General Water Treatment For Cooling Water

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-UTL-900

General Water Treatment
for Cooling Water

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Process Engineering Guide:

General Water Treatment
for Cooling Water

CONTENTS
0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

CHOICE OF COOLING SYSTEM

2

4.1 ‘Once through' Cooling Systems
4.2 Open Evaporative Recirculating Systems
4.3 Closed Recirculating Systems
4.4 Comparison of Cooling Systems

2
3
3
3

5

MAKE-UP WATER QUALITY

4

6

FOULING PROCESSES

4

6.1 Deposition
6.2 Scaling
6.3 Corrosion
6.4 Biological Growth
7

CONTROL OF THE COOLING SYSTEM
7.1 ‘Once through' Cooling Systems
7.2 Closed Recirculating Systems
7.3 Open Evaporative Cooling Systems

4
4
4
5
9
9
9
9


TABLES
1 RELATIVE IMPORTANCE OF FOULING PROCESSES
AND INSTALLED COSTS

3

2

4

WATER QUALITY PARAMETERS

FIGURES
1

PREDICTION OF CALCIUM CARBONATE SCALING

6

2

CALCIUM SULFATE SOLUBILITY

7

3

CALCIUM PHOSPHATE SCALING INDEX

8

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

11


0 INTRODUCTION/PURPOSE
Fouling in cooling systems occurs by four potential processes, viz:
(a) Crystallization scaling.
(b) Deposition of particulate matter.
(c) Corrosion and subsequent transfer of corrosion deposits.
(d) Microbiological growth.
These processes do not occur in isolation and it is frequently the interaction
between them which results in the worst fouling problems. It is, therefore,
essential to identify the potential sources of each and then choose the best
cooling technology and apply the appropriate control, both mechanical and
chemical, in order to minimize the effects of fouling.
The water treatment required for cooling is determined by three factors:
(a) The cooling process in use.
(b) The make-up water quality.
(c) The control of the cooling tower and chemical dosing system.
It is important to understand how these interact if fouling of cooling systems is to
be avoided. Clauses 4 to 6 describe the important parameters before chemical
treatment is discussed.
1

SCOPE

This Process Engineering Guide discusses the factors influencing the choice of
cooling water system, defines the parameters influencing make-up water quality,
explains the four basic processes of fouling and gives general advice on the
cooling water selected.
This Guide does not cover the design of a treatment system nor does it make
specific quantitative treatment recommendations.


2

FIELD OF APPLICATION

This Guide applies to process engineers and water technologists in GBH
Enterprises world-wide.
3

DEFINITIONS

The following definition applies to this Guide.
Concentration Ratio

is the ratio of the concentration (e.g. of Mg++) in the
circulating water to that in the make-up water.

4

CHOICE OF COOLING SYSTEM

4.1

'Once through' Cooling Systems
Water is abstracted, used for cooling and then discharged, generally with
a low temperature rise. The high water usage means that a cheap surface
or groundwater has to be used. Pretreatment may be considered when
serious fouling problems due to silt or river muds occur.
Chemical treatment is difficult because of the quantities involved and the
risk to the environment, but some biocidal treatment may be applied to
reduce fouling due to microbiological or animal life. For this reason, heat
exchangers on such systems are generally made of corrosion resistant
materials, at extra cost, and mechanical methods of controlling fouling are
employed.

4.2

Open Evaporative Recirculating Systems
Water is circulated through heat exchangers and then returned to a
cooling tower in which heat is transferred to the atmosphere by
evaporation. The evaporation loss is generally 1 to 2% of the total
recirculating rate.
The cooled water is reused and the water lost by evaporation is replaced
by make-up water, which will inevitably result in increasing mineral
concentration of the water. This can be kept constant by purging a small
amount of water from the system, generally equivalent to less then
half of the evaporative loss. Chemical treatment of the water is economic
when the sum of the purge losses and involuntary losses from leaks, etc.,
amounts to less than the evaporative loss.


While the capital cost of the Open Evaporative system is higher than for
'Once through' systems, the reduced use of water, the reduced
environmental impact, the ability to chemically treat and the removal of the
need to use corrosion resistant materials make this the system of
choice for most applications.
Recent developments in new packing materials have made Open
Evaporative towers even cheaper to purchase, but have brought a new
fouling problem. The plastic packings come in various spacings, from as
little as 12 mm to 38 mm and higher. At the smaller end, there is a
tendency for the fouling deposits to bridge the gap between packing
sheets, which reduces the cooling efficiency of the system significantly. At
this point the packing has to be replaced as, because of the spacing, there
is no way of cleaning it successfully by water jetting and chemicals cannot
penetrate sufficiently to remove the deposits. Good control of the chemical
regime in the recirculating water can ease the use of such packing.
4.3

Closed Recirculating Systems
Water is circulated through heat exchangers and then passed through an
additional set of heat exchangers where the heat is transferred to air or
water. No deliberate loss of water occurs from the closed loop and hence
little or no make-up is required. Chemical treatment can readily
be applied and control of the water chemistry is easy, particularly if
condensate or deionized water is used. This system is used for critical
heat transfer duties or where elevated temperatures are used. Capital
costs are high, however, and some loss in cooling efficiency has to be
borne from using an indirect cooling method. Treatment of the indirect
cooling water has also to be considered, whether from a 'Once through'
system or an Open Recirculating system.

