Air Compressor Horsepower/Wattage/Amperage

wrote:
On Jun 18, 7:25 pm, GeorgeB wrote:

Based on the units with bigger motors, the HP is a marketing game, not
real. Industry assumes ... ASSUMES ... about 4 cfm (to 100 psi) per
horsepower. Small units will be less efficient ...maybe 3 cfm. Large
(50 hp+) units will be a little better, perhaps 4.4 or so.

It is often missed that capacity is INLET air.

This issue has been beaten to death on the rec.crafts.metalworking
discussion group. Everyone knows that Sears compressors are wildly
overrated. I was in the transportation air brake industry for years,
and me and my guys rebuilt about 12,000 compressors in that time and
tested every one of them on a dyno. We found that, as you have said,
that one HP will pump around 4 CFM. We had the cutout at 120 psi, but
of course, as also mentioned, there's nowhere near 4 CFM being
delivered at 120 psi. The 4 CFM is free air, at atmospheric pressure.
A really good compressor has as little volume as absolutely
possible when the piston is at TDC. This is to drive out as much of
the compressed air as possible; any air left in the cylinder at TDC
will expand as the piston travels downward again and so the intake
valves won't open until the cylinder pressure drops below atmospheric
pressure. A cheap compressor might have so much unswept volume that,
at the higher pressures, the intakes don't open until the piston is
halfway down. Not efficient at all. Unswept volume includes that
between the piston and head, whatever cavities the intake and
discharge valves may have, and so forth.
So the CFM rating is a zero discharge pressure, and it will
drop, depending on the efficiency and overall design of the
compressor, to considerably less as the tank pressure rises. You can't
take cylinder area and multiply it by stroke and RPM to get a reliable
CFM figure, but I think that's what the retailers do. You will be
disappointed if you have a 4 CFM spray gun and expect the 4 CFM
compressor to keep up with it. The spray gun requires 4 CFM at around
40 or 60 psi, the compressor is rated at zero.

Dan


Dan,

While I agree with almost all of what you have said, and you are right
on the mark, I have seen, and often, compressors that are rated at a set
pressure, ie... indicating a set displacement (in cfm) AT a pressure of
xx psi. Curtis and other old line commercial compressors are often rated
this way, cfm @ psi.

George

B A R R Y wrote:


Let's hope the tradition doesn't extend to airplane power plants. G

Oh don't worry, it will.

kontiki wrote:
B A R R Y wrote:


Let's hope the tradition doesn't extend to airplane power plants. G


Oh don't worry, it will.

Well, there was the 150 horse Franklin...

B A R R Y wrote:

Peter Dohm wrote:


(In the future, tools will be rated like sound systems.)



Let's hope the tradition doesn't extend to airplane power plants. G

Well, there was the 150 horse Franklin.

And the 60 horse VW.

Jim, this is interesting:

http://www.truetex.com/aircompressors.htm

(Sorry if it's a repost.)

Don

Thanks, Don. As you say, it is "interesting" but it doesn't speak to the
point. So far I haven't had an on-point reply. This link is technically
good as it tries to relate the impossibility of some advertized horsepowers
pumping out SCFM, but SCFM relates to electrical power not.

Forget I said compressor except for the calculation of wattage/amperage
required in a partially full compressor tank. All I need to do is to
rationally or experimentally match a generator with a motor that HAPPENS to
be connected to a compressor.

To hell with inflated horsepower claims, airplane engines, and the like.
Simple question. One horse compressor motor. How much peak
amperage/wattage to start it zero PSI in the tank, how much to keep it
running to max PSI of (say) 100 psi, and how much to REstart it when the
pressure drops to 80 psi.

Jim

--
"Dream as if you'll live forever. Live as if you'll die today."
--James Dean
"Don Tuite" wrote in message
...
Jim, this is interesting:

http://www.truetex.com/aircompressors.htm

(Sorry if it's a repost.)

Don

RST Engineering wrote:


To hell with inflated horsepower claims, airplane engines, and the like.
Simple question. One horse compressor motor. How much peak
amperage/wattage to start it zero PSI in the tank, how much to keep it
running to max PSI of (say) 100 psi, and how much to REstart it when the
pressure drops to 80 psi.



Jim: I'll check at first light and give you actual model numbers for
my old oilless.
I used to run it off a 4500watt/5000 watt surge genset until one of
the brushes in
genset let go and the compressor motor burned up while on a trip. The
compressor
would start under any load condition in the tank on the genset with
one exception,
and that was with over 100 psig in the tank and ambient air temp over
110 degrees
F. and having been used nearly continously for over an hour. At that
point, I
would have to bleed the tank pressure to under 100 psig toi get it to
start.

Craig C.

Jim: Here are the nameplate specs for the compressor that I have.

It's listed as a 4HP

Model 919.152921

125 psi max

120/240 V 1 phase 15/7.5 amps

Calif. Code 462 (L)(2) / Max RPM 3450

Hope this helps a little at least.

Craig C.

On Jun 19, 10:09 pm, "RST Engineering"
wrote:

Forget I said compressor except for the calculation of wattage/amperage
required in a partially full compressor tank. All I need to do is to
rationally or experimentally match a generator with a motor that HAPPENS to
be connected to a compressor.

To hell with inflated horsepower claims, airplane engines, and the like.
Simple question. One horse compressor motor. How much peak
amperage/wattage to start it zero PSI in the tank, how much to keep it
running to max PSI of (say) 100 psi, and how much to REstart it when the
pressure drops to 80 psi.

A belt-driven compressor relies on its flywheel to drive
it through a compression stroke. The electric motor produces an amount
of torque that probably won't be enough to run a compressor without a
flywheel, or a compressor that has been stopped and has to start
against pressure. For that reason, most compressors have an unloading
system that exhaust the discharge line to let the compressor spool up
before it has to work against pressure.
I've tried to measure peak amps on a starting motor, and it
isn't easy. The mass it is driving affects it a lot, and even with no
load a squirrel-cage motor draws a bunch, maybe double its running
current, but it's so brief that the circuit breaker puts up with it.
Running off a generator means either a limited current supply that
causes massive voltage drop on start, so that the motor won't come up
to speed. And if you are starting against 80 psi with a non-unloaded
compressor you will need both a BIG motor and a BIG generator.
Sorry I can't give you numbers. Most of us won't be able to. There
are too many variables and I don't have even the basic info. Even the
motor type affects it: capacitor or non-cap start, compressor size and
mass, on and on. What is the motor rated to start? A low load like a
jet pump or table saw, or a heavy load like a big fan? I built my own
compressor years ago using an old truck compressor, and drove it with
a 1-horse 3450 RPM jet-pump motor. This motor has next to no starting
torque, so the compressor is set up to run continuously and the
original unloaders in it are engaged when tank pressure is reached. I
have to manually unload it to start it, and shut it off when I'm done.
Truck compressors have a device that hold the intake valves open when
the tank is full so that it can't compress anymore. The crankshaft
keeps turning.

Dan

wrote in message
oups.com...
I built my own
compressor years ago using an old truck compressor, and drove it with
a 1-horse 3450 RPM jet-pump motor.
Dan


How did you supply oil pressure to lube the crankshaft? or did this
compressor have its own closed sump and pump or splash lube system?

TIA