Shell and Tube Heat Exchangers Using Cooling Water

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-HEA-511

Shell and Tube Heat Exchangers
Using 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.

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com

Process Engineering Guide:

Shell and Tube Heat
Exchangers Using Cooling
Water

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

3.1
3.2

HTFS
TEMA

3
3

4

CHECKLIST

¾

5

QUALITY OF COOLING WATER

4

6

COOLING WATER ON SHELL SIDE OR TUBE SIDE

5

7

COOLING WATER ON THE SHELL SIDE

5

7.1
7.2
7.3
7.4
7.5
7.6

Baffle Spacing
Impingement Plates
Horizontal or Vertical Shell Orientation
Baffle Cut Orientation
Sludge Blowdown
Removable Bundles

5
5
5
5
5
5

8

FOULING RESISTANCES AND LIMITING TEMPERATURES

6


9

PRESSURE DROP

6

9.1
9.2
9.3

Pressure Drop Restrictions
Fouling and Pressure Drop
Elevation of a Heat Exchanger in the Plant

6
6
6

10

MATERIALS OF CONSTRUCTION

7

11

WATER VELOCITY

7

11.1

Low Water Velocity

7

11.1.1 Tube Side Water Flow
11.1.2 Shell Side Water Flow

7
7

High Water Velocity

8

11.2

12

ECONOMICS

9

13

DIRECTION OF WATER FLOW

9

14

VENTS AND DRAINS

9


15

CONTROL

9

15.1
15.2

Operating Variables
Heat Load Control

9
9

15.2.1 General
15.2.2 Heat load control by varying cooling water flow

10
10

15.3

Orifice Plates

9

16

MAINTENANCE

11

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

12


0

INTRODUCTION/PURPOSE

This Process Engineering Guide is one of a series on heat transfer produced for
GBH Enterprises.
Many shell and tube heat exchangers use cooling water. There are a number of
design criteria/principles, peculiar to the use of cooling water, which should be
considered if the best design is to be obtained for such a unit.

1

SCOPE

This guide gives good design practice recommendations in the form of a
checklist (see clause 4) for shell and tube heat exchangers using cooling water.
The contents of the checklist are discussed in more detail in the relevant clauses
that follow it.

2

FIELD OF APPLICATION

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

3

DEFINITIONS

For the purposes of this guide, the following definitions apply:
HTFS

Heat Transfer and Fluid Flow Service. A co-operative research
organization, in the UK, involved in research into the fundamentals
of heat transfer and two phase flow and the production of design
guides and computer programs for the design of industrial heat
exchange equipment.

TEMA

Tubular Exchanger Manufacturers’ Association. An organization of
(US) heat exchanger manufacturers. Their publication ‘Standard of
the Tubular Exchanger Manufacturers' Association’ is a widely
accepted industry standard.


4

CHECKLIST

This checklist contains design criteria/principles and should be consulted at an
early stage in the design process for a shell and tube heat exchanger.


5

QUALITY OF COOLING WATER

Properly treated cooling water should be used for shell and tube heat
exchangers. Environmental constraints have largely ruled out the use of the
synergized chromate systems which were the preferred option before the mid
1980s. Current systems generally involve the use of zinc phosphate, but
increasingly tight constraints on discharge are likely to prohibit these also in the
future. A water technologist should be consulted for up-to-date advice.
Poor quality water can give rise to fouling and/or corrosion problems. If in any
doubt, the designer should obtain advice from a water technologist and a
materials specialist as to the quality of the water available on the plant in
question, and the choice of materials of construction.
In many instances it is more cost effective to upgrade the quality of the water
than to design to accommodate poor water quality.
6

COOLING WATER ON SHELL SIDE OR TUBE SIDE

Cooling water is one of the dirtiest fluids to be found on plants. It is also relatively
corrosive, although with careful design and good water treatment this can be
controlled.
Unless the process stream has worse characteristics, the cooling water should
normally be on the tube side because:
(a)

It facilitates cleaning, either mechanically or by high pressure water jetting.

