CM4120CoolTowerLecture
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Transcript CM4120CoolTowerLecture
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:
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–
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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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