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 1oF

4. If 1000 BTU is extracted from 100 pounds, the water temperature will drop by

10oF.

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

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.

For a typical re-circulating system, approximately 1% of the cooling water is

evaporated for every 10oF temperature drop in the cooling water as it passes through

the tower.

E = 0.01 x re-circulating rate x T drop (oF)

10oF

Example:

A cooling system operates at 5000gpm. The temperature drop through the tower is

14oF.

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

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.

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.

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

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.

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

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 & 10oF 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 & 10oF 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 & 10oF 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

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 8oF 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 8oF/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

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

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