Tandem compressors - good idea?
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  1. #1
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    Default Tandem compressors - good idea?

    My shop air has been supplied by a Sanborn 5hp two stage 80 gallon compressor for many years. The only complaint has been the excessive noise that is characteristic of high speed aluminum pumps.

    I just bought an Ingersoll compressor with the same specs except it has a nice, slow, quiet cast iron pump.

    Rather than replacing the Sanborn, I have considered tying both compressors into the shop manifold. There would be check valves on both tank outlets. Setting the pressure switch a few lbs. under the IR should make the Sanborn act as a backup, coming on only during periods of high use. (ie. arc gouging - blasting).

    Has anyone done this and what problems do you see?

    Thanks,

    SCOTTIE

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    seems I just saw someone (mac or snap on, IIRC) selling a compressor with 2 pumps on the same tank, just for what you describe (having 2 motors running during high demand times), the only difference was the 2 motors were identical, and there was some kind of switch built in to alternate which motor ran to fill the tank during lower-need times. So, yes, it can be done relatively easily, but I'm not sure where you could get the parts to 'home-brew' it (or even if they are either special or off-the-shelf common parts)

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    to get it to work right, there are controls that " alternate " which compressor runs first. Without said control you run the nuts off the compressor in line and the second won't engage hardly at all. a second compressor is a great backup, but the control makes the difference in how they work to get proper duty cycle on both instead of 1 wide open 24/7 and one barely coming on.

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    I don't believe he wants to alternate which pump runs. I think he wants his new compressor to run 99% of the time with the old compressor only kicking on when the new compressor can't handle the load. His old compressor was noisy and he doesn't want it to run. But given the choice between not enough air or some extra noise, he is okay with the old compressor running when the extra air is needed.

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    It sounds like you want to run the slow quieter one most of the time and use the other as akicker. I set up two 15 hp units to do this in a shop using lots of air polishers. I tried to get botn machines to turn on at the same time for initial setup then lowered the cut in pressure on one by a few psi. This would let one run most of the tank filling requirements then the other could cut in when the demaned got higher than one could handle. To get them to both come on at the same time, just use one pressure switch for both tanks, pipe the two tanks with no check valve between them.
    The 'back up' unit (Speedair Value-plus series bought new) decided to hammer out the bearings for the second time in a year so I got an old 20 hp Worthington as the main compressor with 1, 15 hp piston Dayton to use when the demand isnt as high.

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    Scottie, Since your Sanborn is old and most likely tired, and you only want it for high demand and back up, you can tie them together just fine. You do not want check valves at each tank...the compressors already have them between the pumps and their tanks, and you will want to use the extra stroage in the Sanborn tank anyway. Each compressor has a pressure switch that turns on the motor when a preset low pressure is reached. You can set each compressor's switch to cycle on with say a 10 psi difference, so the I-R runs most of the time, and the Sanborn only kicks on when the shop air drops 10 psi below the lowest point for the I-R. The pressure switches on recip compresors usually have about a 30 psi differential - i.e. on at 110 psi, off at 140 psi. Set your pressures as low as possible that will still give you the needed performance - high pressure cost more in electricity and wears out the pumps faster...you generally need volume at a certain pressure (~100 psi) for most shop operations anyway. We ran three 10HP compressors this way where I used to work before we stepped up to a couple of 40HP rotary screws (when we installed the second compressor, we did the alternating thing like Willie mentions).

    So, if you want 100 psi shop air all the time, set the I-R switch at the 110-140 range, this will allow for 10 spi drop in your lines. Set the Sanborn switch at 100-130; it will only come on when shop air drops below 100 psi - when the I-R cannot keep up.

    Willie has the right idea for full out production, where you want to have a lead-lag, alternating situation, so your compressors are exercised equally. If you need that much air on a continuous basis, a piston compressor is not the way to go...you would most likely want a rotary screw unit. These controls are not cheap, and probably not what you are looking for anyway...you want the Sanborn semi-retied.

    I think you are best off keeping the Sanborn as you mentioned...you won't get much for it used, and having it as a back up could someday save your butt. Plus, when servicing the I-R, the Sanborn can be on line keeping everything going.

    Steve

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    I used to maintain compressors at an old job. We started out with a 5hp, then added another 5hp then added a 10 hp , all 2 stage 175psi. , added as business increased. I had all of them on a manifold with a air shut off valve on each for maintainance purposes. I just set the cut in pressures staggered depending on which unit I wanted to run first. I forget the order that I set now but it's easy to change as needed.
    The best thing is the second compressor will work even less because you have more storage.

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    20 years ago i got caught with my one ond only pump down.

    no air
    no work

    never again

    i now have 5 pumps

    all plumbed, wired and ready to go

    any 2 can handle the demand most of the time

    your plan sounds good to me
    and i agree with no check valves on tank output
    extra tank will just help with surge

    to be totaly seamless every pump needs it's own service disconnect

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    I agree with wippin' boy.

    I keep 2 compressors in my garage. Good thing cause the motor on one of them crapped out last week. I simply umplugged it and plugged the other and back to work. Took a whole minute to do so.

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    Sounds like a good plan, I cant think of any pitfall's, as long as you've got power covered.

    Compressors suck lot's of amp's, especially starting amp's.
    More than likely a new dedicated line would be required, rather than just adding another disconnect to what you've already got there.

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    this is done all the time, good and bad aspects. Good, is auto back up, also you can run only one during reduced demand, etc. One negative to consider is that you have two machines to maintain.
    The double headed compressors have two applications, one is for critical applications like spinkler presurizing where you can never be with out air, the other is variable loading.

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    I have an older 15 hp Sullair rotary screw and a newer 25 hp Sullair rotary screw running on the same manifold where I work. The 15 is set about 10 psi lower than the 25. The main thing that I did wrong when I hooked them together was NOT putting valves between the compressors and their tanks. If one is off line for maintenance, the tank goes off line with it. I don't know if this could be done with piston compressors or not.

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    One related idea I've had is to mount two electric motors, one on either side of the compressor and belted to the same "fan sheave".

    One motor would be the OEM/stock motor used for "airflow performance" while the other motor would be something around 3/4 to 1 hp and used for "air pressure", of course the smaller motor needs a smaller input sheave diameter.

    My thinking was that the mechanical friction on an extra v-belt and extra motor shaft would be pretty small. Also most air compressors don't have quad "fan sheaves" but some have double-groove "fan sheaves" and the two belts could be separated, 1 per motor, at some reduction in belt life...but the belts last pretty long in my experience.

    The ultimate goal of all this tomfoolery is to have a large motor for sandblasting, etc, and a smaller motor for simple things like blowing up a tire, where you need the air but not as fast and can also live with less noise

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    Piston pump compressors have/need a check valve between the pump and tank, normally located at the tank inlet. Repairing the pump is a non issue with tank storage.

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    Has anyone done this and what problems do you see?
    I started with one, 3hp, 60 gallon vertical ... needed more cfm & added a second unit ... needed more cfm & added compressor #3. This yielded 33cfm + 180 gallon storage, at low cost.

    You'll probably want to stage the compressors so that the best/newest/farthest away starts first, followed by the second one if the pressure drops another 5psi, followed by the third one if the pressure continues to drop. This is easily done using the pressure switch 'cut in' adjustment.

    You do have more oil to check, more filters to check, and more belts to monitor, but it is not a big deal. And if one machine smokes at the worst possible time, you still have enough air to finish the job you're on.


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