash point. A high flash point is necessary for safety.
  3. System compatibility. If the lubricant is incompatible with the solenoids, seals, hoses, gaskets or downstream materials, the user could end up with leaks and/or damage.

Heat and varnish are the two primary enemies of rotary screw air compressors, and the two are directly related. High temperatures create more varnish, and more varnish creates more heat. When the two combine, it becomes a dangerous, vicious circle.

Understanding the five most common base stocks

Rotors with and without varnish. Varnish leads to higher temperatures and inefficiencies and is a leading cause of air end failure. Courtesy: Sullair, LLC

Rotors with and without varnish. Varnish leads to higher temperatures and inefficiencies and is a leading cause of air end failure. Courtesy: Sullair, LLC

There are many choices when choosing a compressor lubricant. There are hundreds of compressor lubricants, dozens of lubricant manufacturers and even more resellers. Start by considering the components of which the lubricant is made. Each lubricant has a primary ingredient, or base stock. Each base stock has different properties and pros and cons.

The five most common base stocks are:

  1. Hydrocarbons (mineral oils)
  2. Hydrotreated hydrocarbons
  3. Synthetic hydrocarbons (PAO)
  4. Polyglycol blends/diesters
  5. Polyglycol/Polyol esters (PAG/POE).

1. Hydrocarbons

Hydrocarbon-based lubricants have been around for as long as long as the rotary screw air compressor. In the past, almost all rotary screw compressors used hydrocarbon-based lubricants. In recent years, technology has improved to the point where hydrocarbons are in the rearview mirror. Most hydrocarbons lubricate well, seal the rotors and have anti-corrosive additives. These products offer a lower upfront cost but do not transfer as much heat as synthetics.

Takeaway: Hydrocarbons were popular in the past but technology has now evolved. Most rotary screw air compressor customers are now opting for other lubricants to better protect against varnish.

High quality lubricant will prevent varnish, has a higher thermal conductivity to help the compressor run cooler, and can be recyclable. Courtesy: Sullair, LLC

High quality lubricant will prevent varnish, has a higher thermal conductivity to help the compressor run cooler, and can be recyclable. Courtesy: Sullair, LLC

2. Hydrotreated hydrocarbons

Hydrotreated hydrocarbons offer good lubrication and a longer life than hydrocarbons. In addition, they offer a lower cost per gallon (than synthetics) but, like hydrocarbons, additional costs will be realized later. Hydrotreated hydrocarbons have the potential to form varnish and have lower thermal conductivity. They also cannot be used in high temperature applications. Hydrotreated hydrocarbons also require condensate disposal.

Takeaway: Hydrotreated hydrocarbons differ from synthetic hydrocarbons because some of the basic hydrocarbon components causing varnish are removed from the lubricant through a hydrotreating distillation process. Ultimately, it still has moderate oil carryover, poor heat transfer and will varnish.

3. Synthetic hydrocarbons (PAO)

PAOs are the best lubricants the petroleum companies have to offer. They are the top-of-the-line hydrocarbon-based products and are good lubricants if changed properly at the correct change intervals. PAOs come with a longer life than hydrocarbons and good air system compatibility. They also offer a moderate flash point (450oF/232o C). While they offer a higher purity level than basic hydrocarbons, they still suffer from many of the drawbacks basic hydrocarbons do.

They have limited heat transfer and will varnish if run at high operating temperatures. The compressor condensate produced while using these products is also not biodegradable and will require costs to remove.

Takeaway: PAOs do offer better performance than hydrocarbons but have the potential to cause varnish and lead your compressor down a path of inefficiencies. Lastly, the compressor condensate must also be disposed of, directly affecting a company’s bottom line.

4. Diesters

Diesters were initially developed for use in reciprocating air compressors to help with carbon build up on the valves and high operating temperatures. Diesters can handle those two issues. However, in rotary screw compressors, they are incompatible with many elastomers and have the potential to form sludge. Sludge is primarily the same chemical reaction process as varnish, but it has a different outcome.

Sludge reduces an air compressor’s efficiency and is expensive to remove once it forms. Courtesy: Sullair, LLC

Sludge reduces an air compressor’s efficiency and is expensive to remove once it forms. Courtesy: Sullair, LLC

Diesters are often used as the primary ingredient in compressor lubricant blends. Beware, as many polyglycol/ester blends turn out to be a diester base with a minimum amount of polyglycol.

Takeaway: Diester-based lubricants have the potential to cause sludge. Sludge can become very costly to a company’s bottom line, reducing compressor efficiency and becoming very expensive to remove from a compressor system once it forms. The compressor condensate produced by diester-based lubricants is not biodegradable, either.

5. Polyglycol

Polyglycol lubricants are the crème-de-la-crème of compressor lubricants. While they have a higher upfront cost, they come with little downside. These lubricants will not varnish in the machine and will remove existing varnish when performing a flush conversion. These lubricants have a higher thermal conductivity, which helps the equipment run cooler, making it easier to stabilize operating temperatures.

These lubricants also offer the highest flash point (505o F), very low carryover and biodegradable condensate. Most polyglycol lubricants are also recyclable due to a high BTU value per pound low ash content. This means there are no additional disposal costs.

Polyglycol lubricants also are compatible with the components in all air compressors sousers can convert compressors to run on these lubricants without any issues.

Takeaway: Polyglycol lubricants offer many benefits. Compressors using these lubricants produce biodegradable condensate, reducing a company’s environmental footprint and costs for condensate disposal. Polyglycol lubricants also do not varnish and remove existing varnish that may have been built up in the compressor. This helps create a more energy-efficient machine and can prolong the compressor’s life.

Why some lubricants like hydrocarbons create varnish

When it comes to varnish, it really depends on what they use to make the lubricant. What is its base stock? Some base stocks create more varnish as they get hot. For instance, hydrocarbons are low in price, but the poor heat transfer can lead to higher temperatures, which leads to machine shutdowns and breakdowns.

All hydrocarbon components are liquid at ambient temperature. Some become solid at higher temperatures. They plate out the metal parts, like the rotors, because there is so little clearance between the two rotors. This causes temperatures to rise and creates more solid components.

Dangers of varnish

The two primary enemies of rotary screw air compressors are heat and varnish. Both are directly related because high temperatures tend to create more varnish. More varnish, in turn, creates more heat. When combined, the two create a vicious circle. Varnish can cause:

  • Air end failure
  • An inefficient machine
    • This can lead to 10% loss of efficiency or 10% higher energy cost
  • Increased operating temperature
    • High temperatures shorten lubricant life and lead to high temp shutdowns, resulting in plant downtime
  • Plug separators, oil return lines
  • Coat and restrict oil cooler.

Flushing it all out

In a day and age where compressor lubricant options can be overwhelming, it helps to understand the pros and cons and compositions of each. While some lubricant options appear to offer cost savings upfront, most equipment owners do not realize the effects of lower cost alternatives on the interior compressor components until it’s too late.


Kyle Treble
Author Bio: Kyle Treble is Senior Life Cycle Manager at Sullair where he is responsible for product management, pricing, inventory and replacement part supply and management. He has held numerous positions of increasing responsibility during his nine-plus years with Sullair including machine assembler, purchasing expeditor and aftermarket sales tech. He holds an associate degree in law enforcement from Vincennes University in Vincennes, Ind. Treble resides in Lawrenceburg, Ind.