When oil-free means just that
Oil removal is not always a good substitute for true oil-free compression

Although it is possible to produce “oil-free” compressed air by removing oil mist downstream of an oil-lubricated compressor, there are many applications where only true oil-free compression will do. For best performance and reliability, such processes need compressed air that has never come into contact with oil.

Many of these highly-sensitive processes are found in the pharmaceutical and food industries, and in semi-conductor manufacture. Even in automotive industry paint shops, compressed air containing even a trace of oil can prevent the paint adhering properly. Applications like these need to get their compressed air from compressors that are completely oil-free.
The easiest way to generate “oil-free” air is by using a conventional oil-lubricated screw compressor fitted with downstream equipment to remove the entrained oil droplets. An oil-injection screw compressor is a simple and reliable machine in which the oil serves two functions: it lubricates the rotors and also absorbs the heat of compression.
In a twin-screw compressor, the presence of a thin oil film on every surface means that the rotors never actually touch. This means that one rotor can be driven by the other, which in turn is driven by the motor. The lubrication system thus allows the compressor to be manufactured without expensive secondary drive systems. The oil also increases the efficiency of the compression process by sealing the gap between the rotors, and limits the discharge temperature to 85–100 °C.

The drawback, of course, is that the intensive contact between air and oil means that the discharge air is saturated with oil vapor and tiny oil droplets. Removing this oil is a somewhat complex and costly procedure. Modern oil-removal filters and absorbers are very effective in generating compressed air that is technically oil-free, but they require regular maintenance such as changing the filter elements. This costs money, and so too does the power absorbed in overcoming the pressure drop across the separation equipment: as a rule of thumb, an extra 1 bar at the compressor discharge means an increase in energy consumption of 6–10 percent. And the possibility of equipment failure or human error during maintenance means there is always a risk of oil breakthrough, with costly consequences.
True oil-free compression
To overcome these disadvantages, it is better to generate compressed air that is oil-free from the start. However, this requires the solution of some demanding technical problems. One of these is the need to prevent contact between the two rotors. In the absence of a lubricating film of oil, a true oil-free compressor requires an expensive constant velocity drive to prevent the rotor surfaces from touching. At the same time, the gap between the rotors must be as small as possible, so this places high demands on tolerances and accuracy.
The absence of lubrication also means that the sensitive rotor surfaces are no longer protected by the oil. Instead, the rotors require a protective coating. PTFE is sometimes used for this purpose, but such a coating can peel off after a few operating hours. As well as leaving the surfaces unprotected, this increases the gap between the rotors and so reduces efficiency. A better solution is to use a tightly-bonded hard coating that will last for the whole life of the compressor.
With no oil to absorb the heat of compression, discharge temperatures in an oil-free compressor may be up to 250 °C. The resulting greater thermal expansion of the compressor parts needs to be allowed for at the design stage. Multi-stage compressor systems also require great care in the design of intercoolers, because the high peripheral speed in the high-pressure stage means that even the smallest condensate droplets can cause severe damage.
Not even an oil-free compressor can run without oil-lubricated bearings, so it is very important to keep the bearing oil away from the compression chamber.
Wear-free labyrinth seals are the usual way to do this. The high discharge temperature tend to increase bearing wear, so the bearings need to be of higher specification than their counterparts in an oil-cooled compressor.
Even monitoring and control of an oil-free compressor are more complex and expensive than for an oil-lubricated machine. Some potential problems do not show themselves until the compressor has been operating for some time; the higher thermal stresses in an oil-free compressor, for instance, can increase the danger of vibration-induced cracking unless the machine is properly designed. Vibration can also create high noise levels. In short, the tight running clearances and high operating temperatures that characterize oil-free compressors pose considerable challenges to the manufacturer. A machine that has not been designed with skill and care may not deliver what is expected of it, or may perform well at first but soon break down and need expensive repairs. Since the earliest days of oil-free compressors, quality-conscious manufacturers have gained much experience in fields such as control, dimensioning and pulsation damping. Newcomers to the field find it very hard to catch up with this lead, so it is not surprising that only a few suppliers have established themselves in this demanding sector. Oil-free compressors are a reliable and cost-effective alternative to conventional systems in critical applications, but it pays to buy from the experts.