In today’s production environment, compressed air is widely used to automate processes, provide motive power, and package products. A clean, dry, reliable compressed air supply is essential to ensure manufacturing facilities maintain efficient and cost-effective production processes to meet output goals.
There are many types of compressor technologies to choose from, including an oil free variant. The promotion of oil free compressors often targets industries such as food and beverage, pharmaceutical and electronics where oil contamination from compressed air is a major concern. Unfortunately, during the selection, purchase and installation of an oil free compressor, the downstream air treatment system is often neglected. For this reason, production and facilities managers are often disappointed that oil and water contamination are still present in their compressed air even after the installation of a new, oil-free compressor.
In this blog we will identify:
- Sources of oil contamination.
- Misconceptions about the installation of an oil-free compressor.
- Contamination risks associated with oil-free compressors.
- How to mitigate the risks by installing the correct purification equipment to deliver clean, technically oil-free compressed air.
Sources of oil in a compressed air system
The two leading sources of oil in compressed air are:
- The ambient air (oil vapor).
- The air compressor (liquid oil, oil aerosols, oil vapor).
If these sources of contamination are not addressed, oil will infiltrate the air receiver and distribution piping, becoming a widespread issue in the manufacturing facility.
Oil in ambient air
Ambient air contains gaseous oil (vapor). It is a combination of hydrocarbons and VOC (volatile organic compounds) which come from natural sources as well as vehicle and industrial exhaust. While oil vapor in air regularly exists in a reasonably low volume, it will be comparatively higher in urban areas, industrial environments, and near parking lots, roads and highways.
How does oil in ambient air contaminate compressed air?
Large volumes of ambient air are drawn in through compressor intakes. The oil vapor in the ambient air cannot be seen in its gaseous state. However, once in the compressor, some of it will cool, condense and form liquid and aerosol oil. This issue increases as the air is compressed (squeezed), creating a higher concentration of oil in the confined space of the compressed air system.
Oil from the compressor
One obvious solution that manufacturing and plant managers turn to in their effort to reduce oil in their compressed air system is to use an oil-free screw compressor.
While there are certain benefits to using this type of compressor, it is a misconception that they will eliminate a potential source of oil contamination completely. While oil-free compressors do not use oil in their compression stage, oil can be introduced from these sources:
Oil carryover from ambient air - Oil vapors in the ambient air are drawn into the compressor intake, compressed and concentrated. These concentrated vapors then enter the compressed air distribution system where they can cool and condense.
Oil carryover from the compressor - The oil used in the closed-loop system to cool and lubricate the bearing and gears heats up and vaporizes during operation. This closed-loop system is vented inside the compressor cabinet, releasing oil vapor. This can often be a source of oil contamination of the compressed air.
Inter-cooling/after cooling - Oil free compressors typically use two compression stages. An inter-cooler is typically placed between the two stages to cool the air down. Before exiting the compressor, the compressed air passes through an after cooler to cool it to a more usable level.
Liquid and aerosol introduction - As the inter-cooler and aftercooler cool the compressed air, it reduces the air's ability to hold water and oil vapors. Cooling condenses the vapors into liquid water and liquid oil which is carried along the air at high velocity. Rough internal surfaces of the piping, elbows, fittings, etc., disrupt the flow of condensed liquids which causes them to atomize and produce droplets or aerosols of water and oil.
Liquid reduction - Many air compressors are fitted with an integrated water separator to reduce liquids. This helps to remove larger volumes of liquid contamination; however, it will not remove fine droplets (aerosols) and oil vapors.
While an oil-free screw air compressor is a common choice of industrial manufacturers, consideration must be given to the placement of the compressor away from high oil vapor concentration in ambient air sources, like parking lots and highways, whenever possible. In addition, awareness of other requirements for oil lubrication, and the threat they present, like venting of closed-loop lubrication on the compressor itself, is critical. Finally, the recognition that treatment of the compressed air for oil and other contaminants is still necessary, even with an oil-free compressor, and is critical to the efficient operation of the plant. Let’s take a closer look at air treatment.