(b)

It is possible to inspect individual tubes for signs of pitting corrosion, using
an intrascope.

(c)

Fewer sedimentation problems occur, because of the simpler flow path.
(Sediments restrict the access of corrosion inhibitors to the metal wall and
thus often promote corrosion in cooling water systems, even with proper
water treatment.)

(d)

Higher velocities are usually possible, which reduce fouling and make it
easier to achieve the required minimum velocity.


7

COOLING WATER ON THE SHELL SIDE

Where it is necessary for cooling water to be contained in the shell side of a heat
exchanger, a number of precautions/considerations should be taken into
account. These are outlined in sub-clauses 7.1 to 7.6.
7.1

Baffle Spacing

Avoid large baffle spacings and large baffle cuts which create low velocity zones
where debris may collect; this may result in loss of heat transfer area and
increased risk of corrosion.
Good design practice usually calls for baffle spacings of between 20-100% of the
shell diameter. Baffle cuts are usually between 17 and 35% of shell diameter for
optimum performance. Avoid large changes in velocity between cross-flow and
window flow.
7.2

Impingement Plates

An impingement plate should be fitted at the inlet nozzle if the velocity in the
nozzle (or the cooling water supply line to the nozzle) is above 1.5 m/s. It may be
necessary to remove tubes from the bundle to give a clearance above the plate
of one quarter of the branch diameter.
In general, high nozzle velocities should be avoided because they lead to high
pressure drops and an increased risk of tube vibration or erosion. On the other
hand, it is preferable, but not essential, to avoid nozzles much larger than one
third of the shell diameter because they can cause problems in design in
complying with the mechanical design codes, and during manufacture in keeping
the required shell circularity.
7.3

Horizontal or Vertical Shell Orientation

Experience indicates that in general, there will be fewer problems of fouling and
corrosion in exchangers with cooling water on the shell side if the shell is
arranged horizontally rather than vertically. This is because dirt deposits tend to
fall to the bottom of a horizontal shell, away from the tubes, whereas in a vertical
shell deposits occur in contact with the tubes on the lower tube plate and on each
baffle.


7.4

Baffle Cut Orientation

In horizontal shells baffles should be cut vertically (rather than horizontally)
wherever possible, to minimize the build-up of sludge deposits. With vertically cut
baffles these can largely be swept away by the water flow.
7.5

Sludge Blowdown

To install sludge blowdown valves at places where debris may collect is
questionable. With a horizontally mounted exchanger with vertically cut baffles, it
could be argued that to be fully effective a blowdown valve should be provided at
each baffle space. If a close baffle pitch has been used to ensure a reasonable
water velocity, this could require blowdown valves every 100 mm, which is clearly
impracticable. Experience would suggest that even if installed, their use is
unlikely in practice. If sufficient sludge could accumulate to make their use
beneficial, then there is a serious fouling problem that should be addressed by
other means.
7.6

Removable Bundles

If possible provide a removable bundle (U-tube or floating head) with square
pitch tube layout to allow regular mechanical cleaning. If this is not feasible, (e.g.
single tube pass required on process side for a vertical condenser) arrange for
regular sludge blowdown (but see 7.5) in conjunction, if possible, with increased
water flowrate, increased level of dispersants and periodic chemical cleaning.
It should be remembered that cooling water on the shell side is liable to result in
local corrosion at ’dead’ spots near baffles, etc. Avoid the use of bellows in the
shell if possible, as they constitute a ’dead’ spot and are prone to corrosion.


8

FOULING RESISTANCES AND LIMITING TEMPERATURES

Recommended fouling resistances for treated cooling waters are given in GBHEPEG-HEA-501.
Systems with good water treatment should in general not have surface
temperatures in excess of 70°C. Bulk temperatures should normally be kept to
lower values, typically 60°C to prevent crystallization. On some plants that have
reasonable water treatment, 60°C is the preferred maximum surface
temperature, with bulk temperatures limited to no more than 50°C, based on
actual fouling observations for water velocities slightly above 1 m/s.
Waters with poorer forms of treatment are more prone to fouling/scaling and, if
they have to be used, should be limited to lower temperatures. Advice should
always be sought from a water technologist.

