1. Open re-circulating cooling towers -summary
Chandran Udumbasseri, Technical consultant
cudumbasseri@yahoo.co.in.
Types
1. Natural draft towers
2. Forced draft towers
3. Induced draft Towers
Fundamental Parts
1. Basin and cold well
2. Louvers
3. Fill
4. Distribution and Fan deck
Objective
1. Cooling tower water has higher temperature and promote scale deposition and
corrosion
2. It is a huge air scrubber and introduces microorganisms, dust and dirt which
promote fouling and corrosion
3. Make up water also brings in scale, deposit and corrosion.
4. Oxygen is drawn into tower increasing chances of corrosion.
Principles
1. It can cool water to 20 to 30% of total cooling by conduction
2. Evaporation cools the water by 1 to 2 % of the re-circulating water.
3. I Pound of water takes 1000 BTU to evaporate. So 1000 pounds of water will
drop temperature by 1o
F
4. If 1000 BTU is extracted from 100 pounds, the water temperature will drop by
10o
F.
5. This accounts for the remaining 70 to 80 % of the total cooling.
6. The water lost by evaporation must be replaced. The lost water as fine mist
must also be replaced.
7. The water must be added to make up for leaks and blow down.
Evaporation, blow down and makeup
M = B + E
M = Make up water
B = Blow down
E = Evaporation loss
Cycles of concentration
C = M + B
Where C is the cycles of concentration
COC Measurement

2. It can be measured from the conductivity of re-circulating water and make up water.
C = Conductivity of re-circulating water = Bμmho or BCl
Conductivity of Make up water Mμmho MCl
Open Recirculating Cooling System
COC control
1. To increase COC, decrease Blow down
2. To decrease COC, increase Blow down
COC and Make up
The Make up requirement decreases very rapidly as the COC is increased to about 4
or 5 with lower reductions at higher COC
Blow down, Evaporation and COC
B = E /(C-1)
COC is not having any unit
So if evaporation is known, the Blow down for a given COC can be calculated.

3. For a typical re-circulating system, approximately 1% of the cooling water is
evaporated for every 10o
F temperature drop in the cooling water as it passes through
the tower.
E = 0.01 x re-circulating rate x T drop (o
F)
10o
F
Example:
A cooling system operates at 5000gpm. The temperature drop through the tower is
14o
F.
E = 0.01x 5000 x 14. = 70 gpm
10
For Air conditioner cooling tower, the evaporation rate is 1.5gallon/hour/T
Deposit formation and control
1. Scale is the deposition on heat transfer surfaces. As evaporation of cooling
water takes place, the dissolved solids concentration becomes greater until the
solubility of a particular salt. This causes crystallization of salts and their
deposition.
2. Most scaling materials are calcium phosphate, calcium carbonate, calcium
sulfate, magnesium silicate and silica.
3. Silica and most salts are more soluble in hot than in cold water.
4. But Calcium salts (phosphates, carbonates, sulfates) are more soluble in cold
water than in hot water.
5. Most likely scale formation region is heat exchanger surfaces.
6. The salt concentration in the make up water can be measured. Also the
evaporation can be calculated. Select the upper limit of concentration of the
solid in the system. Based on these data calculate the COC of the system.
7. Blow down then can be estimated using the relation between, blow down
evaporation and COC.
Determination of scale limits
The amount of dissolved solids is controlled by maintaining COC in the system at a
level that is equal to the lowest COC allowable for any one of the salts. The operating
COC can be increased with proper cooling water treatment.
1. Calcium carbonate COC: This depends on alkalinity and calcium hardness of
the make up, C = √110000/T x MCa: T is total hardness, MCa is calcium
hardness.
2. Calcium phosphate COC: If ortho phosphate is >5ppm, then COC is given by
C= 105 x (9.8-BpH)/ MCa where BpH measured pH in blow down, MCa calcium
hardness in make up.
3. Calcium sulfate COC: C = √1250000/( MCa x Msu) where MCa is the calcium
hardness of make up and Msu is the sulfate (as SO4) in make up.
4. Silica COC: The solubility of silica is about 150ppm at the temperature
encountered in most cooling waters. C = 150/ MSi where MSi silica in make up
water
Determination of COC that control operation

