Cooling Tower Analysis

Transcription

1 University of Tennessee at Chattanooga Process Systems Laboratory ENCH 435 Cooling Tower Analysis Written By Anthony Paolucci September 18, 2002 Instructors Dr. Jim Henry Dr. Frank Jones Red Team Members Ron Vail Troy Hall

2 Abstract The Red Team measured the temperature, relative humidity and velocity of the inlet and outlet air streams on four cooling towers in order to perform an analysis on the tons of cooling produced by each. A psychrometric chart was used to determine the needed enthalpies and a spreadsheet was created to aid in the calculations. The Challenger Center cooling tower, a dry type of cooling tower and the smallest of the four produced 8 tons of cooling. The Administrative Building cooling tower, a wet type, produced 11 tons of cooling. The Central Energy Plant has two wet cooling towers, an old one and new one, approximately the same size. These units were the biggest and thus produced 200 and 110 tons of cooling respectively. The analysis on the new tower was just done on the one running fan. Page 2 of 40

3 Table of Contents Section Page Introduction..4 Background and Theory...5 Equipment 8 Procedure...10 Results 18 Discussion..33 Conclusions 34 Recommendations..35 References..36 Appendix 37 Page 3 of 40

4 Introduction During the first three weeks of class, the Red Team gathered data from four separate cooling towers in order to analyze the performance of each. Velocity, temperature (wet and dry bulb), and relative humidity measurements were taken for the inlet and outlet air streams in order to perform an energy balance to determine the tons of cooling produced by each unit. The four cooling towers are the Challenger Center cooling tower (CC), the Administrative Building cooling tower (AB), the Central Energy Plant (CEP-old) old cooling tower, and the Central Energy Plant (CEP-new) new cooling tower. Both the AB and CEP are wet cooling towers while the CC is a dry cooling tower. The difference between the wet and dry type is, with the dry type, the liquid being cooled is not in direct contact with the air that cools it and therefore evaporation does not occur. Cooling towers are a very important part of many industrial plants. They represent a relatively inexpensive and dependable means of removing low grade heat from cooling medium. Hot fluid from heat exchangers and other sources are sent to the cooling tower. The fluid exits the cooling tower at a lower temperature and returns to the exchangers or other units for further cooling use in the plant. This is normally referred to as a closed loop cooling tower system. Page 4 of 40

5 Background and Theory The general idea of a wet cooling tower is fairly straightforward. Warm water, referred to as a return stream, enters the top of the cooling tower, trickles down the packing in a column, and encounters air flowing upwards, usually from one or more fans. The fan draws air in from the sides of the tower and pulls it up through the center. A small portion of the water evaporates, requiring latent heat, which causes the temperature of the water to fall. The cooler water exits the bottom of the tower through the supply line. A water source is used to replenish water that is lost due to evaporation. Because there is a direct air/water interface, heat transfer is controlled by the wet bulb temperature of the air 1. Figure 1 below is a schematic of a typical wet cooling tower. Figure 1. Wet Cooling Tower The dry cooling tower operates in a similar fashion, but the coolant is enclosed within a piping network, thus there is no direct contact made between the air and the coolant. In fact, in most dry cooling towers as is the case with the CC cooling tower, Page 5 of 40

6 water is not used as the coolant. The coolant can be any type of brine or Freon solution. Heat transfer is based on the dry bulb temperature of the air and the thermal transport properties of the piping material 1. Cooling efficiency is lower for dry cooling systems than wet cooling towers due to the higher dry bulb temperature 1. The theory behind the operation of the cooling tower is the First Law of Thermodynamics, which is the conservation of energy. In simpler terms, the energy that enters the system must exit the system; energy can neither be created nor destroyed, just transformed from one form to another. 2 Energy that enters the cooling tower is in the form of the warm fluid entering the cooling tower from the cooling return line. This warm fluid is cooled by forced convection from the fan by which air is pulled in and forced up through the falling water or across the enclosed fluid in a dry tower. Both the entrance and exit temperatures of the air and/or fluid can be measured. Once this data is recorded, an energy balance can be conducted on the system. An energy balance is a form of bookkeeping that accounts for the energy entering and leaving the system. The main component of the energy balance is enthalpy, which is defined as: H = U + PV H = Q - W (1) Where H is enthalpy, U is internal energy, P is pressure, V is volume Q is heat and W is work. For our purposes, work will be ignored so the equation reduces to H = Q. Enthalpy can be calculated, readily referenced from steam tables or more importantly, be directly read from a pshycrometric chart. Page 6 of 40

