You know your air compressor needs a lubricant to ensure maximum uptime and performance; but with so many lubricant options on the market today, it can be difficult to know which one is best for your specific compressor, application and business environment.
How do you even begin to choose?
Why do rotary screw air compressors need lubricants?
To understand why air compressors need lubricants, you should first understand lubricants primary functions in rotary screw air compressors. To put it simply, a compressor lubricant is to an oil flooded air compressor what blood is to the human body. Lubricants are a compressors lifeline. Just as your heart (your body’s most valuable asset) pumps and circulates blood to keep your body functioning, an air compressor circulates compressor lubricant to keep the machine functioning.
The three primary functions of compressor lubricants are:
Lubricate the Bearings
The lubricant needs to keep the bearings lubricated.
Seal the Rotors
Since the rotors do not touch, the lubricant creates the necessary airtight seal with its film.
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.
Other important functions of compressor lubricants include:
Operate Varnish Free
Varnish is the leading cause of air end failure. Varnish leads to higher temperatures and inefficiencies.
Low Oil Carryover
More top off lubricant per year costs money.
High Flash Point
A high flash point is necessary for safety.
Compatible with System
If the lubricant is incompatible with the solenoids, seals, hoses, gaskets or downstream materials, you could end up with leaks and/or damage.
Understanding the main ingredients
When seeking out a compressor lubricant, you’ll be faced with many choices. There are hundreds of compressor lubricants, dozens of lubricant manufacturers and even more resellers. Start by considering the components of each lubricant as each has its advantages and disadvantages in the way it performs.
We’ll focus on the most common:
- Hydrocarbons (mineral oils)
- Hydrotreated Hydrocarbons
- Synthetic Hydrocarbons (PAO)
Hydrocarbon-based lubricants have been around for as long as the rotary screw air compressor. In the Sullair lubricant line-up, AWF® and HPL 1500 are examples of hydrocarbon-based lubricants.
In the past, nearly all rotary screw compressors used hydrocarbon-based lubricants, but technology has greatly improved. Most hydrocarbons lubricate well, seal the rotors and have anti-corrosive additives but do not transfer as much heat as synthetics.
Many rotary screw air compressor customers are now opting for other lubricants to better protect against varnish.
Hydrotreated Hydrocarbons offer good lubrication and a longer life than hydrocarbons alone. In addition, they offer a lower cost per gallon (than synthetics) but, like hydrocarbons, additional costs will be realised later.
Hydrotreated hydrocarbons have the potential to form varnish and have lower thermal conductivity. They also cannot be used in high temperature applications. Lastly, like with hydrocarbons, hydrotreated hydrocarbons require condensate disposal.
In the Sullair lubricant line-up, SRF 1/4000® and SRF 2/8000® are examples of hydrotreated hydrocarbons.
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 compatibility with air systems. They also offer a moderate flash point (450°F / 232°C). While they offer a higher purity level than the basic hydrocarbons, they still suffer from many of the drawbacks of basic hydrocarbons.
They are limited in heat transfer, like the rest of the hydrocarbon-based oils, 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, directly affecting your company’s bottom line.
Polyglycol lubricants are the crème-de-la-crème of compressor lubricants. While they have a higher upfront cost, they are the premier lubricants and come with virtually zero downside.
Genuine Sullube™ by Sullair is a polyglycol lubricant. These lubricants will not varnish in the machine and will remove existing varnish when performing a flush conversion. They have a higher thermal conductivity, which helps the equipment run cooler, making it easier to stabilise operating temperatures.
These lubricants also offer the safety of the highest flash point (505°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, like with some other lubricants.
Polyglycol lubricants are also compatible with the components in all rotary screw stationary air compressors so you can successfully convert compressors to run on these lubricants.
Why do some lubricants like hydrocarbons create varnish and other lubricants don’t?
It depends on what they use to make the lubricant. What is its primary ingredient? Some 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 then plate out the metal parts, like the rotors, because there is so little clearance between the two rotors. This then 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 then creates more heat. When combined, the two create a vicious circle.
Varnish can cause:
- Air end failure
- An inefficient machine
- Leading to 10% loss of efficiency or 10% higher energy cost
- Increased operating temperature
- High temperatures shorten lubricant life and lead to high temperature shutdowns resulting in plant downtime
- Plugged separators and oil return lines
- Coated and restricted oil cooler
"Flushing" it all out
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 realise the effects of lower cost alternatives on the interior compressor components until it’s too late.
The hidden costs will be realised, and your air compressor will feel the pain.