(60°F [15.6°C] wet-bulb temperature) is used to examine approach temperatures at various load conditions. At 60°F [15.6°C] wet-bulb temperature, the cooling tower approach temperature ranges from 9.0°F (5.0°C) at design load to 2.8°F (1.5°C) at a 29% load (Table 2). Note the approach temperatures at a constant 100% heat rejection load (14°F [7.8°C] range) (Table 3). Between 30°F and 85°F (-1°C and 29°C) wet-bulb temperature, the approach changes by a factor of six-and factor of almost five between 30°F (-1°C) and the 78°F (26°C) design wet bulb! This may be a phenomenon that was previously unknown to many. It's important to understand which mode sets the cooling tower design; summer or water economizer mode. In addition, it must be considered when determining tower setpoints at reduced wet-bulb temperatures. If inaccurate assumptions are made, tower design and/or the method of controlling cooling tower setpoint will be less than optimal. Why do these phenomena occur? They are related to the FIGURE 1 Cooling tower performance. 90 80 70 60 50 40 30 p Range 14 Range 12 Range 10 Range 8 Range 6 Range 4 30 40 50 60 Wet-Bulb Temperature (°F) FIGURE 2 Cooling tower approach temperature. 25 20 15 10 5 0 p Range 14 Range 12 Range 10 Range 8 Range 6 Range 4 70 80 30 40 50 60 Wet-Bulb Temperature (°F) psychrometric properties of air. At lower temperatures, air simply cannot hold as much moisture. Interestingly, at these lower temperatures, a greater proportion of heat rejection is sensible, so the amount of water evaporated is reduced compared to design conditions. So What? What difference can this make when controlling cool- ing towers for optimal system performance or performing analyses? Two examples follow. Example 1 A project team decides that in lieu of full-year analysis they will use a spreadsheet to estimate conditions. They incorrectly assume that the cooling tower TABLE 2 Cooling tower approach temperature at 60°F wet-bulb temperature. RANGE (°F) 4 6 8 10 12 14 PERCENT LOAD 29% 43% 57% 71% 86% 100% APPROACH (°F) 2.8 3.9 5.3 6.4 7.7 9.0 approach temperature remains constant at the design approach temperature of 4.5°F (2.5°C). (The author has seen similar assumptions used in a number of "spreadsheet calculations.") To compare this assumption with actual performance, the 4.5°F (2.5°C) approach and Table 2 data are used to construct Table 4. JAN UARY 2014 ashrae.org A SHR A E J O U RNA L 17 70 80 Approach Temperature (°F) Cold Water Temperature (°F)