4. In untreated systems the smallest calculated COC, using the relationships for these
salts, is the controlling factor. As the system operates, the material with lowest COC
will come out first. So the COC should be kept for the system lower than the smallest
COC.
Example:
Cooling tower make up water data:
Calcium hardness = 100ppm
Total alkalinity = 60ppm
Sulfate = 60ppm
Silica = 14ppm
At what COC the system can operate without water treatment:
CaCO3 = √110000/(60x100) = 4.3
CaSO4 = √1250000/(100 x 60) = 14.4
Silica = 150/14 = 10.7
So the COC for CaCO3 is the lowest. So the system should be operated at lower than
4.3. Scaling will not occur provided no water treatment is carried out.
Use of phosphonate or polymeric anti-sealant is used the COC can be increased based
on calculated scaling index.
Langelier Scaling Index (LSI)
In this method pHs, the pH at saturation point of calcium carbonate is determined.
1. If measured pH is greater than pHs, the water has scale forming tendency.
2. If measured pH is less than pHs, the water has scale dissolving tendency.
LSI = (pH) – (pHs)
 Positive value is scale forming tendency
 Negative value is scale dissolving tendency.
Ryzner Scaling Index (RSI)
RSI = (2 pHs) – (pH)
 A value of 6 indicates stable water.
 A value less than 6 indicate scale forming tendency
 A value greater than 6 indicates scale dissolving tendency.
Practical (Puckorius) Scaling Index (PSI)
Here an adjusted pHeq is used in the equation.
PSI = (2 pHs) – (pHeq)
 A value of PSI 6 indicates stable water.
 A value of PSI, lower than 6 indicates scale forming tendency.
 A value of PSI, greater than 6 indicates corrosion stage.

5.  If PSI is 6 or 7 it is scale-free State.
Scale control
1. Remove scale material from water prior to use.
2. Keep scale forming material in solution
3. Allow scaling material to precipitate as a removable sludge.
CaCO3 control:
Calcium carbonate comes from the breaking of calcium bicarbonate. It depends on
calcium hardness and bicarbonate hardness. Breaking of calcium bicarbonate depends
on temperature.
AMP (Amino tri-methyelene phosphonic acid) and HEDP (1-Hydroxyethylidine 1, 1
Di phosphonic acid) are used for scale controlling.
HEDP is more stable at chlorine levels normally found in cooling water.
The use of 3 to 5 ppm HEDP will increase the solubility of calcium carbonate by a
factor of 3.
So the cooling towers can operate at a PSI of 4 without scaling if HEDP is used.
If Calcium or magnesium is not present, then this HEDP will cause corrosion for both
mild steel and copper.
CaPO4 control
This scale is common in phosphate treated cooling water. If the calcium hardness is
500ppm and pH is 7, it is likely to cause scaling even at low level of 5ppm phosphate.
Its solubility can be increased by a factor of a little less than 3 with the addition of
4ppm of HEDP.
CaSO4 control:
It is soluble in water as compared to other two materials mentioned above. Its scaling
is possible only if some calcium hardness remains after it reacts with all the
carbonates. If calcium hardness is in the range 300 to 500ppm as CaCO3 and SO4 in
the range 500 to 700ppm, then addition of 3 to 5ppm of HEDP will cause the sulfate
scale to remain in solution about three times.
Magnesium silicate control
This occurs under certain conditions. Magnesium has to hydrolyze to magnesium
hydroxide which then reacts with colloidal silica to form magnesium silicate. But as
temperature increase silica goes to solution. This scale is formed in the colder regions.
Silica control
The solubility of silica can not be increased above 150ppm. It is soluble in higher
temperature. It deposits on tower slats rather than on in the heat exchanger.
The slats will show white deposits. If this occurs, then increase blow down, this
causes decrease of COC.
If silica in make up water is 30ppm then silica will be the controlling factor for COC.
Too much high silica in make up water should be removed prior to use.
Fouling & fouling control
Living and dead microbiological bodies cause fouling. Slime producing organisms are
serious source for fouling.

6. Nonliving matter can be controlled by adding dispersants like poly acrylates. 4ppm
addition can suspend the foulants.
Microbiological deposit control
Slime deposits are caused by algae, bacteria, and fungi.
It can grow in the cooling tower and coat pipes and heat exchanger surfaces.
The gelatinous slime produced by these organisms can trap sediments and cause
fouling and scaling.
Algae can be controlled by sprinkling calcium hypochlorite. But this will increase the
calcium in the re-circulating water.
Bacteria can be controlled by chlorine. When chlorine gas introduce to water, it forms
Hydrochloric acid and Hypochlorous acid. Hypochlorous acid favors lower pH.
Chlorine is less effective as a biocide at pH of 9.5 or greater because of lack of
ionization at this pH. pH in the range 6.5-7.5 is the favorable one for chlorine.
If Legionella pneumophila is present in water, then a gallon of chlorine releasing
agent with a pound of tri sodium phosphate should be added to the cooling tower
sump for every 100 ton capacity.
Corrosion in cooling tower
The corrosion occurs when an electric current flows from one part of the metal
(anode) through the water to another part of the metal (cathode).
Galvanic corrosion
When two metals are coupled together, the one higher in the galvanic series will act as
anode and corrode.
Steel and Copper -----steel will corrode
Steel and Zinc--------Zinc will corrode
If steel and copper are present in a alloy then copper may dissolve and resulting plate
out on steel surfaces. This results in galvanic action. Pitting corrosion is the result of
this action.
Crevice corrosion
When two steel plates are bolted, at the contact point a crevice can exist. Flow is
restricted in this crevice. Here oxygen is consumed faster. This results the metal in the
crevice to become anode and corrode.
Deposit corrosion
The underside of slime deposits can act as a crevice causing the metal to become
anode and corrode.
General corrosion

