Cooling water system presentation

Power Plant Chemistry

Cooling Water System

Prepared By : -

Umar Farooq Chemist “ A” SEC MSC ( Chemistry ) MBA ( Marketing )

[email protected]

[email protected]

Shoaiba Jeddah Saudi Arabia

1

Mineral Analysis

Deposit Analysis

Microbiological Analysis

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Table of Content


The Cooling Tower

Fundamental of cooling by Evaporation

Problem in cooling water system

Corrosion and its types

Cooling water system The term cooling water is applied where water is circulated through

an equipment to absorb and carry away heat. Cooling water system

divided into two parts.

1. Primary Cooling Water System

2. Secondary Cooling Water System .

Primary Cooling Water System

a) Once through type

b) Open re circulating system

Once through type.

In once through type system water is passed in

heat exchanger equipment and cooling water is then discharged to

waste is an open cycle . This type of cooling system is adopted only

when cold fresh water available in abundance and also at a low cost

. The source of once through cooling water may be revere and

cannel water or sea water. Umar Farooq Chemist “ A”

SEC Shoaiba Jeddah 4

Once through type.

The cost involved for providing cooling water is that of

pumping only since there is no chance of evaporation . The

concentration dissolved solid in outlet and inlet remains practically

the same.

Deposits in the form of scale and corrosion products may

develop in line and heat exchanger equipment due to heat absorbed

by cooling water and initial low pH of cooling .because of the large

volume of water used in once through cooling system , prior

softening of cooling water is not required.

The most common method to inhibit scale is the use of

an ant nucleating agent ( prevent crystal growth and increase

solubility of hardness producing salts in water ) such as ploy

phosphates, tannins' , lignin's , starches and poly acryl ate. The use

of poly phosphates also prevents corrosion of the lines and heat

exchanging equipment and thus minimizing deposit due to

corrosion. Umar Farooq Chemist “ A”


Open re circulating system The use of recirculating system in which cooling

tower, spray pond or cooling pond serve to remove heat, has been very

economical with respect to water requirement.

In place where fresh cold water is short recirculating system in the

only method for cooling . After circulation through the heat exchanger

equipment , water is cooled in cooling tower. This cooling effect is produced

by evaporation of a portion of circulating water because concentrated.

The evaporated must be reproduced by make up water.

The circulating water becomes more concentrated than the make up water

due to evaporation loss. Cycle of concentration is the term employed to

indicate degree of concentration of the circulating water as completed to

make up water.

For example two cycle of concentration indicates the

concentration of circulating water is twice of the concentration of make up

water . Some water of the cooling tower is also lost due to wind age or drift

loss. Which is the loss of fine droplets of water carried away by circulating

air. the amount of the wind age loss depends upon design of the cooling

towers. The typical wind age loss based on the rate of circulation.

Umar Farooq Chemist “ A”


Secondary cooling water system

Closed cooling water system.

This system also known as “Secondary cooling water system” It is used for

the cooling of a number of power station equipments and its self warmed.

The warmed cooling water is passed through exchanger , where it gives its

heat to primary cooling water which is enter cooled in cooling tower or

derived to waste. Since it is closed system, there is no loss of cooling water

due to evaporation or drainage.

The water once filled in the system can be used again and again. Make up

water is required due to loss of cooling water from gland packing of pumps,

which is essential for lubrication of the pumps, in closed cooling water

system treated water ( soft , clarified or demineralization water ) is used as

coolant. The use of soft or demineralization water eliminates any changes of

cooling in the pipe line.



Secondary cooling water system

Corrosion of the pipe line is controlled by

treating the water with chromate , tennis, nitrate etc. these

chromates forms a protecting coating on the metal surface and act

as a barriers between metal and oxygen present in the water . The

solubility of oxygen gas decreases with increases in temperature on

the other hand oxygen is more corrosive at higher temperature as

compared to low temperature since in a closed cooling system,

There is no way for oxygen to get out , severe corrosion of metal

may take place at elevated temperature.

The metal used for construction of

modern engines, compressor and cooling system include cost iron,

steel, copper alloys' and aluminum corrosion due to galvanic action

may be takes place which is controlled by Cathodic protection.



