Combine air compressors?
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mike93lx
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What are your concerns with a simple T fitting?
Do the compressors have the same max pressure rating?
Do the compressors have the same max pressure rating?
I think the general answer is yes, it can be done safely. You need to be aware of a few things.
For safety the pressure ratings need to be compatible. That means the blow off value of the lowest system should be higher than the peak operating pressure of the highest system. This isn't an issue if the operating pressures are the same for all.
Make sure the electrical system can handle the starting loads. You will likely need each compressor on its own AC circuit. Ideally they should not start at the same time as that's a big demand spike. So when starting from 0psi manually start them in series. When operating odds are the kick in /out pressures are just enough different that they won't start at the same time. To ensure this you might set the kick in pressures to be a few psi different.
Final part, if your sure demand is more than 1 pump can handle both pumps should help with the load. However, if you are lightly loading the system then only the pump that kicks in first (higher pressure) will do all the work. Probably not an issue for a temporary setup but will result in uneven wear if it happens all the time.
For safety the pressure ratings need to be compatible. That means the blow off value of the lowest system should be higher than the peak operating pressure of the highest system. This isn't an issue if the operating pressures are the same for all.
Make sure the electrical system can handle the starting loads. You will likely need each compressor on its own AC circuit. Ideally they should not start at the same time as that's a big demand spike. So when starting from 0psi manually start them in series. When operating odds are the kick in /out pressures are just enough different that they won't start at the same time. To ensure this you might set the kick in pressures to be a few psi different.
Final part, if your sure demand is more than 1 pump can handle both pumps should help with the load. However, if you are lightly loading the system then only the pump that kicks in first (higher pressure) will do all the work. Probably not an issue for a temporary setup but will result in uneven wear if it happens all the time.
nadogail
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I have routinely paralleled two small compressors when spray painting projects larger than what my needs were greater than my resources.
alwaysFlOoReD
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^^^ same.
theoldwizard1
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The biggest problem with doing this plugging them into the same circuit which might trip the breaker if both start at the same time.
Yes as long as both machines are running, you get the combined CFM. limited only by the pressure drop in your air distribution piping.
The most important thing is that both compressor's air tanks have the same maximum allowable pressure. If not, the shutoff pressure and relief valve set pressure for the higher pressure machine must be lowered to protect the lower pressure machine from it.
FWIW, I have specified compressors, air dryers, and receivers and designed the piping distribution system for multiple chemical plant and refinery units over the years. Typical compressor discharge line size for these systems is 4" and we usually have two or three compressors in parallel, but the requirements for a safe design don't change with size. The relief devices simply get larger or smaller as applicable.
The most important thing is that both compressor's air tanks have the same maximum allowable pressure. If not, the shutoff pressure and relief valve set pressure for the higher pressure machine must be lowered to protect the lower pressure machine from it.
FWIW, I have specified compressors, air dryers, and receivers and designed the piping distribution system for multiple chemical plant and refinery units over the years. Typical compressor discharge line size for these systems is 4" and we usually have two or three compressors in parallel, but the requirements for a safe design don't change with size. The relief devices simply get larger or smaller as applicable.
Same max allowable pressure - Is this the label on the tank or the pressure of the the bypass over load valve?
Both tanks have the same MAWP. They both have different HP and tank size. How should I connect the two tanks?
They don't have to be "the same". However, you need to make sure that the highest pressure out of one compressor doesn't exceed the blow off valve pressure (and presumably tank pressure) for the other compressor.
As an example, if you have two compressors, 125 psi max and 140 psi max things might be OK. Both might use a 150 psi blow off valve. If those are the same then both tanks are safe at 150 psi (the tank max pressure will always be higher). However, if you have a 125 psi compressor and a 200 psi compressor odds are they aren't a good pairing unless you turn down the high one. A 200 psi compressor might have a 230 psi blow off valve while the 125 compressor has a 150 psi blow off valve and a 175 psi max tank pressure (all these numbers are invented). This means the 200 psi pump could operate normally and over pressurize the other compressor. Most compressors do allow you to adjust the pressure switch to reduce the peak pressure. Ideally, both compressors would have similar specs so no changes are needed.
