POWER June 2021 - 24

WATER
How Thermal Power Plants Can
Save 80% of Their Water
An adiabatic cooling tower system can save great amounts of water at power
plants compared to typical wet-type cooling towers. While there are challenges to
implementation, understanding how these systems work and the benefits that can
be captured with them will help decision-makers better evaluate their options.
Jagjit Singh Sehra and Michael Kost
here is more concern today than
ever around water usage at power
plants. When water resources were
more abundant, using open-cycle cooling
towers posed few problems. However,
times have changed and cooling tower
water consumption is often heavily scrutinized
at plants today.
It has been observed that between
T
2,900 liters and 3,000 liters of water
evaporate from cooling towers to produce
one megawatt-hour of electrical
power. That adds up to a lot of water at
modern facilities with capacities in the
thousands of megawatts.
Dry Coolers
An adiabatic cooler is an option that has
been developed to reduce water consumption.
To understand the adiabatic
cooling concept, first a person must understand
the concept of a dry cooler, or a
direct cooling tower (DCT).
A DCT, also called a fin fan cooler in the
thermal industry, is a dry system where
the process water flows in a closed loop.
The process water is not exposed to the
ambient air. This type of cooling tower
works purely on the principle of approach
temperature, which refers to the
temperature difference between the outlet
of one stream (such as water being
cooled) and the inlet of the other (such
as ambient air passing across the fins).
The formula for this calculation is:
Delta (A) approach = Water out (from
cooler) - Maximum ambient temperature
The minimum Delta A required has to
be between 3C and 5C. The higher the
approach, the lower the surface area required,
and vice versa. Because air is the
only media of cooling in a DCT, Delta A
cannot be zero or negative.
There are two kinds of dry coolers:
one is an induced draft and the other is a
forced draft. In an induced draft dry cool24
er,
the fans draw the air from the coils. In
the forced draft, the air is blown over the
heat exchanger.
Dry coolers cannot cool the water below
the ambient air temperature. If the
temperature of the water must be reduced
below the ambient air temperature, then
an adiabatic system must be added to the
dry cooler. In the adiabatic system, the
ambient air is first cooled with the help of
water spraying, then it is passed to the drycooling
section of the cooler.
For example, assume the ambient
temperature is 45C (113F) dry-bulb temperature
(DBT) at 15% relative humidity
(RH) at 23.96C (75.12F) wet-bulb temperature
(WBT). If the present opencycle
wet-type cooling tower, which is
operating at 42C (107.6F) and 32C (89.6F)
as water outlet conditions, is to be replaced
by a dry-cooling system, then an
adiabatic system must be installed in
front of the DCT.
Adding Adiabatic Cooling to a DCT
When the adiabatic system is installed
in front of the DCT, the ambient air first
comes in contact and interacts with the
water-cooling system. At this point first,
the sensible load is removed from the
air, which means the dry-bulb temperature
decreases. The principle is based on
evaporation cooling. The hot air coming
from ambient air acts as a heat source,
and when it interacts with the water
droplets, it evaporates, so there is a
phase change in the air. The hot air becomes
colder, saturated air.
For example, to achieve 32C (89.6F) of
water outlet from the cooler, an air inlet
of 27C or 28C (80.6F or 82.4F) is needed.
When the air at 45C DBT (113F) at 15%
RH enters the system, it acts as the
energy that evaporates the water. The
sensible heat gets converted to latent
heat. To evaporate 0.454 kilograms (1 lb)
of water, 284.27 W (970 Btu) is needed.
When the incoming air is at 45C (113F) at
15% RH, the outlet water temperature
has to be at 27C (80.6F) by design. The
airflow passing over the spray systems
has to be calculated for these temperature
profiles. Below 27C (80.6F) there
will be no water required.
When designing an adiabatic system,
caution must be taken to note the temperature
profile of all 8,760 hours in the
year. The water consumption may vary
every hour. Once a detail chart is prepared,
a graph of water versus power
consumption can be generated.
Connecting Adiabatic Cooling to
Open-Cycle Cooling Towers
It is very easy to connect an adiabatic
cooling tower (ACT) in parallel with ex1.
This diagram shows how an adiabatic cooling tower could be added to a plant with an existing
wet-type cooling tower. Courtesy: Kojami Power
www.powermag.com
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POWER June 2021

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