9

PRESSURE DROP

9.1

Pressure Drop Restrictions

An adequate water velocity is essential to avoid severe fouling and potential
corrosion problems. If the velocity is limited by pressure drop restrictions, make
sure that these are realistic and necessary. In some cases, it may be economical
to install a booster pump, particularly where a heat exchanger is mounted high in
a structure.
Economical designs are obtained by making maximum use of the available
pressure drop. Avoid excessive pressure drop in regions of the exchanger away
from where the heat transfer is taking place, such as inlet and outlet nozzles. A
suitable total nozzle pressure drop is around 5 - 20% of the available pressure
drop.
9.2

Fouling and Pressure Drop

Allowance should be made for the thickness of the fouling layer when calculating
a pressure drop. Pressure drop for flow inside a tube varies as the fifth power of
the diameter, so that even a modest fouling layer can have a significant effect.


On the shell side, the fouling layer may block the tube to baffle and baffle to shell
leakage paths. In extreme cases, this can raise the pressure drop by more than
50%.
Unfortunately, the HTFS programs normally used for thermal design do not make
allowance for fouling layer thickness when calculating pressure drop. For water
on the tube side, the effect can simply be obtained by applying the fifth power law
to the fraction of the pressure drop associated with tube friction. For water on the
shell side, it is necessary to adjust the clearances to make allowance for fouling.
A typical thermal conductivity for cooling water fouling deposits is 1.4 W/m.K and
typical fouling layer thermal resistances are 0.0002 to 0.0004 m2.K/W. The
corresponding fouling layer thicknesses are 0.28 to 0.56 mm.
9.3

Elevation of a Heat Exchanger in the Plant

An allowance should be made for the elevation of a heat exchanger in the plant
when estimating permissible feed and return pressures. The exit water pressure
on all heat exchangers should be above atmospheric pressure where possible, or
difficulty may be experienced in venting air from the water side. The exit
pressures on all units have to be compatible with the exit pressure on the most
extreme unit (normally the highest on the plant).
A computer model of the water network is useful.
Where orifice flow meters are installed to measure the water flow, ensure that
they are sited at regions of positive pressure to enable impulse lines to be vented
properly; it is safest to install them upstream of a heat exchanger for this reason.

10

MATERIALS OF CONSTRUCTION

Because of chloride attack (even at ppm levels of chloride) cooling water can be
used with ordinary stainless steels only if stringent temperature restrictions are
used, and attention is paid to particular details of design. Where there is doubt
concerning a particular case, a materials specialist should be consulted.
Carbon steel is normally acceptable for cooling water duties. However, most
materials are susceptible to corrosion if the water velocity is low (<1.0 m/s), or if
there are dead spots where debris can accumulate. This may occur even when
treated cooling water is used; the situation may be considerably worse with
untreated water.

11

WATER VELOCITY

11.1

Low Water Velocity

Water velocities below 1 m/s should be avoided where possible to prevent the
excessive deposition of solids that can lead to local corrosion; this may occur
even with nominally resistant materials or effective inhibitor systems.
Corrosion of carbon steel can occur even in the absence of significant deposits,
and with normal levels of treatment chemicals, if the water velocity is low. Where
water velocity below 1 m/s is unavoidable, a materials specialist should be
consulted.
There are several ways of increasing cooling water velocities at the design stage.
Increasing the total flow of fresh cooling water to a heat exchanger is not always
possible or desirable (see 11.2) but even with a fixed quantity the designer has
several options:
11.1.1 Tube side water flow
Options include:
(a)

Increase the length and reduce the number of tubes. This may not be
possible as it may raise the shell side pressure drop above the allowable
limit. An increased tube or baffle pitch may counter this problem.

(b)

Increase the number of tube passes. This is not always possible as it may
result in too low a value of the ’F’ correction factor to the log mean
temperature difference, or even a temperature cross.

