Chemical Engineering February 2012 - 26

Feature Report
Effective Thermal Design
Of Cooling Towers
A step-by-step approach to cooling-tower design,
with an example calculation to make it clear
Jonny Goyal
Air Liquide Engineering and Construction, Lurgi India
V
arious misconceptions arise
when it comes to the thermal
design of cooling towers. Sometimes
related parameters, such
as range, approach, effectiveness, liquid-to-gas
ratio (L/G), wet-bulb temperature,
cooling water temperature,
relative humidity, number of transfer
units (NTU) and other terms create
a confusion for the designer in effectively
sizing, selecting and evaluating
a particular cooling tower. This leads
to inadequate design.
The objective of this article is to
present a stepwise understanding of
how to calculate the NTU for a cooling
tower, and thus to understand the
basis of thermal design of counterflow
cooling towers for optimizing cost
and performance.
Definitions
First, let's look at some of the basic
terms and briefly describe their significance
and role in cooling tower design
and performance.
Dry-bulb temperature. Dry-bulb
temperature (tdb) - usually referred
to as the air temperature - is the
property of air that is most commonly
used. When people refer to the temperature
of the air, they are normally
referring to its dry-bulb temperature.
The dry-bulb temperature is an indicator
of heat content and is shown
along the bottom axis of a psychometric
chart. The vertical lines extending
upward from this axis are constanttemperature
lines.
Wet-bulb temperature. Wet-bulb
temperature (twb) is the reading when
the bulb of a thermometer is covered
with a wet cloth, and the instrument
is whirled around in a sling. The wetbulb
temperature is the lowest temperature
that can be reached by evaporation
of water only.
Relative humidity (RH). RH is
the ratio of the partial pressure of
water vapor in air over the saturation
vapor pressure at a given temperature.
When the relative humidity
is 100%, the air is saturated and
therefore, water will not evaporate
further. Therefore, when the RH is
100% the wet-bulb temperature is
the same as the dry-bulb temperature,
because the water cannot evaporate
any more.
Range. The range is the difference in
temperature of inlet hot water (t2) and
outlet cold water (t1), t2 - t1. A high
cooling-tower range means that the
cooling tower has been able to reduce
the water temperature effectively.
Approach. The approach is the difference
in temperature of outlet cold
water and ambient wet-bulb temperature,
t1 - tw. The lower the approach,
the better the cooling tower performance.
Although both range and approach
should be monitored, the approach
is a better indicator of cooling
tower performance.
Cooling tower capability. The capability
of the cooling tower is a measure
of how close the tower can bring the
hot water temperature to the wetbulb
temperature of the entering air.
A larger cooling tower (that is, more
air or more fill) will produce a closer
approach (colder outlet water) for a
given heat load, flowrate and entering
air condition. The lower the wet-bulb
24 CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2012
Enthalpy of fin, kJ/kg
F Flowrate, m3/h
L
NOMENCLATURE
t2 Hot water temperature, °C
t1 Cold water temperature, °C
twb Wet-bulb temperature, °C
tdb Dry-bulb temperature, °C
td Dew point temperature, °C
ha Enthalpy of moist air, kJ/kg
h1 Enthalpy of inlet air, kJ/kg
h2 Enthalpy of exit air, kJ/kg
h'
Mass flowrate of liquid, lb/h
G Mass flowrate of gas, lb/h
Q Heat load, kcal/h
Z Altitude above sea level, m
p Barometric pressure, kPa
pws
pw
v
Saturation pressure of water
vapor, kPa
Partial pressure of water vapor,
kPa
Specific volume, m3/kg
W Humidity ratio, kg water/kg air
Ws

C
m
Relative humidity (RH), %
Constant related to cooling tower
design
Slope of tower characteristic curve
temperature, which indicates either
cool air, low humidity or a combination
of the two, the lower the cooling
tower can cool the water. Capability
tests are conducted per the ATC-105
Code of the Cooling Tower Institute
(CTI; Houston; www.cti.org).
The thermal performance of the
cooling tower is thus affected by the
entering wet-bulb temperature. The
entering air dry-bulb temperature
has an insignificant effect on thermal
performance.
Effectiveness. A cooling tower's effectiveness
is quantified by the ratio
of the actual range to the ideal range,
that is, the difference between cooling
water inlet temperature and ambient
wet-bulb temperature. It is defined in
terms of percentage.
Humidity ratio at saturation air, kg
water/kg moist air
http://www.cti.org http://WWW.CHE.COM
Chemical Engineering February 2012

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