wiring two compressors in series?

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semlin

semlin

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i just got a great deal on a big sandblast cabinet. my shop compressor will not be able to keep up with it but i have another portable compressor with a similar pump, and i think that both in series could keep up. i have tried them hooked together and it does work pretty well, but the problem is controlling them both in synch because the pressure switches do not trigger at the same time, so the one that requires lower psi to trip does not go off unless you deliberately drain air and fight the other compressor.

has anyone tried wiring two compressors together like this? my idea would be to install a single large main air storage tank with inputs at opposite ends that would receive air from the stand alone compressors after they fill their own tanks. that tank would then feed a single output to the sandblast cabinet (or anything else)

the trick is how to trigger both compressors to go on automatically at the same time. i want to somehow use a single pressure switch to trigger both so that i don't have to worry about the pressure switches going out of synch. in a perfect world i would also like to have one motor delay triggering for 10 seconds after the pressure switch flips to reduce the momentary load on my garage circuit (they are on separate circuits on a 60 amp subpanel circuit).

any suggestions?
 
Change out the pressure switch that trips at a lower pressure so the two are pretty well matched. It won't matter if they trip at exactly the same time, as long as the lower one trips before you get to your regulated pressure. There are adjustable pressure switches, so you could pretty well match the two together. Of course, this assumes your tanks and compressors are both rated for the higher pressure. You could also tone down the bigger compressor, but what fun is that? :D

On the other hand, if your regulated pressure is lower than both compressor's start pressure, you should just be able to hook the tanks together and only rely on the portable to help keep up under extreme demand. It wouldn't matter if it trips at the same time or not.

I don't know how you would use a standard pressure switch to trigger two motors, mostly because of the pressure release mechanism. You might be able to do it with some creative plumbing and maybe a relay, but it doesn't seem worth it.
 
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-Spike- said:
Change out the pressure switch that trips at a lower pressure so the two are pretty well matched. It won't matter if they trip at exactly the same time, as long as the lower one trips before you get to your regulated pressure. There are adjustable pressure switches, so you could pretty well match the two together. Of course, this assumes your tanks and compressors are both rated for the higher pressure. You could also tone down the bigger compressor, but what fun is that? :D

On the other hand, if your regulated pressure is lower than both compressor's start pressure, you should just be able to hook the tanks together and only rely on the portable to help keep up under extreme demand. It wouldn't matter if it trips at the same time or not.

I don't know how you would use a standard pressure switch to trigger two motors, mostly because of the pressure release mechanism. You might be able to do it with some creative plumbing and maybe a relay, but it doesn't seem worth it.
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i was thinking about a relay and maybe a delay relay too. the problem is that if they don't match and you are not working continuously, you end up having to fire the tool you are using for a while to make sure the second pump kicks in. if you have a big storage tank system that wastes time and pressure.

both pumps are actually the same -- 150 psi rated 2 cyl speedaires, but on the portable it has a 1hp motor and on the shop it has a 2hp motor. They use different pulley sizes to match the pumps to motors.

the existing pressure switches are set to trigger at about 60psi and go off at 100 psi. i am going to change that to 90 and 135 psi.
 
If you're having to 'fire the tool' to get the second pump to kick on, obviously the second pump isn't kicking on before you reach your regulated pressure. What is that pressure?

100 PSI is pretty damn low for a compressor to kick off, usually even cheap compressors kick on at ~90 and off at ~120. Sounds odd, like the springs are weak, or they've been messed with. If you get both of them to kick on somewhere around 90 PSI and regulate your air pressure at the tool to 90 or less, you won't have to worry about the compressors being matched, they'll both be running if you need them. Also, they probably won't kick in at exactly the same moment, so they won't load the circuit at the same time (or both circuits). Trying to get the compressors to kick on and off at the same time is unnecessary and overly complicated. K.I.S.S. :D

If you're trying to maintain 120 PSI at the tool, get a bigger compressor.
 
-Spike- said:
If you're having to 'fire the tool' to get the second pump to kick on, obviously the second pump isn't kicking on before you reach your regulated pressure. What is that pressure?

100 PSI is pretty damn low for a compressor to kick off, usually even cheap compressors kick on at ~90 and off at ~120. Sounds odd, like the springs are weak, or they've been messed with. If you get both of them to kick on somewhere around 90 PSI and regulate your air pressure at the tool to 90 or less, you won't have to worry about the compressors being matched, they'll both be running if you need them. Also, they probably won't kick in at exactly the same moment, so they won't load the circuit at the same time (or both circuits). Trying to get the compressors to kick on and off at the same time is unnecessary and overly complicated. K.I.S.S. :D

If you're trying to maintain 120 PSI at the tool, get a bigger compressor.
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regulated pressure is set wide open for wrenching tools right now. not sure how changing that is going to trigger the second pump any differently. one way or the other, the moment one pump triggers i need to run a tool quickly to get the psi down to trigger the other one, or the first pump is the only one that will trigger on that cycle. since my plan is to use a lot of storage this is not optimum.

the new target tool is a sandblast gun in a commercial cabinet so i am trying to maintain over 100 psi at the tool and the more the better. until i can afford a two stage pump, i am going to try this slightly :hillbilly: two pump fix plus a lot of storage. my neighbour has some large old propane tanks i can use for additional air storage.

incidentally, these are older speedaire (grainger) compressors and are decent quality but the factory non adjustable switches are set low. i have been planning to replace them for a while. i have run a pump manually up to 125 psi and there were no issues with the tanks.
 