7 By utilizing the Chemical Heat & Energy Analysis Picture (CHEAP), the below equation displays the general method for conducting the necessary energy balance for the air and water entering and exiting the system. Σ H in = Σ H out (2) By determining all of the required enthalpies, an analysis of the cooling tower can be conducted. Page 7 of 40

8 Equipment The analysis of the cooling towers required the use of several types of instruments in order to measure the desired properties. Below is a list of the instruments used and their functions. Sling psychrometer This instrument was used to measure the amount of moisture in the air. It consists of two thermometers. One thermometer measures the dry air temperature while the other one measures the wet-bulb temperature. After the wick of the wet-bulb thermometer is dipped in water, the sling psychrometer is whirled around using the handle. Water evaporates from the wick on the wet-bulb thermometer and cools the thermometer due to the latent heat of vaporization. The wet-bulb thermometer is cooled to the lowest value possible in approximately two minutes. This value is known as the wet-bulb temperature. The drier the air the more the thermometer cools and lowers the wet-bulb temperature. The sling psychrometer was used to measure the temperature and relative humidity of both the inlet and outlet air. Once the dry and wet bulb temperatures are known, the relative humidity can be read off of the scale that is printed on the instrument. Anemometer This instrument was used to measure the velocity of the air exiting the cooling tower fans. Several anemometers were used for each cooling tower. Calibrated Wind-Vane: This was used for the new and old CEP towers. It was a wind vane type anemometer that had three dials on it. As the wheel turned, the arms on the dial moved accordingly, and the air velocity can be read from the dials. It was a manual-type instrument because it had to be reset by pushing a lever and started by releasing the level. It read in ft/min. Page 8 of 40

9 Turbo: This was used for the AB and CC towers. This was a digital-type instrument that read in m/s. Mini: This was used for the AB and CC towers, but only for comparison purposes. This was also a digital-type instrument that read in m/s. Digital Thermometer/RH this was used to measure the temperature and relative humidity at the CEP. It is a digital-type instrument. Psychrometeric Chart this was used as a tool to determine enthalpies, volume per pound of dry air and moisture. An example of a paper chart can be seen in the appendix. In addition to the paper chart, a psychrometric calculator was also used. The name of the program is PsyCalc98 3. The calculator was used to verify the readings from the paper chart. Unfortunately the calculator displayed units of grains instead of lbs, so a unit conversion was built into the spreadsheet that was used for the calculations. Page 9 of 40

10 Procedure 1. Inlet Air Data Temperature & Relative Humidity A. Challenger Center B. Administrative Building On the first day this experiment was started, temperature and relative humidity measurements were recorded for the outside conditions and this data was used to approximate the inlet air data for both the Challenger Center and Administrative Building cooling towers. The data that was recorded can be seen in Table 1 below. Avg. Table 1. Comparison of Different Instruments for Outside Conditions for AB & CC towers As can be seen, measurements from many different instruments were taken, and there is a fairly small range among the readings for temperature and relative humidity Page 10 of 40

11 (RH) between each instrument. Because the two digital instruments (Y and T B ) did not have both dry and wet bulb temperatures, nor a RH reading, these measurements were not used. And since the Ω instrument was recorded as a range, and its RH was so different from the other instruments, this reading was also excluded. Seeing as the readings were so close for the remaining instruments (Round, Square A and B), the temperatures and relative humidity results (highlighted in yellow) were averaged to determine the inlet air conditions that were used in both the CC and AB cooling tower calculations. The average temperature and relative humidity are 82 F and 70% respectively. Also from Table 1, it was noted that compressor B on the CC cooling tower was running for hours. Compressor B was for the smaller side of the cooling tower, which only contained four fans. A schematic of the Challenger Center cooling tower can be seen in Figure 2 below. Challenger Center Side B Figure 2. Challenger Center Cooling Tower A reading for the compressor time was taken 24 hours after the initial one, and it was determined that in a 24 hour period, compressor B ran for 22 hours. Page 11 of 40

