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Thread: air/water temp and evaporation

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    air/water temp and evaporation

    i'm trying to balance my indoor environment to help keep energy costs down and comfort level up. here's the situation with my 120k gallon indoor pool in Massachusetts:

    daily ranges:
    air temp - between 82-86 (set 86)
    water temp - 82-84 (set 84)
    humidity - 22-52 (no set point avail)

    on a typical day air will be 82, water 84, humidity 32.

    i'm trying to balance everything but we have a heating/dehumidification system (Johnson controls) that is working against me (I think). it is constantly sucking in cool air (giant whirling sound) through 2 locker room doors and 1 hallway double door. these doors will often stay open due to the air pressure. Is anyone familiar with dehumidification? negative pressure settings?

    I feel like i'm "feeding the need" and wasting a ton of energy by heating the water that i'm losing to evaporation.

    thanks for any help!
    north shore, MA.(non-profit, commercial, indoor,heated) 120,00 gal, plaster, IG, 10hp motor, 600gpm, chlorine, sand

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    mas985's Avatar
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    Re: air/water temp and evaporation

    If it is evaporation that you are concerned with, a pool cover would help significantly.

    The only other way to eliminate evaporation is to have the dew point temperature to be the same as the water temperature. The problem with this equilibrium point is it will be very uncomfortable for swimming or lounging or even spending much time in the room because the relative humidity would have to be very high. Plus it will most likely create mold.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: air/water temp and evaporation

    Well, controlling the humidity in a large, indoor pool is quite critical to the short and long term maintenance of your pool......you just can't keep it "soaking wet" in there for any length of time.

    I would certainly not be willing to give up the dehumidier (air circulation) for a few bucks in water and energy savings.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: air/water temp and evaporation

    thanks Mark and Dave. We used to have problems with corrosion and water dripping down the walls (a few years before me), but any idea how bad the evap might be with 32% humidity? (autofill so I can't tell how much water I'm losing) My building manager is very cautious about messing with the setting on the dehumidifier.
    north shore, MA.(non-profit, commercial, indoor,heated) 120,00 gal, plaster, IG, 10hp motor, 600gpm, chlorine, sand

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    mas985's Avatar
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    Re: air/water temp and evaporation

    The best way to determine the evaporation is to temporarily shut off the autofill (it should have a valve). Then measure the water drop over 24 or 48 hours.

    But this is a common problem with indoor pools. In order to keep the air comfortable (low humidity), dehumidifiers are used and that makes evaporation even worse. If you shut off the dehumidifier, it will become unbearable due to the high humidity so that is not a real solution.

    Again, the only practical way to reduce evaporation is to cover the water. However, you might try one of the liquid solar covers. They work best without wind so it may actually work well in your case.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: air/water temp and evaporation

    32% RH is pretty dry air but the movement of the air across the pool surface will have more to do with exaporation, I think.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    mas985's Avatar
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    Re: air/water temp and evaporation

    RH and wind speed are equally important. The RH determines the saturation vapor pressure of the air which in turn determines the amount moisture that air can absorb and the rate at which it will absorb moisture. The lower the RH, the faster the evaporation. Same is true with air movement. The faster the air moves, the faster the water evaporates. This is because the air movement brings in less saturated air which then increases the rate. But even without any air movement, there is still evaporation albeit slower.

    http://www.rlmartin.com/rspec/whatis/st ... active.htm
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: air/water temp and evaporation

    Thanks again gents. I'm gonna turn off the autofill valve on Monday when I close up and see how much water I lose. a pool cover would be great but we are a boys/girls club in operating on a shoestring budget in a very impoverished community.

    you're both really helping me to articulate what my problems are:

    1) I need to get the RH higher. 50%-ish ???
    2) my air temp being 1 or 2 degrees below water temp isn't helping
    3) there is cool air being drawn in from the hallway and locker rooms so i'm dealing with wind half the time

    as of right now, it seems that the dehumidifier is set too low? any thoughts? the control functions on the console don't seem to correlate to anything (unlike the air temp which goes by degrees). also my building director (my boss) and I are butting heads over this. he doesn't subscribe to my hypothesis about a high evap rate caused by the air temp/water temp ratio. his stance is that we are not evaporating because there would be condensation on the windows and other surfaces. on the other hand, my view is that the dehumidifier is set so dry that it is causing and masking the problem. ugh! he's a good guy and neither one of us is looking to be proven right, we are just looking to tune up the whole system. btw, the pool and building are only 7 years old so all the equipment is relatively new.

    sorry for ranting and all, but I don't know anyone who knows anything about this stuff.

    might anyone be able to tell me anything about the negative pressure setting on the dehumidifier? is that why it's drawing in the air from the corridors like a tornado? it sometimes gets so bad that it will cause the drop ceiling tiles in the locker rooms to fly up into the ceiling and the doors will fly open once the knob is turned.

    again, thanks for everyone's advice!
    north shore, MA.(non-profit, commercial, indoor,heated) 120,00 gal, plaster, IG, 10hp motor, 600gpm, chlorine, sand

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    mas985's Avatar
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    Re: air/water temp and evaporation

    1) Adjusting the RH will help a little bit but you don't want the air too humid.
    2) It is both the air temp and RH (i.e. dew point temp) that matters. Low air temp and high RH reduces evaporation.
    3) If there is a wind across the pool, that will make the evaporation worse.

    But in general, if the humidifier is set too low, then you will lose more heat. So it would probably be a good idea to get a handle on the settings for that thing.

    BTW, this may be of interest:

    http://aquamagazine.com/content/post/In ... pment.aspx
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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