University of Tennessee at Chattanooga
College of Engineering
Engineering 435

Cooling Tower Experiments

Team Members:
Sharon Lewis
Scott Daniels
Austin Newman


September 17, 1996

Introduction

The laboratory cooling tower is a cooling tower unit from a commercial air conditioning system used to study the principles of cooling tower operation. It is used in conjunction with a residential size water heater to simulate a cooling tower used to provide cool water to an industrial process. In the case of the laboratory unit, the industrial process load is provided by the water heater. The laboratory cooling tower allows for complete control of the speed of the fan used in cooling the warm return water and the pump used to return the cooled water to the water heater.

Experiments can be conducted which study how adjustment of one or both of these parameters affects the amount of heat removed from the water provided to the water heater.

The remainder of this report will explain the theory behind the operation of a cooling tower and how the laboratory cooling tower is operated. An example of a mass and energy balance on the laboratory cooling tower will be presented along with the results of experiments in which the rate of heat dissipated by the tower was calculated at full capacity and when the pump speed and fan speed were varied independently.

Theory

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.1

Energy that enters the cooling tower is in the form of hot water. (Other energy contributions such as heat generation from friction of both air and water, energy losses from pipes, etc. are ignored.) This hot water was cooled from temperature T1 to a temperature of T2. The cooling of the hot water was in the form of forced convection3 by which ambient air at T1 was blown over the hot water and exited the cooling tower at some temperature T2. Both the entrance and exit temperatures of the air and water were recorded. 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. (1)

Where H is enthalpy, U is internal energy, P is pressure, and V is volume.

The combined terms U+PV is enthalpy, which means to heat.1 Enthalpy can be calculated or referenced from tables of data for the fluid being used. In the Engineering 435 laboratory, the fluids used by the cooling tower are air and water, whose enthalpy values can be obtained from a thermodynamics textbook. For example: Since both the initial and final temperatures of the input hot water and the output cool water were measured, the temperature Tin can be referenced and the enthalpy (BTU/lbm, or KJ/kg) can be recorded. The enthalpy of the output cooled water can be similarly referenced and an energy balance can be conducted for the water.

The equation below displays the general method to conduct an energy balance:

in = out (2)

where H = H in - H out. A similar method is employed for conducting the energy balance for air entering and leaving the system.

The change in enthalpy for air can be determined form either of two methods. Since the air is at low pressure, it can be treated as an ideal gas and the enthalpy change can be calculated through the use of the following equation:

H = CpT (3)

where H is the change in enthalpy, T is the change in temperature, and Cp is the specific heat with respect to constant pressure.

Since the specific heat relation does not take into account the percent of water in the air, a psychrometric chart is used to determine the enthalpy change between the entrance and exit air. In order for the psychrometric chart to be used effectively, some information is needed about the input and output air.

The information needed to reference the psychrometric chart is the dry bulb and wet bulb temperatures of the inlet and outlet air. Both the input and output air flow is measured with a sling psychrometer. The sling psychrometer is an instrument that has two thermometers. The thermometer for measuring the wet bulb temperature has a wetted cotton sleeve over the bulb end, while the dry bulb thermometer is a regular thermometer. Once the wet and dry bulb temperatures of the inlet and outlet air have been measured, each can be referenced on the psychrometric chart and the enthalpies obtained. Once the enthalpies for the inlet and outlet water and air conditions are known, energy balance can be conducted on the system.

System Description

The basic layout of the laboratory cooling tower system is shown below in Figure 1.

Figure 1 Cooling Tower System Schematic

Water from the water heater, which is denoted cooling water return (CWR), flows through a hose to the cooling tower, where it is then carried by another hose to the top of the cylindrical cooling tower. From here it is collected by a rotating sparger which sprays the water over the area of the tower. The water then flows down the tower over a series of baffles made from a cardboard style material. Ambient air is blown through a duct perpendicular to the flow of water by the fan. This air interacts with the water resulting in a net transfer of heat from the water to the air by the vaporization of some of the water. The cooled water then flows into the reservoir which provides any make-up water required to replenish that lost to evaporation. The reservoir is connected by a hose to the water supply inside the laboratory. The level of the reservoir is kept constant by a float valve which controls the flow of the make-up water from the water supply line. From the reservoir, the water flows to the pump that returns the cooled water, now denoted as cooling water supply (CWR), to the water heater via a hose. For more detailed information on the setup of the cooling tower, refer to the setup/shutdown in Appendix A. Also included in Appendix A is a procedure regarding safe operation of the pump and fan motors and a procedure describing how to handle a mercury spill due to breakage of thermometers.