12 C. Central Energy Plant As for the inlet air data for the Central Energy Plant, a single temperature and relative humidity reading was taken at the bottom of the tower where the air is pulled in from, using the digital Ω instrument. It was a very hot, dry day. The temperature was recorded as 98 C and the relative humidity was recorded as 7%. 2. Outlet Air Data Temperature, Relative Humidity & Velocity Measurements A. Challenger Center Temperature, relative humidity, and air velocity measurements were taken for the air exiting the fans, however they were taken at more strategic locations. Measurements were taken at several different positions around the fan as well at different radii across the fan. Figure 3 below shows a diagram of the different measuring point locations taken for the Challenger Center cooling tower. 0.1 m 0.3 m 0.5 m Figure 3. Challenger Center Cooling Tower Fan As can be seen, measurements were taken at the 4:00, 8:00 and 12:00 positions for each of the three radii. The radius for the CC fan is 0.5 meters. Page 12 of 40

13 The temperature and relative humidity measurements were taken using a sling psychrometer and the air velocity measurements were taken using a digital turbo anemometer. Since the fans were so high off of the ground, a ladder had to be used to be able to reach over the fans in order to take the measurements. When taking measurements with the sling psychrometer, the instrument was held over the fan for approximately 2 minutes. The air moving across the instrument, which simulated the whirling motion of normal operation. Since the thermometer readings changes so fast once the instrument was removed from the fan, the readings were taken while the anemometer was still over the fan. Three different sling psychrometers were used during the experiment. The air velocity measurements were taken in much the same way. The anemometer was held over the fan until display readout stabilized, and the reading was recorded. Again, three different anemometers were used for comparison. B. Administrative Building From the schematic of the Administration Building Cooling Tower in Figure 4 below, it should be noted that this cooling tower has only one fan located in the center. Moist Air Fan Inlet Water Pump Water Evaporation Inlet Air Water Outlet Air Figure 4. Administration Building Cooling Tower Page 13 of 40

14 The AB cooling tower outlet air measurements were taken in the following locations as noted in Figure 5 below. 12 in 19 in Fan radius = 43 in 26 in 33 in 40 in Figure 5. Administration Building Cooling Tower Fan As can be seen, measurements were taken at the 1:00, 4:00, 7:00 and 10:00 positions for each of the five radii. The radius for the CC fan is 43 inches. Again, the temperature, relative humidity, and air velocity measurements were taken exactly as with the Challenger Center cooling tower. Because the bulb on the wet thermometer of the round sling psychrometer broke, the digital Ω instrument was used in its place. C. Central Energy Plant The outlet air data was taken in much the same manner for both the new and old CEP cooling towers as it was for the CC and AB cooling towers with one notable assumption. The outlet air was measured at four different radii, but the measurements were recorded for only one location with the assumption that every spot around the fan at the respective radii would give a similar reading. Figure 6 on the following page shows the locations of the measuring points at the four different radii. Page 14 of 40

15 3.50 ft Fan radius = 9.25 ft 4.67 ft 6.17 ft 7.67 ft Figure 6. CEP New and Old Cooling Tower Fans. The assumption was made for a couple of reasons. One, the skirt around the fan was so high that it was hard to reach over it to take measurements so we picked a location where we could stand on some piping to easily reach over and take measurements. Second, the fan was so big that we could have taken an abundant amount of measurements, so mainly it was done to reduce the sampling time. As a note, a couple comparison measurements were taken around the fan to test our assumption, and the results were within an acceptable range. Both the old and new cooling towers both had two fans each, but the new cooling tower had only one fan operating at the time of measuring. Another difference with these measurements compared with the CC and AB cooling towers was that only one instrument was used for temperature and relative humidity measurements, and only one instrument for air velocity measurements. Since the fans were so large, a special apparatus had to be used in order to take readings out over the fan. A large L shaped metal bracket was used to aid us in our measurements. The calibrated wind vane anemometer was attached to the end of the Page 15 of 40