4.4

Comparison of Cooling Systems

Table 1 shows which fouling processes occur in each of the cooling systems
described. The


TABLE 1

5

RELATIVE IMPORTANCE OF FOULING PROCESSES AND
INSTALLED COSTS

MAKE-UP WATER QUALITY

The "quality" of natural waters can be defined by measurement of the parameters
given in Table 2.
TABLE 2 WATER QUALITY PARAMETERS

6

FOULING PROCESSES

There are four basic fouling processes which can occur in cooling systems, all of
which can be affected by the quality of the make-up water used. These four
processes are now described.
6.1 Deposition
If the make-up water contains a high turbidity or suspended solids, a high
concentration of organic matter and/or a high alkaline hardness, it may be cost
effective to pretreat. In any case, the worst effects of deposition fouling can be
offset by maintaining water velocities in excess of at least 1 and preferably 2 m/s

6.2

Scaling

The solubility of "hardness" salts, i.e. calcium and magnesium salts, decreases
with increasing temperature, thus making any cooling duty a potential cause of
scaling for heat transfer surface temperature constraints). For 'Once through'
systems the temperature rise is generally too small for major problems, and
closed systems are limited to the scale potential of the contained water if there is
no make-up. However, Open Evaporative systems are prone
to scaling with the increased mineral concentration due to evaporation. This is
exacerbated if the make-up water is "hard".
The calcium carbonate scale potential of any water can be estimated using either
the Langelier Index or the Ryznar Index (see Figure 1). Calcium sulfate scaling
may be a possibility where sulfuric acid is added to reduce the alkalinity (see
Figure 2). Calcium phosphate scaling may be a problem, particularly with certain
chemical treatments where control is poor or directly due to poor make-up waters
(see Figure 3). Calcium silicate is not usually a problem unless the silica content
of the recirculating water exceeds 150 ppm.
6.3

Corrosion

For hard waters, the high alkalinity and mineral content make the water less
aggressive towards, for example, carbon steel. However, soft waters are
generally low in both alkalinity and mineral content and tend to be far more
corrosive. Some correlation can be gained by using the Langelier or Ryznar
Indices to predict the aggressiveness of the water, and this is the basis of
some control methods, to hold the water at a neutral Index value to minimize
both scaling and corrosion. In general, increasing the mineral content decreases
the corrosiveness; the addition of acid to reduce scale potential increases
corrosiveness.
6.4

Biological Growth

With the exception of closed loop systems, where a one-off shot of biocide may
be sufficient to control biological activity, no cooling system should ever be
considered to be biologically stable, whatever the make-up water quality.
Waterborne or airborne organisms find the warm, aerobic or anaerobic conditions
almost ideal for growth and can very quickly become established, causing major
fouling problems if not controlled.


FIGURE 1 PREDICTION OF CALCIUM CARBONATE SCALING

Calcium carbonate scaling can be predicted qualitatively by the Langelier
Saturation Index or the Ryznor Stability Index. The indices are determined as
follows:
(LSI)

Lagelier Saturation Index = pH (actual) – pHs

(RSI)) Ryznor Stability Index =

2 (pHs) - pH (actual)

(1)
(2)

The value pHs (pH of saturation) is a function of total dissolved solids,
temperature, calcium and alkalinity; pH actual is the measured pH of the water.
A positive LSI indicates a tendency for calcium carbonate to deposit. The RSI
shows the same tendency when a value of 6 or less is calculated.
The pHs value may be calculated from the nomogram in Figure 1.


FIGURE 2 CALCIUM SULFATE SOLUBILITY


FIGURE 3

CALCIUM PHOSPHATE SCALING INDEX

Calcium Phosphate scaling can be predicted by calculation of the Scaling Index
This is a function of Calcium, pH, temperature and also Ortho-phosphate and is
calculated as:
Scaling Index = pH (actual) – pHs

(3)

where pH actual is the actual pH value of the solution as measured and pHs is
the pH of saturation of calcium phosphate and is determined from the nomogram
in Figure 3.
When the scaling index is positive, precipitation is likely.


