The University of Tennessee at Chattanooga
College of Engineering and Computer Science
Engineering 435
Chemical Process Systems Laboratory
Portable Cooling Tower
Team Members:
Marc Moss
Corita Suber
ENGR 435-001
Dr. J. Cunningham
September 22, 1998
Table of Contents
II.
Introduction
III.
Background and Theory
IV.
System Description
V.
Procedure
A. Fan SSOC
B. Heat Transfer (Steady State)
VI.
Results
VII.
Discussion
VIII.
Conclusion and Recommendations
IX.
References
X.
Appendix
A. Raw Data
B. Sample Calculations
Introduction
The members of this group spent three weeks analyzing the Portable Cooling
Tower located in Room 115 of Grote Hall. The airflow rate was analyzed to construct a
steady operating curve. In addition, the energy flow rate was examined from both the air
and water sides.
This report discusses the procedure followed while performing the experiments.
It begins with a description of the Portable Cooling Tower. Along with the theory of how
it works. It is followed by the experimental procedures and the results obtained. The
report is concluded with a discussion of the significance of the experiment and the
results, as well as the conclusion drawn. An appendix is attached to the end of the report
and includes the raw data charts and references.
Background and Theory
The governing principles behind the operation of the cooling tower is the First
Law of Thermodynamics, which states that energy can neither be created nor destroyed,
only converted from one form into another. The cooling tower does this by transferring
thermal energy stored in hot water to a counter current of ambient air supplied by means
of forced convection. The inlet and outlet temperatures of both the air and water can be
recorded and used to determine the enthalpy changes of the two streams. These enthalpy
changes of the two streams, along with the flow rates, can then be used to calculate the
heat transfer (q) for each stream from the following equation:


q  M H
This is an idealized calculation of the heat transfer, and does not account for
losses in energy due to friction and conduction through the walls of the cooling tower.
These values cannot be easily quantified individually, but the total value of the losses can
be determined by finding the difference between the q value for air and the one of water.
However, losses are considered to be minimal and are ignored, even though this is a poor
assumption.
System Description
The portable cooling tower (See Figure 1) operates by pumping a supply of
cooling water out of a reservoir into the customers load unit. This cooling water supply
(CWS) is then used to cool this load unit. The warmed cooling water (CWR) is then sent
back to the cooling tower. The warm water enters the top of the tower and is sent
through a rotating dispersion mechanism that sprays the water out over a series of baffles
that fill the cooling tower. The water runs down these baffles and returns to the reservoir
at the base of the tower. A fan attached to the side of the tower blows air up through the
baffles, where it interacts with and cools the water flowing the other way. The system is
controlled by means of the “LabView” computer program installed on a personal
computer. “LabView” can be used to independently control the pump and fan motor
speeds to run the systems as desired. “LabView” records the temperatures of the air and
water streams, as well as the flow rate of the water. The humidity readings for the inlet
and outlet air were determined by manually using an electronic hygrometer. The air
velocities used to determine the air flow rate which were also taken by hand.
Procedure 1: Fan SSOC
As the air flow rate is not measured by the computer, it was decided to construct a
steady state operating curve of the air flow rate versus percent fan motor speed. This was
done by dividing the surface area of the top of the cooling tower where the air exits into
three wedges of equal area. It was decided on three since this is how the cooling tower is
partitioned by the construction. Each of these three wedges was divided into five, two
inch bands (See Figure 2). The air velocity was measured on the inside and outside edges
of these bands. This average was then multiplied by the area of the band to give a total
flow rate. This procedure was done four times; at 40%, 60%, 80%, and 100% of the fan
motor speed. It was done for the lower percentages because the velocities only became
measurable in the range from 30%-40%. These four data points were then plotted to
yield the SSOC.
Results 1
Table 1 contains the measured air velocities that were used to plot the fan motor
SSOC, which appears on the next page (Graph 1).
Procedure 2: Heat Transfer (Steady State)
A steady state analysis of the heat transfer was done on the system, with the fan
motor operations as 80% and the hot water being supplied from the hot water line in
Grote Hall. Since the hot water was a continuous supply from a separate source, the
water was allowed to drain out of the reservoir after it had gone through the cooling
tower. The water was turned on and the system was started and allowed to run at steady
state. The computer read the stream temperatures and the inlet and outlet humidities
were read by hand. The system ran for twelve minutes before the computer
malfunctioned and stopped recording data. Only on run was made because of continuing
difficulties with the computer.
Results 2
Table 2 contains the average data for the heat transfer analysis. Plots of the air
and water data can be found on the following pages (Graph 2 &3). The two graphs plot
all of the data for the twelve minute run, but only the steady state range (from t=2min to
t=8min) was used in the calculations
Air In
Air Out
Water In
Water Out
Temp (C) Temp (F) Humidity Enthalpy
27.1
80.9
50%
31.9
38. 7
101.6
91%
62.2
51.1
123.9
52.2
29.0
84.2
91.9
Water
Heat Transfer
3207 Btu/min
Table 2: Heat Transfer Data
Discussion
The SSOC derived from the velocity data could be very useful in further runs
using the system. Having an operating curve would allow the users to determine the air
flow rate without having to measure the air velocity every time the system is used. Even
though data was only recorded for one run, the plots of the air and water temperatures are
useful for demonstrating the general behavior of the system. By sheer happenstance, the
air inlet temperature experienced a sharp increase at a point approximately seven minutes
into the experiment. A comparison of the plots of the air inlet temperatures and the CWS
temperature at this time demonstrate the effect varying ambient temperatures have on the
cooling tower’s ability to cool the water. At the point where the air temperatures
increases, the CWS temperature also increases, revealing that the tower is not able to cool
effectively at higher air temperatures.
Conclusions and Recommendations
The differences in the heat transfer rates for the air and water flows show that the
various heat losses in the system are significant and should not be ignored. One
recommendation for future experiments is to analyze these losses with varying system
conditions (i.e. flow rates and water temperatures).
References
Felder, Richard M. and Ronald W. Rousseau. Elementary Principles of Chemical
Processes, 2nd ed. John Wiley and Sons, New York, 1986.
McCabe, Smith and Harroit. Unit Operations of Chemical Engineering, 5th ed.
McGraw-Hill, New York, 1993.
Appendix