16 bracket using a screw and nut, and the probe of the digital Ω thermometer/rh instrument, which was on a long wire, was run down the bracket and fastened to the end with duct tape. With the apparatus set up as described, we positioned the base end of the bracket on the ground in front of the fan and then we were able to swing the instrument end over the fan. Because of this, we were actually limited to what instruments we could use to take the needed measurements. A 50-ft. tape measure was used to measure the diameter of the fan, being careful to keep it tight as to not let the slack get caught up in the fan blades. The digital Ω thermometer/rh instrument was ideal due to the long cord attached to the probe. The probe could be placed over the fan and the display could safely be read from our standing position. Likewise, the calibrated wind vane anemometer was the perfect instrument. Due to its manual operation, the gage could be started and stopped as needed. The calibrated wind vane anemometer worked a little different than the digital ones. Instead of giving an instantaneous readout of length per time, the gage had to be manually started. Once started, the measurement had to be timed. In order to determine the reading, the length had to be manually read. The face of the instrument contained three dials, one for tens, one for hundreds, and one for thousands of feet. To obtain your length per time velocity, the readout had to be combined with the length of time that was used. Because the anemometer had to be started before it reached the measuring point, a dead time factor had to be established. The dead time is defined as the time it takes for the instrument to reach it destination (once it was started) plus the time it takes to swing Page 16 of 40

17 back, in order to stop the dials. It was determined that a dead time of 10 seconds would be used, and a nominal 150 feet would be added to each measurement. In view of the fact that we were unable to actually measure the radius of the locations used to obtain our data, tape was placed on the bracket before each measurement as a marker. We were able to go back at the end and measure the distance from the end of the bracket to the tape to determine our measuring radius. Page 17 of 40

18 Results A. Challenger Center The temperature and relative humidity measurements that were recorded for the Challenger Center cooling tower can be seen below in Table 2. Table 2. Challenger Center Temp & RH data The error associated with each fan measurement was calculated using a Student T-test. Although the error seems a little high, the measurements themselves are very close. Due to this, the average from each fan was averaged, which is highlighted in yellow, and was used to approximate the temperature and relative humidity of the exiting air. The air velocity measurements that were taken are tabulated in much the same manner, in Table 3 on the next page. The error, also calculated using a Student T-test, looks much better for the velocity measurements than for the temperature. Page 18 of 40

19 Table 3. Challenger Center Air Velocity Data Page 19 of 40

20 To determine the air velocity exiting the cooling tower, the velocity for each position, for each fan, was averaged. These averages are shown in bold in the table. An average was taken of these values to obtain the data seen in Table 4 below. Table 4. Challenger Center Average Air Exit Velocity Now knowing the average velocity and the radius for each measurement, it is possible to Determine the volume of air exiting the tower, otherwise know as the volumetric flow rate (VFR). This is accomplished by plotting the average velocity times the radius versus the radius. This generates a graph as seen below in Figure 7 for fan 1 of the CC cooling tower. Figure 7. Volumetric Flow Rate Approximation for CC fan 1 Page 20 of 40

21 A second order polynomial trend line was generated using Excel to approximate the equation of the line running through the experimental data points. The fit seems to be acceptable. By using a TI-86 calculator, it is possible to integrate the generated equation, using zero and the radius (0.5 m) for the lower and upper limits, respectively, to calculate the area under the curve, which is equal to the volumetric flow rate. The y-axis, in m 2 /sec times the x-axis, in meters yields the units for volumetric flow rate, which are m 3 /sec. This kind of graph was made for each of the four fans. The other three graphs can be found in the appendix. Table 5 below shows the results for all four fans. Table 5. Total Volumetric Air Flow Exiting the CC Cooling Tower. As seen at the bottom of Table 5, the total volumetric flow rate was calculated to be 2.6 m 3 /sec or 92 ft 3 /sec. This was found by adding the individual average volumetric flow rates of all four fans. The psychrometric chart can now be used to determine the pounds of dry air and the enthalpies. This is where the psychrometric calculator came in handy. Instead of approximating the values from the chart, the calculator gave more exact readings. With the outlet air conditions of 97 F and 44% RH, the specific volume can be determined. A value of ft 3 /lb dry air was read from the chart. By dividing this result from the total VFR, a value of 6.37 lbs. dry air/sec is found. This flow rate of the exiting air is assumed to be the same as the inlet air stream, so the inlet air also has a flow Page 21 of 40

