Connecting air compressors

[ironpark]

I'm planning to connect my two air compressors to increase my cfm and volume of compressed air to prepare for some painting and using other tools with a higher air requirements and was wondering about the most efficient way to do it.

The two compressors I have are: 30-gallon, 1.6hp, 155 max psi, rated at 6.3 CFM at 40 PSI which plugs into a standard outlet. This one is new and I use it when I need to bring to remote work sites. The motor kicks in when the tank pressure reaches 120 PSI.

I also have a 60 gallon, 230v, single stage 6.5 HP, rated at 11.5 CFM at 40 PSI, 130 PSI and motor kicks in at 100 PSI.

What is the most efficient way to connect these compressors to achieve what I'm looking for?

Would pressurizing the larger tank to 155 PSI be dangerous or is an extra 25 PSI negligible?

If the smaller compressor is running more than the larger, more powerful one is that a consideration since it kicks on at 120 psi instead of 100? It lists a 75% duty cycle.

Thank you

 

[JamesW84]

I'm planning to do the same with 3 smaller California Air compressors. Your max pressure will be the lowest rated tank. Safety first.

What I think I'm going to try to do is run all 3 of mine from one of the switches. In other words, one pressure switch will tell all 3 motors to kick on.

 

[GeoBruin]

I'm planning to connect my two air compressors to increase my cfm and volume of compressed air to prepare for some painting and using other tools with a higher air requirements and was wondering about the most efficient way to do it.

The two compressors I have are: 30-gallon, 1.6hp, 155 max psi, rated at 6.3 CFM at 40 PSI which plugs into a standard outlet. This one is new and I use it when I need to bring to remote work sites. The motor kicks in when the tank pressure reaches 120 PSI.

I also have a 60 gallon, 230v, single stage 6.5 HP, rated at 11.5 CFM at 40 PSI, 130 PSI and motor kicks in at 100 PSI.

What is the most efficient way to connect these compressors to achieve what I'm looking for?

Would pressurizing the larger tank to 155 PSI be dangerous or is an extra 25 PSI negligible?

If the smaller compressor is running more than the larger, more powerful one is that a consideration since it kicks on at 120 psi instead of 100? It lists a 75% duty cycle.

Thank you

You could put a check valve on the outlet of the lower pressure tank so it would only flow when the downstream pressure was lower than the 130 psi tank pressure, but wouldn't allow the tank from the 150 psi compressor to overpressure the 130 psi tank. That would also kind of prevent the smaller compressor from working overtime to fill the 60 gallon tank.

Othwerise, you could just adjust the pressure switch on the smaller compressor to cut in/out at approximately the same pressures as the larger compressor. Then you just tee them together and go for it. Might be worth the safety precaution of adding a 130 psi blow off valve to your larger compressor if it doesn't already have one. Just in case.

 

[Jswain]

You could adjust the pressure switch on the 30gallon to kick out at 130psi and kick in it 100. If it's not adjustable you could easily replace it

If you want to use it(the 30g) the majority of the time then you could adjust the pressure switch on the 60g to maybe 120 & 90, or vice versa if you would rather use the larger compressor for the bulk of your work.

That way for light duty work only one of them kicks in and when you start heavily using air they both will.

I would run them both unregulated to whatever filter/regulator you intend to use and your hose reel downstream from there.

 

[ironpark]

Thanks for the info, seems like it'll be quite easy to hook them up.

Any difference in having one tank feed into the other and the outlet from there vs having both compressors coming to a T or something like Jswain suggested?

 

[Beerhippie]

Thanks for the info, seems like it'll be quite easy to hook them up.

Any difference in having one tank feed into the other and the outlet from there vs having both compressors coming to a T or something like Jswain suggested?

I prefer both tanks connected--no check valves between. That effectively gives you one, much larger, air reservoir and makes life easier on your compressors.

 

[sparky 1971]

I am doing pretty much the same thing with a 60 gallon 3 HP and a 30 gallon that might be 1 HP. The 60 gallon runs from 105 to 155 psi and the 30 gallon from 120 to 150. My shop is plumbed with 1/2" black iron for the air with a "T" at the beginning for the two compressors to connect to with a 1/2" hose for the 60 and a 3/8 for the 30. At first, I just let them run together, but when I would get down to 120 psi, the smaller compressor would kick in, essentially trying fill both tanks and unless I was using a cutting wheel or die grinder, it was unlikely that the pressure would drop enough for the 60 gallon to turn on, making it seem like forever before the little guy shut off.

