Subject: Re: Air Compressor Etiquette

From: [email-masked] (Martin Schuessler)
Date: 2 Oct 1995 19:09:55 GMT

In article <[email-masked]>, [email-masked] (Hal H. Bybee, Jr.) writes:
>Roger Hoover ([email-masked]) wrote:
>
> > Being new to the world of compressed air, I have a few
> > questions on proper etiquette:
>
> > 3) Is it okay to leave the tank pressurized all the time, or
> >    should I take it down on some interval (daily? weekly? other?)
>
>Just like with SCUBA tanks, if you are going to leave the tank
>unused for several weeks or months, let it down to a low pressure
>(15-20 psi) so that it is harder for more water to form if the
>temperature drops.  This is a bigger concern than the metal
>fatigue due to raising and lowering the pressure.

You never want to drop the pressure in a scuba tank to 15-20 psi. It's impossible
to gauge when you get that low (3000psi is normal), and if you empty your tank
you'll cause corrosion problems from moisture in the air. Air used for scuba
diving is completely dry and filtered. 

A compressor uses humid air, which is why you want to drain the condensed moisture
out of it before you cause corrosion problems.

Martin Schuessler
PADI Divemaster #59146

---

From: Neil Surch <[email-masked]>
Date: 2 Oct 1995 02:43:37 GMT

[email-masked] (Roger Hoover) wrote:
>1) I've been told to drain the water out of the tank every
>   day I use the compressor.  Is it safe to do this at any
>   pressure?
>
>2) The space under the drain valve is kind of tight.  What's
>   a good way to collect the water so you don't have to empty
>   a little pan lots of times and so you don't get water all
>   over the floor?
>
>4) Is it okay to leave hoses pressurized for long periods of
>   time?
>
>5) Quick connect couplers have a lot of kick at 175psi.  Is
>   there a way to control this when disconnecting?
>
>roger
>[email-masked]

Well Roger,

  First I would not try to drain a tank with a pressure higher than say 20 
PSI, especially if you have a tank with just a plug as the plug will fly 
out real fast (not to mention scare the hell out of you).  Secondly visit 
your plumbing store and get an elbow with a big tap or ball valve and 
attach it to the bottom.  This way when you are finished for the night you 
can drain the tank easily (like in about 2 minutes).  

 2.  With a valve or tap installed you can collect the water easily.  If 
drained daily the accumulation will be small (but dirty).

 3.  I try and leave my tank almost empty if not empty of air it just 
relieves the stress on the pistons and seals not to mention anything else 
connected (i.e. question 4 hoses).

 4. see 3.

 5. Hold both ends of the coupler as the female coupler check valve closes 
it kicks the male end out, and the built up pressure in the air tool also 
flows out in the opposite direction.  Another note most tools work within 
the 90-100 PSI range so the hose should not be at 175 PSI.  Spray guns use 
about 20-40 PSI so unless you are using something like a sandblaster or 
blow gun I would not recommend 175PSI for anything.  The main reason 
compressor go to 175 PSI is to increase the volume of air contained in a 
tank,  I read somewhere that a 175 tank holds 2 to 3 times the volume of 
air a 150 PSI tank does with the same capacity (60 gal.).

Anyway I hope this helps you out.

   Neil Surch

---

From: [email-masked] (Insect God)
Date: 2 Oct 1995 19:56:30 -0400

Thanks all who have written on this thread so far - it's been highly
informative.

In other news, here's my QOTD.  I think I'll need a compressor in the next
year or so, and the highest-priority item it will run is a bead-blasting
cabinet for objects up to 24" in size.  It seems from reading catalogs that
I'll need 12-14 cfm @ 100 PSI for a reasonable unit.

However, if "SCFM" is the same as CFM, MSC doesn't list any that will do
that.  Can this be? 

And what's a good brand?  This is one tool that I'd like to buy only once.

Newsgroups trimmed to r.c.m., since I don't read the others.