(c)

The problem can often be overcome by adding shells in series. It may
then be possible to use multi-pass flow on the tubeside of each shell
without incurring an excessive ’F’ factor penalty.

(d)

Reduce the tube diameter. This increases the ratio of heat transfer surface
to tube cross-sectional area and thus, for a constant heat transfer area,
raises the velocity. Note that the minimum diameter for mechanical
cleaning is ¾" NS.


11.1.2 Shell side water flow
Options include:
(a)

Minimize baffle spacing. Spacings down to 100 - 150 mm are quite
practicable, and for small heat exchangers even as low as 25 mm can be
used.

(b)

Keep the tube pitch to a minimum, consistent with mechanical integrity of
the tube tubesheet bond.

(c)

Reduce tube diameter and thus reduce shell diameter for the same tube
count.

(d)

Increase tube length and reduce tube count and shell diameter
accordingly. If this results in an excessively long and thin exchanger,
consider multiple shells in series on the shell side.

(e)

If there is an excessive (>10%) by-passing round the tube bundle or
through pass partition lanes, consider the use of seal strips and seal rods
to reduce these streams. If seal strips were not specified in the original
design, when the mechanical design is known a check should be made
that the actual tube bundle/shell clearance does not lead to an excessive
‘C’ or bundle by-pass stream.

(f)

The use of a longitudinal baffle to give two shell side passes (TEMA ‘F’
shell) is sometimes proposed. This design is not generally recommended
as it is difficult to prevent thermal or even physical leakage across the
baffle, which can lead to inability to meet the design performance.
Satisfactory 'F' shell designs have been made where it is possible to weld
the longitudinal baffle in place. This will, however, prevent removal of the
bundle unless a 4-pass U-tube design is used, arranged so that the Utubes do not span the baffle.

For either shell side or tube side flow, the use of an auxiliary pump to recirculate
water from the exit to the inlet will enable higher velocities to be achieved,
without increasing the flow of fresh water. However, there is a penalty in loss of
mean temperature difference that should be weighed against the gain in
coefficients and lower fouling. This approach is useful as a control scheme (see
15.2.1).


Where the consequences of likely corrosion due to low velocities are
unacceptable, consideration should be given to a secondary cooling circuit with a
non-fouling, noncorrosive fluid, (such as a closed circuit nitrite dosed water), with
a second heat exchanger, (probably a plate type) cooling this secondary circuit
with ordinary cooling water. Here also there is a penalty in loss of temperature
difference, but this does give a system of high integrity and may be particularly
suited to shellside duties where inspection and cleaning is impossible.
An alternative solution, which has been used on critical duties, is to use a
material of construction that is resistant to cooling water corrosion even with poor
water treatment or low velocities, such as Titanium or Hastelloy C. This will not,
however, prevent fouling deposits.
11.2

High Water Velocity

High water velocities may result in erosion, cavitation and tube vibration. With
most alloy/water combinations, velocities of up to 2.5 m/s are safe, and with the
more resistant materials and effectively inhibited water, velocities considerably
greater than this may be used.
A water velocity of 2.5 m/s is, however, too high for copper, and a limit of 1.5 m/s
should be applied in this case.
For shell side flow, TEMA recommends the use of an impingement plate to
prevent damage to the tubes in the entrance region if the product of density and
the square of the nozzle velocity exceeds 2250 N/m2; for water this corresponds
to a velocity of 1.5 m/s.
The safe water velocity is not only dependent on the combination of alloy and
water in question, but also on the details of design (e.g. U-tubes) and factors
such as the chance of debris etc. being present. It is difficult to generalize, and
where it is proposed to operate outside the previously mentioned limits, a
materials specialist should be consulted.
High velocities combined with large baffle spacings can give rise to tube
vibration. This can be very serious, in extreme cases resulting in tube failure
within hours of start-up. The main thermal design programs such as the HTFS
program 'TASC' have an option for performing a vibration analysis. This should
always be done. For meaningful results, the full vibration output option should be
selected. If any potential problems are shown up, a more detailed analysis
should be performed, and/or the design modified. If in doubt, seek advice.


