Industrial Utility Efficiency    

Achieving “Technically Oil-Free” Compressed Air

We are often asked here at Sauer, about how much less compressed air purification equipment is needed with an oil-free air compressor.  As the myth goes, oil-free air compressors will produce high quality compressed air, eliminating the need for a heap of compressed air purification gear. Unfortunately, that is not the case.  While oil-free air compressors do have beneficial attributes over oil-lubricated compressors, the downstream requirements for purification, needed to achieve high quality compressed air, are identical. In this article we will discuss how to achieve actual oil-free air from your air compressor, no matter what type of air compressor it is. 

 

Lubricated vs Oil-Free Compressors

Air compressors of all designs turn mechanical power into pneumatic power by successively concentrating air across compression stages. A rotary screw air compressor, for example, utilizes rotating helical screws to drive air forward, increasing its pressure by reducing the volume of space the air mass takes up.  Mechanical compression of this nature takes quite the force and energy to accomplish, which equates to heat generation and physical wear inside of the compressor. Whether we are discussing an oil-lubricated or an oil-free air compressor, these heat and wear factors are still present, and both require lubrication and cooling to keep the compressor from destroying itself. That is right – an “oil-free” air compressor still uses oil to lubricate its moving parts and manage its waste heat. 

What makes an air compressor “oil-free”, then? Simply put, an oil-free air compressor does not use oil during the compression stage and oil does not come into contact with the discharged compressed air flow. This is a very positive attribute in that significantly less oil is introduced into the compressed air stream than with a lubricated air compressor. Lubricating oil, however, is still used in the crankcase and as an internally recirculated coolant. This is one of three potential sources of air contamination (including hydrocarbons and oil) which can be introduced into any type of air compressor – including an oil-free air compressor.

Neptune compressor    

HAUG Neptune oil-free piston gas compressor.

 

Sources of Compressed Air Contamination

Compressed air has the potential to gain contamination from three sources.  Regardless of if you are using an oil-lubricated or an oil-free compressor, these sources are the same (the oil-lubricated variety just has more internal sources where oil comes into contact with the air).  Likewise, the need for downstream system design considerations and air purification equipment is also the same. 

1)     Environmental Sources

The air we breathe is made up of many gaseous and vaporous constituents. Some of these components are concentrated across an air compressor and build up to form harmful contaminants in our compressed air systems. Volatile Organic Compounds (VOCs), Hydrocarbons, Carbon Monoxide and Dioxide, Sulfuric Oxide, Nitrous Oxide, Organic Particles, Dust, Dirt Particles, and other pollutants are all present in outside air. In addition to organics, various microorganisms and bacteria are equally concerning, especially in medical, food, and other hygienic applications. When brought into an air compressor’s intake, these elements can be present as oils.

2)     Incidental carryover from the oil-free air compressor itself

Recirculating intercoolers and separate gearboxes often have breather valves installed in which hot oil gases can escape into the airspace around the compressor.  This vapor exhaust can be immediately picked up by the compressor’s air intake, indirectly introducing lubricating oil particles into the compressed air. 

3)     Downstream piping and equipment

Air receivers are notorious for contributing moisture and particulates to compressed air streams, but they are not the only culprit.  Piping materials can corrode and scale off into the air stream, vapor separators can fail and recontribute moisture, flexible hoses can leech their composite polymers under pressure, and so on.

 

What is “Technically Oil-Free Air”?

Marketing campaigns and terminology tend to result in misunderstandings around the quality of air you might expect from an oil-free compressor. An “oil-free” air compressor alone simply cannot produce air that is demonstrably free of foreign contaminants. Similarly, reduction of foreign materials in a compressed air stream to absolute zero values is arguably not possible at industrial scales, and regulatory standards governing compressed air systems reflect this.

Depending on your locale, you might refer to International ISO, British BCAS, Safe Quality Food SQF, Canadian FSEP, or other guidance to determine your requirements for air quality.  As an example, one of the highest requirements for food-grade compressed air sets the threshold at 0.01 micron at >99.99% DOP (Dioctyl Phthalate Fog Method) efficiency.  There are functional and measurability limits in both the methods, sample sizes, and instrumentation needed to test air quality, and the regulatory codes allow for these limits above absolute zero. 

Since the objective is typically not the absolute removal of all foreign matter, but instead an extremely high level of removal within practical limits, some firms use the term “Technically Oil-Free Air”.  For them, technically oil-free compressed air may be the lowest total oil level, remaining after an activated carbon filter, measured down to just 0.003 mg/m3

Technically oil-free compressed air is only achievable by the use of compressed air treatment equipment including specific types of compressed air filters and dryers, in addition to the air compressor.  This equipment is required after either an oil-lubricated or oil-free air compressors.

Filter assembly drawing

 Filter assembly drawing for dual coalescing and carbon filters: maximum pressure 5000 psi (340 bar).

 

Reducing Contaminant Levels with Compressed Air Purification Equipment

Let’s run through the compressed air purification equipment categories that can be deployed, in various combinations, depending upon the compressed air dryer type and site requirements, to achieve technically oil-free compressed air.

1.     Water Separator(s)

Reduce levels of liquid oil and water content in the air stream. The goal is to remove bulk liquid before entering a dryer or a coalescing filter.