22 rate of 6.37 lbs. dry air/sec. From the exit air conditions, an enthalpy can also be read from the psychrometric chart. The enthalpy of the exiting air was read to be Btu/lb. By multiplying this result times the air flow rate, a heat rate can be found. The heat rate for the air exiting the tower was calculated to be 266 Btu/sec. With the inlet air flow rate of 6.37 pounds/sec known, we can now calculated the heat rate for the inlet air. This is accomplished by multiplying the flow rate times the enthalpy of the inlet air. The enthalpy for the inlet air was read from the psychrometric chart to be Btu/lb. The result is 241 Btu/sec. The difference between the two enthalpies can now be calculated. The difference between the enthalpies for the inlet and exit air was calculated to be 25 Btu/sec. The inlet and outlet data points were plotted on a psychrometric chart. Figure 8 below shows that the points are located on a horizontal line which displays that the pounds of water per pounds of dry air does not change from inlet to outlet. Inlet Air: 82 F, 70% Outlet Air: 97 F, 44% Figure 8. Challenger Center Data plotted on a Psychrometric Chart Page 22 of 40

23 Due to accuracy limitations with the drawing tools in Microsoft Word, the values plotted are just approximate values. The blue lines can be followed from right to left to read the approximate enthalpies (inlet-38 and outlet-42) from the chart. The tower s cooling capacity can now be found. It is known that 1 ton of cooling is equal to 3.33 Btu/sec. So by dividing the Η value by this conversion factor, the cooling capacity for the Challenger Center Cooling Tower, with four fans running, is calculated to be 8 tons of cooling. B. Administrative Building The Temperature and relative humidity measurements that were recorded for the Administrative Building cooling tower can be seen in Table 6 below. Table 6. Administrative Building RH & Temp Data The error associated with the measurements was calculated using a Student T-test. The error for the humidity was quite large, but the range of the three values is big. We chose to accept the 71% value obtained by the digital Omega Instrument, even though it is quite different from the values obtained from the sling pshychrometers. The error for the Page 23 of 40

24 temperature is much more reasonable. The average values, highlighted in yellow, are the values that will be used for the outlet air. The air velocity measurements that were taken are tabulated in Table 7 below. Table 7. Administrative Building Air Velocity Data Page 24 of 40

25 The error, also calculated using a Student T-test, looks much better than the temperature and relative humidity measurements. The velocity at each radius for each position was averaged to obtain the data shown in Table 8 below. velocity (m/s) Table 8. Administrative Building Average air velocities Now knowing the average velocity at the radius (highlighted in yellow), it is possible to determine the volume of air exiting the tower, otherwise know as the volumetric flow rate (VFR). This is accomplished by plotting the average velocity times the radius versus the radius. This generates a graph as seen below in Figure 9. Figure 9. Administrative Building Volumetric Flow Rate Approximation Page 25 of 40

