wiring two compressors in series?

[tgreening]

1) Buy 1 magnetic starter.

2) Wire 1 compressor mounted pressure switch to the starter. Pick either compressor, it doesn't matter.

3) Wire both motors through the magnetic starter.


When the pressure switch reads your low pressure set point it will fire the contacts in the magnetic starter (basically a heavy duty relay) and both compressors will kick on at the same time. There is no pressure switch calibration needed. When high pressure set point is reached the starter disengages and both compressors shut down.

Done.


I think you will probably play hell trying to calibrate these pressure switches to engage in the manner you wish, and KEEP them that way. If momentary load is an issue run each compressor through a separate set of breakers. If your panel has enough amp capacity to feed both compressors at all you should have enough capacity to either run separate breakers, or just bump up the breaker size to handle the start up draw of both machines. I'd recommend separate breakers though.

 

[-Spike-]

1) Buy 1 magnetic starter.

2) Wire 1 compressor mounted pressure switch to the starter. Pick either compressor, it doesn't matter.

3) Wire both motors through the magnetic starter.


When the pressure switch reads your low pressure set point it will fire the contacts in the magnetic starter (basically a heavy duty relay) and both compressors will kick on at the same time. There is no pressure switch calibration needed. When high pressure set point is reached the starter disengages and both compressors shut down.

Done.

How do you propose to unload the pressure from the cylinder(s) when they shut down?

 

[tgreening]

How do you propose to unload the pressure from the cylinder(s) when they shut down?

How do they unload now? Doing what I suggest should have nothing to do with how the cylinder heads unload. All the pressure switch does is tell the mag starter when to engage and disengage thereby feeding or cutting power to the motors. If the pressure switch has an unload circuit in it it will still function as normal.

Lighter duty compressors usually have the power to the motors wired through the pressure switch. The amp draw is low enough that this is not a problem. Big compressors will have a magnetic starter which like I said is pretty much just a heavy duty set of contacters. When the pressure switch closes it routes power to the contactors, which close, which then allows power to feed to the motors. It has nothing at all to do with how the compressor head does or does not unload.

 

[-Spike-]

They unload via the pressure switch, mechanically. If the two compressors are wired to start at the same time, the one in control will get to it's threshold before or after the other. The other will either blow the valve off early or not at all, unless I'm missing something.

 

[tgreening]

They unload via the pressure switch, mechanically. If the two compressors are wired to start at the same time, the one in control will get to it's threshold before or after the other. The other will either blow the valve off early or not at all, unless I'm missing something.

I'm going with all this via memory as I sit in my chair, which at best is an iffy way to go. I'll head out into my shop tomorrow and have a look at the pressure switches and what not. I have both types, mag starter and non mag starter. I'll post back what I find..

 

[Dirtsquirt]

150 gallons wont buy you much

I am not interested in this problem enough to do the math for you. Things to consider # of cubic feet in the tank arrangement you come up with that are useable.
IE: How many cfm are in the vessels between minimum usable pressure for the job at hand, and Shut off:
In your case 70 and 135. 70psi is a convenient pressure to use for a typical blasting pressure. Dont chime in with all the times you use or have used more or less. this is for the sake of discussion. not about blasting methodologies.

once you determing how much air is in the tanks between 70 and 135. Check your nozzle size (prressure blaster) and or airjet size(siphon) It wont take much looking to find out how many cfm they use. divide the number of cf in the tank by the cfm requirement of the nozzle. and you will know what the benefit will be.
In my estimation without doing any calculation (based on experience) I would say you would lucky to get more than 4 minutes of air usage out of that much tank.

After the 4 minutes you are back to relying on the compressors to keep up.

In short there will be little gain from adding a tank.

Back to the paralell compressing..
I would connect both machines together Join them with a hose or pipe but the important thing is that the tanks are freely joined. Ie no regulators or checkvalves in the joining line. lot of the smaller units have manifolds that have regulators and or check valves in em these are not acceptable
Free flowing connection with no possible restrictions or valves to create unequal pressure build up in one tank or the other.

Once this is done pick a pressure switch to use to control both machines. Its usually best to use one with an unloader on the side of it. Now get some 1/4 copper tube and tee into the unloader circuit on that machine and run to copper to the Compressor discharge of the other machine also.
there are so many variables in possible current configurations that I will tell that the Criteria is to connect into the main discharge pipe of the other compressor between the checkvalve and the head. some heads are ported already for this applicaton.

This will allow both machines to unload off of the same pressureswitch unloader.

now purchase a time delay relay like this oneRelay, Time Delay, Dpdt - Time Delay Relays - Relays - Electrical : Grainger Industrial Supply

Also purchase your self a contactor like this oneContactor, Definite Purpose, 2P, 120VAC, 25A - Magnetic Contactors - Starters and Contactors - Electrical : Grainger Industrial Supply

get the above in the appropriate voltage for your application and
wire it up. let the pressure switch control the start stop of of the first one as it is currently wired.. Wire the contactor in place of the other pressure switch, and wire the Time delay relay (control circuit)in paralell to first pressure switch.

this should net you Both compressors cycling on the same switch with a delay before the start of the second machine.

I want to point out that before attempting any of this. Pull on the tank safety valves to ensure they arent rusted shut.
If no safety valves exist now is the time to put em in.

What you want to do can easily be done with a modest outlay.

My advise. Sell the two you have and buy a larger unit. When I say larger I mean in terms of CFM delivery.

Good luck to you

 

[semlin]

thanks dirtsquirt. that setup makes a lot of sense. my only remaining issue is i have one 240v and one 120 volt motor. do you think i can wire the time relay off one of the 240 volt lines?

btw, i am looking at an extra 80 gallon tank so total 130 gallons. i had actually calculated i would get about 3 and a half minutes of decent blast time for a 10 cfm nozzle so you are very close. i figure that is probably enough to do a small part in one or two cycles until i can source a used two stage compressor.

 

[Dirtsquirt]

if it were me

The controls are avail with many different input voltages
if the pressure switch on the 110v machine doesnt have an unloader on it then you will have to something different.

but if it does, use it as the lead unit that switches the time delay relay which switches the contactor that is in series with the pressure switch on the 2nd machine. (crank the off pressure up 10psi higher than the shut off of the controlling pressure switch
leave it in the circuit for safety sake.