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Dehumidifier and condensation?

  1. Nov 13, 2008 #1
    I did some research on how dehumidifiers work and from what've been able to find out, it takes in room-temperature air, cools it, thus condensation, then runs it back through heated coils so bring the air back to room temperature.

    I got around to taking apart an actual dehumidifier it's pretty much as expected. The only thing that was actually different from my little bit of research was that the air was heated up first and then cooled down rather than vice versa. So the condensation process goes from warm to cool, rather than room-temperature to cool. Would this increased temperature difference make condensation more effective even though the source of the air is the same? Or would warming up the air first dry some of the moisture out?
     
  2. jcsd
  3. Nov 14, 2008 #2
    It doesn't make sense.
    To dehumidify, you neet to cool the air until dew point. Further cooling will not cool the air but will condensate humidity. Afterwards, you need to warm the air up to room temperature.
    If warming comes first, you'll never get condensation.

    By the way, wrming was done with electric resistance or was it using the hot part of the cooling machine?
     
  4. Nov 14, 2008 #3

    russ_watters

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    Well, it wouldn't be done with electric resistance, it woudl be the warm refrigerant. But I always thought the airstreams were separated and that the cool plates just absorbed water via convection/vapor pressure.
     
  5. Nov 14, 2008 #4
    This doesn't make any sense to me either. What part of the machine is indicating to you the direction of airflow?

    [​IMG]

    Looking at the psych. chart its pretty easy to see that if you heat the incoming air, then cool, without heating it again, your just wasting energy. Unless I'm completely missing something.
     
  6. Nov 14, 2008 #5
    Perhaps this issue can be further addressed by looking at the functions of it's opposite; the humidifier... or it's close cousin, the "swamp cooler"

    I realize that a standard home "air-conditioner" has de-humidifying effects. In contrast, a swamp cooler might be an excellent example of a simple humidifier.
     
    Last edited: Nov 14, 2008
  7. Nov 14, 2008 #6

    russ_watters

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    No, I don't think that's helpful. A swamp cooler does not include a thermodyanamic cycle, so there is nothing to analyze. Functionally, a dehumidifier should be exactly identical to an air conditioner.
     
  8. Nov 15, 2008 #7
    Thanks for all the replies, that seems to make sense and helps a lot.

    This was what I look apart: http://www.amazon.com/dp/B000FABESC/

    I opened up the case and spread out it's components, plugged the power back in and I was looking at the fan and it's airflow direction. The fan is infront of that front vent and it seems to be sucking air directly into the warm heatsink. I will reinspect it more carefully when i get my hands on the device again Monday.

    From what I can tell, it's using a Peltier module wedged between the warm and cool heatsinks. There are 2 pairs of wires though, looks like 1 pair per heatsink? I would have expected just one, so I will figure out what those are doing too.
     
    Last edited: Nov 15, 2008
  9. Nov 16, 2008 #8
    Peltier. Interesting. So it seems that the thermic cycle is not performing well enough to get the air warm again after cooling it.
     
  10. Aug 31, 2011 #9
    Hello

    I have a mini-dehumidifier of a similar type to the one that EveryBest took apart. I have just replaced the fan because it had become extremely noisy. Although the new fan is a higher quality and much quieter fan with the added bonus of rubber mounts I suspect it may be somewhere between 1/2 and 1/3 of the speed as it is a 0.15 A one running at 1500 RPM and, whilst I don't know how fast the old one ran, it was a 0.4 A one and some 0.4A ones on eBay are advertised as going as fast as 3800 RPM.

    I'm just wondering, is the work done by the dehumidifier controlled by the throughput of air? Will the dehumidifier overheat if it does not have as fast a fan?

    Kind regards

    Peter
    Scotland, UK
     
  11. Aug 31, 2011 #10
    PS Having had several of these mini-dehumidifiers (of assorted brands and appearances) go wrong over the past few years, some of the old ones should also be fixable by replacing the fan. In some the electronics to the Peltier module presumably failed as water stopped being accumulated despite the fan running, while in some others the transformer failed.

    I've never known how interchangeable such transformers are i.e. which features have to be exactly the same for a replacement transformer to be safe and reliable. Any hints?

    Kind regards

    Peter
     
  12. Sep 4, 2011 #11
    Well ... the fan that you use needs to provide the same amount of cooling as the original fan or the warm side of the peltier device will get too hot and it won't work as efficiently or fail altogether.

    As far as transformers are concerned what matters is the voltage output and the maximum current output. You need to make sure that both match what the original transformer was providing. The transformer may or may not have a label that shows what it's outputs are.
     
  13. Sep 5, 2011 #12
    Thanks. I think the fan needed to be installed upside down (with label underneath) as I have discovered the airflow is in the wrong direction. Will try it later.
     
  14. Sep 5, 2011 #13

    sophiecentaur

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    There is an issue with dehumidifiers that doesn't seem to have been mentioned. Without some pre-heating in very cold conditions, the condensate can form ice at in the output heat exchanger and block the system entirely.

    Having bought a cheap and cheerful one for drying out an uninhabited building, a few winters ago, I found this was what happened - a solid block of ice inside!! I then found out that the more expensive ones have a pre-heater to avoid this effect. I imagine the pre-heater is only used if the exhaust temperature could be low enough for freezing to occur.

    All the above Physics could well be right but real life sometimes takes over.
     
  15. Sep 5, 2011 #14
    Sophie

    Yes, I agree about them icing up! I use a tube heater in conjucntion with the dehumidifier in moderately cold conditions and turn the dehumidifier off altogether in very cold ones. They're only meant for 15 degrees C or above.

    Kind regards

    Peter
     
  16. Sep 5, 2011 #15

    sophiecentaur

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    You can get the same problem with fancy no-frost food freezers. Once it starts to build up, the ice never seems to go away by itself. Hours and hours of defrosting time are needed because the ice is deep inside the guts of the machine.
     
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