12

ECONOMICS

The costs of cooling water systems and their associated heat exchangers are
normally optimized by choosing a high return water temperature from the
exchanger, provided the process duties are above 50-60°C. Pollution
from the cooling tower plume usually limits the return water temperature to the
range 30- 35°C, but often individual items can be beneficially designed with
return temperatures above this, if water quality allows.
The possibility of using water in series through two exchangers on different
duties, where one requires a low temperature and the other does not, should be
considered.

13

DIRECTION OF WATER FLOW

The cooling water (or any other liquid) should, preferably, flow into the heat
exchanger at the bottom and out at the top. This is vital for shell side flow in
vertically installed heat exchangers in order to:
(a)

ensure that air pockets are avoided;

(b)

discourage recirculation, due to natural convection effects.

The lower the pressure drop through the tube bundle (i.e. excluding nozzle
losses) the more necessary this becomes.

14

VENTS AND DRAINS

The design of the heat exchanger should be examined to ensure that:
(a)

high points are adequately vented. It may be necessary to provide a vent
within the tubesheet or an internal stand-pipe to ensure this;

(b)

low points have drains.

For cooling water, 1" NS cocks are usually adequate for both duties. However,
larger drains may be desirable for units over 100 m2 capacity. No vent/drain
branches, with the exception of tubesheet vents, should be smaller than 1" NS;
drain cocks should be full bore to reduce the risk of plugging by debris.


15

CONTROL

15.1

Operating Variables

Heat exchangers cooled with water are usually designed for maximum plant
throughputs with the cooling water inlet temperature at its peak summer value
(typically 21-23°C) and the heat exchanger in its anticipated most fouled state.
However, the actual operating conditions will vary from these values. In winter
the cooling water inlet temperature may be only 10°C or less; when first installed
the exchanger can be expected to have a low value of fouling resistance; the
plant is required to operate under turndown conditions.
On critical duties, performance calculations should be done at the design stage
to assess the likely outlet temperatures of the process streams under varying
conditions, and their effect on the remainder of the process.
15.2

Heat Load Control

In many cases the plant performance is insensitive to the previously stated
variations. In these cases the cooling water flow can be set to the design value
(which will ensure an adequate water velocity) and left at that value. However,
there are occasions when it is necessary to control the heat load on an
exchanger (e.g. when the heat load on a partial condenser is being used to
control the pressure of a distillation column).
15.2.1

General

Except in very special circumstances, controlling the heat load should not
be done by varying the cooling water flow (see 15.2.2). The required
range of water flowrates necessary to accommodate changes in
throughput, cooling water temperature and fouling resistance is likely to be
very great. This results in problems of rangeability of the control valve and
also in it being virtually impossible to ensure that the velocity at all times
lies within the permitted range.
Heavy fouling deposits can be expected during turndown conditions,
which will not necessarily be flushed away under conditions of higher
flowrate, unless the water velocity is maintained above 1 m/s at all times.
Premature failure can be expected from the resulting corrosion.


In general, it is better to control the process exit temperature of coolers by
means of a bypass on the process side. However, problems can occur if
the process fluid becomes very viscous or freezes at temperatures
approaching the water inlet temperature. If by-passing of the process fluid
cannot be allowed and it is required to control the heat load, then one of
two methods is recommended as follows:
(a)

An auxiliary pump recirculating the exit water back to the inlet, with
a controlled makeup of fresh cooling water and a bleed back to the
cooling tower. The control system allows the temperature level of
the water in the circuit to float.

(b)

A secondary cooling circuit with properly treated non-fouling
coolant. This is cooled in a secondary heat exchanger, designed for
constant (high) cooling tower water velocity. The temperature of the
secondary coolant is controlled by by-passing it round the
auxiliary exchanger. The system has a high integrity, but is
expensive and may not be justifiable.

