View Full Version : Twinning air compressors?

12-11-2007, 08:43 AM
Hey guys! Due to my shops ever increasing need for more air, I've just bought a used Coleman 7hp/60 gallon upright to go along with my CH 5hp/60 gallon upright.
Now I'm wondering if there would be an advantage to plumbing both compressors together. This would double the effective volume I need for carbon arc goughing. Wouldn't it?
Or in the end, would I just end up with two crappy air compressors running flat out instead of just the one?
I've been searching for a used two stage compressor locally but missed the only one to come up this year. It sold for peanuts at a business "yard sale" that I didn't hear about til it was over. Arrrgh...and a 10,000 pound forkilift went for $2500.

Steve Steven
12-11-2007, 09:13 AM
I use two compressors to run my abrasive blaster, a 4HP old Craftsman (4 real hp, 220V) and a DeVillbis 5HP one. They are twinned at the source, the tanks are connected by large hose. The two combined can keep up with my nozzle, the 5hp one runs continuously and the 4hp cuts in and out to maintain the 115# set point. On prolonged blasting, they will both run steadily till I stop blasting.

If you need lots of air, I would recommend twinning them together.


12-11-2007, 09:47 AM
Thanks Steve! I forgot to mention..I've just aquired a bead blast cabinet and a good 50 pound sandblaster. I know these two compressors won't be ideal for that but will get me by til I can get an engine powered unit in the spring.
Another question...what about putting the compressors outside in a "lean too" beside the shop? Would i need to keep the small room warm or would the heat from the compressors warm it up enough? I don't like the idea of two compressors running in my small shop. (Wouldn't be able to hear the 8 track :D)

12-11-2007, 12:51 PM
Pretty common. They sell compressor setups with two pumps and two motors on the tank ready to go. I think they stagger the start so the inrush is not so high.

Steve Steven
12-11-2007, 01:36 PM
Thats what I did. I had lots of problems with moisture and noise when I started with the blaster, so I moved the two of them out to behind the house in a small "doghouse" sized for them. I have a large house air filter to keep the bugs out, and a small fan opposite it to pull air form the outside and ventilate the enclosure for cooling.

I piped it up with 3/4IPS galvanized pipe, running thru the crawl space under the house to emerge into the garage, moisture trap, a 45deg rise to just below the ceiling, then over to the blaster with a moisture drop/takeoff/plugged cross, with the takeoff coming off the top to keep moisture at a minimum. Works great! See the diagram at http://www.tptools.com/StaticText/airline-piping-diagram.pdf from TP Tools for ideas.


12-11-2007, 06:36 PM
Russ,well I don't like any compressor in the shop cause of the tanks exploding and what not,but if it must be just plumb up the intakes so they are outside the shop.Intake noise is the bulk of the annoying racket.

They make duplexing controls,but why?For what your doing run the small pump most of the time and throw the big one on when you need a "kicker".That's what we do at work,we have a 5hp Westinghouse for general use and a 7-1/2hp Westinghouse throw on when using the sandblast cabinet.

12-11-2007, 06:45 PM
I have my two compressors plumbed together by just a length of air hose that can be removed any time I want to move the smaller portable one. It helps them to keep up with sandblasting. There isn't any particular safety reason to not do it and as long as both cut out before the safety valve pops on the other it should be fine.

12-11-2007, 11:06 PM
Thanks guys! OK..I'll go ahead with that as soon as I get the power run for the second compressor. Never thought of just using the smaller compressor for day to day stuff.

12-12-2007, 02:34 PM
Russ-- I'd be careful with the outdoor compressor idea...especially up there in the cold north. Yours are splash oiled and as such will not be oiled well if the oil thickens.

I bought a big Ingersoll-Rand 5HP/80 gallon single stage job a few years ago. They warranteed it for two years if you used their synthetic compressor oil. I don't know that I will always use their lubricant, but I will always use someone's synthetic compressor oil....its handles temp shifts much better and doesn't turn to molasses in January.

I heat my shop, but only when I am down there. Temp drops back into the 40's between shop sessions....cold enough to make a noticable difference in the oil viscosity with petroleum products and this affects how well it will splash. Imagine how much worse this will be at freezing....or below zero F! Heat from the compressors is only present after they have cycled a few times...which means that they were run with very thick lubricant the first few cycles each day.

The other issue is that outside air will often become more humid than indoor air. I find that on occasions when I have the doors open in the spring (especiall if the tank is still cool), I have water to drain from the compressor much sooner. I find that I get nothing from my water trap, however...perhaps because of the big tank with room for the air to cool.

Another tip I highly recommend and have mentioned here before....for about $6 you can buy a semi-truck air tank drain valve. It has a pull cable attached and a spring loaded valve. This means that you can give it a quick tug each day when you hit the shop to keep it drained without any stooping. I found that if you do this fairly regularly, the water you do get out, comes out clean rather than rusty....it never sits long enough to corrode the tank bottom. My uncle took my recommendation and later informed me that allowing the works to freeze took his drainer valve apart:D Seems the expansion from freezing disassembled the truck bleeder valve which has a sheet metal shell and is crimped together.


12-13-2007, 01:20 AM
Paul, Thanks for that! I sorta wondered about these being out in the "real" cold. It's been a long winter already. Weeks of -18c so far. I'm thinking I may have to insulate the little lean-too these'll be going into.
The truck valve sounds like a good idea also. Lot quicker than the under tank tap.

12-13-2007, 03:27 AM
Steve Steven,

If the 5 horse carries most of the load, the pressure switches aren't set the same. If it comes on at, say, 90 psi, and the 4 horse doesn't till you exhaust more air, you might should reset the 4 horse switch, either to higher than the 5 horse for lighter duty, or to the same to get maximum makeup when you do heavier duty work.

120 gallons volume would definitely be better, so common plumbing, consider it a manifold, as in 9 Oxy bottles tied to 1 regulator for heavy duty cutting.