On Fri, 21 Dec 2007 12:37:00 -0600, RoyJ
wrote:
wrote:

Ok guys so I need some help. I have an aircompresser that is running
my waste oil burner. The burner needs very little air pressure. I
want to lower the start up pressure so that the compresser runs
longer when it is on and stays off longer. Does anyone know who makes
a pressure switch that is adjustable. Like maybe 35psi lower to 135
upper?

Basic thermodynamics says it is not efficient to run the compressor to
high pressure and then bleed it down to low pressure. Set the stock
pressure switch to the maximum differential (one of the screws), then
lower the max pressure to whatever is required to make the cut in
pressure correct (This is logically backwards, but that's the way the
switches work) This will mean the compressor runs more often but won't
work as hard when it does.

Your logic is sound, but there is another thing to consider - an
electric motor in severe duty (as on an air compressor) is usually
only rated for four to five starts an hour maximum. Even if it's
lightly loaded, as it would be on a 110-135 PSI single-stage
compressor derated to running at 35-50 PSI duty.

When you start an electric motor it's drawing "locked rotor" current
for a half second till it gets up to speed, and that builds up a lot
of heat in the interior of the motor windings. The heat only gets a
chance to start dissipating when the motor gets up to speed and the
cooling fan gets moving. Multiple short cycles per hour will quickly
burn up the motor - it's better to let it run than to have it stop for
three minutes and start again.

If he was going to do this the /right/ way, that really is the
proper job for a small diaphragm or "wobble piston" oilless air
compressor sized to the pressure needs and air consumption of the
babbington burner - say 1/4 or 1/6 HP. Have it start when the oil
burner lights off, and run continuously while the burner is lit. And
when it wears out they are rebuildable or disposable.

Another solution is a continuous-run compressor that leaves the
electric motor running and unload the compressor head for the short
lulls between calls for air. But that makes more sense on a 25-HP or
bigger compressor supplying a large plant than a 2-HP compressor at a
residence.

But if he doesn't want to do that, the large differential is second
best. Then the compressor only has to cycle on once or twice an hour.

-- Bruce --