7

CONTROL OF THE COOLING SYSTEM
Given that the cooling process and make-up water quality are determined,
much can be done to prevent the interaction of the four processes for
fouling by good control of the operation of the cooling system and of the
chemical dosing applied.

7.1

'Once through' Cooling Systems
For 'Once through' cooling systems, care should be taken to avoid a high
temperature rise through the exchangers as this can increase the risk of
scaling and can change the nature of particulate fouling from a sludge to a
baked deposit. Dosing with biocides, if used, should be frequent enough to
prevent deposit build-up. The use of corrosion resistant materials and
regular mechanical removal of deposits should then prevent major
problems.

7.2

Closed Recirculating Systems
At the other extreme, closed loop systems should only use a high quality
make-up water, ideally condensate or deionized water. A one-off shot
dose of corrosion inhibitor and biocide plus the adjustment to pH > 8.5
using alkali should then be sufficient to prevent fouling as long as there
is no significant loss of water. At elevated temperatures (>100 °C), it may
be better to employ a boiler water type of chemical treatment to minimize
corrosion. With either method, the water should be analyzed on a weekly
basis to guard against loss of treatment.

7.3

Open Evaporative Cooling Systems

7.3.1 Concentration Ratio
There is a natural tendency to concentrate minerals in the recirculating
water by evaporation. This can lead to severe scaling problems if the
make-up water is already hard, if the system is allowed to over
concentrate or if the bulk water temperatures are high.
Control is on the basis of Concentration Ratio, which is defined as the
ratio of concentration of the recirculating water to the make-up water,
usually based on the magnesium concentration in each. In practice, the
Concentration Ratio is determined by the rate of loss of water from the
system, either by deliberate purging or through leaks and other involuntary
losses.

In general, the aim is to control the cooling system at a minimum ratio of 3
to 5, but in some systems ratios as high as 8 or 10 can be achieved. This
ratio determines how much water is used and how much chemical
treatment has to be added.
The control range is largely determined by the quality of the make-up
water and the flows and water temperatures in the various plant heat
exchangers. Operation at elevated concentrations can be achieved even
with hard make-up waters by using acid addition to depress the pH, but
failure of the dosing system can have serious consequences. Small
systems with low purge rates may be very difficult to control manually.
7.3.2 Half-life
The rate of change of chemical composition of the water also has an
impact on the ability to control fouling. This is increasingly important with
the introduction of high efficiency PVC packings into cooling towers, which
has allowed manufacturers to reduce the physical size of the towers and
thus the volume of water stored in the sump.
It is best represented by the half-life (or Holding Time Index) of the
system, which is proportional to the ratio of the volume of water in the
system to the purge rate.
For older towers, the half-life is typically 40-50 h, giving a relatively slow
rate of change which can be easily managed given a daily analysis.
Modern towers have the advantage of being considerably cheaper to
build, but operate with half-lives of typically 4-10 h, making manual
control very difficult. Variations in make-up water quality or cyclical
variations in heat load, such as are experienced in batch processes, make
these problems even worse.
7.3.3 Filtration
It may be possible to prevent suspended solids entering the cooling
system by filtering all the make-up water, but this will not remove solids
(several kilograms/day in industrial areas) sucked into the cooling tower
and scrubbed from the air by the falling water droplets. Chemical
dispersants may be able to prevent the finest of these particles from
settling; maintaining high water velocities will avoid the worst of the fouling
problems. Side-stream filtration of 3 to 10% of the recirculating water will
greatly assist the removal of the finer suspended solids (10 – 100 om)
which will otherwise cause major fouling problems.

7.3.4 Chemical Addition
However good the chemical treatment, it will be of little value if it is not
added consistently and at the correct rate. Such treatments will rarely
remove deposits once formed, so the intention is to prevent any
deposition. In view of the interaction between the various types of fouling,
it is essential that the treatment is regarded as a complete program in
which all parts shall be added if it is to work. It is, for example, of little
value to add a corrosion inhibitor when the metal surfaces are covered
with deposits, since it will not be able to penetrate to the metal surface to
prevent corrosion from occurring and adding to the fouling problems.
The dosing system required will depend on the treatment being applied
(corrosion inhibitor, dispersant, biocides, acid, etc.), the make-up water
quality, heat exchanger duty, half-life, etc.. There is no standard system
applicable for all conditions. At one extreme it may be sufficient to
use a manually adjusted purge and add the chemicals as "shot" doses
once per day. For more demanding systems it may be necessary to pump
in treatment chemicals and purge water in proportion to the make-up
water flow while adding two biocides, one continuously and the other
on a weekly basis, and acid on pH control. In this case the control system
is likely to be microprocessor controlled with a full alarm system.


General Water Treatment For Cooling Water

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