Data collection for the tower is provided by a data link from the tower to a Macintosh computer running Labview data collection software. Data collected by the software includes the flow rate of the water through the system; the percent of total electrical load of the fan (0-60 Hz), used to control the fan speed; the percent of total electrical load of the pump (0-60 Hz), used to control the pump speed; the humidity and temperature of the air entering the fan; the humidity and temperature of the air leaving the fan; the temperature of the cooling water supply and the temperature of the cooling water return.

Because of a problem with the flow meter in which the flow detector would become stuck and give erroneous readings, a calibration of the flow meter was performed and is graphically shown in Appendix B. In addition to the flow meter problems, the humidity data collectors on the cooling tower were not used because of questionable readings and a sling psychrometer was used to collect the dry bulb and wet bulb temperatures of the ambient air (air entering the fan) and the air leaving the cooling tower. These values were then used in conjunction with a psychrometric chart to determine the humidity of the inlet and outlet air. A procedure detailing the collection of data is presented in Appendix C.Calculations

After the data has been collected (as discussed in the Cooling Tower Data Collection Procedure presented in Appendix C), an energy balance around the cooling tower can be calculated. First the energy lost by the water can be calculated, and then the energy gained by the air can be calculated. This is shown in Figure 2 below.

Figure 2. Energy Balance for Cooling Tower

The water going into the cooling tower loses energy. The enthalpy of the water going into the tower can be determined by using the enthalpy of saturated liquid water in a steam table. The enthalpy of the water coming out of the tower can be determined in the same way. The data in steam tables are usually not given for every temperature so linear interpolation must be performed to determine the enthalpy at the desired temperature. Then the enthalpy of the water is multiplied by the mass flow rate. A basis of an operation of 1 minute was chosen to make the calculation easier. The change in enthalpy for the water is determined by

. (1)

The change in energy of the air can be determined using the same methodology as was used for water. The enthalpy change is shown as

. (2)

However, the determination of the enthalpy of air is more complicated than the determination of the enthalpy values of the water stream. An important tool that is used for this is the psychrometric chart. On the psychrometric chart, the enthalpy of the air stream can be determined by using the wet bulb and the dry bulb temperature of the air stream. The enthalpy is given with the units of BTU per pound of dry air. By using the output stream, the volumetric flow per mass flow of dry air can be determined. This is a conversion factor that is used to convert volumetric flow of air to pounds of dry air.

Now that the mass flow rate of dry air is known, the enthalpy values of the in and out streams can be determined. The change in enthalpy of the water should have a negative value, and the change in enthalpy of the air should have a positive value. Theoretically, when the two values are added together, the result should be zero. This can be shown by the first law of thermodynamics where

(3)

and

(4)

After collecting the data, the enthalpy values were determined and are shown in the results section of this report. A sample calculation for one experiment is shown in Appendix D.

Results

There were many different experiments that were conducted. However, in most of them, steady state operation was not reached. One set of these experiments was run with a constant pump speed, and the fan speed was stepped up. The data has been plotted and is shown in Appendix E. This experiment proved to be useless to us. There were attempts to calculate an energy balance around the cooling tower for the experiments but the results proved to be inconclusive. On some of the experiments, the change in enthalpy of the air was negative which means the air lost energy to the water. This is impossible according to thermodynamics. In one of the experiments, steady state operation was obtained. The experiment was conducted at a maximum fan speed (60 Hz) and maximum pump speed (60 Hz). The temperature of the cooling water supply and the cooling water return both reached a steady value. A graph of this is shown Appendix E. The results from this experiment are shown in the table below.

Table 1. Steady State Results
Steady State DataIn Out
Temperature of Water (oF)76 71
Mass Flow Rate (pounds)53.1 53.1
Dry Bulb of Air (oF)77 76
Wet Bulb of Air (oF)74 76
Velocity of Air (feet)N/A 600


Using this data, the enthalpy of the different streams were calculated.

Table 2. Enthalpies of Air and Water
InOut Change
Water (BTU/min)2341.2 2075.7-265.5
Air (BTU/min)4756.3 5132.2375.9

Conclusions

There were many different things that were discovered while conducting these experiments. One thing that was learned was to always make sure the system is running at steady state before collecting data. If it is not at steady state, the different measurements that are being made are continually changing. When these numbers are used in calculations, they do not work like they should because everything in the system was changing. Another thing learned was that models do not always fully describe real situations, as is shown by enthalpy changes shown in Table 2 in the Results section. The change in enthalpy for air should be equal to the change in enthalpy for water. Table 2 clearly shows that they are not equal. The change in enthalpy for the air is 42% higher than the change of enthalpy of the water. This is because the cooling tower is not a perfect system. Another variable is the accuracy of the calculations. If the system was perfectly insulated with no energy losses and the thermometers were accurate to an infinite amount of significant figures then the two numbers would be the same.

