Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How effective would an evaporative cooler be when utlized in this way?

  1. Apr 1, 2017 #1
    For a 300 sq ft workshop that you'd not want to add humidity into directly with a swamp cooler, what if you made instead a 4" false ceiling of 2x4's on end, covered with thin plastic sheeting of minimal r-value, to blow cooler evaporative air through a serpentine baffle configuration to achieve even contact all along the whole ceiling and plastic.

    How effective might that be if that evaporative swamp cooler outside was blowing 70F air into that 4" false ceiling space of 300 sq ft when ceiling air temp inside, just under that plastic, before you turned it on, was 90F?

    Let's assume here no heat gain/loss from above the original ceiling, the top of the false ceiling.

    Would this setup likely draw much of that 90F ceiling heat off with 1000 cfm evaporative cooler blower 70F?

    Any guess what the BTU's cooling total range might be for that 300 sq ft with this setup?

    Whatever its effectiveness, would it likely work better just to let hotter air naturally accumulate up at ceiling for largest Delta T, or use some fans to agitate it along underside of ceiling plastic sheeting, even though that might introduce some cooler air from below, lowering Delta T at plastic surface?

    Thanks for any thoughts.
     
  2. jcsd
  3. Apr 1, 2017 #2

    Stephen Tashi

    User Avatar
    Science Advisor

    If you leave the 2x4s or anything with cellulose in it exposed, you're liable to get mold inside the serpentine baffle from the accumulated humidity. The typical swamp cooler needs a strong air flow over the pads in order to be effective so you couldn't make the flow through the baffle very restrictive.
     
  4. Apr 1, 2017 #3
    Good on both points, thank you.

    Maybe to reduce mold risk, I could run it for a few minutes without water before turning off and leaving for the day, that should flush and dry it out some if humidity still in low teens when I did, yes?
     
  5. Apr 1, 2017 #4

    Stephen Tashi

    User Avatar
    Science Advisor

    If you have time, you could rig up an experimental set up to try that.

    In my experience ( southern NM USA) it would take more than few minutes to dry out a swamp cooler by running it without water. The typical swamp cooler doesn't drain as well as a sink, It has a flat bottomed reservoir, which, in practice, will have low spots. Also, to drain the typical commercial cooler you must unscrew the drain pipe because the drain pipe rises above the bottom of the pan and serves to keep water level several inches deep. To drain the cooler conveniently, you could add another drain, but the typical hose-bib type of valve would get clogged. All sorts of debris accumulates in the reservoirs of swamp coolers.
     
  6. Apr 1, 2017 #5
    I was thinking more of the type that pump water to top of pad, and just turning off pump,
    but I'll definitely be looking to capability to somehow bypass water when selecting one.
     
  7. Apr 1, 2017 #6

    Stephen Tashi

    User Avatar
    Science Advisor

    That's also the kind I'm thinking about The typical swamp cooler has a reservoir and the pump pumps water from that reservoir to the top of the pads. If you turn off the pump, the pads will eventually dry out, but incoming air can still pick up humidity from the large surface of the reservoir.
     
  8. Apr 1, 2017 #7
    OK, gotcha, will research that out well before acquiring, thanks!
     
  9. Apr 2, 2017 #8
    If you are keeping the swamp cooled air isolated from the room, you are back to trying to conduct the heat from the room to this cooler space. Air will not transfer heat like water will, and earlier you were talking of 40 F water, now 70 F air. Small T delta.

    Instead of plastic, I think you would want metal ducts for the air, with turbulence and fins or something for added conduction. These would also be easier to get dried out to prevent mold, I would think.
     
  10. Apr 3, 2017 #9
    NTL2009, exploring pipe-in-pipe counter flow utilizing the evaporative cooler there, too,
    just wanted to see what might be expected from 300 sq ft flat plate heat exchanger idea.

    Like to know how much Delta T I need to make it worthwhile fooling with.

    Fortunately, it's not hard or expensive to do, if any reasonable expectations.

    The 40F water mentioned was on another thread for a different project.
     
  11. Apr 3, 2017 #10
    Thinking just a little more about this - even though plastic is obviously a poor conductor, and I mentioned metal ducts, considering that the plastic is thin, and it is cheap/easy to get a very large surface area, maybe it's not as big a deal as I first thought?

    I'd need to review the method and equations again, but it would be child's play for some of the posters here.You look at the thermal conductance of that plastic sheet and the surface area and the T-Delta, and you can determine whether that plastic barrier is a significant problem in this application. I'm guessing now that plastic versus metal isn't a deal killer - but the T-Delta and air medium might be?

    I'm also thinking that condensation might not be a big problem in the ducts - the swamp cooler itself is isolated a bit from the ducts, not right in them, right? I wouldn't think the swamp cooler would be so effective to get the air fully saturated, and as the air goes through your plastic 'plenum', it would be warming and capable of holding even more moisture. So condensation shouldn't really occur to any large degree? Hopefully you get some input from people more knowledgeable than I am on these things, but that should at least get some conversation going.