Using international standards to specify oil free compressed air
ISO8573 series is the most commonly used standard for compressed air. It is made up of 9 separate parts. Part 1 refers to air purity (quality). Parts 2-9 provide details on the equipment and methodology used to accurately measure for different contaminants in a compressed air system. For more information on understanding ISO classification tables, read our blog, "Six Points to Consider When Applying ISO8573-1 in a Manufacturing Facility".
ISO8573-1
ISO88573-1 provides users a way of specifying an air purity required for the entire compressed air system and or for individual usage points for each contaminant (particulate/water/total oil).
What is Class 0?
Class 0 is a reference to an ISO8573-1 air purity classification. It is also often referred to as an oil contamination classification, but it can also be applied to solid particulate and water.
Almost all oil free rotary screw compressors are sold under the banner of Class 0. Unfortunately, the ISO 8573-1 Class 0 classification is often misunderstood or misapplied to air compressors. It is important to remember:
- Class 0 does not mean zero contamination or oil-free air.
- Class 0 does not solely refer to oil contamination.
- A Class 0 specification must be cleaner than the Class 1 specification for the contaminant choices.
- The contamination levels stated for a Class 0 specification must also be within the measurement capabilities of the test equipment and test methods shown in ISO8573 parts 2-9.
- The Class 0 specification must clearly state which contaminants it refers to, e.i., solid particulate, water or total oil (aerosol, liquid, vapor).
- Class 0 requires the user of the equipment supplier to show a contamination level as part of a written specification.
- The agreed Class 0 specification must be written on all documentation to be in accordance with the standard.
- Stating Class 0 without an accompanying contaminant specification is meaningless and not in accordance with the standards.
Oil free compressed air
The implication is that oil free means that produced compressed air is free of any trace of oil. This is misleading and difficult to prove or guarantee in practice. This is due to the fact that 0.003 mg/m30.003 mg/m3 measurement must be conducted within the methodology presented in the ISO8573 series. However, the limitation of accuracy for the measurement of total oil is 0.003 mg/m3, not zero.
Technically oil free air
A definition of technically oil free air has been created to define a compressed air system that has been treated to reduce oil to the low limit of measurability: 0.003 mg/m3.
It is occasionally used to imply an inferior quality of compressed air compared to the so-called oil free air delivery by an oil free compressor — when in fact the treated air will be of a higher quality.
Therefore, technically oil free compressed air is as close to oil free compressed air as possible, with total oil levels down to 0.003 mg/m3.
Can oil free compressors without treatment equipment guarantee oil free air?
In short, no they cannot for these reasons:
- Ambient air contamination: There is a constant source of oil vapor being drawn into the compressor with the ambient air. This must be treated to prevent downstream oil contamination.
- Contamination from the oil-free compressor: Oil-free compressors still use lubricating oil. These lubrication systems need to be vented resulting in another source of oil vapor in the intake air. Wear of seals can also be the cause of leaked oil into the air system.
- Contamination already in the distribution system: Even if a new oil free compressor is installed, residual oil and other contaminants will remain in the piping system and will impact point-of-use equipment and processes.
Should an oil free compressor or technically oil free compressed air be specified to protect my products, processes and brand reputation?
Industries like food and beverage, pharmaceutical and electronics need to be especially concerned with introducing oil and other contaminants to their products via compressed air. Below are some common misconceptions:
- Common mistakes with specifications: If an insistence is made that an oil free - or ISO 8573-1 Class 0 – compressor be installed, there is often a false belief that the compressed air is guaranteed to be oil-free. This is not the case, as there is no means of measuring down to a zero value of oil in compressed air.