BurtEggley
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What Citation wrote.
I have two connected because one could not keep up when I was glass beading something, or spray painting. You do not connect the outputs. You do connect the tanks. That way any surplus by one compressor is used to fill the other tank. If one compressor runs a little longer to a higher pressure, both tanks are filled to that pressure. What you must be careful of, is that both tanks are rated to the higher of the two compressors. That will mean lowering the highest pressure shut off to the same pressure as the lower one. For an example, one of my tanks is certified for 150 PSI and the other for 125 PSI. I had to lower the 150 PSI compressor to shut off at 125 PSI. Also, be aware you are going to be using twice the electricity so make sure your electrical circuit(s) will handle it. The downside of this arrangement is that if you drain your tanks when done, you will have to fill both for small jobs. You could put a shut off between them if that is a common problem for you.
I have two connected because one could not keep up when I was glass beading something, or spray painting. You do not connect the outputs. You do connect the tanks. That way any surplus by one compressor is used to fill the other tank. If one compressor runs a little longer to a higher pressure, both tanks are filled to that pressure. What you must be careful of, is that both tanks are rated to the higher of the two compressors. That will mean lowering the highest pressure shut off to the same pressure as the lower one. For an example, one of my tanks is certified for 150 PSI and the other for 125 PSI. I had to lower the 150 PSI compressor to shut off at 125 PSI. Also, be aware you are going to be using twice the electricity so make sure your electrical circuit(s) will handle it. The downside of this arrangement is that if you drain your tanks when done, you will have to fill both for small jobs. You could put a shut off between them if that is a common problem for you.
engineer2
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If you want to run them one at a time, it can be done automatically using an "alternating relay". They will operate in and A-B-A-B fashion. If you use an alternating relay that can be cross-wired, they normally will alternate, but will also run at the same time if system pressure gets too low. However you have to build a control box for relay mounting and wiring, depending on what type of compressors you have. With magnetic starters, it's easy, but with home-owner type compressors you may need contactors.
It's not quite that simple. If the tanks do not have the same MAWP, then the relief devices on the system must also be set to protect the lower pressure compressor. Merely changing the setting of the high pressure shutdown on the higher pressure machine isn't enough, as failure of this shutdown leaves the lower pressure machine unprotected from the presumably? higher flow provided by the higher pressure machine.
Relief valve sizing calculations can be complicated, and the manufacturers of the small relief valves typically provided on small air compressors frequently don't provide very good documentation. My advice is to stick with compressors that have identical MAWP stamped on the tanks.
TRWham
Well-known member
I would just make up a manifold using iron pipe and fittings and quick connectors of your choice. You could even use a single tee (maybe 3/4" or 1" IPS) as the manifold with the compressors coming in the ends and the outlet off of the side connection.
So if I can find the same compressor I have today I would be ok. I'm still not clear how to connect the air lines between the two compressors and if I would need to configure a relay or just run on the electrical on different outlets.
Everyone is over thinking this. Connect the outlet of both compressors to a tee fitting, set both pressure switches to reasonably the same pressure, the pressures need to be set at what the lower pressure compressor operates at, and plug them into separate circuits.
You will never get both pressure switches to trip on and off at the same time, one will come on first. Within 5 PSI is great. Under light air use only one compressor will run. As the pressure drops the second will start.
I suppose you can get concerned of the relief valves not being the same and replace the relief on the one compressor to match the lower pressure compressor. If this is an occasional deal, I wouldn't worry about. In reality, it the higher pressure compressor keeps running it will pop the relief on the other compressor.
I used to have a tee with two male quick disconnects and one female quick disconnect. Plug the tee into a hose from each compressor and plug in my air hose, plug them in and go.
sparky 1971
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I have a 60 and 30 gallon connected together through a T, it's nothing difficult. In my case, the 60 gallon kicks on at 105 psi and the 30 gallon at 120 so I put a check valve on the 60 to keep the 30 from trying to fill it.
