What Is An Oil-Free Air Compressor?
Mineral oil – the type used to lubricate and cool engines and compressors –will create a health risk if it gets into food or beverage products or pharmaceuticals. If it gets into paints or powders it can adversely affect product quality and reduce yields. The obvious thing to do is to keep oil out, but compressed air systems make this a challenge.
This is because compressed air contains a very small amount of oil carried over from the compressor, unless that compressor was designed to be oil-free. Here we’ll explain why oil can be a problem and what makes an oil-free compressor the answer.
Compressor Air Applications
Compressed air is used as both an energy source and a conveying or transport medium. It powers air tools in countless factories, actuates pneumatic rods and cylinders that operate industrial equipment, and cleans and dries products. Beyond ready availability though, one big benefit of compressed air is that it’s safe for use in areas where there are explosion risks, so it’s used extensively as a power source in chemical processing operations.
Chemical plants also use compressed air as a means of moving powders and granular materials into and out of processing areas. The same approach is used in paint spraying and powder coating operations where compressed air carries the coating to the workpiece.
How Oil Enters Compressed Air
Compressors have two principal components; the airend and the motor. The motor provides the power and motion that drives the compressing mechanism in the airend. Compressing mechanisms take several different forms:
- Pistons reciprocating in cylinders
- Centrifugal designs
- Rotary screw compressors
- Scroll-type compressors
Scroll compressors don’t require oil in the airend because there’s no direct contact between moving parts. The other three designs all have parts in sliding contact. Friction is inevitable in such designs, and oil provides lubrication. It also forms a seal between the surfaces that helps in building air pressure.
A third benefit of oil is its ability to remove heat. Without lubrication, friction would create a lot of heat, but in an oil-lubricated compressor this should be minimal. However, compressing air raises its temperature significantly. This heat needs to be eliminated to prevent expansion causing rapid wear and serious damage.
In all but the scroll compressor, design lubrication leaves a thin film of oil on the walls of the cylinder or chamber where the air is compressed. Inevitably, some of that is picked up by the air and carried out into the air distribution system.
For air-powered tools and other devices there’s an argument that this entrained oil is beneficial: it lubricates the mechanisms within the tool. However, it can build up at the exhaust, eventually becoming a nuisance. In many manufacturing processes and operations though, oil is more than an irritation: it can contaminate the product.
Food and beverage producers are acutely aware of the importance of excluding oil from the product. The same applies to pharmaceutical manufacturers and producers of medical devices. Likewise, in paint spraying operations, chemical processing and even semiconductor manufacturing, it’s essential to keep oil out of the product to maintain quality and yields.
Quantifying the Cleanliness of Compressed Air
Industries where oil in compressed air would be a problem don’t just want to know the air is oil-free, they want to quantify how clean it is. ISO 8573 Part 1 makes this possible.
This standard defines classes of air purity, based on contamination per cubic meter of air. Oil, just one of the contaminants considered, is covered under Classes 0 to 4. Class 4 purity must have less than 5 mg/m3 while Class 1 is less than 0.01 mg/m3. Class 0 is defined as “…more stringent than class 1.”
Using this standard, manufacturers concerned about oil contamination risks can specify the cleanliness of the air they expect from their compressors. Many will settle for nothing less than Class 0.
Clean Air From The Oil-Free Compressor
Oil-free compressors are designed so that the air being compressed never comes into contact with any oil. This entails:
- Applying durable low-friction coatings to surfaces that will be in sliding contact
- Engineering alternative methods of taking heat away
One notable exception is the scroll-style compressor. These use two meshed scrolls, one of which moves in an orbital path. This traps air in pockets that become progressively smaller, so pressurizing the air until it’s released into the distribution system (or receiver.) As there’s no direct contact between surfaces, there’s no need for oil for lubrication or sealing.
Pros and Cons of Oil-Free Compressors
In addition to producing oil-free compressed air, these compressors have one other significant advantage: maintenance costs are lower because they don’t require oil changes at the airend.
Historically, the weakness of these designs has been that the low-friction coatings wear, eventually disappearing to leave metal-to-metal contact. At this point, sealing is reduced while friction increases operating temperature. There’s also the question of where the coating went: presumably into the air distribution system. This meant the early oil-free designs had relatively short operating lives.
In recent years materials science has created more durable coatings. As a result, oil-free compressors offer greatly increased service lives. To learn more about the operating hours you can expect from an oil-free design, speak with a product specialist.
Keeping Oil Out of The Product
The mineral oil used for sealing, lubrication and cooling in conventional air compressor designs will inevitably get carried into the air distribution system. If this is a problem, most likely due to the risk of product contamination, the answer is an oil-free compressor. Produced in various styles, these can deliver high purity air, some even achieving Class 0.
To find out what compressor is ideal for your industry and needs, contact our specialists for more help.