Vane or screw compressors?
Published: 09 July, 2014
When it comes to replacing your air compressor, there are two main rotary compressor options available: vane and screw. Both produce the same end result but how do the two compare in terms of energy efficiency? Andy Jones, general manager at Mattei, weighs the technologies up.
Energy efficiency is high up on everyone’s agenda, not least manufacturers who are increasingly in search of new ways to improve their productivity and save energy. And, with compressed air often considered to be the industrial sector’s fourth utility, a manufacturer’s compressed air system is often a good place to start in terms of making savings. However, not everyone realises that different types of compressors can vary significantly in their performance and therefore efficiency.
Despite the fact that vane and screw compressors were developed at similar times (circa 1950) and are designed to produce the same end result, the two technologies differ quite considerably in terms of energy efficiency, longevity and maintenance.
Vane compressors
A rotary vane compressor is made up of a rotor with longitudinal slots, which house individual sliding vanes. The rotor is offset within a stator (a cylinder), in which it rotates on maintenance-free white metal bushes. While the rotor turns on its axis, the vanes are pushed against the stator wall by centrifugal force, and the air is compressed.
Vane compressors are volumetrically more efficient than screw compressors because their design means they have minimal internal air leakage. The vanes move freely in their slots, but are always in contact with the internal surface of the stator (albeit not directly, due to lubrication), so the air seal is near-perfect. The clearances between the rotor, stator and end covers are also efficiently sealed by oil that is injected into the stator to lubricate the moving parts and to cool the air during compression. This means that a vane compressor typically produces more air per kilowatt and therefore consumes less power.
Crucially, rotary-vane compressors don’t require the roller bearings used in screw compressors, which manufacturers recommend are replaced as often as every 20,000 hours. Instead, the machines’ rotor shafts are supported by simple white metal bushes, ensuring long, inexpensive operating lives. Air-ends in Mattei’s rotary vane compressors can easily run for 100,000 hours without wear and in some cases, have lasted for more than 230,000 hours in the most arduous operating conditions.
Screw compressors
A screw compressor has two parallel rotors, which connect and intersect longitudinally with a critical minimal clearance, maintained by replaceable roller bearings fitted into a stator. During rotation, the profiles mesh into each other, compressing the air.
For a compressor to operate efficiently, the internal air leakage paths must be kept to an absolute minimum – because air lost during compression equals lost energy. If internal leakages are low, the volumetric efficiency is increased, and the power required for a measured unit of delivered air is reduced.
In comparison, screw compressors have an inherent ‘blow hole’, created at the point where the external profiles of the rotors meet, from where air under high pressure returns to the area of lower pressure. Screw compressor manufacturers have tried to reduce the ‘blow hole’, but it is impossible to eliminate it completely.
To compensate for the air leakage, screw compressors run at higher speeds averaging 3000rpm. In contrast, vane machines have always operated at low speeds, from a direct drive coupling, which in turn leads to lower power consumption.
The verdict?
Screw compressors may currently have a large market share but when it comes to energy efficiency, operating speeds, volumetric efficiency, wear and maintenance it seems vane compressors offer advantages. Vane and screw compressors produce the same end result but any comparison of the two technologies will affirm that, in almost all instances, vane compressors will operate more efficiently than their screw counterparts.
What’s more, advances in the technology behind the compression process in rotary vane air compressors continues to develop which means these machines, rather than screw designs will continue to get better and better in terms of energy efficiency.
For further information please visit: www.mattei.co.uk