Important terms relating to evaporation cooling

Cooling Tower : A cooling tower is a semi enclosed device for

evaporation cooling of water by contact with air both dry bulb and

wet bulb temperature are very important in evaluating the

performance of tower.

Cooling Range : Cooling range is the difference in temperature

between hot water entering the cooling tower and cold water leaving

the cooling tower.

Approach : Approach is the difference between temperature of cold

water leaving cooling tower and wet bulb temperature of

surrounding area.

Pumping Head :The minimum pressure required to lift water from

basin to the top of the tower and face it into water distribution

system. Pumping head is equal to static head plus friction loss and

pressure drop through the distribution system.



Important terms relating to evaporation cooling

Drift : Drift is the water lost as mist or fog droplets . These droplets

are entrance by the circulating air flow and discharge to atmosphere

. Drift is independent of water by evaporation and can be reduced to

minimum by good design.

Blow Down: It is continuous or intermittent discharge to waste small

amount of circulating water. evaporation in tower increases its

dissolved solid concentration blow down removes solid

concentration .

Cycle of Concentration : It is equal to the ratio of chloride is

circulating water to chloride in make up water.



Fundamental of Cooling Evaporation

Evaporation : A change of state from liquid to gas is called

evaporation . It can takes place at any temperature . Some

molecules in liquid are escaped into gas. Phase and evaporation

takes place. Because these molecule are with higher kinetic energy.

Evaporation result in cooling of the liquid.

Evaporation Cooling: Cooling water is used at a plant for

condensing . Steam and for products and equipment cooling.

Cooling tower is the unit operation that provides cooling water

recovery thus decreasing water and power requirement , water is

cooled in a cooling tower by evaporation . The driving force for this

phenomenon relates to the difference between the hot influence

water temperature and the wet bulb of surrounding air.

Latent Heat of Evaporation : As the liquid change to vapors heat is

consumed called as latent heat of vaporization . When it about 1000

btu/pond water evaporation . This heat is taken mainly from water

lowers its temperature.




Sensible Heat : It is defined as the heat that changes temperature also

part of cooling picture. because when water is warmer then air ,

there is some tendency for air to cool the water . Air become hotter it

gain sensible heat . Water is cooled as sensible heat is transferred

to air.

About 75% to 95 % of the total heat is removed by

evaporation ( Latent heat ) and 25% by sensible heat transfer.

Temperature Dry Bulb :Temperature of a gas or gas mixture ( air )

indicated by an accurate thermometer.

Temperature Wet Bulb : Temperature at which a liquid by evaporating

into air bring the air to saturation adiabatically at some temperature.




Relative Humidity :

Ratio of mole fraction of water vapors present in air to

mole fraction of water vapors present in saturated air at the same

temperature and pressure.

Relative humidity of air can be determined from a

psychometric chart . When both dry wet bulb temperature are known

. Drier the air. Greater will be the difference between dry and wet

bulb temperature and when relative humidity of air is 100% wet bulb

temperature is equal to dry bulb temperature for any lower humidity

some water evaporates to cool the bulb giving a lower wet bulb

temperature.



Open Evaporation System 1. Cooling Pounds.

2. Spray Ponds

3. Cooling towers.

Cooling Pounds

Water to be cooled is introduced

into pound is called to cool gradually by

natural evaporation , radiation , convention

to a suitable temperature for re use . Since

the method of cooling is slow , large amount

of cooling water and consequently large (

amount ) pound are needed to meet the

normal cooling demands.

Spray Pounds.

These are cooling pounds in

which there is provision for spraying the

warm water by means of pipe and nozzles

or motor drive spry modules . The cooling

rate is faster and size of pounds is smaller

as compared to cooling pounds.

Cooling pond for a power plant in

Grant County, West Virginia.

Spray Pounds for waste water



Cooling Towers Natural draft cooling tower

The water to be cooled is pumped

to the tope of the tower and sprayed

over the tower by mean of nozzle to

increase the surface area of water.

which is further increased by placing

packing in the tower which breaks up

the water droplets more completed

and increases cooling efficiency . The

amount of air entering the cooling

tower is controlled by inlet air on its

side walls, which also helps to prevent

water losses.

As the water falls

through the tower it comes in contact

with the air and evaporates cooled

water is collected in the basin and is

pumped back to the plant for reuse.



