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Old 01-31-2011, 10:08 PM   #1
jomobco
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Default Combining two air compressors

Weird question but I have a problem. My current compressor is a Home Depot husky with specs: 60 gal, Delivers 11.5 SCFM at 40 psi and 10.2 SCFM at 90 psi
135 PSI maximum pressure

It's not big enough. Can I just buy a second of the same compressor and combine the two with a pipe running between the plug on the side of the compressor? This would give me twice the CFM and 120 gallons of tank. Has anyone done this? Why wouldn't it work? I'm thinking because the pipe running between the two would be the weak link?
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Old 01-31-2011, 10:10 PM   #2
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Default Re: Combining two air compressors

While I cannot comment on if this would work or not.

Would it not make sense to sell your current compressor and buying a better one rather then buying a second of the same compressor that is undersized for you and trying to frankenstien them together?
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Old 01-31-2011, 10:32 PM   #3
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Default Re: Combining two air compressors

It would make sense but may not necessarily be the best value if this would work. For ease of discussion the next bigger home depot husky is:
Powerful 5 HP engine provides up to 10,000 hours of extended life
175 psi maximum pressure
Delivers 12.6 SCFM at 90 PSI to provide quiet, abundant airflow to power multiple tools
Steel 80-gallon tan for $1079. Less the $200 I might get for my current unit. That increased PSI would certainly make the 80 gallon tank last a lot longer than the 60 I would imagine.

My current set-up is $439. X 2 it has better specs with the exception of overall PSI.
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Old 01-31-2011, 10:33 PM   #4
CUSTOMMANCAVES.COM
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Default Re: Combining two air compressors

You will not double your SCFM or PSI by combining tanks. You will only double your air supply.
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Old 01-31-2011, 10:41 PM   #5
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Default Re: Combining two air compressors

Quote:
Originally Posted by CUSTOMMANCAVES.COM View Post
You will not double your SCFM or PSI by combining tanks. You will only double your air supply.

If you have two pumps, your total CFM output will be the sum of both their outputs.
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Old 01-31-2011, 10:41 PM   #6
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Default Re: Combining two air compressors

Quote:
Originally Posted by CUSTOMMANCAVES.COM View Post
You will not double your SCFM or PSI by combining tanks. You will only double your air supply.
And you'll double the amount of power you use to do it.
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Old 01-31-2011, 10:54 PM   #7
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Default Re: Combining two air compressors

I am in the process of doing this. I have 2 80 gal comps each has a valve at outlet, then rubber hose to a T, then off to the dryer, then off to the shop.

Not sure why OP says the pipe between would be the weak link. This shouldn't be an issue if done properly with some flexible lines.

The part I put most thought into is the wiring. First thought they should come one separately and off together but then you would have 1 compressor filling 160 gal. most of the time. So my plan is to wire them together on the same pressure switch (with proper contactors to handle the full current for each motor). Still looking for practical ideas on this part of the setup, though.

Of course 1 bigger compressor would be more efficient and simpler but when you get 2 nice units for cheap it is hard to justify the $$$ on a 10HP unit.
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Old 01-31-2011, 10:56 PM   #8
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Default Re: Combining two air compressors

Oh and don't forget the extra $$ for extra wiring, conduit, breakers, piping, fittings, hoses, clamps etc. add a big amount to this project.
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Old 01-31-2011, 11:28 PM   #9
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Default Re: Combining two air compressors

Quote:
Originally Posted by Chestrockwell View Post

The part I put most thought into is the wiring. First thought they should come one separately and off together but then you would have 1 compressor filling 160 gal. most of the time. So my plan is to wire them together on the same pressure switch (with proper contactors to handle the full current for each motor). Still looking for practical ideas on this part of the setup, though.
Each tank inlet has a one-way valve. Air goes out, not back in. One could easily be added to a tank outlet I would think. Might be a restriction though.

