The MGA With An Attitude
Using Two Air Compressors Together -- AR-103

At 09:31 PM 3/13/04 -0500, Ken Law wrote:
" How do I hook up two compressors (120V) without blowing a hose or something. Is it just a matter of 'T'ing them together and let each pressure switch operate as it will?"

Very easy. Just connect a hose directly between the two tanks (bypassing any output regulators). Do not ever try to force air flow backwards through a regulator. And you will need enough electrical supply capacity to run both units at the same time (assuming you intend to do that). Connect them to two different fused circuits if you can (less likely to blow the line fuse).

You will be able to draw air from either unit, and the tank capacity will be the sum of the two tanks. The first one to hit the low pressure switch pressure level will kick on first. If you're not drawing any more air than the one unit can pump, the second one would never kick on, as the first unit can supply all the air you need.

If you are drawing more air than the first unit can supply, then the pressure will continue to drop until the second unit kicks on. Then you get the combined output of the two units working together to recharge the tanks. If you are not using more air than the two can supply together, then the pressure may eventually rise to the point where one unit will shut off. The other unit would continue to run as the pressure drops, and the second one will kick on again at the low pressure switch setting.

If you are using more air than the two units combined can supply, then you run into the duty cycle and trigger time calculations. Add volume of the two tanks and treat it as one. Add output of the two compressors and treat it as one.

At 11:04 PM 3/14/04 -0700, Ken Law wrote:
"If I hook two compressors together that have 120psi each does the pressure add to 240 or just 120?"

No, just 120 psi. They will be in parallel to double the volume at the same pressure.

"When you said add output and treat as one, what terms of output are you speaking?"

Compressor output is expressed in CFM (Cubic Feet per Minute) which is a bit of a misnomer. It is really SCFM (Standard Cubic Feet per Minute). They usually have two noted outputs at different pressure, such as 7 CFM at 40 PSI, and 4 CFM at 90 psi. If both of the units were the same, this would add up to be 14 CFM at 40 PSI and 8 CFM at 90 PSI. Just add the rated outputs and treat it as if it was a single compressor. The tanks are similar, just add the volume of the two tanks and treat it as if it was a single tank on one compressor.

At 01:48 PM 12/2/2008 -0700, John Orlandi (with a professional body shop) wrote:
>>"I would like to know how to hook up two 220V, 2 stage, air compressors. One is a older 19 CFM two stage and the other is a brand new 27 cfm 2stage."

Whatever else you do, always connect the two tanks together to serve as one larger tank. This sounds like you bought the new compressor because the old one couldn't keep up with your overall demand. Connecting the tanks together is a good move for more backup reserve, and longer cycle time (as noted above).

>>"We are wiring them on two separate breakers and want them to come on at separate times in order not to pop a breaker. How would you set the pressure switches on both units so the new one will run most of the time and the older one to come on only as a back up?"

If they are on separate breakers having both run at the same time is no problem and would not pop a breaker, unless your overall building electrical load was high enough to pop the main breaker. Not likely, but if the second compressor running overloads the main breaker you would need major electrical upgrade for larger mains.

Two stage compressors are more efficient at higher pressure and normally operate in the 150-175 psi pressure range. Set:
27 cfm unit, off at 175 psi, on at 150 psi.
19 cfm unit, on at 140 psi, off at 165 psi.

On first start both compressors will run together until the smaller one shuts off at 165, then the larger one shuts off at 175. If you don't want both units to run at the same time on startup, then switch off circuit breakers for the smaller one until pressure gets up to 175, then switch the breakers back on. After that the larger compressor will cycle between 150-175, and the smaller one will remain off (as long a overall air consumption does not exceed capacity of the larger compressor).

Normal usage will run only the larger compressor using both air tanks. If the primary compressor should fail, or over all air consumption is greater than 27 cfm AND pressure falls below 140, then the second compressor will switch on, and both compressors will run to supply the high demand air flow (until pressure gets up to 165 and stops the smaller compressor).

If you start with pressure switch settings noted above and both units on power, but reduce the smaller one's switch on point to about 120, then continued air consumption in excess of 27 cfm would eventually drop pressure farther below the 150 psi switch on point of the larger unit. Consumption could continue in excess of 27 cfm until pressure may drop to 120, in which case the second compressor would kick on to supplement the demand. Difference here is allowing pressure to go a little lower for longer cycle time in event of excess consumption. If consumption is reduced (10 less than 27 cfm) before pressure goes as low as 120, then it can recover on the larger unit only and the second unit would not switch on. This could give a little longer cycle time at reduced pressure in event of excess consumption without using the second compressor, but still have the smaller one on line in case high consumption draws pressure lower than 120.

If you really don't want the two ever running at the same time, then you need to switch off the smaller unit and only turn it on if the larger one fails and will not run. That is the smaller unit would be used only as a backup if the larger one was entirely dead.

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