7. Even a single piece of steel can have cathode and anode areas due to differences in
impurities and stresses. Here cathode area may become anode area and vice versa. So
the corrosion will appear as uniform.

8. Corrosion control
Galvanic and crevice corrosion
These are caused by design problems.
Deposit corrosion
Keep the system clean. Use dispersants to control fouling.
Use biocides to control bacteria and microorganisms.
General corrosion
Use corrosion inhibitors. They form thin films and keep water and oxygen to come
into contact with the surface.
Anodic inhibitors are, orthophosphates, nitrites, molybdate, & silicates. They coat the
anode and control corrosion.
Cathodic inhibitors are Zinc, polyphosphates and polysilicates. They coat the cathode
and inhibit corrosion.
The best way to control general corrosion is to add both anodic and cathodic
inhibitors.
Corrosion control for copper and copper alloys
Tolyltriazole is used as inhibitor for copper. 0.1 pound per day is added per 100ton
capacity of tower.
Make up water treatment
If untreated water can be used with controlled COC, water treatment is not required.
Re-circulating water
This is divided by the size of the tower, small (25 to 100ton) and large (greater than
100 ton). Scale is controlled by adjusting COC through blow down. Treatment is
carried for corrosion, scaling, fouling and microbiological growth.
Scale control by COC
The COC may be controlled by blow down to avoid levels of silica and calcium salts.
This is done by measuring conductivity of make up water and blow down water. The
smallest COC of materials is used as criterion (keep COC below this value by blow
down).Untreated tower may be kept at PSI value of 6 while treated one can go to PSI
value of 4.0.
Small cooling tower treatment
Place a bag containing 15 to 20 pounds of slowly soluble phosphate in the cooling
tower sump. Also add biocide 1bromo, 2chloro, 5, 5 dimethyl hydantoin. Replenish
the bag when the bag is empty.
Medium to large tower treatment
Towers large of the size 100 tons should be treated for scale, fouling, microbiological
growth, and corrosion.
Zinc phosphate program

9. If zinc is allowed in the blow down then this program can be used. The hardness of
make up water should be below 140ppm (as CaCO3) and COC to about 3. It is
necessary to keep pH below 8.0 to avoid precipitation of zinc hydroxide.
Dosage for zinc phosphonate program
Limits Estimated chemical requirement
Pounds/day/100ton Tower
Capacity at 3 COC & 10o
F t
3-5 ppm phosphonate 0.1 HEDP
1-2 ppm zinc 0.1 zinc sulfate
3-4 ppm polymer 0.1 polyacrylate
1-2 ppm TT 0.1 tolyltriazole
Zinc Molybdate program
Use this program when zinc is allowed to blow down. Keep pH below 8.0. Add zinc
stabilizing polymer
Dosage for zinc molybdate program
Limits Estimated chemical requirement
Pounds/day/100ton Tower
Capacity at 3 COC & 10o
F t
10-15 ppm molybdate as Mo 0.9 Sodium molybdate Na2MoO4.2H2O
2-3 ppm zinc 0.1 zinc sulfate
2-3 ppm phosphonate 0.1 HEDP
3-4 ppm polymer 0.1 polyacrylate
1-2 ppm TT 0.1 tolyltriazole
Phosphonate-polymer program
This program is where above two methods can not be applied.
Keep pH from 7.5 to 8.5.
Dosage phosphonate polymer program
Limits Estimated chemical requirement
Pounds/day/100ton Tower
Capacity at 3 COC & 10o
F t
3-5 ppm phosphonate 0.1 HEDP
3-4 ppm polymer 0.1 polyacrylate
1-2 ppm TT 0.1 tolyltriazole
Specification
Inhibitor Chemical Name
Phosphonate HEDP, 60% active
Phosphate Slowly soluble phosphate glassy

