Beware of youtube videos about connecting 2 compressors together

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  1. Bill Crowell

    Bill Crowell FABO Gold Member FABO Gold Member

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    I hope I don't bore you guys to tears with this, but I felt the need to vent. There are quite a few youtube videos where people claim to have constructed a giant air compressor by simply connecting together the output lines of two small compressors. That is clearly not going to work correctly because these 'tubers don't seem to understand an air compressor's theory of operation.

    You can't use two regulators to control one compressor because one of the regulators is always going to shut off before the other one does, even if only slightly before, and then the compressor that's still running will start pumping to the atmosphere, either through the other compressor's intake valve or its unloader valve, and will keep running forever. Maybe you could correct this by using one-way valves, but it would be a kludge.

    Here's how I built a 2-compressor system (picture). I use one regulator for both compressors, and since I doubted its contacts could handle the combined current draw, I use it to control a Worldwide WDP60-3L-230 60-amp "definite purpose contactor" for the two motors. I mounted the contactor in a Square D NEMA box no. 9991UE1, which is about 6 X 3.5 inches, and which was supposed to be the correct size for the contactor, but if I had it to do over again I would have used a slightly larger box because the contactor just barely fit and it was too cramped making the connections.

    I'm not a compressor expert, so if anybody thinks I've made any mistakes in constructing my 2-compressor system, feel free to fire away.

    compressors.jpg
     
    Last edited: Sep 30, 2020
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    • Cuda416

      Cuda416 FABO Gold Member FABO Gold Member

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      Dumb question, why do this at all?
       
    • brian6pac

      brian6pac Well-Known Member

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      I think what you are calling a regulator is actually a pressure switch that turns off the motor when the pressure gets to the high pressure you want.

      You should have 2 contactors one for each motor and use the one contactor to power the other contactor the pressure switch to turn one contactor on and off.
       
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      • Bill Crowell

        Bill Crowell FABO Gold Member FABO Gold Member

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        Because you can get old Campbell-Hausfeld compressors almost for free, parts are readily available, they are easy to repair, pretty dependable, and two of them create plenty of air flow for sandblasting. In fact, I'll go so far as to say 2 C-H compressors kick butt, air flow-wise!

        OK, OK, I'l make this a shameless plug for the old C-H cast iron compressor. Built like a brick outhouse. As George Gobel would say, "they don't make them like that anymore".
         
        Last edited: Sep 30, 2020
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        • Bill Crowell

          Bill Crowell FABO Gold Member FABO Gold Member

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          It's a combined compressor shutoff pressure switch and an air pressure regulator that has tank pressure available on one side and regulated pressure on the other. It's a really nice unit that they used to use on Sears and Montgomery Wards compressors with the C-H compressor back in the '60s through the '80s.

          Maybe I didn't explain it well, but I'm already doing what you suggest. The pressure switch has its own set of fairly hefty contacts inside, which are 'way overkill for the minimal current draw of the second contactor coil.
           
          Last edited: Sep 30, 2020
        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          I think part of the problem comes from the type of pressure switch and unloader. There are three or 4 types of unloaders that I know of, maybe more.

          1...Head realease via the pressure switch. This is popular in the smaller compressors up through 2hp and has a separate tube from the pressure switch to the head. When the switch opens, the switch opens a valve in the switch assembly and drains the head

          2..."Load genie" or other names. This one is incorporated into the check valve from the compressor to the tank, and when the compressor stops, the pressure is equalized between compressor and tank. This causes the "load genie' to shuttle, and opens a bleed hole in the load genie, draining the head. There are no other lines or attachments. One tube from compressor to load genie, screwed right into the tank

          3....Mechanical release at the compressor. This is some sort of mechanical linkage right on/ in the compressor and causes the head release valve (unloader) to open when the compressor stops rotating.

          4...Forget the name.........gasoline........There is a type of control meant for gas engines. They have a piston or other device to operate the governor, and idle down the engine when pressure is met, and an unloader device similar to a load genie.

          No two ways around it you are going to have to have a pair of check valves to isolate the two compressors from whatever comes after, whether the check valves are part of the unloader system "or what"
           
        • RustyRatRod

          RustyRatRod Lemmie see your b00bs. FABO Gold Member

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          No way in hell would I do that anyway. What I would.....and MIGHT do is connect two TANKS together. But that would be it.

          And thanks Bill. You always post good stuff....not boring at all.
           
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          • 67Dart273

            67Dart273 FABO Gold Member FABO Gold Member

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            Bill your photo is really too small, one problem I see is "no strain relief" with the rigid pipe from the upper to the lower compressor pump

            Also the plumbing routing may be wrong. You may HAVE to use one more check valve if there isn't one----from the upper pump.

            Personally I see nothing wrong with the idea, 'run what'cha brung.' I have 3 compressors, all bought used. One works well, but I use it little, it's a high speed direct drive. I use it for "more when I need it." The other is also direct, and the tank leads at one of the plug threads. "I was always gonna fix it" so it sits there. The "main" one is a belt drive I picked up, all that was wrong..........needed a belt and new switch. I think I paid 80 for it 2 1/2 hp
             
          • Bill Crowell

            Bill Crowell FABO Gold Member FABO Gold Member

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            Del, thanks for the reply. When the picture was taken I was working on the system and some stuff was disconnected.