15.2.2

Heat load control by varying cooling water flow

It may be possible to vary the water flow without problems provided that a
minimum stop is put on the valve such that the velocity is never less than
1m/s. However, when designing such a system, remember that the water
pressure drop will rise for a constant flowrate as the exchanger fouls. This
means of control has worked successfully in various locations that have
used non-chromate treated water for several years.
Where the methods outlined in the previous paragraph or in 15.2.1 are not
adopted and heat load control is to be by varying the cooling water flow,
then it is imperative that the heat exchanger be regularly inspected (if
fabricated from material that could corrode). On critical duties this
inspection should include thickness monitoring. The frequency of the
inspection will depend on the quality of the cooling water, but as a guide, it
is likely to be every two years. A materials specialist should be consulted
for advice.
Because of the costs of inspection and the risks of failure, it may be found
to be more economic to install a heat exchanger made from resistant
material (e.g. Hastelloy ’C’).


15.3

Orifice Plates

It is often desirable to fit restriction orifice plates in a cooling water system, to
balance the flows through different units. Although the isolation valves associated
with the exchanger can be used for this purpose, a fixed restrictor is generally
preferable to using an isolating valve because:
(a)

it is more reliable than a manual setting of a valve;

(b)

the isolation valve can be opened fully, which is an unambiguous
operation;

(c)

the risk of erosion damage to the valve, with consequent leakage during
isolation, is reduced.

Against this, as exchangers foul in service, it may be necessary to make
adjustments to maintain the required flow through all units.
Orifice plates (or control valves), if fitted, should be in the exit line from the heat
exchanger to reduce the risk of air degassing and venting problems in the heat
exchanger.

16

MAINTENANCE

Heat exchangers are classified as pressure vessels, and as such are subject to
regular inspection. In addition, there is often a requirement for cleaning. If the
water is on the tube side, mechanical cleaning can often be performed without
removing the exchanger from its berth. The use of TEMA ’A’ or ’C’ front end type
and ’L’ or ’N’ rear head type enables this to be done without disconnecting the
water pipework. However, these head types are more expensive than the ’B’ or
’M’ types. With cooling water on the shell side, mechanical cleaning can only be
done with a removable bundle. The plant layout should allow room for rodding
through on the tube side, or removing the bundle if necessary.
Mechanical cleaning can be performed by rodding, brushing or high-pressure
water jetting. It is generally possible to clean the inside of the U-bend region for
tube sizes down to ¾" NS if the contractor is specifically asked to do so.


In order to reduce the shutdown time associated with cleaning and inspection a
spare heat exchanger is sometimes provided to replace that which is being
maintained. Consideration should be given to the storage of the spare after
cleaning. Chemical cleaning cannot be guaranteed to remove all cooling water
deposits, especially on the shell side. The remaining material is difficult to dry out
completely, and acts as a potential source of corrosion during storage. The
alternative to dry storage, which is to store the exchanger filled with water heavily
dosed with treatment chemicals, presents problems of disposal of the water
before re-installation.
Techniques are now available to measure local wall thickness of the tubes in an
exchanger without having to remove them. A materials specialist should be
consulted for further details.

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
GBH Enterprises PROCESS ENGINEERING GUIDES
GBHE-PEG-HEA-501

Fouling Resistances for Cooling Water
(referred to in clause 8)

OTHER DOCUMENTS
TEMA

Standard of the Tubular Exchanger Manufacturers Association
(referred to in clause 3, 11.1.2, 11.2 and clause 16).

While every effort has been made to ensure the accuracy of the references listed
in this publication, their future availability cannot be guaranteed.


Shell and Tube Heat Exchangers Using Cooling Water

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to Shell and Tube Heat Exchangers Using Cooling Water

Similar to Shell and Tube Heat Exchangers Using Cooling Water (20)

More from Gerard B. Hawkins

More from Gerard B. Hawkins (20)

Recently uploaded

Recently uploaded (20)

Shell and Tube Heat Exchangers Using Cooling Water