Mechanical Draft Cooling towers

Forced draft :

In which air is forced into the tower by

means of a blower.

Induced draft :

In which air is pulled into the

tower by means of a fan located at its

top. This design further divided into

counter flow and cross flow type.

Induced draft ( counter flow) cooling tower

In counter flow design air is

pulled into the tower its base by the air

inlet louvers and comes in contact with

the flowing water from the top of tower

. Drift eliminators are located at the top

just before the fans to prevent wind

age losses. In this design the coldest

water contacts with driest air and the

warmest water contact with more

humid air. Umar Farooq Chemist “ A”


Mechanical Draft Cooling towers

Induced draft ( cross flow) cooling tower .

In cross flow design the air is

pulled along the side of the

tower by the air inlet louver. So

that the air is introduced

perpendicularly to the flowing

water. The drift eliminators' are

placed in the middle of the

tower, because air enters at

90° angle to water flowing as

compares to 180° in counter

flow design , hence here is

less resistance to air flow and

fans power consumption is

reduced.



Specifics Design



Crossflow Counterflow

Ad

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Lower energy consumption Easy maintenance/de-scale More variations of water flow Reduced drift Lower operating costs

Efficient use of air Better tower performance Longer ranges Finer droplet size from spray heads

Dis

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Accelerated algae growth Possible orifice clogging Larger footprint Possible icing on louvers in colder climates

Higher pump head needs Increased operating costs Difficult to clean/de-scale More piping needed High inlet velocities may suck in trash and dirt

Advantage and Disadvantage of Cross flow and counter flow cooling tower's

Cooling Towers

Dry cooling tower.

In place where there is actual shortage of

water and temperature remains very low

through the year, dry cooling towers have

been used for condensing of steam. In this

system the hot condensate from the hot

well is passed through a series of air

cooled spiral tube which resemble like car

radiator.



Langeler Saturation Index Major portion of condensate is pumped to

the Cooling tower . While a small portion is routed into

main steam water cycle. The cooled condensate from

the dry cooling tower is pumped back into the condenser

in the form of s pray, where it mixes directly with steam

coming from the turbine and is condensed

The saturation index I◦ is given by expression

I◦ = pH – pHs

Where pH is actual

pHs= pCa+ pAlk + C

Where pCa is log factor of the calcium hardness expressed

in ppm as CaCo3



Langeler Saturation Index

pAlk: is log factor of M.alkalinity expressed in ppm as CaCo3 .

C : is log factor of the total solid in ppm at the temperature of water

Case # 1 when I◦ is zero i.e. pH is equal to pHs , a saturation

equilibrium exist and there is no scale formation and corrosion

attack is minimized.

Case # 2 : when I◦ is positive i.e. , when pH is grater, than pHs , a

condition of super saturation of CaCo3 exist with respect to alkalinity

and total solid content at existing temperature and scale deposits.

Case # 3 : when I◦ is negative i.e. , when pH is less than pHs, the

equilibrium is unbalanced in the deposited direction . Corrosion of

metal will occur and scale previously formed will be dissolved.



Problem in Cooling water system

Scale Control :

Cooling water may contain carbonates , Chlorides

and sulphate of Ca and Mg as well as salts of Na and dissolved

gases like CO2 and O2 . The tendency of a cooling water to form

scale or cause corrosion depends upon the balance of there

constituents.

Scale is formed by heating of bicarbonate hardiness

or by increasing in an alkalinity .

Ca( HCO3) 2 CaCO3 + CO2 + H2O

Ca( HCO3) 2 + NaOH CaCO3 + NaCO3 + H2O

From equation # 1 it is clear that bicarbonate are

converted into insoluble carbonate and CO2 and if the reaction is

reversed carbonates and CO2 Combine to form soluble carbonates.




This shows that keeping the

carbonates in the dissolved form will make the

water less corrosive because dissolved CO2 is

greatly reduced .exactly at equilibrium , various

form of CO2 ( Free CO2, Carbonates and

bicarbonates ) are so balanced that they cause

no scale formation and corrosion .

This layer of scale acts as a

protective layer against corrosion . But thick

layer of scale binders in heat transfer.




Corrosion Control:

The contact air with water passing over a

cooling tower make the cooling water rich in O2 causing

severs corrosion problem . The air may also contain

gases like SO2,H2S and NH3.If there is industrial area

around there corrosive gases will further enhance

corrosion problem.

