Thread: Fitting needed to run 2 compressors together

Originally Posted by Arron

Hi. I have two small (2hp) compressors and want to run them together - in parallel - to save me the expense and trouble of buying a larger compressor. I need a T join for this. Anyone know where I can buy one ?

cheers
Arron

Bunnings have T-junctions. You can also use galvanise water pipe fittings or even copper plumbing fittings and decent hose clamps.

What are you hoping to do with these? In terms of general compressor use, unless you do some specific wiring to get them both to trigger at the lo=pressure only one will every trigger as the pressure won't normal go low enough to trigger the other. You will need to over draw both tanks to get the second one to trigger. Whether this happens will depend on what you are proposing to do. What happens in the long run is that under normal use the one that triggers first will wear out quickest and the other will not be much used..

Where do you want to join them?

If you connect them after the regulators, you also need to get the regulators to sync up, otherwise the one with the greater output pressure would effectively put the other regulator into cuttoff as it senses that the output pressure is greater than what is dialled up and blocks its tank feed until the output pressure drops to the dialed in value.

Ideally, if I had to do it, I would remove a plug in the receiver end cap of each and join the tanks at that point ( pre regulator), feeding air through a seperate industrial sized filter reg setup, as often the filter reg on the smaller compressors is quite restrictive.

As Bob suggested, getting the presure switches in sync will be important, but you can't allow one compressor to be a Master and switch the second via a relay or contactor, as that would not operate the manifold bleed line part of the pressure switch on the slave unit, causing problems with the slave unit starting, and probably burning out the motor reasonably quickly.

Also for 2 x 2HP motors they would need to be on seperate mains circuits to avoid tripping breakers on start up.

Originally Posted by Arron

Hi. I have two small (2hp) compressors and want to run them together - in parallel - to save me the expense and trouble of buying a larger compressor. I need a T join for this. Anyone know where I can buy one ?

cheers
Arron

G'Day Arron,
I'd encourage you to buy a steel fitting for safety sakes on compressed air.
You could try an engineering suppliers like Bakers at Blacktown or Blackwoods at Smithfield.
Some of the larger plumbing suppliers might also have them too.
I used to work at both Sullair & Atlas Copco a number of years ago and would encourage you to be very safe with the compressed air and your fittings.
Cheers, crowie

Another thing I forgot to add is what sort of improvement in performance do you expect, in terms of continuous air delivery, 2 cheap x 2HP compressors won't even add up to one 3HP quality compressor. The problem is not the HP its the compressor head. A high quality 3HP compressor can approach the theoretical maximum of ~4 CFM/HP where as a 2HP cheapie can only do about 2 CFM/HP.

Thanks for the replies.
I wanted to use it for spray painting. I'm not actually after higher pressure, just trying to get the same pressure as would come out of one 2hp compressor but have it last for longer - about twice as long would be OK as I mainly spray small objects. I think, therefore, its more an issue of tank volume (perhaps ???). I use an Hvlp gun and mostly have the air turned down low. I'm also not worried about one of them wearing out first as both are just cheapies and if I wasnt using them for this then I guess they are no use to me. My main issue is that I dont want anything dangerous.

Still, its starting to sound like one of those things which is more hassle then its worth.

Cheers
Arron

As has been said, I would use one compressor supplying both receivers

receivers are the tanks, right ?

Quick google search indicates it is. That makes good sense. Only one compressor motor running, but filling both receivers, and outletting through one regulator only. Sounds safe and simple, thanks all.

There's a good reason they don't make cheap 2HP compressors with tanks bigger thank about 40L. While the compressor head is running it gets hot. 90% of the wear happens in the last half of the compression cycle when the compressor head is at its hottest. With two tanks it needs to run for twice as long but it also needs to run comparatively longer when it is at its hottest. Sure, it only needs to fire off half as often but it still spends more of its compression time/life at a higher temperature than it otherwise would. Using a compressor this way will wear it out faster than using one compressor until it dies, and then using the other till it dies. The best way to use these two compressors is probably to alternate between using them as individual compressors.

we do this at work sometimes when running to many machines we use x2 large single pahse compressors,

one is setup with a manifold with 4 outlets of it, the other as a simgle outlet. we have a small length of hose which goes out the single and into the manfold.

the 1st compressor cycles more than the second the second only kicks in after the first one is drained and the second is also trying to catch up.

many years ago i also had an old compressor that we used as a storage tank it was in parel to provide more stored air.

hi all,

using compressors in parallel needn't be unsafe, provided both tanks and pump systems can take the maximum pressure of the combined system. The problem of one compressor wearing quicker than the other can also be solved to a large extent.

If i was to make such a parallel system, i would first seek a way to make sure that both compressors start and stop at different times instead of exactly at the same moment. You can manage that through the settings of the pressure sensitive switches (pressostats) that are mounted on every automatic electric compressor. I would first thoroughly measure at what precise level each compressor would start (e.g. when the tank pressure drops below 6 bar) and when it would stop again (e.g. 8 bar). Make sure that you exactly know these thresholds of both compressors. The objective is, to slightly lower the tresholds of one compressor only. The pressostat switch has an adjustment screw for this. Before turning it, mark its original position, then turn it slightly in the "-"-direction. NEVER turn it in the "+"-direction, for this will increase the maximum pressure and thus the upper threshold on which the motor shuts off. Exceeding the factory set pressure max will overload the motor and will drive the pressure on the entire system (joints, weld seams, seals, piping, cylinder head and tank) beyond a safe and factory tested level. Needless to say that you have to have experience with compressors and know your way around their components before you attempt this and be sure of a safe result. Tinkering with pressostat settings may (worst case scenario) result into plumbing failure or tank rupture if you don't know exactly what you're doing.

