Dual Compressors

[Earl Kelly]

OK, I know there are some whiz bang engineers around here, so I have a question. I have 2 compressors a 5hp and a big 3hp. I just bought the 3 hp to use while I rebuild my 20 yr old 5hp. But after I get it rebuilt I would like to hook them up together, so I could use the 5hp as my main compressor but have the 3hp kick in when I need that extra air when sanding a lot( i love my dynabrade RO sander).
I saw a comp on ebay, it was an IR that had 2-5hp pumps and motors set up for this situation. One was the main pump and the other would kick in when the load increased. And yes it was huge, had a 120 gal tank.
Tying the comp together in the air line isn't a problem, but figuring how to control when they come on is. If I set both pressure switches to the same limit then, unless, one cannot keep up, the other would not see the lower pressure and kick in.
It'll be awhile before I'm ready to do this but I thought I'd pick some brains here. Any suggestions or tips would be appreciated.

PS- the power co is gonna love me when I get this hooked up and running.

[Tyler Howell]

I was thinking (yes I know that is dangerous) Just gang them together and set the 3HP at the preasure you want it to kick in. If you want to get fancy, plumb in some Isolation valves so you have the flex to change if you want.;)

This is kind of OT but in this thread I did a test with extra storage to my satisfaction. If two compressors is not the solution just the tank of your 2nd AC will increase your work time. Good luck.

http://www.sawmillcreek.org/showthread.php?p=75725#post75725

[Ken Garlock]

Earl, your idea sounds like a project that Tim Allen would be proud of :)

The question is, will you ever need the 3hp unit. For example I have an I-R 5hp, 60 gal, 2 stage compressor which will produce 15 SCFM per minute at 175 PSI. Your 5hp is probably in the same neighborhood.

Most air tools I have seen like to operate in the 90 to 100 PSI area. This means that you will obviously need a pressure regulator to get down to 100 psi. That also means that you will be able to use more than 15 SCFM at that 100 psi. If we assume your 5hp compressor will produce 15 SCFM at 150 psi, simple ratios of pressures, and volumes yields your compressor is capable of producing 22.5 SCFM at 100 psi. X = (15*150)/100) X= 22.5.

The question now becomes, how many sanders can you run at 100 psi, not whether you need a second compressor. So long as your total air usage is not more than , say, 20 SCFM @ 100 psi, you have no problem with the 5 hp compressor.

[Earl Kelly]

Ken, the sander eats up about 17cfm@100psi. The 5hp will run it but it will only get to take a break for a few minutes out 30-60. It's working really hard. If I didn't already have these I would have just bought a new 7.5hp. I know that would keep up. Yep the 5hp is right on the edge of keeping up with time to cool off. I appreciate the response.

[Steve King]

Earl,
I have two 5hp 80gal compressors at work. One compressor does not pump as fast as the other one. I use the slower compressor for the main, and when needed the faster compressor will kick in and produce the air needed. See if you can determine what compressor will pump the fastest and use it as the auxiliary compressor.
Steve

[Rob Russell]

Earl,

The simplest solution is to have a switch that turns off the 3HP compressor. When you're going to be using an air tool that uses enough air to justify running the second compressor, turn it on. Set the pressure switch on the 3HP unit so it's just a bit lower than the 5HP unit.

It sorta sounds like this isn't going to be an everyday thing.

If you're hard-wiring the compressors in (instead of cord-and-plug), the disconnect switch would serve both the NEC purpose of the disconnect and make it easy to kill that compressor.

Rob

[Earl Kelly]

Steve, I know once I rebuild it the 5hp will pump up faster. Not right now though. How do you have yours hooked up so one can be the main and have the aux. kick in when you need it?

Ken, you may be right on the 5hp feeding the dynabrade, Cause the comp. was already several yrs. old when I first got the sander. Probably wouldn't hurt to rebuild the sander also.

[Ken Garlock]

Ken, the sander eats up about 17cfm@100psi. The 5hp will run it but it will only get to take a break for a few minutes out 30-60. It's working really hard. If I didn't already have these I would have just bought a new 7.5hp. I know that would keep up. Yep the 5hp is right on the edge of keeping up with time to cool off. I appreciate the response.

Then have it :) I would just plumb the two into the same supply line, separating each with a check valve. Some times, the compressor valves aren't as good as we would like, and a check valve will prevent one leaking valve in one compressor from draining both. They are cheap, and an easy precaution to install.

Last, be sure you have sufficient wiring. I would want at least a 50 amp capacity, which, IIRC, is 8 gauge wire. Take the full load amperage on the motors, add them and then add about 25% to arrive at a good total load, and then convert that to wire gauge.

Sounds like a fun project :D

[Steve Jenkins]

I have 3wo compressors hooked up . One kicks on when the pressure gets down to 135 and the other when the pressure drops to 120. It's just a matter of adjusting the pressure switches.I don't have any check valves or anything just both tanks hooked together. Steve

[Jason Roehl]

Just hook 'em together somewhere in the air supply. Set both regulators the same and sand away. Let the compressors themselves worry about when to kick on--if one can't keep up, the other will kick in. If only one ever runs, set the pressure on the other a little higher. Or, you can set the pressure on both all the way up, then add in an aftermarket regulator between the compressors and your sander.

No fluid dynamics dissertations needed.

[Earl Kelly]

Thanks guys. I think I'll just use the pressure switches and try that. See if that will work out correctly. It'll take a while before I get to that point

[Steve Clardy]

tied together on the air line system. My main one runs, but the second one is shut off as the compressor needs rebuilt. So I am basically using the second for a supply tank. Even with both on, with both set on 120#s high side, the second would very seldom come on.
I have plenty of air this way for anything.
As Jason stated, leave the switches set where they are and let them run as is.
Steve