Recommendations

Several recommendations have been generated as a result of the experiences with the cooling tower. The first is a recommendation that the auxiliary heaters always be used during experiments in order to increase the temperature difference between the return water from the water heater and the cool supply water. This increase in temperature difference will allow for a larger enthalpy difference and will decrease the possibility of the enthalpy difference being negligible. Another recommendation is that only a few experiments be planned because of the time needed for the system to reach steady state (approximately 30 minutes). One of the problems faced by this group was that many of the planned experiments involving changes in the pump and motor speed could not be completed in the allotted laboratory periods because of steady state time constraint. A final recommendation is that the humidity recording devices, which were not working properly, be recalibrated or replaced so that more accurate and timely measurements of humidity can be made.

Bibliographical References:

1Boles, M. A. and Y. A. Gengel, Thermodynamics, Engineering Approach , 2nd ed.,

McGraw Hill Book Company, St. Louis, MO, 1994, p. 8-12.

2Harriot, P., W. L. McCabe, and J. C. Smith, Unit Operations of Chemical Engineering, 5th ed., McGraw-Hill Book Company, St. Louis, MO, 1993, p. 330-340.

3Occupational Safety and Health Standards for Genreal Industry, 29CFR Part 1910.303, 1994.




Appendix A

Operating and Safety Procedures



Engr. 435 Laboratory

Setup/Shutdown Procedure for Laboratory Cooling Tower (CT)

SETUP PROCEDURE

  1. Inspect the fan and pump motors per the Electrical Safety Guidelines. If any problems are found, correct them before performing any experiments.
  2. Move the CT outside and position with the water connections facing the window and the wheels closest to the window positioned on the blue marks on the sidewalk.
  3. Set the CT in position by adjusting the leg screws until contact with the sidewalk is made.
  4. Level the CT using the leg screws until the bubble of the level gauge on the tower side of the unit is centered in the circle.
  5. Remove the sheet metal cover from the CT.
  6. Move the pump outside and connect to the cooling tower using the gray pipe coupling.
  7. Place the electrical cords for the pump and the cooling tower fan through the window.
  8. Plug the pump electrical cord into the blue (left) outlet on the laboratory wall under the fan and pump controllers.
  9. Plug the fan electrical cord into the yellow (right) outlet on the laboratory wall under the fan and pump controllers.
  10. Place the data link cord through the window and connect to the "B" input on the computer data input selection box.
  11. Connect the water hoses and set the hose valves as shown in attached Figure.
  12. Place the hose with the "v" pipe connection (connected to the CT unit) in the pipe at the center of the exit air vanes on top of the cooling tower.
  13. Open the red handled pump valve on the gray pipe.
  14. Turn on the water supply and fill the reservoir. Ensure that the float switch will shut off the supply flow by lifting the float manually and verifying that the water flow into the reservoir stops. If the switch fails, shut off the water supply and notify the laboratory instructor of the problem.
  15. Set the computer input data selection box to "B" (also labeled " Cooling Tower").
  16. Turn on the power supply, located on the shelf under the computer.
  17. Select "CT 435 Fahrenheit" file and double click the mouse to open the file.
  18. Set pump and fan slide bars to desired power levels.
  19. Click "START" to begin data collection.
  20. Check to make sure the fan and the pump are operating.
  21. Take data.

SHUTDOWN PROCEDURE

  1. Click "STOP" to end data collection.
  2. Save data to desired location as prompted by the software.
  3. Set pump and fan slide bars to "0".
  4. Click "START" to shut the fan and the pump off.
  5. Verify that the pump and fan are not operating.
  6. Shut off the water supply to the reservoir.
  7. Shut off the valve to the pump and set the hose valves in the opposite arrangement from that in the attached Figure.
  8. Unplug the data collection cable, the pump motor and the fan motor and place the cords through the window.
  9. Wrap the fan motor cord and the data collection cable around the fan motor and secure.
  10. Unhook the reservoir supply hose, the CWS hose from the pump, and the CWR hose from the CT and place through the window into the laboratory floor.
  11. Remove the hose from the top of the CT (leave connected to the side of the CT) and lay it on top of the fan.
  12. Uncouple the pump from the CT and place the pump in the floor of the laboratory against the lab bench.
  13. Loosen the leg screws on the CT.
  14. Carefully move the CT to the edge of the sidewalk until the gray pump coupling is over the sidewalk edge. (DO NOT let CT wheels roll off of the sidewalk!)
  15. Open the pump valve and drain the reservoir into the parking lot. (Note: Not all of the reservoir will drain. About 1 " of water will remain in the bottom of the reservoir tub.)
  16. Close pump valve and replace sheet metal cover on CT.
  17. Push CT back into the laboratory.

