Oil- and germ-free compressed air – causes, problems and solutions

In many production facilities, compressed air comes directly or indirectly into contact with production lines, products or packaging materials. Contamination through residual oil content, micro-organisms and germs then has significant consequences for product quality, consumer safety and market reputation.

Stiftung Warentest: mineral oils in chocolate, Foodwatch denounces the dangers of food packaging. We are all familiar with reports like these about contamination and deficits in the quality of the final product. And we are familiar with the consequences, too: uncertainty among customers and retail traders, a stir in the press, non-objective discussions and last but not least damage to reputations.

The use of compressed air in areas where human health can potentially be harmed by compressed air makes special requirements on the cleanness of the compressed air. For this reason, it is particularly important.

How does compressed air become oil-contaminated?

There are many ways for contamination factors such as particles, oils, germs and humidity to enter the compressed air. They are often already present in the ambient air and get into the compressed air system through the compressor intake air. The reason for the contamination can be a major road or construction site nearby, for example. The risk of humidity occurring in the compressed air system increases with air humidity in the ambient air.

The contamination presents a double risk: on the one hand, it can impair the function of the compressed air plant and lead to premature wear of plant components, on the other it presents a real risk to the quality of the final product and thus the consumer.

ENVIRONMENT

Additional hazards exist depending on the direct surroundings and individual circumstances: in addition to dust and humidity, oil and micro-organisms can also get into the compressed air system via the ambient air.


COMPRESSOR

Not only oil-lubricated but also oil-free compressors (intake air!) can be a source of residual oil vapours in the compressed air network.


FITTINGS

Valves and fittings. Numerous components in the compressed air system are lubricated with greases or silicones to improve their function. These can easily get into the compressed air.


PIPEWORK

Once contaminated, there is a constant risk: over the years, deposits can build up in existing pipelines and influence the compressed air flowing through them.


Problems caused by oil entry

Faulty product quality, machine damage, scrap, rework and recall campaigns for industrial goods that are caused by oil breakthrough are less well known to the general public than the problems mentioned above. However, they cause substantial financial damage, particularly in the following branches and applications:

Pharmaceuticals and laboratory technology

Very stringent hygiene standards are specified for manufacturing medicinal products. It is therefore very important that production takes place in an environment that is free of germs, particles, bacteria and contaminating oils. In addition to utilisation in hospitals, compressed air is increasingly being used in laboratories. In order to exclude the danger of bacterial growth in these highly sensitive environments, compressed air must be absolutely germ-free and dry.

Food industry and packaging industry

This contamination has to be removed or reduced here, too. This is executed in order to protect consumers and to ensure a safe and cost-efficient production process. The condition of the final product must remain unchanged during packing and filling of the products. Beverages and food in particular must be treated very gently, and no contamination may subsequently occur (directly or indirectly).

Direct contact: The compressed air comes directly into contact with the product or packaging material or gets into the respiratory system or onto inner or outer spots not protected by skin (e.g. following an injury).

Indirect contact: The compressed air is discharged to the ambient air during an application. The expanded compressed air only reaches an object over a corresponding distance and having been diluted by normal ambient air.

The primary potential risks are:

  • Contamination of the product by contaminated water (condensate)
  • Contamination of the product by liquid oil (compressor oil)
  • Contamination of the product by oil vapour or general gaseous hydrocarbons and thus undesirable essences
  • Contamination of the product by undesirable metallic or non-metallic solid particles from the compressed air system, e.g. rust, corrosion particles, abrasion, sealing material or other loosened deposits
  • Contamination of the product by undesirable micro-organisms

 

Paintwork

Particles, oil-aerosol and vapour, materials containing silicone as well as condensate are the main causes of faults in the paint shop. The use of compressed air in painting technology makes requirements on the cleanness of the compressed air which even go beyond the classes defined in ISO 8573-1.