You probably want to run them in parallel (hooking the two outlets together) rather than in series (hooking the outlet of one to the inlet of the other). Two stage compressors (series) are used to achieve higher pressures, while parallel pumps will give more volume.
 
Pin_Head said:
You probably want to run them in parallel (hooking the two outlets together) rather than in series (hooking the outlet of one to the inlet of the other). Two stage compressors (series) are used to achieve higher pressures, while parallel pumps will give more volume.
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my idea would be to have two separate feeds into the main tank spaced well apart, to avoid any risk of air flow conflicts and traps. i hadn't even considered the idea of feeding one compressor from the other one and i have no idea how you could plumb it since afaik the pumps source air from the atmosphere from the top of the cylinders. i don't know that a normal compressor could handle a compressed air source.
 
semlin said:
my idea would be to have two separate feeds into the main tank spaced well apart, to avoid any risk of air flow conflicts and traps. i hadn't even considered the idea of feeding one compressor from the other one and i have no idea how you could plumb it since afaik the pumps source air from the atmosphere from the top of the cylinders. i don't know that a normal compressor could handle a compressed air source.
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I doubt it, I know the two stage vacuum pumps have to be specialy design to work that way.

Why don't you just calibrate the two pressure switches to similar thresholds?
 
rusty_tlc said:
I doubt it, I know the two stage vacuum pumps have to be specialy design to work that way.

Why don't you just calibrate the two pressure switches to similar thresholds?
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i think i will give in and do that.

my plan is to use a lot of storage, which means that i don't want to have just one compressor running forever to recharge even if i am using it lightly, and also that a small pressure difference between two switches would take a lot of airtool time to trigger the second pump (2 psi at 100 gallons = 30 seconds of firing an air wrench)

i think the solution is not to use the larger storage except when i need it.
 
semlin said:
i think i will give in and do that.

my plan is to use a lot of storage, which means that i don't want to have just one compressor running forever to recharge even if i am using it lightly, and also that a small pressure difference between two switches would take a lot of airtool time to trigger the second pump (2 psi at 100 gallons = 30 seconds of firing an air wrench)

i think the solution is not to use the larger storage except when i need it.
Click to expand...

What I don't get is this- if it takes forever for the pressure to go down another 2 PSI to trigger the second pump, why do you need a second pump? Apparently the first pump can keep up with whatever you're doing. I guess your problem is that the air pressure is already way too low to run your tool when the first pump comes on. Get the switches fixed so they come on at 90 or 100 PSI or whatever your minimum usable pressure is and you'll be a lot happier. One will kick in, and if it can't keep up the other will kick in.

EDIT- Here's some switches that might work for you. These have an adjustable differential, so you can change when they turn on and off a bit.
http://www.grainger.com/Grainger/items/3EYP3?Pid=search
http://www.grainger.com/Grainger/items/3EYR2?Pid=search
http://www.grainger.com/Grainger/items/3EYT5?Pid=search
Or the whole search, http://www.grainger.com/Grainger/ecatalog/N-/Ntt-compressor+switch?Ns=List+Price|0&op=search&sst=All
 
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-Spike- said:
What I don't get is this- if it takes forever for the pressure to go down another 2 PSI to trigger the second pump, why do you need a second pump? Apparently the first pump can keep up with whatever you're doing. I guess your problem is that the air pressure is already way too low to run your tool when the first pump comes on. Get the switches fixed so they come on at 90 or 100 PSI or whatever your minimum usable pressure is and you'll be a lot happier. One will kick in, and if it can't keep up the other will kick in.
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no, the problem is that the planned amount of storage i want to run a sandblaster with is too much for one pump to recharge alone. it will take forever and overheat the pump, which will in turn wear out the pump and increase the condensation in the lines.

so i wanted to ensure they came on almost together even when i was not using a tool that really sucked the cfm, but not at exactly the same time so as not to double the momentary load. the problem is that pressure switches are not very precise.

so i am going to try and solve the problem by (a) buying two new pressure switches and trying to near but not completely synch them (b) not using the extra storage tank unless i need it for sandblasting or something else that really sucks air.
 
semlin said:
no, the problem is that the planned amount of storage i want to run a sandblaster with is too much for one pump to recharge alone. it will take forever and overheat the pump, which will in turn wear out the pump and increase the condensation in the lines.
Click to expand...

Got it, I didn't think of that. See edited post above for some switches that might work. Note that most switches are not adjustable, those are.
 
semlin said:
the problem is that pressure switches are not very precise.
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Thats why you dont want them to be matched too closely. otherwise, someday they will start at the same time.

I would think you would want at least a 10-15 psi difference between the two.

Square D makes nice pressure switches with unloaders. DONT buy new switches without unloaders if you are expecting the compressors to cycle fairly frequently, meaning there is little time for the pressure upstream of the check valve to bleed off. If it hasnt bled off, you will start the compressor against 120 psi in the cylinder. the electric motor will not have enough torque to get the compressor spinning and it will trip breakers.
 
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