BurtEggley
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That is way over blown as a concern. The failure of a shutdown switch is just that. If one contactor fails to shut down then you have a problem whether the ratings are equal or not. You will be tripping one or both safety valves regardless, and you would still have a problem whether the tanks are connected or not. I will use my two compressors as an example. The 150 PSI unit has a safety VALVE of 175 psi. The 125 PSI unit has a safety VALVE of 150 PSI. If there is an over pressure, the lower safety switch (valve) will trigger first, dumping the pressure. It is unlikely both shutoffs would fail simulataneously, causing both compressors to overwhelm one safety switch, which might not even be possible. Both shutoffs are set to 125 PSI, which is the lower certified tank, so one compressor would already be off unless both failed. Having two shutoffs fail at the same time, and the lower safety valve, along with a tank failure over 25 PSI difference (15%) then you might as well ask God to make it painless because your number has come up. That is like saying the four engines on the airplane all quit, the radio is out to transmit a position and pan pan, no one knows we are here, we are somewhere over the ocean but we don't know where, the lights don't work, and the seas are rough. The life raft is missing too and the water is 33F while we are all in Bermuda shorts. Plus everyone is a diabetic and they haven't eaten in a day so their blood sugar is low. Wish it weren't so but your time is up. A friend of mine used to get a NHTSA report of unusual car and truck accidents monthly. It would be report in it where a motorcyclist impatiently and blindly passed a loaded dump truck about 60 mph only to hit another one head coming the other way about the same speed, just as the one he passed decided to make an ill timed left hand turn in front of the truck that the motorcyclist had gone headon into. Time was up.
That said, the safety valves are only a couple bucks each so if someone wants to replace them to match it is under $20. New ones do not require an engineering degree to buy. Home Depot and Lowes both carry them. Maybe ACE too. AND, in this forum, someone with an industrial compressor is not asking how to hook two heavy duty higher volume and PSI industrial compressors together to spray paint. They are hooking them together because they are both low volume compressors, probably made by Devilbiss 15 - 20 years ago. If they are newer then they are buzzers and those put out even less air.
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what check valve specifically are you using? Is this a one way valve like this? https://airtanksplus.com/product/quick-connect_in-line_one-way_check-valve_w_bleed_valve/
Set one at 150-175, the other at 125-150.. or whatever pressure you're at.
For light duty units,
125-150 and the other 100-125.
2nd pump only kicks on if 1st fails to start or can't keep up.
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sparky 1971
Well-known member
The tank has a 3/4 port on the side and I used a 1" check valve with reducers. That, and I picked it up in the plumbing department at Menards is all I can tell you.
Does the check valve allow air to only move in one direction?
You are assuming the safety valves are the same size. If the lower pressure machine has a smaller capacity the safety valve may not be the right size.That is way over blown as a concern. The failure of a shutdown switch is just that. If one contactor fails to shut down then you have a problem whether the ratings are equal or not. You will be tripping one or both safety valves regardless, and you would still have a problem whether the tanks are connected or not. I will use my two compressors as an example. The 150 PSI unit has a safety VALVE of 175 psi. The 125 PSI unit has a safety VALVE of 150 PSI. If there is an over pressure, the lower safety switch (valve) will trigger first, dumping the pressure. It is unlikely both shutoffs would fail simulataneously, causing both compressors to overwhelm one safety switch, which might not even be possible. Both shutoffs are set to 125 PSI, which is the lower certified tank, so one compressor would already be off unless both failed. Having two shutoffs fail at the same time, and the lower safety valve, along with a tank failure over 25 PSI difference (15%) then you might as well ask God to make it painless because your number has come up. That is like saying the four engines on the airplane all quit, the radio is out to transmit a position and pan pan, no one knows we are here, we are somewhere over the ocean but we don't know where, the lights don't work, and the seas are rough. The life raft is missing too and the water is 33F while we are all in Bermuda shorts. Plus everyone is a diabetic and they haven't eaten in a day so their blood sugar is low. Wish it weren't so but your time is up. A friend of mine used to get a NHTSA report of unusual car and truck accidents monthly. It would be report in it where a motorcyclist impatiently and blindly passed a loaded dump truck about 60 mph only to hit another one head coming the other way about the same speed, just as the one he passed decided to make an ill timed left hand turn in front of the truck that the motorcyclist had gone headon into. Time was up.