    My related experience: I home-brew beer, and home-brewers commonly use an "immersion chiller"; a 25-50' coil of Copper or Stainless Steel tubing, submerged in 5-10 gallons of boiling hot "wort" (the beer liquid before it is fermented). Cool tap water is run through the coil until the wort is brought close to your tap water temperature. I'd often see debates on the beer-brewing forums of the relative merits of SS versus Copper tubing, with many people objecting to SS because its thermal conductivity is much lower than Copper. But I was able to use this math to prove that with all the surface area, and the thin walls of the tubing, that thermal conductivity was a very. very minor factor. I actually got to the point of modelling all this in electrical terms (that's my background), and creating SPICE models so I could play with all sorts of variables and configurations.
     
  12. Apr 3, 2017 #11
    A note on construction - 300 sq feet is fairly small, and plastic sheet (and I'm thinking actual rolls of plastic 'film' here, not hard sheets) is light. Maybe just string fishing line back/forth across the shortest end, and lay the sheeting over that stringing? Use real duct tape (not the common 'fix it' cloth tape) to seal the edges to each other (or maybe devise a way to melt/weld them together?). I think you could tape baffles of film in a similar way.

    What about building codes? All that exposed plastic sounds like a fire hazard?
     
  13. Apr 3, 2017 #12
    It sounds like your are trying to build a two stage evaporative cooler.
    I think I read that they work ok where the outside humidity allows for 55 F dew point.
    Above that the volume of the flow, becomes too great.
     
  14. Apr 3, 2017 #13

    Stephen Tashi

    User Avatar
    Science Advisor

    Based on my homeowner's experience with swamp coolers, the outflow of an inexpensive swamp cooler carries actual droplets of water a considerable distance into the duct work - it also carries bits of material that flake off the pads and debris that is fine enough to pass through the pads. There are brands of swamp coolers like "Mastercool" that use very thick pads and are more expensive machines than the kind of cooler that uses straw or synthetic straw-substitute pads. You can put a furnace filter over the outflow of an inexpensive swamp cooler (which is what I do) , but it will restrict the outflow and the filter gets dirty quickly.

    The duct work in a house that has run a swamp cooler for many years will always show signs of corrosion.

    From a purely economic point of view, a small electric AC unit would cool a 300 sq ft shop and involve a lower fixed cost that constructing elaborate heat-exchange duct work. If a swamp cooler is going to be involved, it would be interesting to investigate the cost of using it an "two-stage" cooling arrangement where the swamp cooler cooled the outside of the electric AC unit. That would keep the outflow of the swamp cooler isolated from the inside of the shop.
     
  15. Apr 3, 2017 #14
    The evaporative cooler would be outside attached to sidewall blowing directly into beginning of baffle space with provision to slide a blocking board at connection point to seal off air infiltration into baffle when not used.

    Also, if inside baffle getting too wet too long seems to create an issue with mold and rot, if I can run it dry at times, as low as the humidity normally is, that might be something that's done at the end of use for a few minutes, drying out the last remaining moisture before shut down for the day.

    The actual plastic I have in mind is fairly stiff at 0.008" (0.21 mm), so while a little less conductive, it will be more robust, less likely to tear through at attachment points and shred in place with blower pressure.
     
  16. Apr 3, 2017 #15

    Stephen Tashi

    User Avatar
    Science Advisor

    It takes more than a few minutes to dry out the pads of a swamp cooler. You could make some arrangement to block off the swamp cooler and use a separate blower to blow air directly from the outside through the duct work. But we need a biologist to tell us if mold is killed or suppressed by periodic drying-out of its environment.

    Swamp coolers are a nusiance! They can get out of adjustment in many ways. Strong winds can blow water off the pads into the blades of blower. If you have hard water, you get mineral build-up. If you have hard water, you need to arrange a "bleeder" for the reservoir or use a pump that periodically flushes it - otherwise the pads become ineffective. The cooler itself gradually corrodes.
     
  17. Apr 3, 2017 #16
    Got one of these, might play with it, was wondering how it might work as blower/mister all in one and it's an adjustable output, of course would need larger water feed...

    1952batomistspacesprayer.JPG
     
  18. Apr 3, 2017 #17
    Unit above mists out forcefully to over 25 feet and 95% of its output is less than 50 microns, most around 20 microns, so it'll evaporate readily. Can be dialed down to about 1.5 gallons per hour. I'm thinking if I did try this, I'd have first channel be a long straight shot for it, before first baffle change in direction, but have to find out first if it can handle extended lengthy operation.
     
    Last edited: Apr 3, 2017
  19. Apr 5, 2017 #18
    I have had a good effect adding a small window box unit to the top of my swamp cooler
    I ducted A.C. exhaust to out side and both hot and cold intake was from the swamp cooler. The cold water that dribbled out of the A.C. Was dumped to the swamp cooler reservoir
    I think some of the inefficiency of one machine worked to the advantage of the other
    I didn't do any math with it I was just bored and hot
     
  20. Apr 5, 2017 #19

    Stephen Tashi

    User Avatar
    Science Advisor

    I don't understand that description. What is the A.C. unit cooling?
     
  21. Apr 5, 2017 #20
    It was further cooling and drying the cool whet air from the swamp cooler
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: How effective would an evaporative cooler be when utlized in this way?
Loading...