Compressor filtration best practices delineated

There are many differences and distinctions of oil-free air that need to be understood because choosing the wrong solution and technology can have adverse effects.

By Camilo Villalobos and Stephanie Roberts January 26, 2022
Manufacturers should know the specific differences between Class 0 oil free air and Class 1, also called “technically oil free” and what they mean for specific applications. Courtesy: Sullair

In the oil-free rotary screw air compressor world, many terms are thrown around: oil free, Class 1, “technically oil free,” Class 0, Certified Class 0 and so on. While they may sound similar, there are many differences that should be understood because choosing the wrong solution and technology can have devastating consequences for a facility.

Manufacturers should know the specific differences between Class 0 oil free air and Class 1, also called “technically oil free.” It’s crucial to know the differences between the two classes.

Why many industries need oil free compressed air

There are certain industries and applications where the purity of compressed air is critical and requires the best quality of air to reduce the risk of contamination on compressed air, on the end product and other items throughout the production process. These industries include food and beverage, pharmaceutical, automotive, medical, electronic, labs, aerospace, semiconductors, refineries and chemical, to name a few.

Oil free air is critical in these industries because even the slightest amount of oil as a contaminant can have damaging effects such as contaminating the end product, damaging production equipment, causing downtime or damaging the brand’s reputation due to the low quality of the end product and/or product rejections.

Knowing the oil-free air classes

The International Organization for Standardization (ISO) 8573-1 Air Quality Classes outlines the contaminants allowed by class. Oil-free applications require Quality Class 1 at minimum, but preferably Class 0. This is because Class 0 is stable; Class 1 is unstable. Class 1 requires inline filtration to eliminate the oil content coming from the compression process, and the filtration depends mainly on two variables that affect the performance of the filtration process: compressed air pressure and compressed air temperature/ambient temperature.

Class 0 air is more stringent than Class 1 (0.01 mg/m3 of oil as contaminant), as agreed upon between the manufacturer and the user.

The lower the class, the lower the concentration of oil contaminant is in the compressed air, and the lower the risk of other contaminants.

Figure 1: Breakdown of the ISO 8573-1 classes. Courtesy: Sullair

Figure 1: Breakdown of the ISO 8573-1 classes. Courtesy: Sullair

The ISO 8573-1 standard consists of purity classes for contaminants in compressed air: particles, water and oil. The ISO 8573-1 purity classes can be used to describe the quality of a compressed air system or to specify the required quality for a precise application.

For purposes of this article we will focus only on oil as a contaminant.

What does ISO 8573-1 Class 1 mean?

Class 1 air is achieved using oil flooded air compressors with inline filtration to capture some of the oil passing downstream. For this reason, Class 1 air can be thought of as “technically oil-free” air.

For example, consider a 1000 cfm oil flooded machine. It releases about 14 gallons of oil downstream per year (at 86°F and 3 mg oil/m3 of carryover on oil separator). At minimum, three filters are needed to remove the oil, including activated carbon filtration for oil vapors – but this can be a risky and precarious proposition, and here’s why. The performance of inline oil filters decreases with the inlet temperature.

Downstream filters are specified at 68 to 70°F (20 to 21°C) of compressed air inlet temperature. This is a very precise situation. If this is not the case, the performance of filtration decreases exponentially at higher temperatures (see graph #2). Imagine what happens if the compressed air reaches 80°F or 100°F or more. It can easily happen because the compressed air temperature at outlet of the compressor is approximately 27°F over ambient temperature. Here is what happens: as the temperature increases, the potential of oil going downstream significantly increases. This means oil can get into the equipment and contaminate the end product, production equipment and the environment.