Presence of chlorides and sulphates in

increased concentration arising from continuous

recirculation of cooling water enhance the corrosion rate

and make corrosion control difficult by corrosion

inhibitors.




Corrosion problem also arise due

to galvanic action , if the metals , Example of

such corrosion is observed in heat exchangers ,

where steel tubes are used along with admiralty.

Sand and dust may be blown into

the open recirculation water besides loose fibers

from cooling tower wood . Sludge and slime may

collect in the tower pumps, deposits of all these

contamination on metallic surface give rise to O2

concentration and result in severs localized

pitting. Umar Farooq Chemist “ A”


Prevention CaCO3 Protective Scale

The Method of treatment has been discussed

under langelier saturation Index.

Mechanical De aeration .

Since O2 must be removed from the entire

volume of circulating water to avoid corrosion . It

is not possible practically for economic reason ,

In some small cooling towers system ,

vacuumed de aeration of cooling water is

practiced.



Corrosion Inhibitors The corrosion inhibitor are chemicals that reduce and in some cases

almost stop corrosion of the metals when used in the cooling water.

Most frequently used corrosion inhibitors are poly phosphates,

Chromates, Tannins, Lignin's, and Starch, other corrosion inhibitors

like Nitrites, Silicates and Amines are less frequently used, These

act as a barrier between metal surface and acid . It dose not allow

an acid ( H2SO4 ) to attack metal surface. An anodic inhibitor

restrains the anodic corrosion reaction.

Fe= Fe + 2e-

Examples of anodic inhibitors are , Sodium chromate, Potassium

chromate ,Nitrite, Phosphate and hydroxide.

Example of Cathodic inhibitors are salt of Zinc and Nickel , such as

Hydroxide , Oxides and Carbonates, which are sparingly soluble in

water.

O2+2H2O+4e=4(OH-)



Slime and there Prevention The word slime is

used vaguely and all fouling except corrosion and

scale is usually called slime. However in many

cases slime occur to gather with corrosion and

scale.

Slime is caused buy the adhesion

and accumulation of soft muddy material which is

formed by mixing “micro Organism” like

bacteria ,fungi ,algae , etc. which grow by utilizing

the dissolved nutrients in water with inorganic

matter like mud sand and dirt.

As a result slime causes not only

a drop in thermal efficiency of exchanger and

deterioration of water flow but also cause local

corrosion in the equipment and piping.

Prevention : Application of slime control agents or chemical such as

chlorine ,phenolic and ammonium compounds.

Side stream filtration

Improve of heat exchanger operating conditions.



A heat exchanger in a steam power

station contaminated with macro

fouling

Water treatment plays vital role in Cooling

Tower Operation Cooling tower maintenance can not run

high unless the water is treated to prevent corrosion , biological

growth , deposits and to protect the cooling tower wood from

chemical attack .

Ca( HCO3)2 normally present in all raw water breaks

down to form relatively in soluble CaCO3 . Scale is the most

common type of water form deposit cooling system. The langelier

index measure tendency of CaCO3 to precipitate under given

condition of Calcium hardness ,alkalinity ,pH , Temperature , TDS ,

A Positive index mean tendency water to deposit scale while

negative index tends to dissolve scale therefore causing corrosion.

Chemical treatment with H2SO4 Keep scale

forming salt of calcium and magnesium in soluble form by lowering

pH of circulating water chemical inhibitors are then need to check

corrosion surface active chemicals or chelating agents.



Water treatment plays vital role in Cooling

Tower Operation

Such as sodium hexa meta

phosphate prevent crystal growth and there fore scale formation

.They increase solubility range of scale forming salt. For controlled

scale treatment . Adjust composition of water so as a thin in

previous layer of CaCO3 Scale deposits on the surface of

circulating water system .

Scale must be thin enough to prevent any

corrosion but not thick enough to effect overall heat transfer.

Microbiological growth like slime and algae retard cooling , drop

plant efficiency and rise cost of cooling system maintenance.

Mechanical cleaning in the best way to get rid of accumulated

growth. But to keep slime and algae from getting into bigger masses

use chemicals such as chlorine , phenolic and ammonium

compound.





Engineering

Cooling water system presentation