Fouling Resistances for Cooling Water

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-HEA-501

Fouling Resistances for Cooling
Water

Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
good faith, but it is for the User to satisfy itself of the suitability of the information
for its own particular purpose. GBHE gives no warranty as to the fitness of this
information for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE accepts no liability resulting from reliance on this
information. Freedom under Patent, Copyright and Designs cannot be assumed.

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Process Engineering Guide:

Fouling Resistances for
Cooling Water

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

GENERAL

2

5

COOLING WATER FOULING

2

6

CHROMATE SYSTEMS

3

6.1
6.2
6.3
6.4

General
Constraints
Requirements
Fouling resistances

3
3
3
3

7

NON-CHROMATE SYSTEMS

3

7.1
7.2
7.3

General
Requirements and Constraints
Fouling resistances

3
4
4

8

UNTREATED COOLING WATER

4

9

MATERIALS OTHER THAN MILD STEEL

4


APPENDICES

A

FOULING RESISTANCES FOR COOLING WATER

5

B

FOULING FILM THICKNESS

6

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

7


0

INTRODUCTION/PURPOSE

The selection of fouling resistances for use in heat exchanger design is a highly
subjective matter. As far as process streams are concerned reliance is generally
placed on previous knowledge of the process or similar processes. However, in
the case of cooling water more information is available and it is of more general
applicability.

1

SCOPE

This Engineering Guide outlines a standard procedure for estimating fouling
resistances for cooling water, and covers both chromate and non-chromate
systems.
The procedure is based on experience of operating both chromate and nonchromate systems within GBH Enterprises. It is supplemented with the results
from fouling test rigs at European and Gulf Coast plant locations.
Design fouling resistances and temperature limitations are given for treated open
evaporative cooling water systems.

2

FIELD OF APPLICATION

This Guide applies to the process engineering community in GBH Enterprises
worldwide.

3

DEFINITIONS

For the purposes of this Guide no specific definitions apply.

4

GENERAL

There are many problems in the design of water cooled heat exchangers, some
of which can have a strong influence on fouling and/or corrosion; for further
details, see GBHE-PEG-HEA-511 “Shell and Tube Heat Exchangers Using
Cooling Water “ prior to commencement of design of such a unit.


The fouling resistances recommended in this Engineering Guide are, in many
cases, significantly lower than values used in the past for design purposes.
These values presuppose a well designed heat exchanger with adequate water
velocities and good water treatment.
In deciding the design fouling resistance the designer should assume that if
water treatment is installed the operations are maintaining it correctly. At the
commencement of a project the Process Engineer, in consultation with the
appropriate Water Chemist, should confirm that operations are maintaining a
suitable water treatment program. If these conditions are not met, then much
higher fouling resistances may occur, and corrosion of mild steel equipment is
likely.

5

COOLING WATER FOULING

The principal causes of cooling water fouling are:
(a)
(b)
(c)
(d)

corrosion
biological growth
scaling/crystallization
sedimentation

They can all be controlled to a greater or lesser extent by correct materials
selection, good cooling water treatment and good heat exchanger design.
Sedimentation fouling is strongly influenced by the water velocity; and scaling by
temperature. However, provided that temperatures are kept below certain limiting
values and a good program of water treatment is maintained, velocity is the only
significant factor.


6

CHROMATE SYSTEMS

6.1

General

Provided that local environmental constraints allow their use, low chromate
synergized systems are the cheapest and best available.
6.2

Constraints

Chromate based treatments give very low corrosion rates on carbon steel
equipment provided that water velocities are not less than 1.0 m/s. This applies
equally to water in tube and water in shell exchangers. Pitting corrosion occurs at
lower velocities (eg 0.5 m/s) even if the metal surface is free of all deposits. If dirt
deposits do occur then pitting corrosion is likely beneath these deposits
regardless of the quality of water treatment, as the deposits prevent access of
the treatment chemicals to the metal surface. Velocities of less than 1.0 m/s
should be avoided if at all possible; if this is not possible then a materials
scientist and a Water Chemist should be consulted.
A properly applied chromate treatment works equally well on unfiltered raw
makeup water as it does on a potable makeup supply, providing that the heat
exchangers are of sound design. See GBHE-PEG-HEA-511, “Shell and Tube
Heat Exchangers Using Cooling Water”.
High surface temperatures (up to 80-100°C) can be used with chromate based
treatments.
6.3

