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Re: Air Compressor HP?

To: Shop Talk <shop-talk@Autox.Team.Net>
Subject: Re: Air Compressor HP?
From: Douglas Shook <>
Date: Mon, 08 Jun 1998 13:08:29 -0400
George Zhookoff wrote:
> A while back I started a round of talk on Puma Air Compressors.
>  Someone commented to watch out for Sears and the way they rate
> compressors.
> Well this weekend I borrowed a neighbors Sears 6 HP compressor.  It
> was better than my small 1.5 hp Campbell-H but it did not blow my
> socks away.  Is a DeVilbiss, Puma, any other 6 HP compressor better
> than a Sears, or is that as good as it gets?

George and Listers,

This horsepower rating business is getting a little absurd, if that is
even possible :) I have a household vacuum cleaner that draws 8.4 amps
at 115VAC (that really ought to produce about 1/2 horsepower) and is
rated at 4 HP!

Though there are differences in design efficiencies, RPM, motor frame
dimensions, service factors, etc., the key to the power a compressor
will deliver is the amount of power it consumes--the wattage, or if you
are comparing 220VAC motors, just the amperage.

A typical "6 to 6 1/2 HP," $400 to $600 compressor will use a #56 frame,
220 VAC, 15 amp motor with a 5/8" shaft and a 1.0 service factor. These
motors are compact (lighter duty) and weigh around 40-50 pounds.  Though
the pump efficiencies will vary, they will deliver about 9-11 CFM at 90
PSI, and 11-13 CFM at 40 PSI. These motors typically run at 3450 RPM and
make quite a racket (the "oilless" designs are the loudest).  They are
produced with the home user in mind, generally are single stage (max
pressure of around 125 PSI) and are good enough for most casual uses,
short of larger nozzle sandblasting. They run almost continuously under
steady use like painting or blasting.

The heavy duty 5 HP compressors typically use the larger 184, or 184T
frame motors (motors weigh around 80-90 pounds), consume 20-28 amps,
have a 7/8" to 1 1/8" diameter shafts and run at 1740 RPM.  They will
produce about twice as much output as the light duty "6 HP" models
(around 17-20 CFM at 100 PSI), are dual stage, and tend to not be so
annoyingly loud (heavier construction and lower speeds contribute to
this?). Because they usually have peak pressures of 175 PSI, they store
about 40% more air in a given tank size, are more efficient due to the
dual stage pumps used and will run longer with less maintenance than the
"6 HP," single stage units.  The only downside is they typically cost
about twice as much as the single stage units ($800-$1,200).

To get back to your question (sorry for the lengthy response), though
some compressor pumps are a little more efficient or longer lasting or
quieter (please don't disregard the noise factor if you are going to use
it regularly), there will be marginal differences in output from any of
the "6 HP or 6 1/2 HP" units as long as they are using the 15 amp
motors. These units really are not 6 HP (this rating, I believe is based
upon the power they can put out for a very brief timespan).

A basic approach would be to compare the amperage consumed and look for
heavy-duty, cast iron pumps. The V-pump used by Puma hypothetically
should run cooler and be better balanced than the single block, vertical
twin pumps used by many manufacturers. Ingersoll-Rand has been using
this approach for 60 years, and last I knew, were the largest supplier
of industrial compressors.

On the otherhand, Quincy makes some of the finest compressors around,
and they use the single block style for all of their models other than
the 40 CFM, 10 HP, three phase models.  They use very heavy construction
and low pump speeds (830 RPM) for many models.

To compare compressors, of course you need to look at the advertised CFM
ratings at comparable air pressures, but I look closely at the amperage
consumed (best measure of `actual power available to the pump), the
speed that the motor and pump turn (the slower the better for a given
CFM--less noise, less wear, etc.), and the weight of the compressor
(unlike automobiles or motorcycles, the heavier the better).


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