Administrative Building Cooling Tower University of Tennessee Chattanooga Ben Dalton Lab Partner: Murat Ozkaya ENCH 435 Dr. Jim Henry December 2, 2008
Abstract Experimental data was taken at the air inlet and exhaust of the cooling tower at the Administrative Building at UTC to determine the heat removed by that cooling tower. The experiments we conducted on two consecutive weeks, September 23 and 30, 2008. The ambient dry bulb temperature for the two weeks were 26.5 C and 28 C respectively and the relative humidity of the ambient air was 45% and 38% respectively. The tower was found to have 306 tons of cooling on September 23 and 540 tons of cooling on September 30. 2
Table of Contents Introduction.......... 4 Theory........... 6 Equipment.......... 8 Procedure.......... 9 Results........... 12 Discussion of Results......... 14 Conclusion.......... 15 References.......... 16 3
Introduction The cooling tower attached to UTC Administration Building is a wet cooling tower. The tower acts to cool the Administration Building by facilitating the evaporation of warm water from the building. Air is pulled across baffles on the side of the tower by the exhaust fan on the top. As the air moves across the baffles, it evaporates some of the warm water. The warm humid air is then vented out of the top of the tower by the exhaust fan. A schematic of the tower is shown below in figure 1. Figure 1: Cooling Tower Schematic The purpose of the experiment was to determine the heat removed by the tower. To accomplish this, data regarding the temperature and humidity of the air entering and leaving the tower was collected. Additionally, measurements of the velocity of the air leaving the tower were taken at different radial distances from the center of the fan. This 4
data along with literature values and a psychometric chart were used to volumetric and mass flow rates of the air, which contains water from the tower, exhausted out of the top of the tower. These calculations were in-turn used to calculate the total heat removed by the tower. 5
Theory During the analysis of the system, it was assumed that the cooling tower did no work. While in operation, water and air, which contains some water vapor, enter the cooling tower. Some of the water evaporates into the air stream which is then exhausted out of the top of the tower. It is this mass transfer and related heat transfer which provide the cooling effect. A block diagram of the system is below as figure 2. Figure 2: Cooling Tower Block Diagram Exhaust velocities were measured at six distances from the center of the fan. Each measurement was assumed to represent the average annular velocity all of the way around the fan at that radial distance. Those annular velocities were then used to calculate an average velocity (v avg ) for the entire fan. v avg is calculated using equation 1 below. vavg = vxdx r 0 (1) Multiplying the average velocity by the area of fan yields the volumetric flow rate of the air through the tower as seen in equation 2. 6
v = A vavg (2) Multiplying the volumetric flow rate by the density (ρ) of the air yields the mass flow rate of the air, as demonstrated in equation 3. The density of the air is found using the measured wet and dry bulb temperatures and a psychrometric chart. m = v ρ (3) Performing an energy balance on the system using the mass flow rate of the air, the intrinsic property of enthalpy of the air, the change in the mass flow rate of the water, and the latent heat of vaporization yields the heat (Q) removed by the cooling tower as seen in equation 4. The change in the mass flow rate of the water in the tower is read from the psychrometric chart by comparing the wet and dry bulb temperatures of the air in and out of the tower. Q = mair, OUT h Air, OUT mair, IN h Air, IN + ΔmW Δ H V (4) 7
Equipment The equipment used to conduct this experiment, consisted of two sling psychrometers, an anemometer, and a stopwatch. The anemometer simply gives a velocity for the air, therefore a time interval was needed to calculate a velocity. The equipment can be seen in the photo, figure 3 below. Figure 3: Anemometer bounded by two sling psychrometers 8
Procedure Wet and dry bulb temperatures were taken at both the inlet to the tower and at the exhaust of the tower. On week one, this measurement was only done once; however, these measurements were repeated throughout the experiment on week two to monitor any changes in temperature over the time of the experiment. The temperature data and corresponding psychrometric chart data for week 1 are summarized in table 1 below. Table 2 contains the temperature sample data from week 2. Note: the average temperatures in week two were used with the psychrometric chart to yield the data in chart 3. Dry Bulb ( C) Week 1 Air Temperature Data Wet Bulb ( C) Relative Humidity (%) Absolute Humidity (kg H 2 O/kg air) Specific Enthalpy (kj/kg dry air) Humid Volume (m 3 /kg dry air) Tower In 26.5 18 45 0.0095 51 0.86 Tower Out 25.5 24 89 0.0182 72.5 0.87 Table 1: Air Temperature Data Week 2 Air Temperature Samples Sample 1 Sample 2 Sample 3 Average Uncertainty Dry Bulb In ( C) 28 28 28 28 0 Wet Bulb In ( C) 18 18 18.5 18 0.5 Dry Bulb Out ( C) 24 22 22 23 2 Wet Bulb Out ( C) 21 22 23 22 2 Table 2: Air Temperature Samples 9
