FEATURE COMPRESSED AIR
AIR COMPRESSOR DESIGN IS THE KEY TO UNLOCKING TRUE LIFECYCLE COSTS
With vane compressors, efficiency can actually improve over an initial running- in period. With no roller and thrust bearings to experience wear within the unit, from the moment a rotary vane compressor is turned on the compression unit undergoes a polishing process. The assembly consists of a single
offset rotor rotating within a cylindrical stator. The compression unit is sealed with two end covers that house two white metal bushings. The rotor has machined longitudinal slots, into which fit free sliding blades or vanes. The rotor is generally directly driven usually between 1000 and 1500 rpm (50Hz) causing the blades to make sealed contact with the stator wall thereby forming compression pockets. Air is drawn in, along the length of the
by Andy Jones, managing director at Mattei I
t’s clear that energy saving and energy recovery in industrial compressed air
systems is an important part of any plan to reduce greenhouse gas emissions. However, whilst most major compressor manufacturers have successfully reduced their specific energy over recent years, there is much more to compressor efficiency than meets the eye. Mattei has long argued that traditional
discussions fail to take into account changes in efficiency across the lifespan of a compressor – therefore, a true Life Cycle Cost (LCC) cannot be reached. Relying on the standard life cycle cost calculation does not factor in fundamental design issues and long- term wear and tear. As a result, buyers are being misinformed on the true running costs of units, preventing accurate analysis of energy usage. So, why is the design of vane and screw compressors so important?
WEAR AND LEAKAGE FLAWS In the case of screw compressors, tests have shown that efficiency deteriorates from the beginning of operation. The design of a rotary screw compressor consists of a pair of meshing helical lobed rotors. The rotor shafts are supported by roller and thrust bearings and generally one rotor drives the other by means of the helical profiles. Screw compressor manufacturers advise
the substitution of all major rolling and thrust elements after a certain number of operational hours – often between
8 SEPTEMBER 2018 | FACTORY EQUIPMENT
40,000 and 50,000. During rotation the screw profiles
uncover an intake orifice at one end of the stator, through which the air enters and fills the volume between the profiles. On the opposite side, the profiles penetrate each other, thereby reducing the volume, which compresses the air until the delivery ports are uncovered. Lubricant is injected to seal, lubricate
and cool the compressed air. The lubricant is subsequently removed in the reclaimer tank followed by a final coalescing element. The compressor is started and stopped through the system pressure switch set to the maximum and minimum settings. Within this design, the clearance path between the rotors and the end clearance on the high-pressure side are important for performance. However, these provide significant leakage paths between the high and low pressure working chambers, meaning the potential for leakage is very high. Whilst the size of the radial and
interlobe clearances is determined by the size and tolerances of the main compressor parts, the axial clearance is set during the machine assembly. And, given that the performance of a screw compressor is highly affected by leakage, any modification of the clearances is likely to have a significant impact on efficiency.
VANE EFFICIENCY In contrast, the engineering principles of a rotary vane compressor are very different to that of the screw equivalent.
Mattei
www.mattei.co.uk T: 01789 450 577
Buyers are being misinformed on the true running costs of units, preventing accurate analysis of energy usage
stator at the point of greatest volume, becomes trapped in the pocket and the volume reduced (pressure increased) through one rotation. At the point of smallest volume, air is discharged from the compression element (maximum pressure setting). Internally generated air pressure is used as the lubricant pump. The vane compressor has no axial
thrust pushing the rotor against either end cover. The axial clearance is set during the machine assembly, and, as the rotor is free to move axially, it is kept equally spaced from both the end covers by means of the lubricant which is injected, under pressure, through dedicated injection ports in the end covers, thus preventing contact and providing efficient sealing. As there are no wearing roller and thrust
bearings inside a rotary vane compressor, the manufacturing set clearances are constant throughout the lifetime of the compressor. There will never be any loss in volumetric efficiency over time, and the compressor will never require an overhaul to substitute the worn bearings. This allows Mattei to extend their compression unit (airend) warranty to 10 years with unlimited 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 LCC and energy efficiency to be reached. Whereas the current method of calculating lifetime costs may present both technologies as being similar in terms of efficiency and cost effectiveness, the reality is that the vane compressor can prove a far better choice in the long run.
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