Cooling Towers: Overview
CM4120
Spring 2008
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Topics
Introduction
 Definitions
 Operating Conditions
 Basic Components
 Water Cooling Systems
 Types

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Introduction

Boxed shaped collection of multilayered wooden
slats

Air flow breaks up water as it falls

Design ensures good contact between water and
air

Used to remove heat from water
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Key Definitions

Wet-bulb temperature = air temperature measured by a
wet-bulb thermometer
– simulates effect of evaporative cooling

Dry-bulb temperature = air temperature measured by a
dry-bulb thermometer

Approach = difference in wet-bulb temperatures between
inlet and outlet called “the approach to the tower”

Latent heat = heat associated with change in state of
matter (e.g., liquid to gas phase)
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Operating Conditions

10-20% of heat (sensible heat) removed
from contact between water and air

80-90% of heat removed following
evaporation

Evaporation is most critical factor affecting
tower efficiency!
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Operating Conditions

Factors which affect cooling tower
performance:
–
–
–
–
–
–
relative humidity
Let’s discuss these!
temperature
wind velocity
tower design
water contamination
equipment problems (pump failure)
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Basic Components

Water distribution system = includes header which
distributes (sprays) water from top of tower over splash
bars

Fan = induced and forced draft towers use fans to push or
pull air

Air intake louvers = louvers on side of towers which
direct air into tower (fixed or movable)

Water basin = collects water at bottom of tower prior to
discharge
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Basic Components

fill = material inside a tower which redirects air flow and
water

column = wooden or metal post which supports tower

stack = hyperbolic towers and chimney towers have huge
stacks located at top

make-up water = water which is added due to evaporation
and blowdown

splash bars = used to redirect the downward flow of water
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Parallel vs. Series Flow
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Classification of CTs

By direction of air flow
– crossflow (airflow is horizontal )
– counterflow (airflow is vertical) designs

By how the air flow is produced
– naturally (hyperbolic or chimney towers)
– mechanically (forced draft or induced draft)
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Induced Draft, Cross Flow CT
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Atmospheric Cooling Tower (Natural Draft)

Use natural forces (wind)
to move air through CT

Air flows in through the
sides, and out the top

Drift eliminators on the
top
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Hyperbolic Cooling Tower

Also called chimney CT

Often seen at power plants

Very high flowrates

Air flows up, creating a
draft
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Forced Draft Cooling Tower

Fans used to create a draft

Air forced in the bottom,
and flows out the top

Typically solid sides

Some recirculation of air
possible, harming
efficiency
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Induced Draft Cooling Towers

Fans located at the top of
the CT

Lifts air out of the CT,
preventing recirculation

Probably the most
common type used in
chemical plants and
refineries
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Troubleshooting

Water dissolves many things (especially hot
water!)

Water is cooled and results in deposits in
tower

Solids concentrate in cooling tower basin
Trivia Question: Are Cooling Towers equipped with automatic sprinklers?
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Problems Faced by Operators

Scale formation suspended solids form
deposits

Corrosion electrochemical reactions
with metal surfaces

Fouling - due to silt,
debris, algae

Wood decay - fungi
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Water Composition Control

Suspended solids levels checked by operators (ppm)

Measured values compared to make-up water
concentrations

Problem controlled by “blowdown” (i.e., old water
replaced with new)

Note: 100 ppm = 100 lbs. suspended solids/1,000,000 lb
water
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Water Composition Control (Solutions)

Scale formation
– remove scale forming solids with softening agents
– prevent scale forming materials by addition of
chemicals
– precipitate scale for removal
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Water Composition Control (Solutions)

Corrosion
– add chemical inhibitors (adds thin film to metal)

Fouling
– use filtering devices
– use dispersants with filtering devices

Wood decay
– use biocides (chlorine or bromine)
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Water Testing (by Operators)
pH of water
 total dissolved solids (TDS)
 inhibitor concentration
 chlorine or bromine concentration
 precipitant concentration
 filter and screen checks
 temperature and humidity

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Humidity Measurements
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Humidity -- Background

Humidity is the amount of water vapor in the air

Humidity is described in different ways
– "relative humidity," which is the term used most often
in weather information meant for the public
– Relative humidity is the amount of water vapor in the
air compared with the amount of vapor needed to make
the air saturated at the air's current temperature

Dewpoint temperature gives a much better estimate of the
amount of moisture actually present in the air
– very important in determining precipitation amounts and even how
comfortable you feel
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Definitions

Absolute humidity: Mass of water vapor in a given
volume of air( i.e., density of water vapor in a
given parcel, usually expressed in grams per cubic
meter)

Dewpoint: Temperature air would have to be
cooled to in order for saturation to occur (Assumes
there is no change in air pressure or moisture
content of the air).
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Definitions

Wet bulb temperature: Lowest temperature that can be
obtained by evaporating water into the air at constant
pressure.

Name comes from the technique of putting a wet cloth
over the bulb of a mercury thermometer and then blowing
air over the cloth until the water evaporates. Since
evaporation takes up heat, the thermometer will cool to a
lower temperature than a thermometer with a dry bulb at
the same time and place. Wet bulb temperatures can be
used along with the dry bulb temperature to calculate dew
point or relative humidity.
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Definitions

Relative humidity: The amount of water vapor
actually in the air divided by the amount of water
vapor the air can hold. Relative humidity is
expressed as a percentage and can be computed in
a variety of ways.

One way is to divide the actual vapor pressure by
the saturation vapor pressure and then multiply by
100 to convert to a percent.
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Sling Psychrometer
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Humidity Determination

From wet and dry bulb temperatures

Use psychrometric charts
– find intersection of wet and dry bulb
temperature lines
– can read humidity from chart (y-axis)
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END LECTURE!
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