So, which compressor system provides the most cost-effective way of producing oil-free compressed air? Leaving aside the claims of individual manufacturers, there is no doubt that premium quality, oil-free compressed air can be achieved both with dry-running and fluid-cooled compressors. Ideally therefore, the deciding factor to consider when selecting an air system should be efficiency.
1. What does “oil-free compressed air” mean?
According to ISO 8573-1, compressed air can be described as oil-free if its oil content (including oil vapour) is less than 0.01 mg/m³. That is approximately four-hundredths of that contained in normal atmospheric air. This amount is so minute as to be barely measurable. But what about the quality of the compressor’s intake air? Of course, this depends greatly on local ambient conditions. Even in normally contaminated zones, the hydrocarbons in the air caused by industry and traffic emissions can lie between 4 and 14 mg/m³. In industrial areas, where oil is used as a lubricating, cooling and processing medium, the mineral oil content can be far in excess of 10 mg/m³. Other impurities such as hydrocarbons, sulphur dioxide, soot, metals and dust are also present.
2. Why treat air?
Every compressor, regardless of type, draws in contaminated air, concentrates the contamination by compression and, if no measures are taken to remove it, passes it on to the compressed air network.
a) “Oil-free” compressors
This especially applies to so-called “dry-running”, or “oil-free” compressors. Because of the pollution mentioned above, it is impossible to produce oil-free compressed air with a compressor that is equipped only with a three-micron dust filter. Other than these dust filters, so-called “oil-free” compressors have no further treatment components.
b) Fluid- or oil-cooled compressors
In contrast, aggressive matter is neutralised and solid particles are partly washed out of the air by the cooling fluid (oil) in fluid-cooled rotary compressors.
3. Non-defined compressed air quality without treatment
Despite the higher degree of achieved compressed air purity, the same applies here too: It’s a no-go without compressed air treatment. With “dry” or oil-cooled compression alone, under normal intake conditions and with the associated air contaminant levels, it is not possible to achieve defined oil-free compressed air quality in accordance with ISO 8573-1. As to how efficient compressed air production is, depends on the pressure and delivery range, as well as on the required compressor type. Sufficient drying forms the foundation for all application-tailored compressed air treatment. Energy-saving refrigeration drying is usually the most efficient method (Why do we need to dry compressed air? - read HERE).
4. Treatment with the KAESER Pure Air System
Modern fluid-/oil-cooled rotary screw compressors are approximately 10% more efficient than “dry-running”, or “oilfree”, compressor models. The Pure Air System, developed by KAESER for fluid-/oil-cooled rotary screw compressors, and for dry-running compressors, provides further cost-savings of up to 30%. The residual compressed air oil content achieved by this system is less than 0.003 mg/m³ and is therefore far below the limit for Quality Class 1 (regarding residual oil content) stipulated in the ISO standard. The system includes all the treatment components needed for achieving the required air quality. Depending on the application, either refrigeration or desiccant dryers are used together with various filter combinations. Air qualities ranging from simple dry air through to particle- and oil-free air to sterile air are reliably and cost-effectively achieved in accordance with the quality classes set out in the ISO standard (Fig.).