Figure 2: Effects of temperature on filtration of activated carbon filters. Courtesy: Sullair

Figure 2: Effects of temperature on filtration of activated carbon filters. Courtesy: Sullair

Additionally, there are maintenance factors that should also be considered when using oil flooded compressors with filtration to achieve Class 1 “technically oil-free” air. In addition to the big risk of contamination, users should consider the work and costs associated with maintaining oil separators, in line oil filters, oil vapor filters and condensate treatment equipment including:

  • More frequent, and often more expensive, maintenance costs
  • Increased energy costs because the pressure drop over the filters increases
  • Increased risk of oil passing through the filters due to potential damage or faults on the filters. This can cause contamination and other damages in a user’s production equipment and instrumentation.

A filter’s lifespan is short. If the user consistently maintains 68-70°F (20-21°C), the activated carbon filters – which remove oil vapors up to 0.01 mg/m3 – will last a little under six weeks (41 days). With any temperature shifts to around 100°F, the lifespan of the activated carbon filter could be less than four days (90 hours).

Variation of temperature makes oil-free Class 1 unstable and means air quality can easily jump from Class 1, or “technically oil free,” to Class 2, which doesn’t even qualify as oil-free air.

Class 1 is not always technically oil free because the quality/performance of filtration depends on the temperature of the compressed air. The higher the temperature, the more the compressed air quality decreases. The compressed air can jump between Class 1 to Class 2 depending on the temperature.

Even under optimum conditions and with several layers of oil removal (inline filters), “technically oil free” solutions are no longer considered a best practice. This is because of the risk of instability of the quality of compressed air (jumping from Class 1 to Class 2 and vice versa), due to the performance of inline filters fluctuating with temperature.

What does Class 0 mean?

Class 0 is more stringent than Class 1 (or 0.01 mg/m3 of oil as contaminant). Class 0 air is a safer solution than Class 1 because it is stable, doesn’t change with temperature and doesn’t require inline filtration to remove oil contamination coming from the compressor.

As the most stringent air quality class, Class 0 ensures a compressor discharges compressed air free of oil aerosols, oil vapors or oil liquids coming from the compressor. Class 0 oil-free compressors help eliminate potential contamination as no oil or lubricant is introduced into the air compression process. While Class 0 oil-free air compressors may have a higher upfront cost, the costs outweigh potential risks.

Class 0 compressors are safer for sensitive applications such as food and beverage, pharmaceutical, electronics and medical. Class 0 certification lowers the risk of air contamination, which can result in damaged products, significant revenue losses and more.

Figure 2: Effects of temperature on filtration of activated carbon filters. Courtesy: Sullair

Figure 2: Effects of temperature on filtration of activated carbon filters. Courtesy: Sullair

What is the difference between Class 0 and Certified Class 0 air?

Class 0 meets ISO 8573-1 requirements. Certified Class 0 has been certified by an outside lab to ensure compliance with all Class 0 requirements. Certified Class 0 is like getting an extra check on the air quality by a 3rd party.

Table 1: Differences between class 0 and class 1 oil free and what they mean for manufacturers. Courtesy: Sullair

Table 1: Differences between class 0 and class 1 oil free and what they mean for manufacturers. Courtesy: Sullair

Know what class is best for your application

There are many terms thrown around in the oil-free rotary screw compressor world. While they may sound similar, they have huge differences and can cause severe consequences if the wrong one is chosen. Today, we’ve outlined why oil free air is important to begin with, as well as the differences between Class 1 “technically oil free” air and Class 0 air.

A lot is on the line each day in a manufacturing facility and there are many risks. Cutting corners on a compressed air system can lead to many long-term problems. Using ISO 8573-1 certified Class 0 oil-free air compressors helps improve your business operations and profitability by reducing overall risks.


Camilo Villalobos and Stephanie Roberts
Author Bio: Camilo Villalobos is director of OEM and engineering solutions at Sullair. He manages Sullair and Hitachi OEM and product solutions used in mining, medical, industrial, transportation, e-mobility, and oil and gas sectors. Stephanie Roberts is Director of Communications at Sullair, LLC. She has been with Sullair for nearly six years and is responsible for executive and external and internal company communications, public relations and social media for the Americas and Hitachi Industrial Equipment Systems.