Most body shops have 2 compressors (well, the big ones have a large rotary screw compressor). I have talked to compressor salesmen and they state that it is often recommended to run 2 smaller compressors v/s a one large one. The starting load of 2- 5 hp compressors, with a staged start (2 different start psi switches) is FAR less than starting a 10 hp compressor.

(i.e.... the lights wont dim every time it turns on)

Last edited by CARS; 01-31-2011 at 11:32 PM. Reason: spelling
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Old 01-31-2011, 11:31 PM   #10
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Default Re: Combining two air compressors

I guess if I'm running hard lines in the shop I could just t the hard line and run both compressors into the supply. Both are 110v so wiring isn't an issue. I think it would be best to have them on separate breakers in case they both kick on at the same time. Good points about shut off. My biggest issues are when I plasma and I think the double supply of air might get me close. Heck I could add a 3rd compressor for slightly more than the next bigger one at HD. The noise would suck though. I wonder how much louder it would be than a comparable single unit?

Last edited by jomobco; 01-31-2011 at 11:34 PM.
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Old 01-31-2011, 11:44 PM   #11
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Default Re: Combining two air compressors

There is a 100A breaker feeding my compressor shack and I can turn both compressors on at once and the lights don't dim, there is no problem with that.

Other reason I am starting and stopping the motors on the same switch is because with 2 separately switched motors it would be tough to stop them at the same time. So one compressor runs for a long time to reach it's off pressure. It's kind of stupid because when I have the plasma table running for 45 minits straight and I'm running a die grinder or whatever else at the same time; that higher 'off pressure' compressor runs all the time.

Even 2 120V compressors don't seem like enough for plasma cutting if you ever get into gouging or long cuts you will probably never be happy with them. I would strongly suggest getting a 240V unit and proper sized wire/breaker.
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Old 01-31-2011, 11:54 PM   #12
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Default Re: Combining two air compressors

I run staggered cycle systems at work all the time. Set one compressor to come on at a slightly higher pressure than the other. This means one will start to make up air and will be the only one to come on if you only draw a little bit off. The other one kicks in when the first cannot maintain the second starting pressure point. Every so often switch the pressure settings so the other compressor comes on first. You can also just leave it set so the same compressor always runs first. In the home setting you don't need to worry much about cycling them both equally. Do not run both on one switch as most pressure start switches also have a cylinder unloader built in.

No fancy wiring, no high load from two motors coming on together, less noise. Simple plumbing, a T fitting. There is nothing wrong with a long run time for a compressor. In fact, like cars, it is good for it as it heats the oil and gets moisture out.
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Last edited by Vicegrip; 01-31-2011 at 11:56 PM.
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Old 02-01-2011, 02:04 AM   #13
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Default Re: Combining two air compressors

I used two together with a T arrangement for my air hydraulic log splitter. Made a ton of racket but got the job done. Finally bought a 60 gallon vertical oil lubricated and the noise difference is like night and day.
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Old 02-01-2011, 05:30 AM   #14
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Default Re: Combining two air compressors

Quote:
Originally Posted by jomobco View Post
Weird question but I have a problem. My current compressor is a Home Depot husky with specs: 60 gal, Delivers 11.5 SCFM at 40 psi and 10.2 SCFM at 90 psi
135 PSI maximum pressure
There is no reason this can't (or even shouldn't) be done. In another forum there is a whole thread on this.....I just can't seem to find at the moment.

Is your problem working time or max PSI?

As one poster said above, your PSI will not increase at all in this set up....the motors aren't strong enough to compress the air any more than 135 PSI; however, you will effectively double the working time you have at whichever setting you're working with.

