In regard to the volume of air required for a particular cooling tower requirement and a particular tower at high altitude when compared to sea level, the effect of altitude is two folds. Since it is the mass (or weight) of air and not the volume that is vital to the tower performance, the first effect is that, because of altitude and corresponding reduction in air density, it takes a larger volume to obtain the required pounds. On the other hand, because of the higher water partial pressure which increases the evaporative tendency, the actual mass of air required for the duty is reduced.

Although it has been recognized by most cooling tower manufacturers that, for the majority of operating conditions, the more significant of these two effect is the evaporative effect, it is generally assumed they are sufficiently canceling to ignore elevation in the calculation of the volume of air, and consider only the reduced static pressure and corresponding reduction in horsepower resulting from the lower air density.

At the reduced atmospheric pressure associated with high elevation, the higher partial pressure of water results in a higher moisture content in the air at any temperature. This higher moisture content increases the heat content of the air, or the temperature-enthalpy plot of saturated air at high elevation is above the sea level curve.

Example 15-1. Discuss about the effect of altitude on cooling tower rating and performance at 2000 feet in the altitude using the example 13-1.

(Solution)

First, find a dry bulb temperature at 2000 feet in the altitude for 80% of relative humidity corresponding 80^oF of wet bulb temperature.

First Step: find a dry bulb temperature for 80% of relative humidity corresponding 80^oF of wet bulb temperature at the tower inlet.

Second Step: Find an exit air temperature and air volume of fan. The procedure is exactly same as the contents described in the example 14-1. Refer to it for the details.

Third Step: Calculate the tower characteristic for the results obtained above per the performance data provided by the fill manufacturer.

Fourth Step: Iterate until NTU satisfies the value of tower characteristic by changing of approach figure.

Therefore, Actual Cold Water Temperature = Wet Bulb Temperature + Approach = 80 + 8.425 = 88.425^oF

Download the example file, Version ID-THERMAL/TOWER (idthermal.zip)
This file is same as the example file discussed in example 13-1.