            For the life of me I am not understanding why you say I need to use two check valves. I'm just using one, and it seems to work, but of course maybe I am overlooking something. I sure would appreciate it if you would explain that.
             
          • Bill Crowell

            Bill Crowell FABO Gold Member FABO Gold Member

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            Rusty, thanks for the kind reply, but it seems to me that connecting two tanks together, each with its own separate compressor and pressure switch, would create the same problem: one compressor would shut off first, maybe with its intake valve open, and certainly with its unloader valve open, so the tank pressure would bleed off to the atmosphere and the other compressor would run forever. Am I wrong? Am I wrong??!! I'm sure we will figure it out.
             
          • RustyRatRod

            RustyRatRod Lemmie see your b00bs. FABO Gold Member

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            I said two tanks. Not two compressors. lol

            ...and no, you're not wrong.
             
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            • 67Dart273

              67Dart273 FABO Gold Member FABO Gold Member

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              As I said I cannot really see your photo, and it depends on the unloader. But since you are cutting power at same time, and "I guess"? the pump outputs are connected together, this means either/ both or just one unloader should drain both heads. If that is the case then just one check into the tank would do. So main issue I see is the rigid pipe. Don't use hose, but you need something for vibration/ heat expansion which is an issue. "Usual" solution is a coil or U bend of copper tube (or aluminum on smaller stuff)
               
            • Professor Fate

              Professor Fate Push the button, Max...

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              I think what Rusty is saying is he would connect two TANKS together- not two compressors- and luxuriate in having double the volume available... :)
               
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              • dartnabout

                dartnabout FABO Gold Member FABO Gold Member

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                I'm stupid here ….but interested....would not connecting the 2 tanks together create more volume for a compressor that already cannot keep up with say a sand blaster. I have to blast and wait, blast and wait, blast and wait
                 
              • mopowers

                mopowers Well-Known Member

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                This is interesting. In theory, would the CFM capacity of the compressors be additive? In other words, would two 10cfm compressors provide 20 cfm in this configuration?

                Also, is there a good schematic out there that shows the dummies like me how everything is actually connected? Or is it really as simple as running the two motors off of the same pressure switch, so when the tank comes up to pressure, it shuts them both off at the same time?
                 
              • 67Dart273

                67Dart273 FABO Gold Member FABO Gold Member

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                I'm sure capacity would add. I see no reason it would be diminished.

                So far as electrical, re-read what Bill did. The original pressure switch is not heavy enough to operate both motors. IF that were the case, yes, you could simply wire both motors to the same switch. What he did was use the pressure switch to operate a larger contactor (relay) which DOES have large enough contacts

                Of course doing so, at some point, you need enough "oomph" with your electrical service to the garage!!!!
                 
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                • mopowers

                  mopowers Well-Known Member

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                  Good point. You'd have to add the amp draw from both motors and make sure the circuit is sized accordingly. Not sure my lowly 30 amp circuit would do it.
                   
                • RustyRatRod

                  RustyRatRod Lemmie see your b00bs. FABO Gold Member

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                  RIGHT! One compressor, two tanks.
                   
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                  • Cuda Al

                    Cuda Al Well-Known Member

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                    Personally I'd just segregate the two systems and switch between them, one runs low, unplug hose move to the other or do it with ball valves, seams so much easier.


                    Alan
                     
                  • 68gtxman

                    68gtxman I used to reMember

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                    I’ve done this with two simple/unsophisticated compressors with tanks and interconnecting them with a flexible air line. Each compressor shuts down when it’s pressure switch shuts It down, but the other runs and charges both tanks until it too shuts down. Neither compressor has an unloader. Both are protected by pressure blow-off safety valves. I also added in an additional pressure tank too. I’ve been doing this for years.
                     
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                    • jas0162

                      jas0162 Well-Known Member

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                      I have two pump/motors that run simultaneously from one switch operating a contactor. Both pumps are unloaded by the one unloading valve and pump through the one check valve into a single tank. Works great, I can sandblast all day with no waiting.
                       
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                      • Tooljunkie

                        Tooljunkie King of cobble/master of the broken bolt FABO Gold Member

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                        I plan to run 2 compressors/ tanks. Second one will only kick on at 80 psi. When i blast its usually only for a few minutes.
                        Longer sessions and i need to keep it from dropping below 80 psi. If you pair both tanks together, the check valves in each one prevent back flow. Be it a load genie or whatever.
                         
                      • toolmanmike

                        toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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                        They do make a alternator switch for dual compressors. (not the kind of alternator you may be thinking about). It runs on compressor, shut;s it off and then runs the other. It alternates between the two to spread out the wear. Otherwise with just pressure switch settings, one compressor runs all the time and the second only when needed.

                        duplex.jpg
                         
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                        • sireland67

                          sireland67 FABO Gold Member FABO Gold Member

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                          I have had my compressor set up this way for years, it really increases reserve volume.
                           
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