University of Tennessee Chattanooga

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

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 110tons of cooling respectively. The analysis on the new tower was just done on the one running fan.

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

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.

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 air1. 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, 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 material1. Cooling efficiency is lower for dry cooling systems than wet cooling towers due to the higher dry bulb temperature1.

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.

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.

Hin = Hout(2)

By determining all of the required enthalpies, an analysis of the cooling tower can be conducted.

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.

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 PsyCalc983. 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.
Procedure

1. Inlet Air Data – Temperature & Relative Humidity

1. Challenger Center
2. 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.

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 (RH) between each instrument. Because the two digital instruments (Y and TB) did not have both dry and wet bulb temperatures, nor a RH reading, these measurements were not used. And since the  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 34137 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.

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.

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  instrument. It was a very hot, dry day. The temperature was recorded as 98°C and the relative humidity was recorded as 7%.

1. 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.

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.

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. Figure 4. Administration Building Cooling Tower

The AB cooling tower outlet air measurements were taken in the following locations as noted in Figure 5 below.

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  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.

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 bracket using a screw and nut, and the probe of the digital 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 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 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.

Results