Electrical Safety Guidelines for Laboratory Cooling Tower
  1. Inspect the fan motor on the CT and the pump motor for any obvious signs of damage such as3:
  2. If any signs of damage are found inform the Engr. 435 instructor and either the mechanical or electrical shop supervisor and arrange any necessary repairs/replacement. DO NOT OPERATE the CT until the equipment is returned to a safe operating condition.
  3. If no obvious signs of damage are found, proceed with the setup per the "Setup Procedure for Laboratory Cooling Tower (CT)".
  4. During operation of the CT, if any problems arise regarding the fan or pump motor operation (i.e. motor will not start, fluctuations in motor performance, overheating, smoking, etc.), SHUT THE UNIT DOWN IMMEDIATELY and inform the Engr. 435 instructor and either the mechanical shop or electrical shop supervisor of the problem. DO NOT ATTEMPT any repairs or testing of the malfunctioning motor without the approval of the Engr. 435 instructor.

Mercury Spill Procedure

In the event that a thermometer is broken in the lab, follow these steps:

  1. Inform the instructor of the breakage.
  2. Keep others away from the spill area.
  3. Obtain a mercury spill kit.
  4. Put on a pair of disposable gloves.
  5. Following the mercury spill kit instructions explicitly, collect the mercury and place in the appropriate container. Also place any items (such as the thermometer bulb) which have mercury on them in the container. DO NOT ALLOW THE MERCURY TO CONTACT THE SKIN!!
  6. Contact the UTC Safety Director for pick up of the sealed mercury waste containers.



Appendix B

Flow Meter Calibration Curves


Appendix C

Data Collection Procedure



Cooling Tower Data Collection Procedure
  1. Outlet Air Velocity

The outlet air velocity readings were obtained using the anemometer/thermometer instrument provided by the instructor. The top of the cooling tower where the air exits was divided into eight equal sections as shown by in Figure 1 below. Four velocity readings were taken at equal intervals in each section. A weighted average of the velocities in the eight sections was then calculated to provide an overall average velocity.

Measurement Point


Figure 1. Velocity Measurement Profile

  1. Inlet Air Velocity

The inlet air velocity was obtained by taking one velocity measurement in the center of each of the three sections of the fan inlet using the anemometer/thermometer instrument. An average of these three measurements was then calculated.

  1. Ambient Air Conditions

The dry bulb and wet bulb readings of the ambient air were measured using a sling psychrometer. The wick on the wet bulb thermometer was wetted with reservoir water and the psychrometer readings were taken within five feet of the fan inlet. To obtain accurate readings of the ambient conditions, readings were taken away from the open windows containing the hoses to the unit. This precaution minimized the influence of the humidity controlled air from the building. Four readings of the wet bulb and dry bulb temperatures were obtained during the course of an experiment, and the average of these readings was calculated and used in conjunction with a psychrometric chart to obtain a relative humidity value for the ambient air.

  1. Outlet Air Conditions

Outlet air dry bulb and wet bulb readings were obtained each time experimental conditions, such as pump or motor speed, were changed. These readings were obtained by placing the psychrometer (with the wet bulb wick wetted with reservoir water) in the exit air stream on top of the cooling tower. One set of readings was taken each time an experiment was changed and these readings were used along with the psychrometric chart to obtain relative humidity readings for the exit air.

5. Flow Rate

Flow meter data collected by the LabView computer program was taken and used in conjunction with the flow meter calibration curves in Appendix ## to obtain the water flow rate measurements.

  1. Pump and Fan Speeds

The fan and pump speeds (full power equal to 60 Hz) were taken directly from the LabView computer program.

  1. Cooling Water Supply and Cooling Water Return Temperatures

The cooling water supply and cooling water return temperatures were taken directly from the LabView computer program.


Appendix D

Sample Calculations

Sample Calculations

The data collected from one of the experiments is shown below.
Steady State DataIn Out
Temperature of Water (oF)76 71
Mass Flow Rate (pounds)53.1 53.1
Dry Bulb of Air (oF)77 76
Wet Bulb of Air (oF)74 76
Velocity of Air (feet)N/A 600

By using the temperature of the water streams, the enthalpy of each of the streams was determined from the steam tables. The stream going in has a value of 44.09 BTU/lb. The stream coming out has an enthalpy value 39.09 BTU/lb. The mass flow rate is 53.1 lbs.











The enthalpy can be determined by:

The enthalpy of the air going in can be determined from the pyschrometric chart. The enthalpy of the input air was 36.7 BTU/lb DA. The enthalpy of the output air was 39.6 BTU/lb DA. The volumetric flow rate of the air coming out was determined by using an anemometer to determine the velocity of the air. The velocity was then multiplied by the area of the output duct of the cooling tower. This is shown by 2.98 ft2*600 ft = 1788 ft3.

The volume of the output air is 13.8 ft3/llb DA. The volume is used to determine the mass flow rate by

.

The mass flow of the air is then multiplied by the enthalpy of the input and the output air streams.

These calculations are used for any combination of any pump speed and fan speed.