Paints and varnishes react extremely sensitively to certain contamination in the compressed air. This results in coating wetting problems in the form of craters and bubbles, linked to corresponding rework and thus additional expense. The compressed air must be paint compatible i.e. free of substances which interfere with coating wetting (these include graphite, waxes, metal soaps, paraffins, talcum, teflon and plastic abrasion).

Paint-compatible compressed air is required when the compressed air comes directly or indirectly into contact with still wet paints or varnishes or surfaces to be painted. Whereas indirect contact can be avoided by cabins, direct contact is unavoidable wherever painting nozzles are used.

For this reason, paint-compatible compressed air must always be

  • dry,
  • free of liquid contamination, oil and aerosols,
  • free of vapour phases which may condense,
  • free of dust and other solid particles down to tiniest residual amounts.

Like greases or oils, the silicones dreaded in painting technology can be present in different phases (solid, liquid, gaseous) in compressed air. Contamination containing silicone can be removed using appropriate filters. However, gaseous and thus volatile silicone compounds can only be removed from the compressed air by means of catalytic oxidation. For this reason, the use of at least one catalytic converter is essential for the production of paint-compatible compressed air.

Chemical industry

Wherever there is direct contact during the processing of raw materials (powders and granules), the compressed air must be absolutely dry and oil-free. This is the only way contamination and the formation of agglomeration can be excluded. In order to guarantee process reliability, both permanent monitoring and complete documentation of the compressed air quality are indispensable.

Electronics

Chip manufacturing processes must take place under clean room conditions i.e. the quality of the compressed air must be adapted to these requirements.  Another field of application for compressed air is the application of solder paste on the circuit board as well as cleaning of circuit boards, electronic printed circuit boards and wafers. The compressed air utilised here must be free of particles, oil and moisture.

Producing oil-free compressed air

Generally speaking, there are three ways of producing oil-free compressed air. The special requirements must be given careful consideration during planning. This starts with the question of whether oil-free compressed air has to be available centrally or whether treatment can be decentral, since only part of the flow is subject to particularly high requirements.

1 - Treatment through catalysis

With a catalytic converter, not only oil-free but also germ- and bacteria-free compressed air can be produced in an environmentally friendly way. This procedure, which is independent of intake conditions, is significantly more reliable than filtering. while at the same time requiring less maintenance effort. It is the most innovative method and can also be retrofitted downstream from oil-lubricated compressors.

Oil-contaminated compressed air is routed into the catalytic converter. There, hydrocarbons in the compressed air are broken down into carbon dioxide and water in one single process step. The thermal energy necessary for chemical breakdown provides an effective monitoring option. If oil breakthrough occurs in the compressed air pipe upstream from the converter, the temperature rises sharply, causing a solenoid valve to close the outlet. This effectively prevents oil getting into the downstream compressed air pipe. Oil discharge caused by too low temperature is also excluded by the integrated temperature monitoring.

The procedure achieves constantly oil-free compressed air with a maximum residual oil content of a barely measurable 0.001 milligrams per cubic metre. The residual oil content exceeds the requirements for compressed air classes 0-1 according to ISO 8573-1. The incidental condensate from the cooling of the compressed air is also absolutely oil-free and can be channelled into the sewer system without treatment. It must be noted that a bypass or redundant system is urgently required for 24/7 operation.

Our catalytic converter is the ideal solution wherever constantly oil- and germ-free compressed air is required. It combines economic efficiency with process reliability and is independent of intake conditions. It can easily be retrofitted to existing compressed air stations. The BEKOKAT stands for high efficiency and constant compressed air quality particularly when used with sensitive products such as food or pharmaceuticals. 

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2 - Treatment through adsorption

Technically oil-free compressed air still contains hydrocarbons as well as various fragrances and essences which would lead to loss of quality and odour pollution. Smaller partial volumes can be treated with microfilters or activated carbon filters. Larger volumes are treated using activated-carbon adsorbers. A filter can only remove oil droplets from the compressed air, whereas an activated-carbon adsorber can also remove hydrocarbon vapours from the compressed air. An activated-carbon adsorber always has to have an upstream high-performance filter and dryer for preliminary filtering.