That said, the safety valves are only a couple bucks each so if someone wants to replace them to match it is under $20. New ones do not require an engineering degree to buy. Home Depot and Lowes both carry them. Maybe ACE too. AND, in this forum, someone with an industrial compressor is not asking how to hook two heavy duty higher volume and PSI industrial compressors together to spray paint. They are hooking them together because they are both low volume compressors, probably made by Devilbiss 15 - 20 years ago. If they are newer then they are buzzers and those put out even less air.
Considering the OP needs help on how to connect the two compressors, advising anything less than industry accepted overpressure protection seemd unwise.
I know my answer is conservative, but I PE stamped hundreds (maybe over a thousand) of relief device calculations after OSHA 1910 made industry have documentation that they were sized correctly. I know a bit about how this works.
Yes, if it fully seals off. If not, it would allow the higher air pressure to bypass it towards the the lower pressure.
sparky 1971
Well-known member
Yes. Don't put it on backwards.
I have never seen a CFM rating on a compressor relief valve.
IMO, most relief valves for a compressor that most of us will see as long as the relief valve has he correct pressure it will work just fine, say up through 5 Hp.
Excluding the guys with a 50 CFM scroll compressor.
That is part of the problem with the inexpensive small brass relief valves. The valve orifice size is generally unknown or unmarked. I've dug through relief valve manufacturer's catalogues and can assure you that they are absolutely not all the same size.
I agree that several of the shortcuts suggested above would "probably" work, but "probably" isn't good enough in my opinion. I would never design a system that would "probably" be safe.
It should.
If you plumb two together, without check valves, the air pressure will equalize between the two tanks.
What is it that you are trying to accomplish, just more reserve air ? If this is your goal, I would get a second tank, similar to the specs of what you have and plumb it into the system.
I need to increase cfms. Trying to run an outdoor sand blaster.
BurtEggley
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I added the second tank and compressor to glass bead. At the time my one compressor could not keep up. I read many posts here that described how to safely join two compressors. As to safety valves, yes there are different pressures. It takes less than $20 to replace both with identical safety valves. Both pressure range valves show in stock at my local big box stores. There is a 25 psi difference between the two consumer safety valves sold. Go with the lower if you are fragile. If 25 psi makes a big difference to you, it is probably time to replace the compressor and tank. And if you do that, be sure to cut the old tank up so no one can blow themselves up with a 25% overpressure. 
Most likely a 125 psi tank is certified close to 300 psi. To be really frank, I have been trying for five years to find someone to hydo test my just under 30 year old compressor tanks, and no one will do it for residential type units. Only big commercial tanks. They gave me some number at the state OSHA who will inspect them VISUALLY to see if they look Ok. No one will hydo test them.
Most likely a 125 psi tank is certified close to 300 psi. To be really frank, I have been trying for five years to find someone to hydo test my just under 30 year old compressor tanks, and no one will do it for residential type units. Only big commercial tanks. They gave me some number at the state OSHA who will inspect them VISUALLY to see if they look Ok. No one will hydo test them.Are you referring to this item?I added the second tank and compressor to glass bead. At the time my one compressor could not keep up. I read many posts here that described how to safely join two compressors. As to safety valves, yes there are different pressures. It takes less than $20 to replace both with identical safety valves. Both pressure range valves show in stock at my local big box stores. There is a 25 psi difference between the two consumer safety valves sold. Go with the lower if you are fragile. If 25 psi makes a big difference to you, it is probably time to replace the compressor and tank. And if you do that, be sure to cut the old tank up so no one can blow themselves up with a 25% overpressure.
or this one way check valve?
BurtEggley
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the brass one. It pops and lets out air if the contactor ever turns permanently on like the contact points weld. The contactor is adjusted to come on below a certain pressure and off at a set pressure. If two compressors are being used, and there is a big difference between the two pressures, one compressor will be running close to 100% of the time under heavy load. The brass safety valve prevents overpressure.