- Offsetting cost: When the cost of an oil-free compressor is considered, there is often a desire to reduce costs elsewhere. Attention is often focused on reducing compressed air filtration in the compressor room and downstream due to the belief that the compressor will produce oil-free compressed air for the entire facility. As described, intake air and the air distribution system will often prove this to be untrue.
In conclusion, technically oil free compressed air, i.e., ISO 8573-1:2010 Class 1 for Total Oil or ISO8573-1:2010 Class 0 (0.003 mg/m3 for Total Oil should be specified.
How do you achieve technically oil free compressed air?
In order to achieve technically oil free compressed air, the following purification equipment must be installed.
- Liquid separators
- Coalescing filters
- Adsorption filters
These will treat the oil vapor drawn into the compressor intake and residual oil in the distribution lines throughout the facility. They will also remove other solid and liquid contaminants.
Should air purification equipment be installed in the compressor room or at the points of use?
Achieving technically oil free compressed air requires a careful approach to system design. Treatment in the compressor room should be robust enough to protect the downstream piping system. Then at the points of use, additional treatment should be installed to address the specific needs of each individual critical application. The ISO air purity specified should be reflected at the last stage of filtration on the line ahead of use.
Beyond oil
Oil in a compressed air system is not the only concern for users. There are a minimum of 10 contaminants found in a compressed air system, oil being one of them, that require treatment:
- Microorganisms
- Water vapor
- Atmospheric particulate
- Oil vapor (hydrocarbons & VOC)
- Water aerosols
- Condensed liquid water
- Liquid oil
- Oil aerosols
- Rust
- Pipescale
To operate any compressed air system safely, efficiently and cost-effectively, contamination must be reduced to acceptable limits. Failure to control contamination can result in a myriad of problems for manufacturers and consumers including:
- Contaminated product/failed audits
- Increased manufacturing costs
- Reduced production efficiency
- Legal actions
- Brand damage
- Equipment failure and downtime
- Unplanned maintenance
Contamination control
Ensuring effective compressed air contamination control requires a number of purification technologies. In fact, the purification equipment required downstream of an oil free compressor is identical to that of an oil lubricated compressor. The table below highlights filtration and drying technologies and the contaminants they reduce:
A solution for every contaminant
- Water separators - usually the first piece of equipment installed downstream of an after cooler or wet air receiver, water separators are used to protect coalescing filters from liquid contamination. They have no effect on water or oil in an aerosol or vaporous phase.
- Coalescing filters - vital for the cost-effective operation of any compressed air system, coalescing filters (installed in series) treat water aerosols, oil aerosols, atmospheric particulate, micro-organisms, rust and pipe scale.
- Adsorption filters - activated carbon filters ensure technically oil free air by treating oil vapour and hydrocarbon odors.
- Compressed air dryers - water vapor (water in gaseous form) will pass through both water separators and coalescing filters. Compressed air dryers are used to treat water vapor.
- Dry particulate filters - provide particle reduction down to 0.01 micron with a removal efficiency of 99.9999%. When combined with a -40°C pressure dewpoint, dry particulate filters can provide a significant reduction of microbiological contaminants.
- Sterile air filters - provide absolute (100%) removal of solid particulates and micro-organisms.
Conclusion
Due to the financial and commercial impact of a contaminated product, many companies specify the use of an oil free compressor, in the mistaken belief that this will deliver oil-free compressed air to critical applications. Oil free compressed air systems are typically installed without downstream purification equipment intended to treat oil, as they are deemed unnecessary accompaniments. While oil-free compressors do not use oil in their compression stage, oil can be introduced from other sources. Therefore, treatment of the compressed air for oil and other contaminant removal is still necessary, even with an oil-free compressor. Failure to control contamination can impact production, company image and financial performance.
To learn more about how to assure technically oil free air, remove other contaminants and about key components of a compressed air treatment system, please download our white paper, "How To Get Oil Free Air From an Oil Free Compressor"
This article was contributed by Mark White, compressed air treatment applications manager, Parker Gas Separation and Filtration Division EMEA
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