COOLING TOWER
Hampa Energy Engineering & Design Company
Mohammadreza Malek Senior Engineer Machinery
Department
ABSTRACT :
The method by which heat is removed from an open
recirculating cooling water system is evaporation (E) of some of the water over
a tower.
The
amount of temperature reduction that can be accomplished by evaporation at any
time is limited to the wet-bulb temperature or simply, the relative humidity.
A
high relative humidity lowers the evaporation rate; dry conditions encourage a
higher evaporation rate. Seasonal humidity conditions are an important
consideration in tower sizing, design, and placement.
The
recirculation rate and the temperature drop across the cooling tower are the
two pieces of data needed to calculate the amount of water lost from the open
recirculating cooling system (due to evaporation).
Following
items will be discussed and calculated in this article :
- Evaporation
- Temperature Drop
- Recirculation Rate
- Concentration Ratio or
Cycles of Concentration
- Make Up Water
- Holding Capacity or System
Volume
- Blowdown Rate
Evaporation:
- Evaporation losses will vary depending upon
temperature and humidity, but a general rule is that for every 100 F. (60 C.)
temperature drop across the tower, approximately 0.85% of the recirculation
rate will be evaporated.
- Evaporation (Estimation: See Value Added
Troubleshooting Guide
- Cooling Section for precise method )
- E = ∆T×R×0.00085 when T measured in Fahrenheit
- E = ∆T×R×0.00153 when T measured in Centigrade - Where:
R = Recirculation Rate, gpm (m3/hr)
- Units will be equivalent to the R value. Typically
gpm or m3/hr.
- This figure can be used for estimating purposes, but
should not be used when more exact information is required (e.g., in a
proposal, problem solving, etc.).
Details
for calculating evaporation rate based upon temperature and humidity conditions
are provided in the PAC-3 section of the Value Added Troubleshooting Guide.
Temperature Drop :
The temperature drop (∆T) for a cooling tower can be
measured by taking the temperature of the tower return water (TR) and
subtracting the temperature of the basin supply water (TS).
This
difference can be used to calculate the approximate amount of evaporation that
has occurred in the cooling tower:
∆T = TR
− TS
Recirculation Rate
To maintain a flow of water through the heat
transfer equipment, water must be pumped or recirculated.
The
recirculation rate can be determined from information on pump performance,
tower hydraulics, etc.
A
detailed description of how to determine recirculating rate is given in the
PAC-3 section of the Value Added Troubleshooting Guide .
It
can be grossly misleading to simply use the pump name plate data to determine
recirculating rate.
Many
times throttling valves, pipe restrictions, and head pressure restrictions
interfere and can produce deviations as great as 50-75% from the name plate
values.
Concentration Ratio or
Cycles of Concentration
The concentration ratio of an ion carried in a
recirculating system is merely the concentration of that ion in the
recirculating water divided by the concentration of the ion in the makeup
water.
Concentration
ratio is also referred to as the cycles of concentration.
C R = Specific Ion Concentration in the
Recirculation Water
Specific
Ion Concentration in the Make Up Water
Theoretically,
evaporation from a cooling tower is pure water. All of the dissolved ions are
left behind to concentrate in the system.
If
the only system water loss was through evaporation, the dissolved ions in the
recirculating water would continue to concentrate (from the ions left after
evaporation) until the solubility of each ion in the water was exceeded and
massive scale/deposition resulted.
Most
systems cannot tolerate any scale; therefore, the level or concentration of
critical scaling-prone ions in the water is usually controlled by a combination
of bleeding off a certain portion of the recirculation water and adding
anti-scaling compounds.
The
rate at which water is bled from a system (in gpm; m3 /hr) compared with the
amount of fresh water being introduced in the system (in gpm; m3 /hr) will also
determine the concentration ratio.
C R = MU
BD
To
check the concentration ratio in a system, select and monitor a soluble ion
(such as silica or magnesium) that is present in sufficient quantity, stable,
and easily tested.
Compare
its concentration in the makeup water to its concentration in the recirculating
water by dividing the tower content by the makeup content.
Repeating
this same testing for scaling species (e.g., calcium) will provide an
indication if scaling is occurring or if the system is in chemical balance.
If
the cycles of calcium concentration are consistently lower than the cycles of
magnesium concentration, for example, the calcium can be assumed to be
precipitating in the system.
(There
may also be scale forming in the heat transfer equipment, thereby impeding
production.)
Entry
of ions from sources other than the makeup water can invalidate any ratio being
developed. These sources include chlorination, chemical additives, process
leaks, acid additions, and airborne gases.
Make Up Water :
Water that must be added to replace water lost from
the recirculating system by evaporation and bleed-off (or blowdown) is called
makeup water (MU).
The
amount of water entering the system must be equal to the amount leaving the
system.
MU = E
+ BD
Where:
MU = Makeup Rate, gpm (m3 /hr)
E =
Evaporation Rate, gpm (m3 /hr)
BD = Blowdown Rate. gpm (m3 /hr) includes drift,
leakage, filter wastage and export
If
the temperature drop across the tower and the recirculation rates are known,
the amounts of water loss through evaporation can be calculated.
If
the concentration ratio is also known then the makeup water requirements can be
calculated as follows.
MU = E
× CR
CR − 1
The
expression was developed from the following fundamental cooling tower water
balance relationships.
MU = E
+ BD
CR =
MU/BD
Substituting
BD = MU/CR in the first equation.