My solution was a 1" check valve with reducing bushings right after the ball valve on the 60 gallon. Now the 30 gallon still comes on first, but only has to fill itself up, but when the 60 gallon is running, it can help fill the smaller one.

 

[Citation]

Just connecting the two compressors together with an open hose/pipe could be fine but there are a few things to consider.

1. Make damn sure the safety valves and tank pressure ratings of both compressors are good for a 155psi shut off. It would be annoying if one of your blow off valve was set to 160psi and occasionally tripped. Far worse is if the tank's rated pressure was say 150 psi. So make sure both the tanks and blowoff valves won't be tripping all the time. As others have noted you could adjust the 155 psi compressor down if the pressure is too high for the other unit.

2. Connect both with a simple, open hose/pipe. From a pure safety POV this is fine assuming you followed #1. The next issue is which pump does the most work? If both tanks are empty then both pumps will turn on. Together they will raise both tanks to the lower cut out pressure (130 psi of the big tank). After that you will have to wait for the smaller pump to raise 90 gallons to 155 before it cuts out. So long as your demand is light the smaller pump will be the only one doing any work. That could overwork the pump which might be only rated for 50% duty cycle. Only when the load is really high would the larger pump kick in. And, like before it will stop before the little pump.

With the proposed setup, I think the little compressor/pump is going to be working really hard since it will be the only pump that works when dealing with lighter loads. If you draw the system down from 155psi to 115 psi then the little pump alone with pressurize a 90 gallon combined tank. I would personally want the reverse. If possible I would adjust the small compressor so it's kick in/out are just a bit lower than the big compressor. If the little one uses a traditional pressure switch it should be a straight forward change. It also be default ensures that rule #1 is addressed.

 

[Hohn]

I'm planning to connect my two air compressors to increase my cfm and volume of compressed air to prepare for some painting and using other tools with a higher air requirements and was wondering about the most efficient way to do it.

The two compressors I have are: 30-gallon, 1.6hp, 155 max psi, rated at 6.3 CFM at 40 PSI which plugs into a standard outlet. This one is new and I use it when I need to bring to remote work sites. The motor kicks in when the tank pressure reaches 120 PSI.

I also have a 60 gallon, 230v, single stage 6.5 HP, rated at 11.5 CFM at 40 PSI, 130 PSI and motor kicks in at 100 PSI.

What is the most efficient way to connect these compressors to achieve what I'm looking for?

Would pressurizing the larger tank to 155 PSI be dangerous or is an extra 25 PSI negligible?

If the smaller compressor is running more than the larger, more powerful one is that a consideration since it kicks on at 120 psi instead of 100? It lists a 75% duty cycle.

Thank you

If you want to know what's safe for the 60 gallon air tank, check the plate. It should have an ASME plate on it showing what it's rated to handle.

The problem I see with just plumbing them together is that your small compressor will almost certainly bust its 75% duty cycle when trying to pump a total of 90 gallon of tank up from 120psi to 150. As Citation said:

With the proposed setup, I think the little compressor/pump is going to be working really hard since it will be the only pump that works when dealing with lighter loads. If you draw the system down from 155psi to 115 psi then the little pump alone with pressurize a 90 gallon combined tank. I would personally want the reverse. If possible I would adjust the small compressor so it's kick in/out are just a bit lower than the big compressor. If the little one uses a traditional pressure switch it should be a straight forward change. It also be default ensures that rule #1 is addressed.

The easy way to balance the loads I think would be by pressure switch manipulation. With only 75% duty cycle on the smaller compressor, you want to keep it in a smaller pressure window. You also want to set the cut in/out so that the smaller compressor isn't kicking on the minute you use a tiny bit of air.

If your compressors have the same duty cycle rating, then use have one pressure switch (the lower pressure one) control both compressors via Relay. Don't wire both compressors through the same switch or you'll be burning up contacts all the time.
If you have a disparity of duty cycles, use the lower rated duty cycle one as the "backup." That means your little one would only kick in when the big one is already up and running, so you'd have to set the cut in of the smaller unit way down at 95psi or so.


100psi is an almost uselessly low cut-in pressure. Most home shop setups will have more than 10psi of line losses, meaning you'll have no chance of keeping 90psi dynamic pressure at the tool before the compressor cuts in.

 

[Beerhippie]

If you want to know what's safe for the 60 gallon air tank, check the plate. It should have an ASME plate on it showing what it's rated to handle.