Your spec above looks like an oiless version - 2 of these would be LOUD.
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Old 02-01-2011, 11:31 AM   #15
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Default Re: Combining two air compressors

Industrial settings frequently use multiple compressors for factory air supply. They are always setup with a "Lead" unit and "Slave" units. The "Lead" unit is set to come on first and typically carries all the load until the load exceeds the capacity of the "Lead" compressor then the "Slave" compressors come on to help out. If you are concerned about one compressor running more than the other then just routinely alternate the "Lead" compressor with the "Slave" units to balance out the run time of the individual units. I would set the units to come on in 5# differences with the same delta P the units will also cascade with this same 5# drop on shut off pressures.
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Old 02-01-2011, 11:33 AM   #16
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Default Re: Combining two air compressors

Quote:
Originally Posted by isaac338 View Post
If you have two pumps, your total CFM output will be the sum of both their outputs.
EXACTLY...+less down time between Start ups on the compressors.........
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Old 02-01-2011, 11:41 AM   #17
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Default Re: Combining two air compressors

Best advice...Sell it and upgrade....
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Old 02-01-2011, 12:36 PM   #18
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Default Re: Combining two air compressors

I have my shop compressor, and a portable IR compressor. The shop one is about 8 to 10 CFM which is not enough for a few things I do. The IR portable is 4.5, which combined with the shop gives me just enough air.

I made up a short hose with two male connectors, and a ball valve in the middle. Close the valve, connect the two compressors (with the valve closed there is no leakage) then open the valve. Takes about 30 seconds to do. I use this configuration about once a year (maybe twice, not not often...)
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Old 02-01-2011, 02:23 PM   #19
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Default Re: Combining two air compressors

if you're planning on running 2 oil-less compressors i sure feel bad for you neighbors. i can't work around those things. they are just soooo much louder than low rpm oil-bath unit.

the only benefit to getting a second cheapy compressor is that replacement cost when they die is a smaller bite. but you'll be replacing them more than twice as often.

if you figure in the cost of an enclosure to cut down on noise then i bet the better compressor is a better deal.
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Old 02-01-2011, 03:28 PM   #20
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Default Re: Combining two air compressors

There's a few more issues that haven't been addressed.

To make a long story short, I see no problem with adding another compressor. However, you'd be wise in this setup to switch between the two units separately (some sort of two way valve could be easily arranged to switch one out, and the other in).

Without some sort of rather complicated schemes, you could wind up having only one unit pressurize all 120 gallons. The problem there is that units in this quality range usually DO NOT have a hundred percent duty cycle. That's what limits them to whatever size tank they are on, can't fill up a lot more than that without a lot of undue stress on the thing.

If you are using single stage units rated for 100 percent duty cycle, just go for it. Staggering the pressures a bit will be fine.

With two units, and a simple valve arrangement, you can switch between the two, and keep your volume and pressure up, AND reduce the duty cycle time on both units. A win win, and is nice that you don't have to sell a unit at some reduced price to upgrade.

I've found that in ANY high use air situation, sitting at the sand blaster, running my die grinder (it's a hog...), taking a break as often as you'll need to anyway to switch between the units supplying your shop piping wouldn't be a problem at all.

By the way, running a compressor to the point everything warms up is bad in a way. The warm air (inevitable because of compression, boyles law) eventually warms up the tank. THEN, all the moisture in the air, stays in the air and winds up in your tools, sandblaster or paint. When it first starts up and everything is cold, the moisture condenses inside the tank (not great for the tank, but it keeps it out of everything else) and you get nice dry air at the point of usage.

Keep in mind that the water separators common to most systems are really water PARTICLE separators. Water VAPOR separators use either a dessicant or a heat exchanger (usually CRAZY expensive using refrigeration).

Lastly, and I'm sure this has been hashed and hashed and hashed here at some point, is that in deriving what you really have for horsepower on top of these things, you need to figure out the 110v amperage, and divide by 15 (as a very rough rule of thumb). A 15 amp motor (running on 110v) is 1 hp. Many air compressors will list this thing as a 5hp or whatever... it's nonsense. You're not gonna get but about 4 or 5 cfm at around 90psi out of 1 hp. It's all fairly linear in the ranges we play with these things.

All generalities, but will serve better than relying on whomever's advertising department figures.

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