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  1. #1

    Hmm cooling tower, can it be too large?

    we have two 300 HP vertical discharge, cross flow cooling towers. the location is the Sonora desert (HOT). the original use is condenser water for two HVAC chillers.
    A varity of equipment using the tower water has been added without much thought to tower capacity.
    I'm thinking to purchase a new larger tower(s) and wonder of there are any operational problems of low flow over the fill, reduced capacity etc. caused by oversizing the tower by a factor of 2 or 3.
    thanks

  2. #2
    Join Date
    Dec 2008
    Location
    Dixiana, AL
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    2,611
    Only condition that would create low flow is if you added tower cells and didn't increase pumping capacity. What purpose would that serve?

  3. #3
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    5,520
    Quote Originally Posted by Steve Rhodes View Post
    we have two 300 HP vertical discharge, cross flow cooling towers. the location is the Sonora desert (HOT). the original use is condenser water for two HVAC chillers.
    A varity of equipment using the tower water has been added without much thought to tower capacity.
    I'm thinking to purchase a new larger tower(s) and wonder of there are any operational problems of low flow over the fill, reduced capacity etc. caused by oversizing the tower by a factor of 2 or 3.
    thanks
    Couple points:

    1) "HOT" is relative. To a cooling tower dry bulb temrpatures are pretty much irrelevent except to the fan motor and gearbox. It's an evaporative cooler, it cares about wet bulb temepratures. Most are speced at a nominal wet bulb of 75F. You gain capacty below that to a point and lose some capcity above that and will have higher outlet temperatures. A tpical aprpoach temp is about 10F above wet bulb, form what I've heard. So it it's a 75F wet bulb outside, a 85F outlet temp is pretty good.

    Looking at Yuma and Pheonix, you dewpoints are really low, in the 50's. You woudl ave WAY WAY WAY more capacity than a cooling tower almost anywhere else in teh country right now. With your dewpoints, we could shut down 1/4 of our cooling towers here in Iowa right now. Our dewpoint is aroudn 70-75F today... oh and it's going ot get to 100F. Its' way hotter here to. I'd love to have 108F and 50F dewpints right now, it would feel pretty good. It was 82F at 6AM this morning, but unlike your wimpy 46%RH, it was 63% RH... and climbing.

    2) Cooling towers as mentioned above are designed for a range of flow rates, too mcuh or too little water and you lose capacity. At very low flows, you'd probably have trouble wetting the fill and it you ran the fan at full speed I suspect you could have scaling issues even with good water treatment. You might also see higher than usual amps on the fan at full speed since static pressure would decrease a little. Nozzle pluging/fouling could be come a problem too.

    You can upsize the piping and pumps or stage multiple pumps and use VFD's or a control valve to target a pressure setpoint.

    3) You have a lot of capital and expense in maintaining towers that are 2-3X more capacity than you need. Fill isn't cheap to eventually repalce. Towers cost money to clean and treat that water. Evne if you put in spare capacity, you'd still need adeqaute GPM to each tower/ Theo pumps need electricity to operate.


    This is sort of like installing two condenser coils so you have serserve capacity on hot days. It's going to be really inefficient to run twice as many condenser fans. At some point, you coil tmpe will equal the outdoor air temp and you're gaining nothing afterthat.

    Similarly with excess tower capaicty at some point you outlet water temp nearly equaly your outdoor wetbulb.

    4) Are these cooling tower feeding water cooler chillers? IF so, keep in mind that chiller usually don't like water temps much below 70F. IF you regularly have cooler water you actually need a primary, secondary condenser water loop ( a good thing ot have anyway) and a 3 way valve to regulate the temprature. Below 60F, in many cases you can't even get a chiller to start-up becuse it's inverted. (evaporator is warmer than the condenser and refrigerant migrates).

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    For what you're discussing I'd be looking at separating the HVAC tower and process tower loads.

    With your current configuration, do you have times where when both the HVAC and process heat rejection demand is high, the temperature of the water supplied from the tower rises sufficiently to cause problems either with the chillers or process loop loads?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

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