26 A third order polynomial trend line was generated using Excel to approximate the equation of the line running through the experimental data points. The fit looks to be fairly good. By using a TI-86 calculator, it is possible to integrate the generated equation, using the measuring points for the lower and upper limits, to calculate the area under the curve, which is equal to the volumetric flow rate. The y-axis, in m 2 /sec times the x-axis, in meters yields the units for volumetric flow rate, which are m 3 /sec. It should be noted that the AB cooling tower only has one fan. The total VFR for the AB cooling tower was calculated to be 1.96 m 3 /sec or 69 ft 3 /sec. Now the volumetric flow rate has been found, the psychrometric chart can now be used to determine the specific volume of the air, lbs. H 2 0/lbs. Dry Air, and enthalpies. Again the psychrometric calculator was used to obtain more accurate results. The following equation displays the composition of the moist air: Moist Air (MA) = Dry Air (A) + Moisture(in) (M) + Evaporation (E) (3) Using the outlet conditions of 86 F and 88% RH, the specific volume can be read from the chart. The specific volume was ft 3 /lb. Dividing the VFR by the specific volume will give the value for the mass of dry air per second, which is 4.83 lbs./sec. This will be used as the mass of air for both inlet and outlet. The moisture content in the outlet air can now be found, (lbs. H 2 O/lbs of dry air) and is read directly from the chart. This was found to be lbs. H 2 O/lbs of dry air. The outlet air moisture flow is calculated by multiplying this value times the mass of the dry air. The moisture flow rate of the outlet air was calculated to be 0.12 lbs. H 2 O/sec. The Enthalpy of the dry air is found by multiplying the mass of dry air times the specific Page 26 of 40

27 heat of air times the change in temperature from inlet to outlet. This was calculated to be 3.5 Btu/sec. The inlet air stream is now used to determine the moisture content of the inlet air. This value was read directly from the chart and was determined to be lbs. H 2 O/lbs of dry air. The inlet air moisture flow is calculated by multiplying this value times the mass of the dry air. The moisture flow rate of the inlet air was calculated to be 0.08 lbs. H 2 O/sec. The evaporation rate can now be found by subtracting the moisture out by the moisture in flow rates. The evaporation rate was calculated to be 0.04 lbs. H 2 O/sec. The Enthalpy of the moisture in is found by multiplying the moisture out times the specific heat of water times the change in temperature from inlet to outlet. This was calculated to be 0.15 Btu/sec. The enthalpy of evaporation is calculated by multiplying the evaporation rate times the heat of vaporization of water. The enthalpy of evaporation was calculated to be lbs. Btu/sec. The total change in enthalpy is calculated by adding the enthalpies for the dry air, moisture and evaporation. This was found to be Btu/sec. The tower s cooling capacity can now be found. It is known that 1 ton of cooling is equal to 3.33 Btu/sec. So by dividing the total Η value by this conversion factor, the cooling capacity for the Administrative Building Cooling Tower is calculated to be 11 tons of cooling. This entire process was automated using an Excel Spreadsheet. All the user needs to do is to enter the inlet and outlet temperatures, the volumetric flow rate, specific volume, and the grains of moisture per pounds of dry air. All the calculations and conversions will done automatically. The specific volume and grains are read directly Page 27 of 40

28 from the psychrometric calculator. Figure 10 below shows this spreadsheet with all of the prompts highlighted in yellow. Note the constants and conversion factors in the upper right corner. Figure 10. Cooling Capacity Spreadsheet Administrative Building. C. Central Energy Plant Since the cooling towers at the Central Energy Plant are also wet cooling towers, the exact procedure and spreadsheet that was used for the Administrative Building is also used for both cooling towers at the Central Energy Plant. There are a few minor differences in the way the volumetric flow rates were found for the towers at the Central Energy Plant, but other than that, the calculations are the same. Figure 11 on the following page shows the data that was collected for the outlet air streams of both the new and old cooling towers. The old tower had an average outlet temperature of 60 F, average relative humidity of 81% and an average air velocity of 1189 ft/min, with the new tower having values of 85 F, 53%, and 1249 respectively. Page 28 of 40

29 Figure 11. Central Energy Plant Data For the CEP cooling towers, both a 2 nd and 3 rd order polynomial fit was used and the two were averaged to determine the volumetric flow rates. As seen for the CEP North fan on the following page in Figure 12, the 3 rd order fit yielded a volumetric flow Page 29 of 40

