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Tool Talk Discussion Forum

piggy backing two air compressors

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pop

01-11-2004 14:45:56




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I have two 3 hp 20 gal air compressor has any body ever run an air line between them to come up with more air pressure for hvlp painting, sandblasting and air tools ?




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steve

01-12-2004 15:42:01




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 Re: piggy backing two air compressors in reply to pop, 01-11-2004 14:45:56  
The best way to set two compressors up to run together is a load sharing alternator. Compressor #1 always starts first when you start but #1 and #2 then alternate every other cycle, if you use more air than one compressor can supply the other one comes on line automatically to meet the demand. Biggest problem is $$$$$. Can be built with 3 control relays, 2 pressure switches and 2 motor starters. Steve

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buickanddeere

01-12-2004 08:27:25




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 Re: piggy backing two air compressors in reply to pop, 01-11-2004 14:45:56  
We assume you want to increase volume, not pressure. The compressor will discharge to a common header. Best to avoid starting both compressors simultaneously. The current inrush may trip the hydro service or at least dim the lights, trip computers, drop out metal halide lamps and possibly stall other electrical motor loads. That would be an inrush of 150 to 225 amps per motor on 240vac for the 1st cycle or two, then 75 amps each until rated speed is reached. Should drop to about 15 fla each depending on motor efficiency and power factor.

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George

01-11-2004 21:18:18




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 Re: piggy backing two air compressors in reply to pop, 01-11-2004 14:45:56  
Saw a commercial setup that used two compressors and two pressure switches -- the unique thing was that between the compressors and the switches was a set of relays that would alternate the lead/lag compressors to equalize wear and keep compressor temperatures down.



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ScottT

01-12-2004 09:47:49




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 Re: Re: piggy backing two air compressors in reply to George, 01-11-2004 21:18:18  
This isn't all that unusual in an industrial setting. We have 3 - 300 hp rotary screw baseload compressors and a variable speed 100 hp rotary screw trim compressor at work. They all operate based on pressure settings to maximize effciency and delivery. Lead / lag settings are based on the computer controller.

Scott



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Larry E

01-11-2004 18:25:43




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 Re: piggy backing two air compressors in reply to pop, 01-11-2004 14:45:56  
I run 2 compressors and use a check valve on outlet on each unit,so air can go out but not in.



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Roger

01-11-2004 18:18:00




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 Re: piggy backing two air compressors in reply to pop, 01-11-2004 14:45:56  
Lots of places use two seperate compressors running into the same line. One is usually the lead compressor (higher cut in) and the other is the lag or "trim" compressor (lower cut in).This is common when a company has outgrown the capacity of an existing compressor and purchases a new, bigger unit. The old compressor is usually delegated the lag compressor role. The lead compressor runs the show until it can't keep up with demand. The lag compressor cuts in when the pressure drops to its cut in pressure. Both compressors run until the lag compressor reaches its cut-out, whereupon it shuts down and the lead once again runs by itself. The lag compressor's range lies entirely below the lead compressors's, i.e, it cuts in lower and it cuts out lower. That should not be a problem in your application. The only thing you should be aware of is that the cut-out pressure of either machine must not exceed the pressure rating of the other's tank or safety valve. Put a valve on both tanks as well, so you can isolate them when you want to remove a line or something. As another poster stated, this will give you more volume (which is what you want) not more pressure.

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Paul Janke

01-11-2004 17:03:17




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 Re: piggy backing two air compressors in reply to pop, 01-11-2004 14:45:56  
The first time I saw this done, it was a body shop which chose to have two smaller compressors rather than one big one. If they had trouble, instead of being out of business, they could still operate at reduced capacity. They had both hooked to the same pressure switch. I later learned it is better to have separate switches so the first one to start is the last to shut off. By setting them up that way, one should seldom shut off, and the other should seldom start. They should both last longer, because starting is always the most wear causing thing they do.

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Bus Driver

01-11-2004 16:56:58




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 Re: piggy backing two air compressors in reply to pop, 01-11-2004 14:45:56  
If you are considering putting the output fom one compressor into the intake of the other-NO-NO-No. Back in the early 1950's, I had no money and used two old belt driven refrigeration compressors feeding into one tank. For one of the compressors, I had no motor large enough to drive it, so I used two motors of the same speed, belted them together to drive one of the compressors. One tank, two compressors, three motors. A relative still owns that property and the setup is still working.

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Wayne

01-11-2004 15:10:01




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 Re: piggy backing two air compressors in reply to pop, 01-11-2004 14:45:56  
Hey Pop, piggybacking two isn't gonna give you any more pressure, but it will give you more volume. That is if you run a big enough line between the two tanks as well as using a bigger service line to whatever tool your using that's gonna be capable of flowing the higher volume. Before we got our P185 IR compressor we used to piggy back four piston compressors together for our big 300 gal pressurizes pot type blaster. We used two 35CFM/30gal tank compressors on our service trucks with our 45CFM/80gal tank shop compressor with another big 80CFM/100gal tank conmpressor. Each one ran and operated off it's own independant controls loading and unloading at whatever they were set at. If you were going through a regulator with a set pressure lower than the set pressures of any of the compressors this starting and stopping wouldn't make any difference. However if you were drawing straight tank pressure it will cause a pulsation in the line, especially when sandblasting. We discovered this as soon as I fired the blaster up. Other than having to channel the flow through a big regulator we never had any problems. With only two compressors though it shouldn't be that hard to syncronize the load/unload points so they work together all the time wether they are start/stop types or contiunous run types.

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Scott

01-11-2004 15:05:31




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 Re: piggy backing two air compressors in reply to pop, 01-11-2004 14:45:56  
I've done it with my new 6hp 60 gallon and my old 3hp 20 gallon. Gives more capacity to run my sand blaster with out stopping. Just make sure they are both set not to excced the lowest pressure rating of the two tanks. If one is set at 125 and the other is set at 175, the 175 one's pressure switch would have to be turned down to 125.



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BFarr

01-11-2004 14:56:51




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 Re: piggy backing two air compressors in reply to pop, 01-11-2004 14:45:56  
I did it several years ago and I if I remember right I got away with it. Am going to try the samething this month to sandblaster an F6 but I may install check valves in the lines to each tank



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