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This may be Air compressor 101 but if you have an AC that produces 15cfm at 90psi, and you have another AC that produces 20cfm at 90psi and you run them together would that give you 35cfm at 90 psi or some other number? Sorry if this is a stupid question, but an inquiring mind wants to know.

Kenny B.

551st3800,

I think it would be the sum of both..just like you indicate. The sticky wicket would be trying to plumb two pumps into a common receiver when each compressor had its own control system.

Stuart

robin,

Both compressors would be plumbed into a common tank with only one pressure switch controlling both pumps. The switch would simply be set at the maximum pressure desired, just like any compressor. What you gain from this setup, if it would work, is the combined output of two pumps in CFM.

I don't think this would be practical for a home shop as you would have to modify the electrics to control and start the compressors at the same time. You would have to have some serious circuits to handle the inrush as both pumps kicked on. The switch contacts on the little pressure switch wouldn't handle this load, so a motor starter would have to be incorporated into the system.

This might work, but it isn't a five minute project and could be more trouble than it is worth. Save your scrilla and buy an appropriately sized compressor and be done with it.

Stuart

robin58,

Most small, and even some medium air compressors start across the line using contacts inside the pressure switch...just like turning on a light switch on the wall. If you plumb both compressor outputs into a common tank and install one switch to control both compressor motors, it will probably be too much load for the switch contacts. You need to make the pressure switch a control mechanism for another larger switch...a motor starter, that can be wired to start both compressors at once.

Plugging the compressors into separate circuits is easy, but you still need to close some contacts (pressure switch) to start both motors.

One pressure switch can handle the current to start one motor, but if you hook the second motor to it, you might overpower the little contacts, electrically speaking. If you look on the pressure switch, you might see a horsepower rating as well as a current rating (amps). If the combined HP of both electric motors doesn't exceed the rating of the pressure switch, you are all set.

This all sounds very anal, but it is the approach you need to take on something like this. The other method would be to stick all the things together and see what happened. This has been know to work just fine too.

Motor control stuff can be difficult to understand...why can't I just plug it in? You won't electrocute yourself, and with the circuit breakers you are pretty safe. Dink around and see what develops, then let us know.

Stuart

OK maybe I can get my wife to type what happens when I die from electrical shock. I was thinking that it would be just as easy as plugging in two compressors and hooking up a T. From the way you guys are talking it might be a little harder to get this to work. It was just a thought that I had. I have a friend that is thinking about giving me his old compressor. I will probably try it if he actually gives it to me. He is one of these guys that will tell you something and than say "Well I gave it to So and So" :rolleyes:

Kenny B.

551st3800,

If that old so-and-so gives you the compressor, plumb those suckers together and see what happens...let us know.

Stuart

Hey I was just thinking of the same process and ran on to this post. I have a central air line in the shop with compressor #1 in shed outside. I want to try sandblasting and plan to borrow another compressor and just plug it in to the air line. Looks to me like the amount of air used would determine when both compressors came on. First to try this post results
Bob

Hay automarc, if you ran one pressure switch for this set up you could use the points in the switch to operate a coil to a larger contactor that would handle both compressors. I think that this would work. The coil in the contactor would have to be of the right voltage.

Not an engineer so what the bleep do I know? Many moons ago I worked in an old beat up truck repair shop. It had a "small" bay that held maybe 4 tractors (as in KW, White Frieghtliner, Peterbuilt) and a jumbo spray booth and a working yard that was maybe an acre where we worked on 40 foot trailers. The shop had two very large industrial air compressors that were simply plumbed into a common 3/4 pipe and ran all thru the shop and out to the acre lot where everybody and his brother used the air. It worked fine (those compressors got one heck of a daily workout.) We used a tremendous amount of electricity too with monster welders all over the place so the juice box was probably enormous.

A sandblasting guy once told me that you can indeed just plug multiple air compressors into a common line and go for it. The more the better he said. Just be prepared for moisture issues and he proceeded to talk me thru the subject of dryers, another subject.

The only caveat would be the power supply for multiple units. In a home, you would need to tap separate circuits with adequate breakers. (probably 20 to 30 amps each depending on the compressor.) Also, extension cords are a no no with air compressors because they act as a resistor and suck up much needed amperage. Short cords. Separate circuits. Should work n'est pas?...

235 Fat Fenders,

Your thinking is right on. Using a small switch to control a large capacity switch is the basis for a starter, or motor control starter.

orchidskip,

'Been there, done that' beats speculation every time. Your experience proves that plugging several compressors into a common line will work.

Stuart

It will work, and to simplify things you don't have to run both compressors at the same time all the time. Set the low side of the pressure switch for the second compressor about 10 psi lower than the first compressor. On normal demand the first compressor operates normally but has a larger storage capacity. On above normal demand the first compressor kicks on and as the load increases the second compressor will kick in. This will save both electricity, and wear on one of the compressors. Also the high amperage draw from both motors at one time could affect other sensitive electronic equiptment such as Computers, Sterios, TV's, and Alarm Systems.

Spareparts is right about not starting both compressors at the same time. Running current stamped on your compressor motor is far lower than starting current. Locked-load current for two motors at the same time on home wiring would cause a big voltage drop.
The other thing that you want to consider is that the two compressors should not be mismatched to any great extent. Every component in a compressor has a max pressure rating. My compressor runs between 150 and a high of 180psi. I'd be a little leary of pushing 180psi down the throat of one rated at, say 140psi. Can the pressure relief valve or the unloader valve handle the pressure from the larger compressor? Is the tank rated at the higher pressure?

Our water works has two compressors plumbed together.
They each have their own pressure start switch, one set a few pounds lower than the first. The compressors are the same units. During high demand both compressors run. Normally only the first operates. These have been in service 60 years. There is a lot of moisture it the air lines, but some of that is poorly designed piping.

ive seen compressors with 2 motors and pumps on one tank for fast fill