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| A
screw compressor is most efficient when it is new. The clearances
are minimum, leakage is minimum and MEP (mean effective pressure)
is minimum. As the compressor wears the clearances enlarge. Leakage
of air to intake and to preceding pressure cells increase, which raises
the MEP. The effect is to reduce the air delivery and increase the
H.P. required to compress the reduced quantity of air. The vane compressor will not degrade with wear but will actually improve. As the blades seat in on the stator and rotor slots, leakage and power are reduced and the compressor stabilizes at a level 5 to 10% higher efficiency than when new. This level is maintained by the clearances being maintained at minimum throughout the life of the compressor wear. The rotors of the screw compressor are made of steel of a very easy machinable grade. This is because of the large amount of metal removal and numerous machining operations. This type of steel is prone to galling under marginal lubrication conditions. A pair of rotors that have been in service for less than 2000 hours will have gaps in the sealing lands on the O.D.’s of the lobes, and numerous scars on the flanks of the lobes. As a result of metal to metal contact during each and every start up, galling occurs that is the result of the welding that takes place between similar malleable metals without lubrication. The surface of the metal is not porous and does not retain oil for start up. The vane compressor has the rotor, stator and end covers of high quality cast irons. Cast iron is a superb bearing material because of the free carbon, hard iron carbides and porous surface which retains oil. There is no contact between these parts, only between them and the blades. If the blade is made of a material that is dissimilar and wear resistant then galling is eliminated and wear is reduced. The Davey Permavane blade is made of such a material and processed to produce a highly wear resistant blade. |
| It is impossible to produce lobe forms, which mesh perfectly throughout the cycle. During most of the cycle leakage paths exist, which allow air to escape from the H.P. zone to the intake. The leakage through these paths is constant regardless of speed, and the displacement of the compressor is a function of speed. The effect of this is that the percentage of leakage lessens at higher speed and increases at lower speed. This low speed characteristic will cause the screw compressor to absorb high H.P. at low rotational speeds. Because the vane compressor does not have these paths the torque demand of the compressor will decrease as the speed lowers. |
| The
space available for bearings in a screw compressor is limited by the
close spacing of the rotors. The bearings have to be a type to absorb
radial and axial loading, generally of an angular contact ball bearing
type, which has limited capacity in both directions. This is further
worsened by the high speed that these machines run at to obtain reasonable
efficiency. The effect of bearing failure is often complete destruction
of the compressor. The vane compressor has no limits on bearing size, and produces radial loading only. By using parallel roller bearings of sufficient size it is probable that the majority of vane compressors will use only one set of bearings throughout their life. |
| The auxiliary components of the vane and screw machines are similar. They both use an oil tank, separator, oil cooler, thermal bypass, minimum pressure valve, high temperature switch, and some means of controlling capacity. The maintenance of these items i.e. oil, filter and separator changes, would be similar. However, if repair is needed on the compressor alone then the screw compressor is more difficult and costly. In most cases it is handled on a factory exchange basis. The vane compressor can be serviced easily in the field. Typically the blades, which are the only wearing part, can be replaced in one to two hours depending on size. There are no critical clearances, which have to be measured and adjusted. |
| The failure of an air-end in a screw compressor can cost twenty to forty percent of the cost of a new unit. A set of blades for a vane compressor will cost less than ten percent of unit cost new. |
| When a screw air-end fails, a wait of several days or weeks is entailed because these are expensive and not carried by most dealers. Parts for the vane compressor are less costly and can be carried by dealers. |
