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CE427 - CHEMICAL ENGINEERING LABORATORY III
FALL 2005
COOLING TOWER
Objectives:
- Determine at various ratios of water to air mass flow rate, the tower characteristic for
the cooling tower located in Jarvis 116.
- Determine at a water to air mass flow ratio of ~1, how the range, and the approach
vary with increasing water flow rate.
- Estimate the evaporation rate of water (water loss) for the tower.
System:
In the cooling tower system located in Jarvis 116, warm water is brought into contact
with unsaturated air over the surface of horizontal redwood slats. During this process,
part of the water evaporates, lowering the water temperature. The water flow rate is
measured with a rotameter while the air flow rate is measured using a pitot tube. Wet
and dry-bulb temperatures for inlet air, and dry-bulb temperature for outlet air, as well as
inlet and outlet water temperatures are measured by thermocouples which are
interfaced with a data acquisition board controlled by LabView software.
Theory:
Cooling towers are commonly used in industry to reduce the temperature of utility
cooling water to allow its reuse in heat exchangers. Inside a cooling tower, a liquid
warm water stream is exposed to unsaturated air. The temperature of the water is
decreased by the simultaneous transfer of mass and heat at the gas-liquid interface.
The theoretical background for this assignment is contained in the first two portions of
Section 12 (Psychrometry, Evaporative Cooling, Air Conditioning, and Refrigeration) of
Perry’s Chemical Engineers Handbook and the experimenter is advised to read this
material.
Physical properties of air-water vapor mixtures
Analysis of the operation of a cooling tower requires the determination of the physical
properties of air-water vapor mixtures. A psychrometric metric chart represents a
concise compilation of a number of physical properties for a particular gas-vapor
mixture. It is recommended that you review the psychrometric chart tutorial located at
the following site:
www.uwsp.edu/it/tlrn/LOs2003/paperlo
The key definitions:
- Wet-bulb temperature - the steady-state, non-equilibrium temperature reached
by a small mass of liquid exposed to a continuous gas stream. In the lab, the
web-bulb temperature is measured with a thermocouple that is covered with a
water-saturated wick. This thermocouple is located in the inlet air stream and
the wet-bulb temperature is used to determine the moisture level and other
physical properties of the incoming air
- Absolute Humidity (Humidity ratio)- mass of water vapor per mass of bone-dry
air.
- Humid Volume (Specific volume)- volume of the humid air per mass of bone-dry
air.
- Enthalpy - the measure of the heat content of the humid air. Enthalpy of the air
is the saturation enthalpy minus the enthalpy deviation.
Operating Line
For a short section, dZ, of a counterflow cooling tower, an enthalpy balance can be
written as:
y x
Gdh = d Lh (1)
Here h denotes the enthalpy (Btu/lb or J/g). The subscript x is associated with the liquid
phase, while y with the gas phase. The mass velocity of the air is G , the mass of
vapor-free air per hour per unit cross section of tower. The mass velocity of water is
denoted by L.
Assuming that only a small fraction of the liquid evaporates in this process compared to
the total amount of liquid fed into the tower, we can assume L to be constant. The
datum plane (or reference temperature, T 0
) for all enthalpy calculations is arbitrary and
it eventually cancels out on both sides of the equations. For water, it is usually taken at
a temperature of T o
=32°F (according to Perry’s Handbook). For liquid water then,
x l x o
h = c T! T where c
L
is the specific heat of water. Thus, the differential form of
Equation (1) becomes:
y L x
Gdh = Lc dT (2)
Using the convention that the subscript a depicts the top and b the bottom of the cooling
tower, and integrating from the bottom of the column to any particular point the column,
one obtains:
y yb L x xb
G h! h = Lc T! T (3)
where K is the overall mass-transfer coefficient, a is the contact area per tower
volume, V is the effective cooling volume per tower cross sectional area, L is the water
mass velocity,
'
y
h is the enthalpy of saturated air at water temperature,
y
h is the
enthalpy of the air stream, and
xa
T and
xb
T are the entering and exiting water
temperature, respectively.
Equation 4 is similar to the definition of the number of transfer units (NTU) and thus the
tower characteristic represents the change in temperature of the water stream divided
by the average driving force. The tower characteristic is determined by numerical
integration. One could use the trapezoidal rule or Simpson’s rule, but according to
Perry’s Handbook, the tower characteristic is normally determined used the Chebyshev
rule. In this case the integral in Eqn. 4 is approximated by
1 2 3 4
xa xb
KaV T T
L h h h h
where
1
! h = value of
'
y y
h! h at 0. 1 ( )
xb xa xb
T + T! T
2
! h = value of
'
y y
h! h at 0. 4 ( )
xb xa xb
T + T! T
3
! h = value of
'
y y
h! h at 0. 4 ( )
xa xa xb
T! T! T
4
! h = value of
'
y y
h! h at 0. 1 ( )
xa xa xb
T! T! T
While the tower characteristic varies with
L
G
, knowing the tower characteristic at fixed
flow rates can be used to predict changes in tower performance with changes in
ambient air conditions.
Comments and Items to be Addressed
- While performing the experiment, make sure the wick of the wet-bulb
thermocouple is completely wet at all times.
- Examine at least three water to air mass flow rate ratios (approximately 1, 2 and
- at each of three water flow rates.
- Ensure that the system has reached steady state before starting acquisition of
temperature data.
- Report the approach and range for the cooling tower under your experimental
operating conditions. How does the approach and range vary with changes in
operating conditions?
- Use the Chebyshev method as described in the Evaporative Cooling section of
Perry’s Handbook to carry out the integration to help determine the tower
characteristic (also called the Mass-Transfer Coefficient Group in an example in
Perry’s).
- How does the tower characteristic change with water/air ratio? Theoretically, the
tower characteristic should remain constant at a constant water to air mass
ratio. Do your data indicate this? Explain any differences.
- In the theoretical analysis, it was assumed that the evaporation of water is
negligible to the water phase. Estimate the rate of evaporation (water loss) in
the cooling tower in gal/min or l/min. What percentage of the total water stream
is this? (Hint: Assume exiting air stream is saturated, do a mass balance to
determine how much water the air has gained) Was the assumption valid?
References
- McCabe, Smith, and Harriott, Unit Operations of Chemical Engineering, 6
th
Edition , McGraw-Hill, 2001.
- Felder, R.M and Rousseau, R.W., Elementary Principles of Chemical Processes,
John Wiley &Sons, 1978.
- Perry, R.H. and D.W. Green (eds.), Chemical Engineer’s Handbook , 7
th
ed.,
McGraw-Hill, 1997.
Pre-lab Homework for Cooling Tower Experiment (to be completed individually)
- Using a psychrometric chart determine the enthalpy of humid air at a dry-bulb
temperature of 75 F and a wet-bulb temperature of 60 F.
- At the air conditions described in problem #1, determine the air mass velocity of
dry air in
2
min
lb
! ft
when the pitot tube indicates a flow rate of 100 ft
3
/min
(Assume a square foot cross sectional area).
- Draw an equilibrium curve like that shown in Figure 1 (except include specific
values for air enthalpy), for air saturated at water temperatures between 70 and
o
F. Use 4 temperatures in this range. (Hint: use psychrometric chart)
- Draw an operating line on the figure in problem #3 using the inlet air conditions
described in problem #1 if water enters the tower at 100 F and exits at 70 F and
the L/G ratio is 1.
