How to produce oil-free air for controlled manufacturing environments

Today’s state-of-the-art, oil-free compressors not only produce the highest quality Class Zero air, but they also deliver significant savings in lifecycle costs. Oil-free air technology helps to avoid expensive filter replacements, cuts the cost of oil condensate treatment and disposal, and reduces energy loss from pressure drops.

However, in selecting the most suitable oil-free compressor; there is an important distinction between ‘Class 0’ oil-free compressors, which guarantee absolutely no contamination of air, and what is known as ‘technically oil-free’ category where oil as a contaminant is introduced to the compressed air system and needs to be filtered out and removed downstream of the compressor.

Compressed air has its own set of standards set by the International Organisation for Standardisation (ISO). The ISO 8753-1 standard governs the ultimate air purity determined by the number of particles per cubic metre as a function of particle size, with compressors ranked from ISO Class 0-5:

ISO Class 0 - Oil-Free air: This class is the cleanest and least risky choice.
ISO Class 1 - Technically oil-free air: This class allows for a certain level of oil contamination to be introduced into the compressed air stream, which then needs to be filtered and removed.

Oil-free compressors operate without oil in the compression chamber, which eliminates the chance of downstream contamination with oil in the compressed air supply. Rather than using oil for lubrication, sealing and cooling, these types of compressors utilise other technologies, such as water jackets and advanced, coated compression elements, engineered with microscopic tolerances to deliver oil-free compressed air.

In oil-free screw compressors, which make up the bulk of the market for this type of product, external gears synchronise the position of the counter-rotating screw elements. As the rotors do not come in contact, nor create friction between each other, no oil is required for lubrication within the compression chamber.

Precision engineering of the housing and screw elements minimises air leakage and pressure drops from the pressure side to the inlet. As the internal pressure ratio is limited by differences in air temperature between the inlet and discharge ports, oil-free screw compressors are frequently built with several stages and inter-stage cooling to maximise the pressure reach.

In addition to saving on through-life costs, using modern oil-free compressor technologies can also deliver significant energy savings. For large-scale, high-energy applications, like glass processing, food production, steel manufacturing, automotive, and petro-chemicals, centrifugal compressors offer significant energy efficiency in process air or bulk air applications presenting a flat load, or when providing a baseline flow in mixed installations with VSD screw compressors.

A VSD compressor, which matches output to load, saves on average 35% of energy consumption, while advanced models can save as much as 50% compared to fixed speed units even at full load. It is also possible to equip oil-free compressors with an energy recovery unit, where up to 94% of the compressor’s input power is converted into usable energy. This can be used to heat water for sanitary purposes, space heating or process applications and is an important consideration for companies looking to become carbon neutral.

Finally, oil-free compressors can also take advantage of advanced central controllers and remote monitoring tools to further boost energy efficiency. A master controller will optimise the operation of a compressed air system with multiple compressors. Furthermore, the compressors can also be easily connected to the cloud for mobile monitoring over a secure network, thereby providing a real-time means of avoiding breakdowns and production losses

Oil-free compressed air systems require fewer items of ancillary equipment and less maintenance compared to a technically oil-free system meeting the same ISO air quality standard. The gulf between the two solutions is further widened when looking at specific applications such as those in the food industry.

Maintenance cost savings start with the oil itself. It is important to rectify a common misconception, namely that oil-free compressors are all oil-less. Most oil-free compressors still require oil for lubrication of the drive train gear, but smart sealing technology prevents this lubricating oil entering the compressed air stream.

An oil-free system typically includes aftercoolers, a dryer, and dust filters before being distributed for use. In the technically oil-free solution, this equipment is also fitted downstream, but with the addition of oil separators, coalescing filters, and carbon capture towers − all of which adds to the level of maintenance required, increases the pressure drop and, with many more failure points, the risk of malfunction.

It is worth noting that in a typical compressor installation, operating for ten years, the cost of increased maintenance involved over this time could easily dwarf the higher initial capital investment for an oil-free compressor installation.