Champion

The Myth:

Lubricant and varnish is a harmless by-product of all compressor lubricants and has little effect on the performance of the compressed air system.

Analysis:

Rotary-screw air compressor lubricants are required to perform more than just a lubrication function; they provide a mechanical seal between the rotors and the stator, remove the heat of compression and lubricate the rotary element bearings.

Over time, hydrocarbon-based lubricants degrade through oxidation, thermal deterioration and chemical breakdown. This lubricant degradation causes the formation of varnish - contaminants that combine and adsorb onto metallic surfaces, forming a thin, insoluble film. The presense of varnish is counter-productive to the lubrication process, causing blockages, friction (due to reduced component clearance), higher operating temperatures and increased component wear on components. Varnish production increases at high operating temperatures and extended intervals between lubricant change-out.

The formation of lubricant varnish is particularly damaging in air compressors. Attacking the rotary-screw airend, oil cooler and separator element, varnish causes a wide range of problems that can be expensive to remedy.

The rotary-screw airend presents a particularly challenging lubrication scenario, as the rotary-screw bearings are supplied with lubricant via relatively small bearing apertures. Varnish tends to block these apertures, restricting the flow of lubricant. The result is disastrous, with the rotary-screw mechanism eventually sticking or seizing completely. The rubbing of these metal parts produces microscopic metallic particles that circulate in the oil, grinding and eroding other compressor components. While the lubricant filter removes many of the particles, eventually it becomes blocked. Regular air compressor mainenance is required: specifically change-out of both the filter and the lubricant.

Furthermore, varnish layers on rotary-screw components reduce the clearance between rotors and cause increased friction. As a result, the motor is required to work harder to overcome this extra load, consuming greater energy. In some cases, varnish-stricken compressors can experience energy consumption increases of up to 10 per cent. The simplest means of avoiding lubricant varnishing is to use air compressors that employ non-hydrocarbon lubricants. Such synthetic lubricants offer long life and zero varnishing. Finned-tube lubricant coolers within the compressed air system are also susceptible to lubricant varnish. The varnish clogs the gaps between the fins (typically only 3mm apart), reducing the heat-transfer performance significantly. The reduction in heat-transfer performance causes the compressor to "run hot". The hotter the lubricant gets, the quicker the varnish forms--a vicious cycle is developed, particularly in Australia's hot climate.

Varnish formation can be minimised by running the compressor "cool". This can be achieved by employing air compressors with "stepped output control". This is achieved using a compressor with a motor/drive combination featuring a 20 per cent de-rated drive speed--a drive speed that would ordinarily be offered with a machine that supplies compressed air at a higher pressure. As a result, the compressor doesn't work as hard to produce the required compressed air output.

A "stepped output control" ensures that the air compressor runs cooler, producing 20 per cent less heat than it would if fitted with a smaller motor/drive combination operating at full capacity. Additional benefits are a longer lasting motor and airend, and less wear on all compressor components.

Conclusion:

Lubricant varnish in air compressors results in higher maintenance costs, shorter equipment life and higher electricity consumption. Varnish formation can be minimised by running the compressor "cool" and changing out hydrocarbon lubricants more frequently. Using synthetic compressor lubricants present an ideal solution, offering long life and zero varnishing.