You can use screw type compressors for these ranges but they are expensive. Among many other trade-off's we do it is cheaper to buy a 7 bar compressor and regulate the pressure down. Indeed the energy is wasted in doing so. ■

Shenjie,

My choice would be to use screw compressors for 3.5 bar operation. They are easy to maintain and operate. You can compare their total costs to other types such as reciprocating or centrifugal, but you will find them more economical.

Regards,

Amrit T. Agarwal

Consulting Engineer

Pneumatic Conveying Consulting

Emailolypcc@aol.com

Ph and Fax: 304 346 5125 ■

Dear Shenjie

Using an oil filled screw compressor with load/ no-load regulation and pressure controlled oil separator is no problem at all for pneumatic conveying.

You can use a standard oil filled screw compressor, f.i. a mobile compressor as used on a building site, to supply work air.

Between the compressor and the pneumatic conveying installation, there must be a

Pressure reducer of sufficient air capacity.

The setting of this pressure reducer should be slightly higher than the maximum conveying pressure.

As reducing is a throttling process at constant enthalpy, losses are not that much.

BR ■

Dear Shenjie,

Compressor efficiency is a rather complicated issue.

It is thermo dynamics to the top.

Compressors can be : isothermal, adiabatic, isochoric or a combination of these.

Pressure ratio influences the efficiency (definition?)

A reducer after the compressor will of course reduce the efficiency, but as far as I can remember from my study (long time ago) is that reducing pressure is a throttling process, whereby the enthalpy is conserved and therefore not much energy is lost.

All the turbulence energy is converted into heating the air.

(although the air expands, it does not cool down)

Better check with compressor suppliers.

Good luck ■