Cooling tower water consumption per ton of refrigeration
How much water is expected to be consumed by a cooling tower per ton of refrigeration?
From the wikipedia article:
At first I misread the article as saying 15,000 Btu/hour would evaporate 3 gallons per minute. It is actually saying that 15,000 Btu/hour equals delta T of 10F at 3 gallons per minute.
A ton of air-conditioning is the rejection of 12,000 Btu/hour (12,661 kJ/hour). The equivalent ton
on the cooling tower side actually rejects about 15,000 Btu/hour (15,826 kJ/hour) due to the heat-equivalent of the energy needed to drive the chiller's compressor. This equivalent ton
is defined as the heat rejection in cooling 3 U.S. gallons/minute (1,500 pound/hour) of water 10°F, which amounts to 15,000 Btu/hour, or a chiller coefficient-of-performance (COP) of 4.0 (or EER of 13.65).
To estimate evaporation rate needed to reject heat for per ton of refrigeration, I would do:
(15000 btu per hour) / (2260 kJ/litre) = 0.030 US gallons per minute
or 0.1167 litres per minute
or 0.168 m^3 per day
since 15000 Btu/hour is the heat dumped out by a chiller of COP:4.
and since 2260 kJ/litre is the heat of vaporization of water
and since I assume that all the heat rejection is due to evaporation... this is a big assumption, since cooling tower exhaust air is at higher temperature than inlet too.
I've never seen the formula you're using. The one I've used in the past is:
Evaporation Loss = 0.00085 * water flowrate(T1-T2)
flow in GPM and T in F. Some use 0.001 instead of 0.00085 it just depends on how close you want to get. Basically, evaporation works out to about 1% (or 0.85%) of the flowrate for every 10 degree temp change.
There are 3 components that determine cooling tower make-up:
- rift loss due to the wind
- evaporation due to the heat transfer process
- blowdown required to keep maintain an acceptable level of TDS (total disolved solids)
You can go to just about any cooling tower mfrs website to get more info & understand the calc. You'll end up seeing that blowdown is the largest contributor to make-up water flow rate. A rule of thumb is about 1% of the condenser water flow rate, assuming a 10 deg Delta T, of course it depends of the quantity of TDS in the make-up water. If you have well water, you'll use more if you have city water & treat it w/ a softener you'll use less.
The rule of thumb was 3 Gallons per ton per minute MAX for sizing water makeup for cooling towers.