The problem I see with just plumbing them together is that your small compressor will almost certainly bust its 75% duty cycle when trying to pump a total of 90 gallon of tank up from 120psi to 150. As Citation said:


The easy way to balance the loads I think would be by pressure switch manipulation. With only 75% duty cycle on the smaller compressor, you want to keep it in a smaller pressure window. You also want to set the cut in/out so that the smaller compressor isn't kicking on the minute you use a tiny bit of air.

If your compressors have the same duty cycle rating, then use have one pressure switch (the lower pressure one) control both compressors via Relay. Don't wire both compressors through the same switch or you'll be burning up contacts all the time.
If you have a disparity of duty cycles, use the lower rated duty cycle one as the "backup." That means your little one would only kick in when the big one is already up and running, so you'd have to set the cut in of the smaller unit way down at 95psi or so.


100psi is an almost uselessly low cut-in pressure. Most home shop setups will have more than 10psi of line losses, meaning you'll have no chance of keeping 90psi dynamic pressure at the tool before the compressor cuts in.

Good points I hadn't thought of. I'm used to our two comps of different types but the same size and output.

 

[ironpark]

Interesting points, thanks for sharing.

My smaller compressor (rated at 155 psi) was very easy to turn down as suggested. It has a set cut-in/cut-out gap with a single screw so I just turned it down to match the larger one for now.

The larger compressor (60 gallon tank) has a continuous duty rating while the smaller has 75% as mentioned so I'll likely turn it down to 95 PSI as suggested so the bigger one can be the primary. The larger tanks tag says the M.A.W.P. is 145 psi by the way so pressurizing to 155 is out of the question regardless.

100psi is an almost uselessly low cut-in pressure. Most home shop setups will have more than 10psi of line losses, meaning you'll have no chance of keeping 90psi dynamic pressure at the tool before the compressor cuts in.

Hmm, does that mean my setup is simply underpowered for what I'm trying to achieve?

 

[Jswain]

It's not the end of the world, IF you plumb it with the right sized line. If you must use 3/8" air line then no longer then 25ft. If you need longer then that use 1/2". Milton V style high flow fittings or equivalent.

Definitely all 1/2" to a blast cabinet. Impacts etc are mostly removing bolts quick enough when you plumb the lines correctly that you can get a lot of work done with the air stored in reserves.

If you plumb it with a small 1/4" npt regulator and 50ft(or more) 3/8" airline and some **** fittings yes it would perform poorly. - this is where people go wrong*

If you are using a ~125psi compressor and a regular amount of appropriate sized air hose there is simply no need to regulate the pressure down any lower for impact/ratchets/air hammers etc. so let it flow as freely as possible.

 

[Hohn]

Hmm, does that mean my setup is simply underpowered for what I'm trying to achieve?

Not at all. I should not have overstated the case as I did.

It just means that it's possible that the stars will align in such a way that you will lose tool performance under some conditions. Line losses are always a function of air flow demand and relative restriction. So, if you have a modest demand air tool with sufficiently sized plumbing (3/4" pipe) and a short final run of hose through low restriction fittings, you'll probably have no issues at all.

In the opposite case of trying to pull higher airflow through a long (50ft) run of 3/8" hose with a high demand tool, you might see dynamic pressures drop as low as 70-75psi. That could cause a performance issue.

Thankfully, most really high demand tools don't need a long steady demand at 90psi. A really high demand tool (like a blast cab) can work pretty well (albeit slower) at lower pressures. Most impacts and fastening tools just need a short blast, and you'll have the bolt loose before line losses really become an issue.

I've pulled my small compressor down to 40-50psi with an air angle grinder and still gotten useful work out of it. But I wouldn't want that to be my default.

 

[cannuck]

You can rig this easily and effectively without check valves to use both tanks at once. Just set the big compressor shutoff at max value for lowest peak rating of the tanks. Just for now assume the big one remains 100/130, so now you need to be able to set the small one for 110/120 IF it is adjustable for high and low points. Reasoning is the big one is carrying the high duty cycle load with the little one only used when big one is at capacity. Problem is: if you air needs exceed the combined total you now have the little one running at 100%, so you might want to put a timer or temp limit switch on it. Since the small one is able 155 it will be at lighter load if not duty cycle at 130.

 

[ironpark]

The tools I plan to run off this setup in the immediate future are a DA sander and a straight-line air sander (8.5 CFM consumption, 90 PSI working pressure) which could be running continuously for quite a bit of time. As well as running an HTE paint gun for painting a car and I worry that pressure drops or getting ahead of the air supply could be catastrophic on my pearl paintjob..

