Compressor selection: Take a long term-view
16 November 2018
When making the choice between a vane or screw compressor, there are a number of different areas for consideration. Amongst these, the most important are the long-term factors, as these will paint the most accurate picture of which technology is likely to offer the greatest benefits. Andy Jones, managing director at Mattei, examines four key areas to take into account when making a compressor selection
In today’s market, there are two main types of industrial rotary compressor – vane and screw. Although both are designed to produce the same output, it is important to know how they work, as these differences affect the maintenance costs, energy efficiency and levels of reliability associated with each technology.
‘Blowholes’, where air under high pressure can be driven to areas of low pressure, are inherent in screw compressors. To compensate for these leakage paths, they need to run at high speeds. The design of rotary vane compressors eliminates this issue, which means they can operate at low speeds from a direct drive coupling. This in turn improves energy efficiency and reduces power consumption.
A rotary-vane compressor comprises a rotor, with longitudinal slots which house individual sliding vanes. This assembly is offset within a cylinder, or ‘stator’, and rotates on white-metal bushes. Oil is injected into the stator to form a film on which the vanes slide, sealing clearances between the rotor, stator and end covers and to lubricate the bushes. With no direct metal-to-metal contact between the vanes and the internal stator wall, there’s no detrimental wear. In fact, the oil film polishes the various surfaces during use, which results in less friction, and consequently, better operation and reduced energy requirement.
The polishing effect within a vane compressor’s airend means that its efficiency actually improves for the first 1000 hours or so and then stays constant throughout the rest of its operating life, whereas typically a screw compressor's efficiency falls away over time as its bearings wear. If a rotary vane compressor is looked after in accordance with manufacturer’s instructions (which often only require compliance to a standard and a less than onerous servicing regime), the operating life of its airend can easily extend to over 100,000 hours without wear, even in dusty operating conditions.
Absence of roller bearings
The absence of roller bearings in a vane compressor means lower maintenance costs and a reduced risk of component failure. In fact, some vane compressors have been known to run for over 200,000 hours without the need to replace the white-metal bushes used in this type of compressor. This poses a serious contrast with roller bearings, which are typically required to be changed out at every 30,000-40,000 hours.
With these fundamental engineering differences between vane and screw compressors in mind, the change in compressor efficiency across its lifespan must be considered for a true analysis of energy efficiency to be reached. I believe the reality for businesses is that the vane compressor can prove a much better choice in the long run.