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Thread: Greenhouse Heat Sink / evaporative cooler / humidifer

  1. #9
    Steve Booth's Avatar
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    Hi Manders

    See what you are doing now, in which case what you propose will work, i've never used a system like it but there are people (I think even in this forum, but cant find the link) who have created evaporative coolers with pumps and wetted media but not gone the extra stage and used a large reservoir of water as a heat sink. The exchanger design and air volumes will need thinking about. I presume you intending to speed control the fan on humidity rather than temperature, just allowing the heat sink to dissipate its heat by conduction to the surroundings.

    Take your point on the gravel mix, I was just thinking of the mass rather than the thermal properties, apologies.

    cheers
    Steve

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    Quote Originally Posted by manders View Post
    Came up with this design, anybody using anything similar? Did it work?
    Put warm water into the lower tank and get cool water out? How that?

    If the lower tank is placed in a refrigerator you can put warm water into it and take cool water out.

    Otherwise you put lukewarm water into the lower tank and take lukewarm water out. I cannot detect a big cooling effect in the outlined design.

  3. #11

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    Jesse, the cooling effect is by transfer of sensible heat from the air to the cold water and further by evaporation of the water into the air. Of course if the air is cooled enough it may be 100% saturated anyway and hence the air will be cooled close to the water temperature entering the vertical heat exchanger.

    The lower tank will have 'warm' water entering, mixing with a much larger volume of cold water and hence the water exiting will be cooler than the feed water. At night the conditions are reversed.

    I think the trick will be to balance the water and air flows to get it working in the right way and hence both pumps need to be variable flow.

    Hot air actually contains very little energy compared to water / deg C.

    So hopefully instead of having say a 20 C swing in temperature over a 24 hour period, it can be controlled to a 10 C swing in temperature (at very low cost).

    ---------- Post added at 10:03 AM ---------- Previous post was at 09:48 AM ----------

    Quote Originally Posted by Steve Booth View Post
    Hi Manders

    See what you are doing now, in which case what you propose will work, i've never used a system like it but there are people (I think even in this forum, but cant find the link) who have created evaporative coolers with pumps and wetted media but not gone the extra stage and used a large reservoir of water as a heat sink. The exchanger design and air volumes will need thinking about. I presume you intending to speed control the fan on humidity rather than temperature, just allowing the heat sink to dissipate its heat by conduction to the surroundings.

    Take your point on the gravel mix, I was just thinking of the mass rather than the thermal properties, apologies.

    cheers
    Steve
    Plan is to just adjust both the water and air rates manually and see what happens over 24 hours, a few dataloggers will come in handy... Probably will adjust air rate to change the rate the heat is released back into the greenhouse at night but until i try and see what happens its hard to say exactly.

    Agreed, Water/air distribution in the exchanger could be tricky and have a huge impact on how well it works...

  4. #12
    Steve Booth's Avatar
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    High Manders

    Without making a fine spray which would be best, the air will need to pass over wetted medium for maximum cooling effect, the more medium the better obviously, but this may impose too great a resistance on your fan, depending obviously on what it is and how much of it there is.

    Taking an arbitrary greenhouse size of say 3m x 2.4m x say 2.3 high and say 10 air changes per hour that gives an airflow of 0.04 m3/s (100mm duct will do), or mass flow of about 0.056 kg/s. Original air temp (within greenhouse, full recirculation on the air side?) say 30C and 38-40% saturation, then passing this through your kit (assuming 100% efficiency) would give an off plant (pardon the pun) condition of 20C saturated absorbing 0.766kg/hr of water. Excess water pumped will obviously return to the reservoir.

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    Thanks Steve, thats an interesting drop in air temp, I can allmost envisage chucking out the autovents and living with 30-35C daytime maximum and high(er) humidity than what i get now. Even if the evaporative cooling effect is reduced, should still be able to absorb much of the air temp into the water resevoirs.

    started building a spreadsheet model of the heat balance over an 'any time of year' 24 hour period ( with lots of simplifying assumptions) But had so far neglected the evaporative cooling effect, have to fix that one when I get time...

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