Chemical Engineering February 2012 - 28

0.06
Feature Report
the enthalpy of the air at the surface
of the water and the main stream of
the air. Thus, the driving force at any
point is the vertical distance between
the two operating lines. And therefore,
the performance demanded from the
cooling tower is the inverse of this
difference. The solution of the Merkel
equation can be represented by the
performance demand diagram shown
in Figure 3. The KaV/L value is equal
to the area under the curve, and represents
the sum of NTUs defined for a
cooling tower range.
An increase in the entering twb
moves the air operating line towards
the right and upward to establish
equilibrium. Both the cold water temperature
(CWT) and hot water temperature
(HWT) increases, while the
approach decreases. The curvature of
the saturation line is such that the
approach decreases at a progressively
slower rate as the twb increases. An
increase in the heat load increases
the cooling ranges and increases the
length of the air operating line. To
maintain equilibrium, the line shifts
to the right increasing the HWT,
CWT, and approach. The increase
causes the hot water temperature to
increase considerably faster than does
the cold water temperature. In both
these cases, the KaV/L should remain
constant. However, a change in L/G
will change the KaV/L value.
Cooling tower design
On the basis of the above discussion,
it is clear that there are five parameters
that, in combination, dictate and
define the performance of a cooling
tower, namely:
1. Hot water temperature, HWT
2. Cold water temperature, CWT
3. Wet bulb temperature, twb
4. Water mass flowrate, L
5. Air mass flowrate, G
The first four parameters are determined
by the user of the cooling tower.
It is the fifth quantity, G, that is selected
by the designer of the cooling
tower. Once these five quantities are
available, the tower characteristic
(KaV/L), can be calculated through
the Merkel equation.
The first step in designing a cooling
tower is the generation of a demand
curve. In this curve, the KaV/L val0.055
0.05
0.045
Performance
demand
is area under curve
0.04
0.035
0.03
20
25
30
Temperature, °C
FIGURE 3. Solving the Merkel equation (Equation 2), is usually done graphically,
where the integral is equal to the area under the curve
3
Tower characteristic curve
2.5
2
Design L/G
1.5
1
Design NTU
0.5
0.2
0.4
0.6
0.8
1
1.2
L/G
FIGURE 4. The intersection of the tower characteristic curve and the design NTU
curve gives the design L/G ratio
ues are plotted against varying L/G
ratios. The next step is to superimpose
fill-characteristic curves and
demand curves. The Cooling Technology
Institute has tested a variety of
fill configurations and generated fill
characteristic curves for each type;
CTI's Technical Paper TP88_05 can be
referred to in this regard.
Cooling tower design is basically an
iterative process. The factors that effect
the selection of design L/G and
consequently the fill height are: cell dimensions,
water loading, air velocities
across various cooling tower sections
and pressure drops, and fan selection.
The classical method of thermal rating
of cooling towers is to estimate the
ratio of liquid to gas first and then find
the proper tower volume by the means
of trial and error using the tower perfor26
CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2012
mance curve. The L/G is the most important
factor in designing the cooling
tower and related to the construction
and operating cost of cooling tower.
Finally we can summarize the importance
of the L/G ratio with the following
points.
A high L/G ratio means:
* More water to less air
* Air is more saturated - driving
force is reduced
* More residence time of water needed
* Less cooling in given time
* Increase in required fan power
* Decrease in height of tower
* Low evaporation loss (under same
water flowrate)
An example makes it clear
As an example, let us design a cooling
tower with the following data:
1.4
1.6
1.8
2
2.2
35
40
45
1/ (hw - hA)
KaV/L
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Chemical Engineering February 2012

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