-Sheba

---

From: [email-masked] (John Porter)
Date: 2 Oct 1995 14:36:30 GMT

Roger Hoover ([email-masked]) wrote:
: 5) Quick connect couplers have a lot of kick at 175psi.  Is
:    there a way to control this when disconnecting?

You can get couplers that let you relieve the pressure before you
mechanically disconnect the hose. They're compatible with the
"industrial interchange" pattern for the plug end. Grainger's has
them. Probably a good idea if you're using unusually high pressures
or large diameter couplers or have a lot of volume to relieve. They
also make inserting the plug easier.

--
John Porter
[email-masked]

---

From: Mike Schechter <[email-masked]>
Date: 3 Oct 1995 17:31:48 GMT

In article <[email-masked]> Andy Dingley,
[email-masked] writes:
>The extra amount it will hold illustrates a simple scientific
>principle that is worth knowing about. For any given quantity of gas,
>the value (P * V / T) will remain constant. P is the pressure, V the
>volume and T the temperature. 

and what all this means, is that as you increase pressure the temperature
will rise.
if you do it very quickly, it will rise faster than the container holding the
gas will dissipate it..
likewise, when pressure drops quickly, it will get cold..

---

From: [email-masked] (J. Kimberlin)
Date: 3 Oct 1995 15:56:38 -0700

>I'll need 12-14 cfm @ 100 PSI for a reasonable unit.

Sounds about right, IMHO.

>However, if "SCFM" is the same as CFM, MSC doesn't list any that will do

SCFM means cubic feet per minute at 29.92" barometric pressure and 60 
degrees F temperature.  If you are at a different altitude your produced 
volume may be less than stated.  The temperature is less of an effect, 
but the colder the air, the more CFM you get.

>And what's a good brand?  This is one tool that I'd like to buy only once.

Any good industrial  air compressor such as Ingersol Rand, Gardner 
Denver, Sullair, Curtis, Kellogg, and others.  5 hp should do it fine 
but 5 hp in the imports won't hack it at all.

>-Sheba

JerryK

---

From: [email-masked] (Andy Dingley)
Date: Mon, 02 Oct 1995 22:41:26 GMT

Neil Surch <[email-masked]> wrote:

>I read somewhere that a 175 tank holds 2 to 3 times the volume of 

Well the _volume_ it will hold will be exactly the same, as a gallon
is a volume measure. It will hold a greater _mass_ of air though (and
thus a greater volume of room pressure air).

As it's a simple ratio, this constant relationship will hold for any
units, whether imperial or metric. The temperature does have to be in
a unit like Kelvins though, which has its zero at absolute zero,
rather than at an arbitrary temperature like the freezing point of
water. To convert, just add 273 to the Celsius temperature, i.e 0C =
273K, 17C = 290K, etc.

There's a more complex statement of this relation, of the form 
      PV = nRT
where n is the number of gas molecules, and R is the Boltzmann
constant (the same for all gases, everywhere). Another relation
(Avogadro's Law) shows that the mass of a quantity of gas is equal to
the mass of each molecule, multiplied by the number of molecules. As
the masses of molecules can be simply calculated from the mass of
their constituent atoms, and the masses of these atoms are listed in
standard tables, then it's easy to calculate the volume of a mass of
any gas, at any temperature and pressure, simply by knowing its
chemical formula. Any standard scientific textbook can explain
further, if you want the details.

---

From: [email-masked] (Chris Chubb)
Date: 4 Oct 1995 13:22:22 GMT

Lots of good things being said about air copmpressors here:

But, I have a slightly different question.

I want to buy my first compressor. It needs to be portable,
and it needs to be cheaper than $500. 

I want to do painting, impact wrench, sander, some light sandblasting
with it. Mabye some house painting, etc.

I have been looking and it looks like a 5 HP compressor with a 
20 gallon tank should cover it.

I have looked carefully at the Craftsman and the Campbel-Hausfield (sp?)
compressors. The Craftsman compressor hos a slightly higher airflow
rating, (11 SCFM@90) is oilless, and direct drive. 
The C-H compressor has a slightly lower rating (9.5 SCFM@90PSI)
but boasts as cst iron pump and belt drive.