2.     Storage Tank

Present to store compressed air, storage tanks will also reduce levels of bulk liquids in the air stream. “Wet tanks” are installed before the dryer and “dry tanks” after.

3.     Coalescing Filter(s)

Reduce levels of oil and water aerosols, atmospheric particulates, microorganisms, and metallic/non-metallic particulates from the piping system (such as rust flakes or mineral scale). Goal is to remove liquid contaminants and solid particulates. Some coalescing filters are designed to focus on particulate filtration while others are designed for oil and water aerosols.

4.     Compressed Air Dryer

Reduces water vapor levels that are suspended as a vapor in the air stream to prevent it from condensing into a liquid downstream when it cools. Moisture removal also eliminates an environment where microorganisms can grow.  Removal of water and water vapor is also necessary to allow oil aerosol and oil vapor removal filters to perform optimally. Dew point requirements will vary by application and will determine the type(s) of compressed air dryers to be used.

5.     Activated Carbon Absorption Filter or Tower

Reduces oil moisture from the air stream by absorbing oil vapors via an activated carbon media bed.

6.     Dry Particulate Filter

Removes particulates from the air stream by use of a porous mechanical filter, with filtration options down to 0.01 microns. Used only with adsorption type dryers.

7.     Sterile Filter

Removes even further levels of solid particles, including microorganisms, from the air stream by use of a sieve retention or membrane filter.  Sterile filters are intended for frequent cleaning to assure sterility.

8.     Condensate Drains and Oil-Water Separators

Filters, storage tanks and some types of dryers will remove gallons of liquid water, mixed with oil, via the use of condensate drains. This condensate is sent to an oil-water separator to responsibly remove the oil before the condensate is sent to drainage.

Sauer Mistral Series

Sauer Mistral Series WP65LB with oil-free filtration to provide ISO 8573:2010 Class 1 Oil Content.

 

ISO 8573-1:2010 Classes for Oil Content

When using the ISO 8573-1:2010 Standard for compressed air contaminants and purity classes, it’s important to note the “Class Level” system it deploys are applied to three types of contaminants. They are (1) Particles, measured by particle size in microns or by mass (2) Water, measured by vapor pressure dew point and liquid (3) Oil liquid, aerosol & vapor measured by mg/m3.  A common error made in specifications is to use “Class 1” and not specify which of the three contaminant types it’s making reference to.

Referring to the international air quality code ISO 8573-1:2010, “Technically Oil-Free Air” could be equated to Class 0 or Class 1 thresholds for total oil content (liquid, aerosol & vapor).  Class 1 describes the highest expressed quality values covered by the standard, at  £ 0.01 mg/m3 (0.008 ppm). 

The definition of Class 0, applicable to all three contaminant types is, “As specified by the equipment user of supplier and more stringent than Class 1.”  Class 0 is mainly for a customizable, use case specific condition, and does not state absolute zero contamination exists in the air stream. 

To achieve Class 1 or Class 0 thresholds for oil content, the typical compressed air system installation will consist of an air compressor (either lubricated or oil-free), a water separator, a coalescing filter set, a dryer, a receiver tank, an activated carbon absorption filter, condensate drains and an oil water separator.  This kit will properly protect the outbound air from containing any liquid oil or oil vapors.  From here, the overall air distribution system throughout the facility must be considered for additional protection depending on downstream risk points.  Very likely, you will want to add additional filtration at each point-of-use, protecting from any re-contamination to the air coming from the piping system itself. 

With the above information, you can see the installation of an oil-free compressor alone is not enough to properly deliver “oil-free” compressed air.  The entire air distribution system must be taken into consideration, and additional post-compressor air purification equipment is always required to treat environmental, carryover, microbial, and piping contaminates.  For more information or to ask any questions, please visit our website at www.sauerusa.com/

 

About Sauer Compressors USA

Sauer Compressors USA specializes in the manufacturing of medium and high-pressure air and gas compressors for naval, commercial maritime, offshore, research and development, and demanding industrial applications. In addition to compressed air, Sauer Compressors works in the CNG, N2, He, and inert gas markets. Sauer USA, located in Stevensville, Maryland, is an affiliate of J.P. Sauer & Sohn, headquartered in Kiel, Germany.

The four product lines - SAUER, HAUG, Girodin and EK - focus on specific fields of application. The SAUER line comprises oil-lubricated high-pressure compressors for a wide variety of applications, while HAUG stands for oil-free and hermetically gas-tight compressors. The Girodin and EK lines offer special compressors for the naval market. Sauer Compressors’ modern reciprocating compressors for the compression of air and various gases reach pressures of 290 to 7,000 psi. Besides standard products, it offers customized solutions for individual customers, OEMs and companies that operate on a global stage. With a global network of agents and representatives, Sauer maintains close proximity to its customers. By supplementing the compressor range with high-quality accessories, engineering services, assembly and service concepts, Sauer offers system solutions right up to complete turnkey installations. For more information, visit www.sauerusa.com.

To read similar Air Compressor Technology articles visit https://www.airbestpractices.com/technology/air-compressors or for Compressed Air Standards articles visit https://www.airbestpractices.com/standards/iso-cagi.