On Sat, 25 Aug 2012 23:28:26 -0700, "Bruce L. Bergman (munged human
readable)" wrote:
On Sat, 25 Aug 2012 09:37:55 -0400, Stuart Wheaton
wrote:
In an effort to increase both storage and CFM capacity when running
several air tools, we obtained a second compressor.
I can tee the two outputs together, but I feel that I will want to do
something with the pressure cut-off switches to ensure that one
compressor does not end up doing all the work. Is there any reason not
to use one switch on one of two teed tanks to trigger the contactors for
both compressors together? Both tanks will have the factory pressure
relief and safety systems left intact.
First, this is a Small Shop strategy for 10 HP MAX compressors - if
you've got a Big Plant, you need to use small piston compressors as
backups and get either a pair of Screw Compressors or Big Piston
Compressors that can run in Continuous Run Unloader mode.
The power Demand Charges from starting and stopping 25 HP or larger
compressors many times a day will kill you - Continuous Run means they
start once in the morning, then you leave the motor running and just
cut the pump in and out as needed. The power utility will be pleased,
and so will your accountant. Okay, enough with the detour...
I would mount the two compressors side by side, and plumb them into
one main riser with separate shutoff valves to each compressor tank -
and a union in the plumbing so you can take one compressor out of the
system with the other one still up and running. Leave space for a
third compressor, and extra physical space so you can go several sizes
larger without rearranging all the furniture.
If they are 3-HP or under and use direct pressure switches, upgrade
them to Combination Magnetic Starters. You need Pilot Duty control
capability.
Put your pressure switches on a little manifold that can also be
valve isolated from the main output riser, set the switches 10 to 20
PSI apart, and wire them to an Alternator - The Higher switch will
call for First Compressor, and the alternator makes sure Compressor 1
starts first this time, then Compressor 2 starts first next time. If
the demand is heavy enough, the lower switch will kick the second
compressor and they both run at once - #2 will drop out when demand
catches up.
Old-school Mechanical:
http://www.hubbell-icd.com/icd/components/ac5247.asp
Electronic - with pilot lights and all:
http://www.time-mark.com/SearchResul...x?categoryID=6
And if you want to go really fancy, they make process controllers that
will monitor actual system pressure and cut them in and out as
programmed, but save that for when you win the lottery.
4 simple pressure switches - no unloaders, just accurate:
140 PSI All Pumps Off
130 PSI Leading Start
110 PSI Lagging Start
90 PSI ALARM - Buzzer and a flashing light.
Make the alarm obnoxious enough to get your attention - and tell the
folks in the Shop to knock it off for a minute till the air system
catches up to them. Most tools and actuators are fine till it drops
below 90 PSI, then things might get erratic.
If it gets so low it hits the Alarm regularly, you need more
compressor volume.
And if your control system blows up, you can go back to the pressure
switches on the individual compressors. Put toggle switches for
"Independent/Off/Remote" between pressure switch and starter.
I would also have an extra Inlet valve on that master manifold, or
better yet plumb it through the wall and put a Dixon Air-King /
Chicago / Claw style 1" compressor coupling port outside the wall
there.
If the entire system goes to chit on you, you run down to the local
rental yard and get an engine-driven Jackhammer Screw Compressor, park
it near that outside air inlet and let it run the place for the day.
And then AFTER that main manifold you can put a set of three
Bypass/Isolation valves for your refrigerated air dryer - same thing,
if it breaks you throw the valves and cut it out of the main loop.
And then you have shutoff valves for each of the lines headed out to
different areas of the plant, so you can shut one section down to work
on it while the rest of the place keeps going.
Ball Valves are cheap, extra pipe to make all the feeders into Ring
Feeds that can be split with more segment valves to just shut off one
area where the leak/break is at (so you can fix it with the plant up)
is cheap.
Downtime is not cheap. If it hits at the worst possible moment, it
can put you out of business. Machines stopping in the middle of a
cycle and ruining a batch of parts, expensive castings, etc.
Using Copper Pipe for all your air lines isn't cheap, but it avoids
problems with rust in the lines, and you don't have to play with a
pipe threader up in the rafters. Moves and changes are a whole lot
easier - Cut, Clean, Braze in a tee, DONE. Do it if you can afford
it.
And Silver Brazed joints is a clue to the Plumbers that's not a water
line, just in case they can't read the "COMPRESSED AIR" tags.
Don't mix black steel pipe and copper pipe, or you get galvanic issues
- one or the other.
And NEVER USE PVC FOR AIR LINES - it will shatter into shrapnel when
it breaks, and someone can get maimed. They do make special green
CPVC Pipe for chemical plant air lines, but it's Mega Spendy.
-- Bruce --
Excellent posting!
One thing I always always do...is install a check valve at the output
of each tank before it goes into the system. And a regular valve.
This prevents some problems that can arise with starting if your tank
check valve locks open among other things.
Gunner
One bleeding-heart type asked me in a recent interview if I did not
agree that "violence begets violence." I told him that it is my
earnest endeavor to see that it does. I would like very much to ensure
- and in some cases I have - that any man who offers violence to his
fellow citizen begets a whole lot more in return than he can enjoy.
- Jeff Cooper