Okay, here goes, these are quotes from McQuay's Chiller Plant Design Application Guide. This guide is available on the web at McQuay's site for downloading. If you or anyone involved with towers and chillers haven't read this thing yet, I recommend doing so:
Approximately 1% of the design condenser water flow is evaporated. A 1000 ton chiller operating at design conditions can consume 1800 gallons of water per hour. The specific amount can be calculated by reviewing the psychrometric process.
Doing the math, a 1000 ton chiller at 3.0 gpm per ton: 1000 x 3 = 3000 x 60 = 180000 x 1% = 1800 gallons per hour lost to evaporation when chiller and tower are operating at design conditions.
The following quote makes more sense if the info in it is plotted on a psychrometric chart. Cooling towers are rated by ARI at 95 degrees dry bulb entering air, 78 degrees wet bulb entering air, 95 degrees entering water temperature, 85 degrees leaving water temperature. A common design tower approach temperature for a cooling tower is 7 degrees above the wet bulb temp of the air entering the tower (tower approach is difference between wet bulb temperature and temperature of water leaving the tower):
Assume 1 lb. of water is cooled by 1 lb. of air. The water cools from 95 degrees F to 85 degrees F and releases 10 Btus of heat to the air (1 Btu = the amount of heat required to raise the temp of 1 lb. water 1 degree F). The 10 Btus of heat raises the enthalpy of the air from 42.4 btu/lb to 52.4 Btu/lb and saturates the air. The leaving air condition is 87.5F and 100% RH. The moisture content went from 0.018 lb/w (118 grains/lb) to .029 lb/w (203 gr/lb). This means .029 -.018 = .011 of water was evaporated, which is why it is common to hear that cooling towers lose about 1% of their water flow to evaporation. The latent heat of vaporization for water at 85F is about 1045 btu/lb. Multiplying the latent heat times the amount of evaporated water (1045 x .011) results in 11.45 Btus of cooling effect. Cooling the water required 10 Btus, the rest was used to cool the air sensibly. The air entered the tower at 95F and left the tower at 87.5F
To answer your original question, you will have to break out a psychrometric chart, plot your points, and do the math. All I have done via the quotes above is prime the pump.