MU
= E + MU/CR
(MU)(CR)
= (E)(CR) + MU
(MU)(CR)
- MU = (E)(CR)
MU =
E × CR
CR − 1
BLOWDOWN RATE :
The blowdown (bleed-off) rate is generally defined
as the water lost from the system for all reasons except evaporation.
In
very tight (low water loss) open recirculating systems, the two primary areas
for system water loss are evaporation and water blowdown.
In
practice, however, a lot of water may also be lost through system water leaks,
by water combining with the product or process, or by tower drift.
For
calculation purposes, all of these water losses, except for evaporation, are
generally considered together and called tower water blowdown.
The
blowdown rate is normally measured in gallons per minute (m3/hr).
System
blowdown (BD) rate can be calculated from the following expression:
BD = E_ E
CR − 1
Where: BD = blowdown rate, gpm (m3 /hr)
E = tower evaporation rate, gpm (m3 /hr)
CR = concentration ratio or cycles
This
expression was derived from the following cooling tower water balance
relationship: MU = BD + E
Substituting MU = (CR)(BD) in MU = BD + E :
(CR)(BD)
= BD + E
(CR)(BD) - BD
= E
(BD)(CR-1)
= E
BD = E_ E
CR − 1
Non-Blowdown
Water Losses Included in Blowdown [ Drift, Leaks, Filter Wastage, Export ]
If
cooling system is operated under ideal conditions all water removed from the
system would be due to evaporation or blowdown.
Unfortunately
the ideal cooling system only exist in concept and in operating systems we find
other water losses that need to be understood and factored into the overall
cooling system materials balance equation.
Drift - Tiny droplets of water
that become entrained in the airstream and carried out of the unit in the
leaving airstream.
Unlike evaporation, drift is a droplet of water and
contains solids and bacteria.
Drift is the primary mechanism for transmission of
pathogens from a cooling system to a host.
Drift is usually estimated based on a percentage of
recirculation. Estimates vary from 0.002 to 0.01% of recirculation.
Splash fill towers tend to have higher drift rates
then film fill towers.
Drift eliminator design, unit maintenance and air
flow also have an influence on the amount of drift that is released from a
cooling system.
Leaks - Uncontrolled water lost
from a system. Leaks should be identified, quantified and corrected where
possible.
Leak identification and management is a valuable service
to any client operating a cooling system.
Possible sources: pump seals, valves that do not
seal, overflow, tower containment or splash out, exchanger failures . . .
Filter /
Separator Wastage - Water wasted from a system due to separator flush or filter back
wash.
Export - Water intentionally
removed from the system and used in another system.
Holding Capacity or System
Volume :
The holding capacity of a system is the amount of water in the system expressed
in gallons (cubic meters).
Normally,
most of the capacity of a system is contained in the cooling tower basin; the
exact amount, however, can be determined only by conducting a TRASAR diagnosis
or an ion concentration study.
This
technique is described in detail in the Value Added Troubleshooting Guide,
PAC-3.
Assumptions
about holding capacities can be dangerous and may lead to incorrect dosages for
biocides, including biological control programs that are ineffective or too
costly.
Holding Time Index or
Half-Life
The holding time index (HTI) is a calculated figure
that indicates the time required to reduce the chemical or makeup water added
to a system to 50% of its original concentration.
It
is essentially the half-life of a chemical added to the system.
The
basic method of calculating the holding time index is as follows:
HTI = 0.693 × HC
BD
Expressed
in the time units used for blowdown BD. Usually reported in hours.
Where: BD = Blowdown Rate. gpm (m3 /hr) includes
leakage
HC = Holding Capacity or Volume, gal (m3 )
The
holding time index is important in choosing a chemical treatment program.
Very
long holding time indexes may preclude the use of certain chemicals, such as
polyphosphates, because of excessive reversion of the polyphosphate species to
orthophosphate and subsequent precipitation as tricalcium phosphate (a compound
that has a very low solubility in water).
A
short holding time index may limit the use of some chemicals because of the
higher amount of chemical required to maintain the necessary treatment levels
(and the accompanying increased costs).
Further,
not all chemical inhibitors will prevent scale, corrosion, and fouling for the
same length of time.
Therefore,
the particular chemical program chosen and the level at which the chemicals are
applied are influenced by the holding time index.
Finally,
the holding time index is used to determine the required amount of some
biocides to achieve proper control of microorganisms.
This
is particularly true when slug feeding slower-acting biocides.
Short
holding time indexes may not allow enough time to maintain critical biocide
concentration for kill and can result in developing resistance.
We
can manage holding time index to some degree by preblowing down prior to
biocide dosing to increase the holding time.
TIME PER CYCLE :
The time per cycle is defined as the time it takes
all the water in a system to make one trip around the recirculating loop (from
the discharge side of the recirculation pump back to the suction side of the
pump).
Time
per Cycle = HC
R
Expressed
in the time units used for recirculation rate R.
Where: BD = Blowdown Rate. gpm (m3 /hr) includes drift
and leakage
CR = Concentration Ratio
E = Evaporation Rate, gpm (m3 /hr)
HC = Holding Capacity or Volume, gal (m3 )
HTI = Holding Time Index MU = Makeup Rate, gpm (m3
/hr)
R = Recirculation Rate, gpm (m3 /hr)
∆T = Temperature drop across tower, Measured as 0 F.
or 0 C.
HC = Holding Capacity or System Volume, gal (m3 )
CONCLUSION : For selecting a cooling
tower we shall calculate all the characteristics at first in order to meet all
process demands and fulfill the requirements in the best way.
http://idochp2.irandoc.ac.ir/FulltextManager/fulltext15/se/150/150637.pdf
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