After having turned the screw a quarter turn or so in the "-"-direction, test the compressor again. Bleed air from the tank until the motor starts and note the gauge needle's setting. Also note the moment and the needle setting when the motor shuts down again. 7.5 bar and 5.5 bar (instead of the earlier 8 and 6) are what you are aiming for.

When both compressor are used in parallel, in fact you use both reservoirs in parallel, doubling the amount of tank liters, which is good for spraying (nice buffer). To start the system up, you switch on the compressors on by one, with a few seconds in between. Switching on both motors at the same moment would cause one massive amp peak, which will blow a fuse. Both compressors are now jointly building up the pressure in their shared tanks. The needle will eventually move up to 7.5 bar and the pressostat which was adjusted down a bit to 7.5 bar will be tripped first, shutting down its connected motor. The other motor will continue running until both tanks are topped up to 8 bar; after that the motor connected to the pressostat with its original 8 bar setting, will also shut down.

Spraying consumes a lot of air, often more than a single 2 HP compressor can cope with (that's why you thought up this setup in the first place). So the tank pressure will drop fast, and the motor connected to the 8/6 bar pressostat will start up first, because on the way down the gauge needles will pass 6 before they reach 5.5, meaning that the 8/6 pressostat will be tripped before the 7.5/5.5-one. But if the air consumption is larger than the pumping power of the single compressor running at that moment, the needle will rather sink further down than climb back up beyond 6 bars, so when the tank pressure drops below 5.5 bar, the second pressostat kicks in and starts up the second compressor. The time lapse between 6 and 5.5 bar will serve as a time lapse between the two start-up peaks, limiting overall startup current in a dependable way. When the tank pressure again reaches 7.5 bar, the "7.5/5.5"-compressor will stop first and the other one will again keep running to top up the tanks to the 8 bar max.

Indeed the "8/6"-compressor will need to work harder (against higher pressures during its duty cycles) and the "7.5/5.5"-compressor will have a much easier life. But you could alternate that every few months or so. Screw back up the 7.5/5.5 pressostat to its original 8/6 values (the original marking, make sure not to exceed 8 bar!) and reduce the pressostat settings of the other compressor from 8/6 to 7.5/5.5. The 8/6 is in fact the main pump and the 7.5/5.5 is the back-up, kicking in when the main one can't deliver fast enough. By marking both the 8/6 and 7.5/5.5 settings, it is easy to find the proper positions back quickly. Be advised, though, that these screws are not very precise and are also not laid out for tinkering too much with them. By which i mean to say that you can't rely on your marking still guaranteeing a proper 8 or 7.5 bars after some ten or twenty times tinkering around with (due to screw tip and thread wear). To be sure, do a test run after each pressostat adjustment session to at least make sure that the upper threshold of 8 bar is not exceeded. A further safety measure is to secure the adjustment screw (Loctite fluid or sealant laquer).

When i were to build this parallel array, i would choose to leave both compressors to remain in as original a state as possible. Leave on their pressure reducers and moisture traps and couplers. Take two lengths of hose and apply fittings that will fit the coupler bajonets. Let the other ends of the hose lengths come together in a T- or Y-manifold, with the joint outlet of the manifold screwed into a third pressure regulator and moisture trap. Set the regulators of both compressors in a high setting (6 bar or so) and regulate the final working pressure (to the spray gun) with the third regulator. It is advised to use a small buffer tank in combination with the third regulator, to dampen any pressure fluctuations, improving air feed stability to the gun.

By simply uncoupling the home made "tank parallelling contraption", you are left with the two original compressors, which can each be used on their own, as before.

greetings

gerhard

What a marvellous and detailed explanation! Thank you Gerhard

Been there done that, as has been explained the difficulty is in getting each comp. to operate as one so you get the extra air output, Gerhard's tuning of the pressure switches is an option if you can tolerate a lower pressure switch on point of the second compressor,

In a previous life I setup a system using just one pressure switch mounted in the factory pipework manifold, this switched three compressors on in succession using delay on timers (so they didn't all start at once), it was also possible to alternate which compressor was the master and the other two were "slaves" to even up wear, your twin system could be simpler than this but done along the same thinking, you will need a sparky to put in a timer and wiring mods.

Also be aware of water and oil in your air coming out the gun, not good for spraying finishes


Pete

I am with Gerhard on this. Makes good sense. A number of things of concern are mentioned here which I believe are not applicable. Wear of one compressor over the other one. If only one compressor is used then it will wear out much quicker anyway. Settings? I would just try it and see what happens without changes first.

I have the same issue with a 2.5hp unit. I have connected a 45kg gas bottle as an extra receiver for use only when doing high volumn jobs such as spray painting. When using this setup with 2 compessors one is going to be running full time anyway while the other one may cut in and out or may run full time.

Dean