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  1. #1
    Join Date
    Dec 2006
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    39

    Tower water usage

    Is there a calculation to figure water usage on a cooling tower?? I have a CVHF 600 ton chiller thats run at 100% daily. The chiller specs are 85 entering, leaving 99.44. 1200 GPM, 12.19 ft. pd. Customer thinks the tower water usage is too much, 17,942 gallons per day. Sounds like alot to me but I guess depending on the wet bulb that would affect the evaporation rate. Any help would be greatly appreciated.
    Thanks

  2. #2
    Join Date
    Dec 2008
    Location
    Dixiana, AL
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    1.5 gallons per hour/per ton, more or less, is the generally accepted rule of thumb makeup rate (not including bleed-off) for a standard 10* delta T design hx working loaded. At 15* delta, that will be a little different, but conditions will dictate how much. Sounds like you're running unloaded and with very little bleed, (or your tower's not making capacity) based on 24 hr/day run time. May want to check the cycles of concentration on the tower water and see if your controller is working as advertised.

  3. #3
    Join Date
    May 2001
    Location
    Western Wa
    Posts
    1,842
    One pound of water evaporating rejects 1000 BTU.
    8.3 pounds per gallon. 12,000 BTUH per ton.

    Then there's bleed, which will vary greatly depending on the water being used, and wind drift. You'll end up somewhere between 1.6 and 2.2 or even more, gallons per hour per ton. The divide that by 60 for GPM of make up.

    Wet bulb mainly affects the amount of fan energy required, and the leaving temperature possible with your type of tower.
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  4. #4
    Join Date
    Dec 2006
    Posts
    39
    So that water usage seems low??? Is that what you are saying??

  5. #5
    Join Date
    Dec 2008
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    Dixiana, AL
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    600 tons x 1.5gph/ton = 900 gph x 24 hrs = 21,600 gallons/24 hrs (not including bleed). This would be a minimum rate if the machine is loaded.

    Feel free to correct the math if there's a mistake. It won't be the first time.

  6. #6
    Join Date
    May 2001
    Location
    Western Wa
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    1,842
    I would guess that your chiller is running at less that full load. Those make up numbers, based on a 24 hour operation, and without any allowance for bleed or drift, come out to an hourly average of about 517 tons. Allowing for bleed and drift, with an average of say 1.8 GPH per ton, would lead to a guess of around 425 to 450 tons of actual load at the chiller.

    That's without knowing anything about the quality of your make up water, or how many cycles of concentration you run in the tower.

    Chill barrel GPM times the delta T, in degrees F, divided by 24, will give you the chill barrel tonnage.
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  7. #7
    Join Date
    Dec 2006
    Posts
    39
    Thanks for the good info!!! Much appreciated!!

  8. #8
    Join Date
    Sep 2009
    Location
    San Juan, Puerto Rico
    Posts
    11
    Trying to learn something here with you guys, but quick question! Are those numbers desing? or actual?

    We have a site with the same unit (or close to it, same model and capacity at least) and the only way we can get it to work fine is with a 10 degree split on cond with 1800 gpm's!! And that is what it calls for on the desing specs and drawings, it seems to me that 14 to 15 split might be low on cond water gpm's.

    Will be good to know this unit refrigerant pressures and temps. Sorry to get somewhat out of topic but the change in flow must also change your tower evap rate, correct me if I am wrong.

  9. #9
    Join Date
    Oct 2004
    Location
    Metro ATL
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    454
    Quote Originally Posted by LyJoDrFk View Post
    Trying to learn something here with you guys, but quick question! Are those numbers desing? or actual?

    We have a site with the same unit (or close to it, same model and capacity at least) and the only way we can get it to work fine is with a 10 degree split on cond with 1800 gpm's!! And that is what it calls for on the desing specs and drawings, it seems to me that 14 to 15 split might be low on cond water gpm's.

    Will be good to know this unit refrigerant pressures and temps. Sorry to get somewhat out of topic but the change in flow must also change your tower evap rate, correct me if I am wrong.
    Not sure if I completley understand your question but here goes....

    Your tower should be designed based on the tonnage of the equipment it serves. In the case of this CVHF it shoud be rated for the tonnage of the chiller plus mechanical cooling.

    If the chiller was designed for a 10 degree split with 1800gpm, that is what you need to provide. The factory can make changes in the cond tube bundle that allow for different applications, thus different design specs even on machines with the same model number i.e. CVHF600.

  10. #10
    Join Date
    Sep 2009
    Location
    San Juan, Puerto Rico
    Posts
    11
    Thanks Jemawalton for your answer, I considered the job specifics to be the reason for the diference, but still the doubt lingered. Randy S and Klove mentioned the posibility of a reduced capacity on the tower, wondered if any of this apparent reduced capacity might be related to flow. Might be a problem with the chiller or pumps not the tower itself.

    Thanks again

  11. #11
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,357
    Quote Originally Posted by LyJoDrFk View Post
    Thanks Jemawalton for your answer, I considered the job specifics to be the reason for the diference, but still the doubt lingered. Randy S and Klove mentioned the posibility of a reduced capacity on the tower, wondered if any of this apparent reduced capacity might be related to flow. Might be a problem with the chiller or pumps not the tower itself.

    Thanks again
    Check the tower sump screens and the pump intake screens for debris?

    Tubes punched on chiller's condenser barrel recently?

    Regardless the OP was concerning water usage of the tower(s). Does this chiller run 100% loaded 24/7? Or just part of the day? Heat load on the chiller as well as wet bulb will affect evaporation rate. If the towers have VFD blowers or fans that will also affect evaporation rate, as will staged blowers/fans.
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  12. #12
    Join Date
    Dec 2008
    Location
    Dixiana, AL
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    2,611
    Quote Originally Posted by LyJoDrFk View Post
    Thanks Jemawalton for your answer, I considered the job specifics to be the reason for the diference, but still the doubt lingered. Randy S and Klove mentioned the posibility of a reduced capacity on the tower, wondered if any of this apparent reduced capacity might be related to flow. Might be a problem with the chiller or pumps not the tower itself.

    Thanks again
    Reduced capacity on the chiller, not the tower. Evaporation rate goes down as heat input to the condenser water goes down due to running at reduced capacity on the chiller. No matter the load, it takes about 1000 btu's to evaporate one pound of water.

    *GPM x deltaT / 30 = condenser tonnage (a small addition to some good info given by Randy)
    Last edited by klove; 10-02-2009 at 06:36 PM.

  13. #13
    Join Date
    Aug 2002
    Location
    Southold.calm
    Posts
    5,838
    With 2) 500-Chillers and 2) 375-Ton chillers (only 3 machines running at a time) last August I would average 30,000 gal per day usage. Rough #’s 10,000gal per machine. Too many variables to get it closer. These #’s are in line with the last 3 years I’ve recorded them.

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