10. plate or lump
Polymer Sodium poly acrylate or polyacrylic
acid Mol Wt 2000-4000, 50% active
TT Sodium tolyl triazole, 43% active
Zinc Zinc sulfate, Zn SO4.H2O
Molybdate Sodium molybdate, Na2MoO4.2H2O
Problems
These programs are to keep calcium in solution. If not properly controlled severe
scaling will occur, if treatment chemicals are lost.
Treatment chemical calculation
The chemical requirement can be estimated based on:
1. The desired level in the cooling system.
2. The sum of chemical loss in the blow down, chemical consumed by reaction
and other losses in the system
Treatment = Loss
Treatment = chemical added, lb/day
Loss = blow down + reacted + other losses
3. The chemical addition calculated from blow down losses is increased by an
amount that will replace these losses
4. Blow down loss can easily be calculated
B x Level x Conversion Factor
Where B is blow down, gallons per minute
Level = concentration of chemical in blow down, ppm
Conversion Factor = 8.33 pounds per gallon x 1440 (minutes/day)
1000000 gal
= 0.012
Example
1. A cooling tower with 5000 gpm recirculation has 8o
F temperature drop. The
conductivity of blow down is 1200 micromhos and that of make up water is
300 micromhos. It is desired to maintain 100 ppm treatment in the system.
What is required blow down? What will be the make up? How much chemical
must be added each day?
2. Evaporation in the system
E = 0.01x recirculation rate x T drop/10
= 0.01 x 5000 gpm x 8o
F/10
= 40 gpm
3. Determine COC using conductivity data
COC = Bμmho = 1200/300 = 4.0
Mμmho
4. Estimate system blow down, based on COC and Evaporation

11. B = E/(C-1) = 40/ (4.0 – 1) = 40/3 = 13.3 gpm
5. Determine make up water requirement
M = B + E = 40 + 13.3 = 53.3 gpm
6. Determine treatment chemical required
Blow down = Loss x Level x 0.012
= 13.3 gpm x 100ppm x 0.012
= 16.0 lb/day
Feeding treatment chemicals
The three chemicals in the program may be mixed and added with a continuous
system. Biocide may also be needed.
Example
1. A cooling tower is treated program1: 5ppm phosphonate, 2ppm zinc, 4ppm
polytriazole, and 2ppm tolyl triazole.
2. Foe 100ton tower
HEDP = 0.1lb/day
Zinc sulfate = 0.1 lb/day
Polyacrylate = 0.1 lb/day
Tolyl triazole = 0.1 lb/day
3. The proper mixture concentrations are
HEDP = lbs x 100/lbs total = 0.1x100/(0.1+0.1+0.1+0.1+0.1) = 25%
Zinc sulfate = 25%
Polyacrylate = 25%
TT = 25%
4. To prepare a 3% solution in 55 gallons (416.5 pounds0 = 3% x 416.5 = 12.5
25% x 12.5 = 3.1 lbs of HEDP
25% x 12.5 = 3.1 lbs of zinc sulfate
25% x 12.5 – 3.1 lbs of polyacrylate
25% x 12.5 = 3.1 lbs of TT
5. This blend is added at 0.3lbs/day for each 100toncapacity of a cooling tower
operating at 3COC.
6. Adjust the feed rate of the mixture to maintain the recommended levels. It
should be within 10% of the target level.
7. Add appropriate biocide.
Ozone in cooling towers
1. Ozone is a very effective biocide for cooling towers.
2. But it increase corrosion
3. Ozone systems are expensive.
4. maintenance of ozone system may be a problem
What to check in a Cooling tower
1. Distribution deck: plugged water distribution holes.
2. Green slime: indicates algae growth in areas not shielded from sunlight
3. Scale: calcium carbonate scale is white but it will appear as grey

12. 4. Scales appear on heat transfer surfaces and not visible in the bulk circulating
water.
5. Scale formation is a severe problem.
6. Scale is formed on the air intake slats of the CT due to evaporation and not a
corrosion problem.
7. Slime deposits are found at the bottom of the distribution deck and walls of the
tower.
8. Dispersants keep the solids in suspension and prevent settling. The tower
water will be turbid if the dispersant is doing its job.
Corrosion test coupons
1. Always note the visual appearance of the coupon when removed.
2. Suspended solids can deposit on the coupon. This indicates for more
dispersant.
3. Inside surface of Heat exchanger tubes:
1. The scale deposits should be in the form of thin layer and may be
visible when the tube is dried. The scale may appear from white to
grey.
2. Fouling deposits are usually loosely adhered to the tube surface.
3. In chiller, the scale formation indicates the degradation or poor
efficiency of the chiller.
Note
Other types of water treatment equipments will be explained in the next session