On Fri, 06 May 2011 13:53:15 +0100, Roger Mills wrote:

On 06/05/2011 08:33, Bob Minchin wrote:

Aldi/Lidl type compressors I have seen already have a dump valve
arrangement fitted.



Mine has a ring-pull valve to de-pressurise the receiver - is that what
you mean?

It *might* work with the genny if I turn it on and off manually,
completely exhausting the pressure each time - but that wouldn't be very
efficient.

In normal operation the motor is controlled by a pressure switch with
some hysteresis between on and off pressures - so it cuts out when it
gets to max pressure, and turns it on again when the pressure falls a
bit (but still way above zero).

My experience is with big process gas compressors (a small drive motor was
around 300kW, a large one 8MW!) The typical way of starting such
compressors is to remove the load either by forcing the compressor's inlet
valves to remain open or by bypassing the discharge back to the inlet. For
an air compressor, you do not need to do this, instead, you just isolate
the receiver and vent the compressor's discharge.

Don't think of the compressor and its receiver as one unit, think of of it
as a compressor feeding a receiver via a pipe. A valve in that pipe can
isolate the feed to the receiver, preventing air escaping and at the same
time vent the compressor discharge so there is no back pressure. Once the
motor is up to speed, the valve can operate, closing the vent and
reconnecting the compressor to the receiver. This will allow the normal
stop start nature of small compressors, while starting the compressor with
virtually no back pressure and therefore little load every time.

SteveW