Some questions that nobody (At the store) seems to be able to 
answer:

Cast Iron:If the pump is not cast iron, what is it, aluminum? Is cast iron
going to last much longer than the alternative? What does it get
me for the higher price.

Direct Drive: Other than compactness, what is the difference between
directr drive and belt drive, other than the belt? Are the belts
quieter or more noisy?

Oilless: My engineering training makes me cast the hairy eyeball on 
something that says 'lubed for life.' Without the oil, I should not
need a seperator if I want to paint, etc. But, a seperator is not
that expensive. Does the oil let the pump last longer, run cooler,
what? Or, does it just make up for the shortcomings of cast iron?

The C-H compressor seems much heavier. I think the tank walls are 
stronger, and the pump is definately heavier. It is also about
15% more expensive, and about 10% less powerful (by the specs).

Is there another widely availible compressor that I should be 
looking for? I only want to spend the money once, for a loooong
time, but I am limited to about $500 including a seperator, drier,
etc. that I need to make it functional.

Respond here, but please cc: by e-mail because my news feed
runs faaaar behind.

Thanx for your attention.

PS: I drain the water from the tank each day (under pressure)
and remove the pressure in the hose, but leave it in the tank
by zeroing the hose pressure dial. (On my Dads compressor)

-- 
Chris Chubb -- [email-masked] (Alexandria, VA)
** Leading a life of noisy desperation. **
My opinions and $0.75 will get you a cup of coffee.

---

From: [email-masked] (gary preckshot)
Date: 3 Oct 1995 17:50:26 GMT

In article <[email-masked]>, [email-masked] (Insect God)
wrote:

>....  I think I'll need a compressor...[for]... bead-blasting
......

> However, if "SCFM" is the same as CFM, MSC doesn't list any that will do

yes.

SCFM = CFM at STP (remember chemistry?).  It's the volumetric pumping rate
referred to intake air at STP.

And yes, MSC (I've never seen their catalog) may not offer a compressor in
the 12-14 cfm range.  This is getting up to industrial size, about where
you'd do better to get a 2-stage.

> And what's a good brand?  This is one tool that I'd like to buy only once.

Good idea.  Look for a Quincy, Puma, or C-H 5 hp 2-stage industrial unit
with a cast iron pump and continuous duty rated motor.  With an 80 gal ASME
tank, these units weigh over 600 lbs, but the reliability and capability is
worth it.  Don't go by brand name, because all of the good names have
prostituted themselves to get market share in the mass market.  If you want
a good machine, go with the specs.  Most of the good names, including the
ones I mentioned, make machines in the industrial range.

---

From: "John R. Johnson" <[email-masked]>
Date: Tue, 3 Oct 1995 15:55:54 -0500

They call them double cut off couplers and they are readily available
>from  Hansen as well as a number of other commercial coupling makers.
You probably will NOT be able to get double shut off couplers from your
local hardware store.  They are normally only used for industrial apps.
John

On Sat, 30 Sep 1995, JT wrote:

> Andy Dingley ([email-masked]) wrote:
> 
> : You should hold both parts of the coupling when disconnecting. A lot
> : of the kick comes from the "dead space" inside the coupling still
> : having pressure in it when disconnected. Some brands of coupling have
> : smaller volumes than others. You can even get "instrumentation"
> 
> Maybe I'm missing something here, but all of my couplers only seal
> on the female end.  The "dead space" in all of my stuff is anything 
> outboard of the disconnection.  This could be 50 feet of hose or
> more.  It can be quite a kick if he was really running 175 psi.  I
> personally don't run that much pressure but it still can get away
> from you if you dont hold on tight.
> 
> Unless Andy has couplings that seal on both the male and femal end,
> which I dont believe are commonly available.
> 
> YMMV
> 
> James
> 
> 
> 
> 
> -- 
>                                              [email-masked]
> 
>