Activated carbon is also used to remove the vaporous residual oil. Cleaning of compressed air by adsorption is a purely physical process. Oil molecules are bound by the adhesive forces of the surface of the activated carbon and the compressed air thus cleaned. No chemical compound is formed. The quality of the activated carbon is not obvious, yet it is of decisive importance, because activated carbon does hard work. Differences only become apparent when the service life does not meet expectations. As large an inner surface and as fine a pore system in the activated carbon as possible are required to prevent the accumulation of undesirable pollutants.

The dried and filtered compressed air is routed by a diffuser into the loosely shaken up activated carbon bed. This enables long contact times and optimum utilisation of the adsorbent. At some point, the activated carbon is saturated and can no longer carry out cleaning. From this time onwards, the residual oil content of the compressed air increases again. In other words, activated carbon is a consumable that cannot be regenerated and has to be replaced after approx. 8000 to 10000 operating hours. If the filters are serviced regularly, only a small residual oil content remains in the compressed air, which complies with the requirements of compressed air classes 1-2 in accordance with ISO 8573-1.

For safety reasons, a high-performance filter should be integrated downstream of the adsorber as an afterfilter, because the compressed air sweeps along tiniest carbon dust particles (smaller than 1 µm) from the activated carbon bed. If these preconditions are fulfilled, the activated-carbon adsorber CLEARPOINT V protects your system from oil entry, at the same time standing out on account of a low differential pressure and long filter standing times.

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3 - Oil-free compressors

This is the direct way of avoiding additional contamination of compressed air by oil through the compressor. Compressed air is produced by oil-free piston or screw compressors without the compressed air coming into contact with liquid or vaporous oils because the compressor chamber is not lubricated and pairs of screws run without touching each other. Compressed air produced in this way is often described as being “technically oil-free”. This is only possible thanks to perfect sealing and maximum precision. This involves high investment costs and limited operating pressure.

Does an oil-free compressor offer complete safety?
It is true that use of an oil-free compressor leads to no additional oil getting into the compressed air system. Since contamination (such as exhaust gases from vehicle combustion engines or heating systems, oil aerosols and micro-organisms) is already contained in the intake air, however, and is present in concentrated form downstream of the compressor, the compressed air always has to be treated. The oil content is often ≥ 0.01 mg per cubic metre (Class 2 or an even poorer rating).

Comparison of compressor designs
Compressor design Residual oil content at the compressed air outlet Oil entry in the network with a volume flow of 1000 m³/h
Piston compressor, oil-lubricated 10 - 180 mg/m³ 240 - 4320 g
Rotary vane compressor, oil-lubricated 1 - 180 mg/m³ 24 - 4320 g
Screw compressor, cooled by oil injection 1 - 20 mg/m³ 24 - 480 g
Compressor with oil-free compression 0 - 3 mg/m³ 0 – 72 g
Intake temperature 20° C, intake pressure 1 bar (a), load time 24 h/d (source VDMA Standard Sheet 15390-1: 2014-12) .

Maximum safety through residual oil concentration monitoring

Manufacturers who keep to guidelines such as GMP for critical compressed air applications and carry out a hazard analysis and risk assessment have created important pre-requisites for safe compressed air application. Those who monitor their compressed air quality 24/7 are one step further. Quality assurance concepts such as HACCP require maximum controllability of even extreme limit values of 0.001 mg/m³ of the oil vapour content in compressed air. METPOINT OCV makes permanent online monitoring possible.

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Intelligent compressed air treatment requires expertise

In practice, the subject of compressed air is often a critical point for product managers and quality managers. The problem is that the requirements are often not worded clearly in the guidelines, or the effects on the entire plant are not mapped sufficiently. Detailed knowledge is required for the specification of the necessary compressed air quality class and the suitable – i.e. safe and energy-efficient – design of the compressed air treatment.

Plant Optimising and Air Audit…