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Cooling compressed air from pump

  1. Jul 14, 2014 #1
    I have a fairly large (for household use) 7.5HP electric motor driven two-stage air compressor, which charges the receiving 80 gallon tank to a limit of 175PSI.

    There is a short 3/4" diameter copper pipe that connects the pump to the tank. I have not measured, but guesstimate that the temperature of that pipe gets above 150F while the pump is running.

    The tank is collecting a significant amount of water. I drain it daily, but would like to try to keep water out of the tank if possible.

    Question: If I replace that short length of copper pipe with a long coil or loop of copper with integrated cooling fins, and maybe even add a fan to blow through the fins -- will that help keep the water vapor from condensing into the air tank? Or will it just move the problem somewhere else?
  2. jcsd
  3. Jul 14, 2014 #2


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    If you can cool your pipe below the dew point of the compressed air, you can catch some water in the pipes there. I'm not sure if that helps, the tank certainly has more space for water.
  4. Jul 14, 2014 #3
    Yes, what you describe is called an aftercooler. Normally you would also install a separator between the aftercooler and the receiver tank. The separator (typically a cyclone device) has a drain line to carry the liquid away to a drain. Without the separator the liquid condensed in your aftercooler will just drain to the tank.
  5. Jul 14, 2014 #4
    ok thanks. I really want to keep the water from getting into the main 80 gallon tank.

    So, would it look like: pump head->long loop of copper pipe->separator->tank (?)

    And is the separator something I can purchase - would you happen to have a recommendation on type/brand or any hint where to look for it?

  6. Jul 15, 2014 #5


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    Someone has to mention health and Safety when we are discussing such a large volume of air at 12 Atmospheres of pressure. Are you planning to use the right equipment for this? There is a lot of potential energy involved, which could be released all in one go! PF doesn't like to get involved with 'dangerous' projects without some reassurance about competence etc..
  7. Jul 15, 2014 #6
    what are you running with this compressor? A compressor that size could power a medium sized machine shop.

    You should realize that the tank is supposed to collect water; draining it daily (or more often) is normal. How old is this tank? They corrode from the inside so they don't last forever.

    Yes you can buy them. Look at industrial supply places like Grainger or McMaster-Carr. Like this: http://www.mcmaster.com/#43775k53/=sughda Note these are not cheap. It's easier (?) to just blow down the tank as needed. You should already have a filter-regulator on the tank discharge line. They could be manual blowdown (open the petcock at the bottom to blow out the water / oil / junk). If you're using the air to spray paint, you should have a filter-separator just before the spray gun too.

    The safety advise from sophicentaur is important too. A piping failure in a system like this can kill you or anyone else who happens to be nearby. Don't run piping like this unless you know what you're doing. Please don't use PVC for compressed air service. If you don't have a regulator on the tank discharge, shut that machine off until you get one - you shouldn't be running the pipe or hose at 175 psi. If you aren't using the machine to its capacity, consider adjusting the pressure switch to shut off at a lower pressure (say 125 psi). It might cycle more often, but for intermittent use in a home shop that wouldn't matter. As an added benefit, you'd get less condensate too. Is the compressor motor wired properly? A 7.5 hp motor is a big load, normally it would be 3-phase which you don't find in home shops.
  8. Jul 15, 2014 #7
    I understand the safety concerns, trust me I do know what I'm doing even though my water removal question sounds fairly pedestrian. The system is completely piped with 3/4 copper, soldered with 95/5 lead-free solder. There are downstream regulators and filters. The unit is 230V/1ph and is wired into a 50A breaker using 6 gauge copper. I have a 60A shutoff switch in the garage and a local service disconnect at the unit which is installed in a machine room underneath my garage. I am doing all this at home but it is a small automotive restoration setup.

    Anyway, I know you are right about the tank collecting water by design. It's just that I'm getting brown rusty water out of the tank drain each day and (as an auto restoration guy) it is just disconcerting to me to see rusty water coming out of my new $2500 air compressor :-(

    I would really like to try to remove the water as the compressed air exits the pump head. So if I can run a length of copper from the pump head, with integrated cooling fins -- then down-sloped into one of those water separators (with automatic drain) - before entering the tank, then perhaps I have a chance of improving this condition?

    Thanks guys...I do appreciate the help and advice.
  9. Jul 15, 2014 #8
    I would be careful to avoid the potential for the liquid condensate to run back towards the pump outlet valves. The compressor vendor routed that short pipe to the tank carefully, think twice before you change it.

    If you do add any new piping, don't put any valves in there that could isolate the pump from the safety valve or the pressure switch.

    If the pressure switch is on the tank you might need to think about the extra pressure drop your new cooler piping/separator adds. It would make the pressure at the pump outlet higher, maybe you want to turn it down a little.

    The more I think about this the less I like the idea of changing it. Especially since you said the compressor is new. Any changes like this would void your warrantee. And depending on the state you're in, there might be code issues, particularly if you are running a business there.

    You could instead put an automatic drain on the tank drain line, to keep the tank from collecting too much water. That way you don't have to remember to blow it off from time to time during the day.

    There are several machine shop / home machine shop forums where you can find nearly endless threads on compressed air system designs and piping layouts.
  10. Jul 15, 2014 #9
    ok thanks. I wanted to understand the physics around "what happens to the water" through a compression cycle, and how to remove it - so that's why I posted here. I agree it's turned more into a best practices on compressor plumbing discussion.

    Thanks for the feedback I appreciate the advice given!
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