Requirements

The treatment is easy to monitor using simple analytical techniques and
'forgiving' with respect to minor upsets in pH value or chromate concentration.
However, it is essential to have a good chlorination program in order to obtain the
best results from chromate systems. Ideally, chlorination should be once a shift
to give a free chlorine residual of 1 mg/l and a maximum viable bacterial count of
10,000/ml. Provided there is no significant process contamination in the cooling
water circuit, then no further biocide is required.
6.4

Fouling resistances

Providing that the previously mentioned criteria are observed, design fouling
resistances of 0.0002 m2°C/W should be used with velocities in excess of 1.0
m/s.

Note:
Although there is some evidence that even lower values can be used for
velocities of 2 m/s or above, a constant value is recommended. The cost penalty
for this is low as at this levelfouling is usually a small part of the total heat
transfer resistance.

7

NON-CHROMATE SYSTEMS

7.1

General

In Continental Western Europe, chromate use is, in general, prevented because
of river pollution concerns. If environmental constraints prevent the use of
chromate, then it is imperative that the hard won experience of continental users
of non-chromate systems be applied, whether the treatment be a high pH scale
inhibitor program or a low pH phosphate program. The following factors are
common to all continental users who have been operating large critical
single stream cooling systems on non-chromate treatment for up to 30 years.
7.2

Requirements and Constraints

The make-up water to the cooling system shall be free of suspended solids and
have minimum organic material to avoid annual cleaning of the exchangers. This
normally necessitates pre-clarification by flocculation or lime softening, followed
by filtration, or the use of high cost potable water. Raw untreated river or lake
water shall not be used as make-up. Suspended materials absorb the treatment
formulations making them less effective.
Natural organic material acts as a nutrient for microbiological growth or as a
foulant itself. The high concentration of chlorine required to combat the above
may also oxidize the inhibitors and dispersants, making them non-effective.
Supplementary biocides are therefore added with controlled chlorination.
Between 5 and 10% of the circulation water requires to be filtered through sidestream filters in order to remove precipitates and any corrosion products.
Very good analytical control is required to avoid gross fouling or corrosion, which
can take place in a matter of days. Roughness of the heat transfer surface
enhances fouling.
Surface temperatures should not exceed 70°C.


Water velocities below 1.0 m/s should never be used. If they are, fouling rates of
2 to 4 times those for chromate treatment are likely, and corrosion is a strong
possibility.
7.3

Fouling resistances

Provided that the previously mentioned criteria are met, a fouling resistance of
0.0002 m2°C/W should be used for water velocities of 2.0 m/s. For velocities of
1.0 m/s the fouling resistance is 0.0004 m2°C/W (see Appendix A).

8

UNTREATED COOLING WATER

Untreated, raw water should never be used for cooling purposes unless:
(a)

the heat exchanger is fabricated from corrosion resistant materials

(b)

the water in question has a low fouling and scaling potential

Materials scientist and a Water Chemists should be consulted.
9

MATERIALS OTHER THAN MILD STEEL

Strictly, the fouling resistances recommended in this Engineering Guide are for
mild steel equipment; lower values may be realized in practice for other
materials. However, unless an adequate water velocity is maintained, problems
of fouling and/or corrosion may still occur even with more corrosion resistant
materials, albeit at a lower rate.


APPENDIX A

FOULING RESISTANCES FOR COOLING WATER


APPENDIX B

FOULING FILM THICKNESS

Based on assumed thermal conductivity of 1.385 w/m°C.


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
ENGINEERING GUIDES
GBHE-PEG-HEA-511

Shell and Tube Heat Exchangers Using
Cooling Water (referred to in Clause 4 and 6.2).


Fouling Resistances for Cooling Water

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