 

[Jswain]

Connect the two now and try it out with the air tools. If you are unsatisfied then sell both and buy a "real" 5hp ~18cfm 80g 2stage compressor

If you are happy enough with your setup for now, but not forever, you have the benefit of time to search for a good used unit depending on the area you live in.

 

[GeoBruin]

The tools I plan to run off this setup in the immediate future are a DA sander and a straight-line air sander (8.5 CFM consumption, 90 PSI working pressure) which could be running continuously for quite a bit of time. As well as running an HTE paint gun for painting a car and I worry that pressure drops or getting ahead of the air supply could be catastrophic on my pearl paintjob..

Unfortunately there is a difference between the rated consumption of your DA sander, and the actual instantaneous consumption. There are lots of discussions about this online, including on this forum, but in short, air tools are often rated using a working factor which assumes that over a given period of time (say, 1 minute) you are only going to be squeezing the trigger for an average of (for example) 15 seconds. In this scenario, you would need to multiply the rated consumption by a factor of 4 to get the actual instantaneous consumption.

Here is a video I made testing the actual consumption of a hand full of different air tools and comparing to the rated air consumption.

Here is another video where I was testing a couple different quick couplers, but I happened to be using a DA sander to test at 90 psi, which is exactly the use case you describe. As you can see, my old DA sander draws somewhere between double and triple your advertised 8.5 CFM. A modern version of this model is still sold and they actually list the max air consumption at 23 CFM. If more manufacturers did this, I think people would be less confused about why their compressors can't keep up with their tools.

 

[cosmokenney]

There is a video on YouTube -- which I cannot find but if you have better google fu you might be able to -- that details how to string several together in a way that one trips on a second or so after the one before it. It prevents them from all kicking on at the same time and blowing fuses.

 

[GeoBruin]

There is a video on YouTube -- which I cannot find but if you have better google fu you might be able to -- that details how to string several together in a way that one trips on a second or so after the one before it. It prevents them from all kicking on at the same time and blowing fuses.

They have a 240 volt compressor and a a 120 volt compressor, so likely not on the same breaker, though arguably you could still stagger the cut in pressure on the pressure switches to minimize the likelihood of a big hit to the main breaker/subpanel main.

 

[ironpark]

Update on this thread; I ended up going a different route and bought an 80-gallon tank on FB marketplace, cleaned it out the best I could and mounted a new Chinese electric motor and Harbor Freight 2-stage pump to it. The pump is rated at 15 cfm at 90 psi so it should be great for my needs but I'm not sure about the electric motor..

I believe I have everything lined up the best I can and have a properly sized belt but the electric motor seems to get very hot while running. The motor is rated for continuous use and is a 25 amp motor so I think it should be plenty good for this pump but when I ran it for 1 hour to break in the pump it got really hot and I actually noticed a small amount of smoke/vapor before I shut it off. It hasn't shut off on it's own for the thermal trip yet which is good but it's getting so hot that I'm afraid to run it much and risk burning out the new motor in case something isn't right. It's rated for 40C which is 104F and I'd bet it's gotten a lot hotter than that. Not sure when the thermal safety thing trips.

The pully diameter should be correct so it isn't that but the pump is rated to use either A or B style belts and I have an A belt on it now. The only thing I can think of is that the pully for the motor and pump still isn't perfectly lined up or that I might need to change out the motor pully to one that can accommodate a B-style belt and try that. The belt doesn't seem to get hot and I don't think it's slipping but could a taller/wider belt help move the pump more efficiently?

Not sure if I can upload a video but the belt is tight at the top while running but has a bit of bounce at the bottom. And like I said, it seems to be lined up well although I'm considering drilling some additional holes or slotting them out to get a little more adjustment and messing with that to see what difference it makes. Any ideas?

Oh, and I replaced the check valve as the one in the tank was bad which is what probably what burned out the old motor and the reason the guy was selling it. So it's not that.

 

[GeoBruin]

When you say you ran it for an hour, you mean you had the valve on the tank open you let it literally it run the pump for an hour straight? That's pretty intense. Under normal circumstances it would at least cycle off when the tank was full. That said, at least if there wasn't pressure building in the tank, it shouldn't have been operating near it's peak power.

How the belt tension?

All new compressors smoke a little at first. I assume its just some of the oil on the surface getting hot, but I would expect that to occur on the pump.

