Fall 2002
Technical Development
Program
Cooling Tower
Applications
Heat Rejection
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How do cooling towers fit into an air
conditioning system?
A cooling tower is a heat transfer device.
A cooling tower will reject heat from water
that is circulated through a chillers shell &
tube condenser.
A shell & tube condenser is the refrigerant to
water heat exchanger used in an open water
circuit like a cooling tower.
How Cooling Towers Work
How Cooling Towers Work
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Warm water leaving the condenser is pumped to the
top of the tower.
This water is then distributed through spray nozzles
or troughs through the wet deck surface (fill media)
so that it may be brought in contact with outdoor air.
This causes a small portion of the water to be
evaporated. This evaporation removes heat from the
remaining water.
The cooled water collect is a sump at the bottom of
the tower where it is returned to the condenser to
pick up additional load and repeat the cycle.
Definitions
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Entering Wet Bulb
Temperature:
The lowest temperature that
water theoretically can reach
by evaporation.
Entering Wet Bulb
Temperature
Entering Wet-Bulb
temperature is an extremely
important parameter in
tower selection and should
selected for your specific
climate zone in your exact
location.
Definitions
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Approach:
The difference between the
temperature of the entering
wet- bulb temperature of the
air and the cold water
leaving the tower.
Establishment of the
approach fixes the operating
temperature of the tower
and is an important
parameter in determining
both tower size and cost.
7º approach is common in
HVAC.
Approach
Approach
Definitions
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Cooling Range:
The difference in
temperature between the
hot water entering the tower
and the cold water leaving
the tower.
10º is Cooling Range is
common in HVAC.
Cooling Range
Cooling Range
Definitions
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Heat Load:
The amount of heat to be
removed from the circulating
water within the tower.
It is of primary importance
that an accurate heat load
determination be made.
If the heat load calculations
are low the cooling tower
will be undersized.
If the calculations are high,
oversized more expensive
equipment will result.
Definitions
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Pumping Head:
The pressure required to
pump the water from the
tower basin, through the
entire system and return
back to the top of the
tower.
3GPM per Ton of
Refrigeration
Or
90GPM = approximately 30
ton tower
Definitions
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Make-up:
Is the amount of water
required to replace normal
losses caused by drift
evaporation and bleed off.
Make-up Water
Definitions
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Drift:
Water that is entrained in
the airflow and discharged to
the atmosphere
approximately 2%.
Drift loss does not include
water lost by evaporation.
Drift
Definitions
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Evaporation:
For each pound of water that
a cooling tower evaporates,
it removes somewhere near
1000BTU’s from the water
that remains.
The more evaporation that
takes place, the more heat
that is removed.
Evaporation
Definitions
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Bleed Off:
Water contains impurities, when
water is evaporated these
impurities are left behind.
If nothing is done about it, the
concentration of impurities in
would build up rapidly.
Bleed off of some of the water
is continuously required to limit
this build up.
The bleed off rate required is
best determined by a water
treatment specialist who is
prepared to make the necessary
tests and recommendations.
Bleed Off
Cooling Tower Design
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When air circulation is provided by a fan or
blower the tower is called a mechanical draft
tower.
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Induced draft: Draws air through the tower
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Force draft:
Blows air through the tower.
Cooling Tower Design
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Induced Draft
Counterflow Cooling
Tower
Air goes up
Water goes down
Cooling Tower Design
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Induced Draft
Crossflow Cooling
Tower
Air goes across
Water goes down
Cooling Tower Design
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Hyperbolic Natural
Draft
Counterflow Cooling
Tower
Cooling Tower Design
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Atmospheric Spray
Tower
Cooling Tower Design
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Forced Draft
Counterflow Cooling
Tower
Application of Cooling Towers
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1. When selecting the
location, sufficient
clearance should be
allowed for the free Wind
flow of air to the inlet of
the tower and for its
discharge from the
tower.
Obstructions will reduce
airflow causing a
reduction in capacity.
Application of Cooling Towers
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2. Cooling towers should be
located so that noise
created by air or water is
not a source of annoyance.
3. Cooling tower location
should be such that the air
discharge will not cause
Wind
condensation on nearby
surfaces or wetting because
of drift.
Water
Noise
Application of Cooling Towers
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4. The tower should be
located away from
source of exhaust heat
and contamination.
Application of Cooling Towers
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5. Each cooling tower should be
located and positioned to
prevent the introduction of the
warm discharge air and the
associated drift. This drift may
contain chemical or biological
contaminants including
Legionella, which may get into
the ventilation systems of the
building on which the tower is
located or those of adjacent
buildings.
To select a cooling tower,
the following must be known
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Wet Bulb Temperature
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Cold Water Temperature
(this is the year round
environmental temperature for your exact location).
(this is the desired water
temperature to return to your condenser)
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Hot Water Temperature
(the water temperature
leaving your system)
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The Gallons Per Minute requirements of
your system.
To select a cooling tower,
the following must be known
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Selection of design
wet-bulb temperature
must be made on the
basis of conditions
existing at the site
proposed for a cooling
tower, and should be
that which will result in
the optimum cold water
temperature at, or near,
the time of peak load
demand.
Cooling Tower Ratings
Condenser
Entering
Water Temp
80.0
85.0
90.0
95.0
CAPACITY KW
TONS
INPUT
110.8
106.3
101.6
97.1
71.6
76.0
80.4
86.5
Based on 30HXC106 Screw Chiller, R134A
45F Leaving Chilled Water Temperature
To select a cooling tower,
the following must be known
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A wet bulb chart is arranged to show the summertime
frequency of occurrence.
Anaheim would have a 68 degree wet bulb that is exceeded
.5% of summertime hours
Generally, the designer engineer would select the design wet
bulb for a specific installation, but some installations aren’t
critical allowing the use of a reduced design values and smaller
cooling towers. Other installations may work only in the winter
or at night when the wet bulb temperature is low.
The designer must select the design wet bulb for his/her
project. When in doubt, select the highest anticipated wet bulb
temperature to insure satisfactory year around operation.
Cooling Tower Sizing
Information
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EWT
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85
___
95
Leaving Water Temperature
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LWT
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75
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85
Entering Wet Bulb (this is the year
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EWB
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68
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78
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GPM
___
150
___
150
Entering Water Temperature
(the hot water temperature leaving your shell and
tube condenser and entering the cooling tower)
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(this is the desired cold water temperature to
return to your shell and tube condenser)
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round environmental temperature for your exact
location).
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The gallons per minute
requirements of your system.
(3 gallons per ton)
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Cooling Tower Model
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___ ST80
___
ST100
Cooling Tower Sizing
Information
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Spending a little
more for a tower
with a VFD and last
a little longer is
almost always the
wisest decision than
selecting a tower
that is too small.
Cooling Tower Capacity
Control
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Capacity of a cooling
tower can be reduce by
airflow thru the tower
by using a on/off
contactor or variable
frequency drive (VFD).
Temperature of the
towers water basin can
be used for operating
the fan control.
Condenser Temperature
Control
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Three way valve
For condenser temperature control on “Carrier Screw
Chillers” a three way valve should be specified to
maintain temperatures above Carriers minimum 70°
water temperature for the condenser.
The water loop between condenser and valve should
be kept as small as possible so the water will heat up
quickly.
Condenser Temperature
Control
In conclusion
Application checklist should include:
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Correct selection using actual Entering Wet Bulb
Acceptable location on site
Wet deck compatibility
Piping (inlet, outlet, drain, make-up, overflow,
support, equalizer)
Capacity control
Pan water freeze protection
Sound