30 rate of ft 3 /min and the 2 nd order fit yielded a ft 3 /min volumetric flow rate. The average of the two is ft 3 /min. Figure 12. Volumetric Flow Rate Calculation CEP Old Tower North Fan This average was compared to finding the area using the trapezoid rule as well. The graph for the area found by the trapezoid rule can be found in Figure 13 on the following page. As seen in Figure 13, the area was found to be ft 3 /min. The two methods proved to be fairly close in value. Due to the ease of generating a polynomial fit in Excel and using the TI-85 to calculate the area, it was determined that the values obtained using this method were accurate enough for our calculations. The graphs using both methods for the other two CEP fans can be found in the appendix. Page 30 of 40

31 Figure 13. Volumetric Flow Rate Trapazoid Rule CEP Old Tower North Fan Figure 14 on the following page shows the spreadsheet used to calculate the cooling capacity for the old and new towers at the Central Energy Plant. The old tower had a cooling capacity of about 200 tons and the new tower had a cooling capacity of about 110 tons. The difference in capacity is due to the fact that the new tower only had one fan running at the time the data was collected. It could be approximated that if both fans had been running that the new tower might have had a slight edge in performance versus the old tower. Page 31 of 40

32 Figure 14. Cooling Capacity Spreadsheet CEP Old and New Towers. Table 9 below summarizes the results for the cooling capacity of the four cooling towers that were evaluated. Table 9. Cooling Capacity of the Four Cooling Towers Page 32 of 40

33 Discussion We first started this experiment wondering how cooling towers worked and how we were going to analyze them. After doing a little research and speaking with Dr. Henry, we went out to take measurements of the different towers. Through experimental trial and error, we learned that different types of instruments yield different and that it may be hard to determine which instrument is giving the correct reading. It is very important to use calibrated instruments in order to have confidence in your readings. When not known if an instrument is calibrated, it is best to take several measurements and average the results. A Students T-test can be used to evaluate the error for the measurements. At the beginning of the experiment it was hard for us to imagine that by just measuring the inlet and exit air conditions, that a cooling tower can be analyzed. Water temperatures are not needed in order to perform an energy balance. The psychrometric calculator aided us greatly in interpreting the paper chart. We did however verify the calculator results on the paper chart. Page 33 of 40

34 Conclusion It was determined that as the volumetric flow rate increases, the cooling capacity also increases. This is usually achieved by using a larger fan, thus a bigger cooling tower. Although wet and dry cooling towers appear quite different, the calculations to determine their cooling capacity are very similar. Table 10 below summarizes the conditions for the four cooling towers that were evaluated. As can be seen with the wet cooling towers (AB and CEP), the larger the volumetric flow, the larger the cooling capacity. Also from the table, the statement that was made previously in the report; that the dry cooling tower is not as efficient as the wet cooling tower, is confirmed by comparing the CC and AB towers. The CC tower has a larger volumetric flow rate, but the cooling capacity is slightly less than that of the AB tower. Table 10. Comparison of the four cooling towers. Page 34 of 40

35 Recommendation Many improvements can be made on the experimental technique in evaluating the cooling towers to achieve a more accurate analysis. Ensuring that reliable instruments are used is the first step. The initial data is very important because it is used to determine the cooling capacity of each cooling tower. If you are not confident in your initial measurements, then you will not be sure if the values you obtain are accurate. Now that we have created spreadsheets with all of the needed formulas, it is the measured data that is plugged into the worksheet that now becomes very significant. The more data points taken on each fan, the better the average measurement will be. Page 35 of 40

36 References Boles, M. A. and Y. A. Gengel, Thermodynamics, Engineering Approach, 2 nd ed., 3 Page 36 of 40

37 Appendix Challenger Center fan 2 Volumetric Flow Rate Approximation Challenger Center fan 3 Volumetric Flow Rate Approximation Page 37 of 40

38 Challenger Center fan 4 Volumetric Flow Rate Approximation Volumetric Flow Rate Calculation (polynomial fit) CEP Old Tower South Fan Page 38 of 40

39 Volumetric Flow Rate Calculation (polynomial fit) CEP Old Tower South Fan Volumetric Flow Rate Trapazoid Rule CEP New Tower South Fan Page 39 of 40

40 Volumetric Flow Rate Trapazoid Rule CEP New Tower South Fan CEP Inlet & Outlet Conditions Page 40 of 40