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Feature Compressed air
The taming of the screw
When specifying an industrial compressor, the choice is
usually between a vane or screw machine – but what are
the differences between the two? Andy Jones, general
manager of Mattei, puts forward the case for vane
compressors, with a focus on energy efficiency, operating
speeds, wear and maintenance and oil carryover
T
here are two types of industrial between the rotor, stator and end
rotary compressor available – covers are also efficiently sealed by
The Maxiima rottary vane
vane and screw – and, despite the injected oil.
compressors are suiittablle
fact they are designed to produce In comparison, screw compressors
ffor iinsttallllattiions where
the same end result, they differ quite have an inherent ‘blow hole’, where
compressed aiir demand iis
considerably.
consttantt and conttiinuous
A rotary vane compressor com-
prises a rotor with longitudinal slots, is supported by simple white metal
which house individual sliding bearings, which ensure a long and
vanes. The rotor is offset within a inexpensive operating life.
stator (a cylinder), in which it rotates. In comparison, oil-injected screw
While the rotor turns on its axis, the compressors cannot offer the same
vanes are pushed against the stator levels of longevity. Because the rotors
wall by centrifugal force, and the air have to operate with high thrust and
is compressed. radial loads while maintaining mini-
Meanwhile, a screw compressor has mum clearances, screw compressors
two parallel rotors – a male and a use roller bearings by necessity. Screw
female – which engage into each other. manufacturers recommend these bear-
These two rotors intersect longitudi- the external profiles of the rotors meet.
Andy Jones argues
ings are replaced as part of the mainte-
nally with a critical minimal clearance, The air under high pressure returns to
the case for vane
nance schedule as early as 24,000
and in turn are fitted into a stator. the area of lower pressure through this
compressors
running hours, but the cost implica-
During rotation, the profiles mesh into hole. Manufacturers of screw compres- tion of this is often overlooked at the
each other, compressing the air. sors have tried to reduce the ‘blow time of purchase.
hole’ effect, but it is impossible to
Common misconceptions eliminate it completely. Addressing oil carryover
Because screw compressors have To compensate for these air leakage Finally, there is oil carryover to con-
larger market share, there is a common paths, screw compressors need to run sider. Many years ago, vane compres-
misconception that vane technology is at high speeds. In contrast, vane sors were criticised for their relatively
the older, more outdated of the two. machines have always operated at low high oil carryover, but this has been
However, in reality, modern vane and speeds, which in turn leads to lower addressed, with vane technology being
screw compressors were both devel- power consumption. Some of Mattei’s at least as good as the best screw
oped during the 1960s and 1970s, and, fixed speed compressors operate at design. For example, in Mattei com-
in recent years, advances have been 1,000rpm, whereas the average screw pressors the oil quantity carried over is
made in vane technology. compressor needs to work at 3,000rpm less than three parts per million (ppm)
The widespread use of screw com- to achieve the same result. as standard. Oil content can be further
pressors has more to do with the Another important factor to address reduced to less than one ppm with fil-
machinery being easier to replicate is wear and maintenance. Providing a tration – so these machines can be
than technological advantage, and, machine is looked after in accordance used where an ‘oil-free’ compressed
when choosing between the two, a with the manufacturer’s instructions, air supply is required.
number of factors should be consid- the operating life of a vane compressor Screw compressors might currently
ered – in particular energy efficiency, is virtually unlimited, and mainte- have a large market share, but it
operating speeds, wear and mainte- nance is minimal. An air end can shouldn’t be assumed they are supe-
nance and oil carryover. easily run for 100,000 hours without rior to vane compressors, or that they
In a compressor, internal air leakage wear – in fact, some Mattei machines are a more modern solution. Vane
paths need to be kept to an absolute have lasted over 230,000 hours in machines can outperform their screw
minimum; air losses equal lost energy, arduous operating conditions. counterparts, especially in terms of
and they increase the power required There will only ever be negligible energy efficiency, operating speeds,
to deliver a measured unit of com- wear on the vanes, because the use of a wear and maintenance and oil carry-
pressed air. quality lubricating oil means there is over, and Mattei is actively promoting
With a vane compressor, internal air no direct metal-to-metal contact these benefits with the aim of redress-
leakage is minimal. The vanes are between the vanes and the internal ing the balance.
always in contact with the internal stator wall. Most significantly, a vane
surface of the stator (albeit not compressor does not require roller
Mattei
directly, due to lubrication), thus the bearings as used in a screw compres-
T: 01789 450577 Enter 202
air seal is near-perfect. The clearances sor. The vane compressor’s rotor shaft
www.mattei.co.uk
Process & Control JUNE 2009 9
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