Dry Bulb ( C) Week 2 Air Temperature Data Wet Bulb ( C) Relative Humidity (%) Absolute Humidity (kg H 2 O/kg air) Specific Enthalpy (kj/kg dry air) Humid Volume (m 3 /kg dry air) Tower In 28 18 38 0.0088 51 0.864 Tower Out 23 22 90 0.0162 65 0.865 Table 3: Air Temperature Data The anemometer was placed on top of the fan 40 inches from the center and three velocity measurements were taken over a 1 minute time interval. This was repeated moving 5 inches closer to the center with the final measurement taken 15 inches from the center. The blades of the fan attached to a disc at the center of the fan that was roughly 12 inches in radius. This disc prevented the collection for velocities any closer than 15 inches from the center as readings became very erratic. The results of both weeks anemometer measurements are given in tables 4 and 5 respectively. Radial Distance (in) Radial Distance (ft) Week 1 Exhaust Fan Velocity Sample 1 Sample 2 Sample 3 Average Velocity Uncertainty 40 3.3 781 795 785 787 14 35 2.9 1085 1082 1090 1086 8 30 2.5 1136 1149 1161 1149 25 25 2.1 1112 1102 1103 1106 10 20 1.7 1050 1044 1052 1049 8 15 1.3 307 314 363 328 56 Table 4: Measured Exhaust Velocities 10
Radial Distance (in) Radial Distance (ft) Week 2 Exhaust Fan Velocity Sample 1 Sample 2 Sample 3 Average Velocity Uncertainty 40 3.3 1240 1285 1268 1264 45 35 2.9 2342 2322 2321 2328 21 30 2.5 2478 2468 2462 2469 16 25 2.1 2436 2421 2427 2428 15 20 1.7 2182 2159 2155 2165 27 15 1.3 1090 1111 1074 1092 37 Table 5: Measured Exhaust Velocities 11
Results The average volumetric flow rate of the fan was calculated by multiplying the velocity at each radial distance and its corresponding radial distance and plotting it versus the radial distance. A graphical integration of the resulting curve yields the average volumetric flow rate for the exhaust fan. The graphs for week one and two are given in figures 4 and 5 respectively and the results are summarized in table 6. Average Volumetric Flow 3500 3000 2500 Velocity * r (ft 2 /min) 2000 1500 1000 500 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Radial Distance (ft) Figure 4: Average Volumetric Flow Rate Week 1 12
Average Volumetric Flow 3000 2500 2000 Velocity * r (ft 2 /min) 1500 1000 500 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Radial Distance (ft) Figure 5: Average Volumetric Flow Rate Week 2 Average Volumetric Flow Rate Week Flow (ft 3 /min) +/- (ft 3 /min) 1 31,900 445 2 69,200 460 Table 6: Average Volumetric Flow Rate Once the average volumetric flow is calculated, the psychrometric chart is used to obtain the density of the air, making a simple calculation to convert the average volumetric flow rate to an average mass flow rate. With the average mass flow rate of the air known and with enthalpies read from the psychrometric chart, it is possible to solve for the heat (Q) removed by cooling tower as shown in equation 4 above. The results of this heat removal are listed in table 7 below. Heat Removed by Tower Week BTU/hr +/- (BTU/hr) Tons +/- (Tons) 1 3.67 x 10 6 5.10 x 10 4 306 4 2 6.52 x 10 6 4.32 x 10 4 540 3 Table 7: Heat Removed by Cooling Tower 13
Discussion of Results While the ambient air temperature and relative humidity did not differ much from week 1 to week 2, the fan was spinning much faster on week 2 as shown in the much higher average volumetric flow rate of air from the fan. Despite the difference in average volumetric flow rate, the errors in those measurements are very similar. As expected, with similar ambient conditions, the fact that the average volumetric flow of week 2 is roughly double that of week 1 results in a corresponding approximate doubling of the heat removed by the tower in week 2 over week 1. 14
Conclusions For a given set of ambient conditions, the speed of the fan of a wet cooling tower has the biggest impact on the amount of heat removed. However, since a wet cooling tower relies on the evaporation of water to facilitate the heat transfer, a wet cooling tower will be most efficient when the ambient relative humidity is lower. The water feeding into the tower can more readily evaporate into drier air. 15
References 1. Felder, Richard M. and Rousseau, Ronald W. Elementary Principles of Chemical Processes, 2nd edition. John Wiley and Sons, 1986. 16