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 110 tons of cooling respectively. The analysis on the new tower
was just done on the one running fan.
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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
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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.
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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,
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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 è ∆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.
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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∆Hin = Σ∆Hout
(2)
By determining all of the required enthalpies, an analysis of the cooling tower can be
conducted.
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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.
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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.
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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
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(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 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 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.
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.
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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.
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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
Air
Water Outlet
Figure 4. Administration Building Cooling Tower
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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.
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3.50 ft
4.67 ft
Fan radius = 9.25 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
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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
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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.
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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.
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Table 3. Challenger Center Air Velocity Data
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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
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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 m2 /sec
times the x-axis, in meters yields the units for volumetric flow rate, which are m3 /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
m3 /sec or 92 ft3 /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 14.41 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
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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 41.71
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 37.84 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.
Outlet Air:
97°F, 44%
Inlet Air:
82°F, 70%
Figure 8. Challenger Center Data plotted on a Psychrometric Chart
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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
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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
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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
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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 m2 /sec times the x-axis,
in meters yields the units for volumetric flow rate, which are m3 /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 m3 /sec or 69 ft3 /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. H2 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 14.29 ft3 /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. H2 O/lbs of dry air)
and is read directly from the chart. This was found to be 0.024 lbs. H2 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. H2 O/sec.
The Enthalpy of the dry air is found by multiplying the mass of dry air times the specific
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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 0.0156 lbs. H2 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.
H2 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. H2 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 38.86 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 35.22 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
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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.
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Figure 11. Central Energy Plant Data
For the CEP cooling towers, both a 2nd and 3rd 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 3rd order fit yielded a volumetric flow
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rate of 38583 ft3 /min and the 2nd order fit yielded a 48384 ft3 /min volumetric flow rate.
The average of the two is 43484 ft3 /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 41500 ft3 /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.
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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.
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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
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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.
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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.
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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.
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References
1
http://www.netl.doe.gov/coalpower/environment/water/policy/cwis.html
2
Boles, M. A. and Y. A. Gengel, Thermodynamics, Engineering Approach , 2nd ed.,
3
http://www.linric.com/
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Appendix
Challenger Center fan 2 – Volumetric Flow Rate Approximation
Challenger Center fan 3 – Volumetric Flow Rate Approximation
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Challenger Center fan 4 – Volumetric Flow Rate Approximation
Volumetric Flow Rate Calculation (polynomial fit) – CEP Old Tower South Fan
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Volumetric Flow Rate Calculation (polynomial fit) – CEP Old Tower South Fan
Volumetric Flow Rate – Trapazoid Rule – CEP New Tower South Fan
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Volumetric Flow Rate – Trapazoid Rule – CEP New Tower South Fan
CEP – Inlet & Outlet Conditions
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