Good it didn't trip the thermal overload protection. Theoretically that means it didn't exceed what it considers it's safe temp. Without more information, I'm guessing you just pushed it hard so it got hot but it doesn't seem like it got excessively hot given the circumstances.

Maybe just try to use it under normal conditions for a while and monitor it?

 

[ironpark]

Thanks, I'll just use it normally and keep an eye on it. Maybe I'm just being paranoid as the reviews on these Vevor electric motors are really mixed from what I read and a lot of people had issues with them failing quickly. And yes, I ran it for an hour with it open after putting oil into the pump to make sure it was all good. Seeing it get so hot and smoking worried me but I figured it was a good sign it didn't actually trip the thermal overload.

I think the belt tension should be good. It's quite tight and is straight across the top with a little bouncing on the bottom of the belt in operation.

I was also wondering if the pump was being restricted as I had to use a 1/2" OD flexible copper pipe to connect to the resivoir. I was originally planning to use 3/4" but the tank only accepts 1/2". The pump was sold with both adapters so I figure it's fine but it is something I thought about.

 

[u3b3rg33k]

Update on this thread; I ended up going a different route and bought an 80-gallon tank on FB marketplace, cleaned it out the best I could and mounted a new Chinese electric motor and Harbor Freight 2-stage pump to it. The pump is rated at 15 cfm at 90 psi so it should be great for my needs but I'm not sure about the electric motor..

I believe I have everything lined up the best I can and have a properly sized belt but the electric motor seems to get very hot while running. The motor is rated for continuous use and is a 25 amp motor so I think it should be plenty good for this pump but when I ran it for 1 hour to break in the pump it got really hot and I actually noticed a small amount of smoke/vapor before I shut it off. It hasn't shut off on it's own for the thermal trip yet which is good but it's getting so hot that I'm afraid to run it much and risk burning out the new motor in case something isn't right. It's rated for 40C which is 104F and I'd bet it's gotten a lot hotter than that. Not sure when the thermal safety thing trips.

The pully diameter should be correct so it isn't that but the pump is rated to use either A or B style belts and I have an A belt on it now. The only thing I can think of is that the pully for the motor and pump still isn't perfectly lined up or that I might need to change out the motor pully to one that can accommodate a B-style belt and try that. The belt doesn't seem to get hot and I don't think it's slipping but could a taller/wider belt help move the pump more efficiently?

Not sure if I can upload a video but the belt is tight at the top while running but has a bit of bounce at the bottom. And like I said, it seems to be lined up well although I'm considering drilling some additional holes or slotting them out to get a little more adjustment and messing with that to see what difference it makes. Any ideas?

Oh, and I replaced the check valve as the one in the tank was bad which is what probably what burned out the old motor and the reason the guy was selling it. So it's not that.

what do you mean by "pretty hot?"

that's not rated for 40C, it's rated for 40C RISE. as in 40C over ambient. so if your shop is 26C/78F, then the motor would be 66C/150F. motor winding insulation is rated higher than the case temp. e.g. a nice inverter duty motor with class F insulation is rated for 155F (winding temp, not motor housing temp).

you'd burn yourself real nice touching something that hot and it'd be in spec.

example:

 

[ironpark]

Thanks for the information, that's good to know. It did get hot enough that you can't really touch it but based on the information you've just shared and the previous post I guess it's probably working as designed.

 

[u3b3rg33k]

you should also expect a compressor head to get hot. I agree with the above comments on duty cycle (it's worth knowing what it is), but even a big SF1.0 compressor head is still going to get to instant burn temps on the heads/discharge line.

what's important is that the oil in the compressor stays at a temperature that doesn't degrade it, or that an oilless compressor stays at temps that don't degrade the seals.

My little big box store oil-free "silent" compressor says no more than 1 hour of continuous runtime. since all I do with it is fill tires and clean things occasionally, I'll likely never hit the 4000 hour lifetime it claims to be rated for.

In contrast, the variable speed unit at work gets an oil/filter change every 1000hrs and runs nearly nonstop.

 

[Jswain]

TBH running for an hour to break in the pump with an open drain valve is pretty easy work, I wouldn't expect the motor to get that hot. Although you do get what you pay for with electric motors, the cheap one on my old IR SS3 would get smoking hot as described but only during normal use.

Is the compressor pulled far enough away from the wall?

Only way you will know for sure is watching amps when motor is running, ensure the amp draw isn't more then FLA x SF at cutout.

With 0 pressure it should be significantly less amps & less heat.