I have a trusty old Webster two cylinder compressor which runs great, bought
used after it had a useful career supplying air to drive tools in a dentists
office. It's on a portable type 20 gallon tank, and I have a 2 hp motor on
it ( probably what it needs, it wouldn't always start when it had a 1 hp
motor). I'm going to measure it's CFM today, using the method gathered from
a link on another thread.

I have in mind putting a 60 - 80 gallon tank under it, since the original
tank is so small and very old. I expect that the cycle time will be a lot
longer, both between cycles and for pump-up time. Generally, would putting
a bigger tank on be a good thing or a bad thing? any significant down-side?
Is there a rule of thumb for sizing tanks to compressor head capacity?

Brian

Brian

Brian writes:

Generally, would putting
a bigger tank on be a good thing or a bad thing? any significant
down-side? Is there a rule of thumb for sizing tanks to compressor
head capacity?

As I explain (see http://www.truetex.com/aircompressors.htm), "the tank
should be sized to be at least 1 gallon of volume per CFM of the
compressor". Bigger is OK, costs more, more overhead, but lets you run
tools bigger than your continuous capacity for short bursts, and cuts down
cycling for lesser loads.

Most systems have some slow leaks, and if your use is only occasional, then
you wind up refilling a huge tank and slowly leaking it out just to get a
puff of air for something now and then. The ideal would be to have a bank
of several sizes of tanks with valves so that you can select an optimal
aggregate tank size for the tasks at hand. But if like most you have just
one tank, one compromises with something a little bigger than what
optimizes the average task.

I've got a 60 Gallon tank under a 2HP compressor. Yes, it takes a while to
pump up and 'recharge' times are long however it also means the contactor
has NEVER been replaced and the whole unit seems 'happy'.
I put a bigger tank on my water system too.
It's better to have the motors run constant every once in a while than
on-off-on-off all the time.

hth
jay

A good rule of thumb is to size your air PUMP according to your largest
need. You should buy a big enough air pump so that your pump can run at no
larger than 50% duty cycle when running your largest air tool, sandblaster etc.
How big your air tanks are is a factor of how much power you want to waste on
small jobs or how much capacity you want to increase air supply for a short
time for peak loading.

When I found out the above I was bummed. I had always wanted a bead blaster
and the ability to run big air tools. Now I realize I'd need a HUGE compressor
to do these things. The solution is actually simple - buy a trailer-mounted
compressor that runs on gas or diesel, or rent one as needed. Those can often
supply air sufficient to run several huge jackhammers simultaneously. Then, in
your shop, put in a much smaller unit. I have found a 5 hp. 2-stage Quincy unit
to be enough for most shop uses. It wasn't enough for a 9" air grinder, so off
went the air grinder. I don't do blasting at home either. Sigh.

Grant

Brian wrote:
I have a trusty old Webster two cylinder compressor which runs great, bought
used after it had a useful career supplying air to drive tools in a dentists
office. It's on a portable type 20 gallon tank, and I have a 2 hp motor on
it ( probably what it needs, it wouldn't always start when it had a 1 hp
motor). I'm going to measure it's CFM today, using the method gathered from
a link on another thread.

I have in mind putting a 60 - 80 gallon tank under it, since the original
tank is so small and very old. I expect that the cycle time will be a lot
longer, both between cycles and for pump-up time. Generally, would putting
a bigger tank on be a good thing or a bad thing? any significant down-side?
Is there a rule of thumb for sizing tanks to compressor head capacity?

Hi Richard. I went out to the shop and timed my air compressor, using your
method, and I got between 6 and 7 CFM from 80 psi to 120 psi. It's on a ten
gallon tank. From what you said, that's about the right size, and I might
gain a bit by going to a 20 or 25 gallon tank depending on my typical
usage - big air or little air. I use it mostly for smaller jobs, and once
or twice a year I get into a big usage situation for a day or so.

Maybe I'll just think about this for a while longer. It seems I could go to
a 1.5 hp motor, and regain the flexibility of running it on 120 volts,
though. It came to me with a 1 hp motor that was fine in warm weather but
couldn't reliably start it in below zero temps.

Brian
"Richard J Kinch" wrote in message
. ..
Brian writes:

Generally, would putting
a bigger tank on be a good thing or a bad thing? any significant
down-side? Is there a rule of thumb for sizing tanks to compressor
head capacity?

As I explain (see http://www.truetex.com/aircompressors.htm), "the tank
should be sized to be at least 1 gallon of volume per CFM of the
compressor". Bigger is OK, costs more, more overhead, but lets you run
tools bigger than your continuous capacity for short bursts, and cuts down
cycling for lesser loads.

Most systems have some slow leaks, and if your use is only occasional,
then
you wind up refilling a huge tank and slowly leaking it out just to get a
puff of air for something now and then. The ideal would be to have a bank
of several sizes of tanks with valves so that you can select an optimal
aggregate tank size for the tasks at hand. But if like most you have just
one tank, one compromises with something a little bigger than what
optimizes the average task.

Richard J Kinch wrote: (clip) The ideal would be to have a bank of several
sizes of tanks with valves so that you can select an optimal aggregate tank
size for the tasks at hand. (clip)
^^^^^^^^^^^^^^
If the tank you have now is not unsafe, you can achieve the above goal by
adding a tank of either 10 gal or 40 gal capacity, with a couple of valves.
The former would give you a choice of 10 gal, 20 gal or 30 gal. The latter
would give you a choice of 20 gal, 40 gal or 60 gal.

Leo Lichtman wrote:

Richard J Kinch wrote: (clip) The ideal would be to have a bank of several
sizes of tanks with valves so that you can select an optimal aggregate tank
size for the tasks at hand. (clip)
^^^^^^^^^^^^^^
If the tank you have now is not unsafe, you can achieve the above goal by
adding a tank of either 10 gal or 40 gal capacity, with a couple of valves.
The former would give you a choice of 10 gal, 20 gal or 30 gal. The latter
would give you a choice of 20 gal, 40 gal or 60 gal.

Plus the second tank will help get moisture out of the compressed air. Also,
having valves to shut off the flow will reduce the usage of the motor and pump.
I have my compressor on an 80 gallon tank, followed inline by a 120 gal with
ball valves between the two and after the second tank. Closing those at night or
when leaving the shop for a few hours makes a major difference in running time
to fill them. With a couple very small leaks at the tanks, it takes about 2
minutes to bring them up in the morning. When I forget to shut the valves, it
takes a lot longer. Only under very heavy usage in warm weather do I get a hint
of water when doing a major amount of continuous die grinding, for example. And
that only rarely.

michael

Depends a little on whether or not you keep air up all the time or
not. If you don't, then (as Richard points out) you have the delay
of pumping up a larger tank before you get useful air. If you do,
then that isn't a factor. In that case, the fewer fittings and valves
the better because leaks are usually at fittings.


On Mon, 08 Mar 2004 09:28:05 -0600, Richard J Kinch
wrote:


As I explain (see http://www.truetex.com/aircompressors.htm), "the tank
should be sized to be at least 1 gallon of volume per CFM of the
compressor". Bigger is OK, costs more, more overhead, but lets you run
tools bigger than your continuous capacity for short bursts, and cuts down
cycling for lesser loads.

Most systems have some slow leaks, and if your use is only occasional, then
you wind up refilling a huge tank and slowly leaking it out just to get a
puff of air for something now and then. The ideal would be to have a bank
of several sizes of tanks with valves so that you can select an optimal
aggregate tank size for the tasks at hand. But if like most you have just
one tank, one compromises with something a little bigger than what
optimizes the average task.