Subject: Re: Sandblaster - Pressure vs Siphon

From: "John R. Johnson" <[email-masked]>
Date: Wed, 4 Dec 1996 11:33:44 -0600

On 2 Dec 1996 [email-masked] wrote:

> In article <[email-masked]>, [email-masked] (Fitch R.
> Williams) writes:
> 
> >I also checked the current rating on my 20+ year old 2 HP
> >oil lubricated Sears compressor - 11.0 amps at nameplate
> >minimum voltage of 230V.  The newer compressors have *magic*
> >motors that can get 6HP our of 15 amps.  I wonder if they
> >have *magic* cfms also?  Maybe you need to move to high
> >a
> 
> This has been a curiousity to me too. I always 'believed' there one horse
> power equalled 745 watts. At 110v that should be around 6.77 amps so how
> can my 'small' compressor have a 3hp motor running on 220 volts with a
> rated 7.5 amps? My trusty calculator says it is 2.2hp.
> 
> The 'big' compressor runs on 220v at 15 amps and is claimed at 6.5 hp. But
> my calculator gives me just 4.4hp????
> 
> Maybe the American horses are smaller and cuter than the rest of the
> world?
> 
> Peter
> 

Sears Roebuck started the trend.  People want more power!  Actually, 
one horsepower is 746 watts according to my physics text.  In practice,
I have always figured twice that for input to get 1 HP output.  

Checking against some of my single phase motors, I find that my 1 HP
Gould motor draws 7 amps at 240 volts.  That is pretty close to 2 HP
input to give 1 HP output.  I see also that my Sears Roebuck Compressor
that is rated at 5 HP also draws 7 amps at 240 volts!  Interesting.
Also the CFM rating is appropriate for a 1 HP compressor and NOT a
5 HP compressor!  Interesting.

I got curious about these ratings and was told that they are 
"peak power" ratings.  They measure the stalled current, multiply it
by the voltage and divide by 746!  Very useful rating.  Your equipment
can run at that level for from one to three seconds before it stops
permanently!

John

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From: Tom Krause <[email-masked]>
Date: Wed, 04 Dec 1996 12:35:53 -0600

> can run at that level for from one to three seconds before it stops

Sears is good at doing this with their vacuums also,big
horsepower/amperage ratings for motors no bigger than your fist!
I told a Sears salesman once that I figured that a motor only the size 
of my fist that draws 7amps was most likely a piss-poor design/very
inefficient motor.Boy ,he didn't think much of that statement!
 Has anyone actually put a current meter on one of these machines?
HMMMMMMMM!
Tom

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From: Robert Bickers <[email-masked]>
Date: 3 Dec 1996 00:01:38 GMT

Fitch, that was almost more than I wanted to know about turttles, high vrs low altitudes, at 
higher levels I would expect the compressor to run longer, but be loaded lighter[ thinner air 
= easier to compress], at the Dead Sea it would have its work cut out for it. Cheers Bob

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From: Mark Winlund <[email-masked]>
Date: 4 Dec 1996 03:14:15 GMT

You've got to remember there are "copywriters horses" and then there are the horses 
that the rest of us ride! The former are much more numerous and profitable; hence 
the preference for them in advertising.

Regards, Mark

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From: [email-masked] (Filip M Gieszczykiewicz)
Date: 5 Dec 1996 14:56:47 GMT

In Article <[email-masked]>, through puissant locution, [email-masked] (James P. Riser) soliloquized:
>In article <[email-masked]>, Robert Bickers
><[email-masked]> wrote:
>> The only reason I know to cool compressed air is to condence water.
>> After coolers are well worth the trouble if you are having moisture problems.
>
>Right.  Warmer air can hold more moisture.  Cooler air can hold less.  If
>you cool the air in your lines, the water vapor will condense out and can
>be drained off.

Greetings. I would imagine that it would be better to remove the water
before it gets into the compressor (when it's at STP). A possible
solution would be to strip off a outer casing of a small-ish de-
humidifier and constuct a sheet-metal funnel/tunnel - such that
you would have a cylinder that would encompass the chilled coils of
the dehumidifier and then would taper off and go into the compressor
intake. Some scheme of removing the condensed water off the coils
would have to be developed but I would consider just the effect of
cooling the air some xx degrees would be a bonus. These suckers go
for ~$100 new and if I had more than the little cheezy tank-less
oiless, I might consider it. Anyone?

Take care.
-- 
+-->Filip "I'll buy a vowel" Gieszczykiewicz | E-mail: [email-masked]
| http://www.paranoia.com/~filipg/ |SCI.ELECTRONICS.REPAIR FAQ + LOTS MORE!
| Enjoy your job, work within the law, make lots of money : Choose any two.
| I think for myself. I listen. I make decisions. I speak what I believe.

---

From: [email-masked] (Fitch R. Williams)
Date: Fri, 06 Dec 1996 06:28:56 GMT

>= easier to compress], at the Dead Sea it would have its work cut out for it. Cheers Bob

Yup.  That about says it.  If you'd written the handbook, I
wouldn't have a head ache.

Was more than I wanted to know too, just couldn't seem to
stop till I had it figured out.  I hate being outwitted by a
few cfms and psigs - on the surface they don't seem more
complicated than a deep breath, but if you dig ....  bettter
idea, don't dig.

Fitch
Considering a new rule - If you find yourself in a hole, the
*most* important thing is to stop digging.  yeah ... thats
it.