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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.
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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 HTFS 3 3.2 TEMA 3 4 CHECKLIST 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 Baffle Spacing 5 7.2
Impingement Plates 5 7.3 Horizontal or Vertical Shell Orientation 5
7.4 Baffle Cut Orientation 5 7.5 Sludge Blowdown 5 7.6 Removable
Bundles 5 8 FOULING RESISTANCES AND LIMITING TEMPERATURES 6
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9 PRESSURE DROP 6 9.1 Pressure Drop Restrictions 6 9.2 Fouling
and Pressure Drop 6 9.3 Elevation of a Heat Exchanger in the Plant
6
10 MATERIALS OF CONSTRUCTION 7 11 WATER VELOCITY 7 11.1 Low
Water Velocity 7
11.1.1 Tube Side Water Flow 7 11.1.2 Shell Side Water Flow 7
11.2 High Water Velocity 8 12 ECONOMICS 9 13 DIRECTION OF WATER
FLOW 9 14 VENTS AND DRAINS 9
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15 CONTROL 9 15.1 Operating Variables 9 15.2 Heat Load Control
9
15.2.1 General 10 15.2.2 Heat load control by varying cooling
water flow 10
15.3 Orifice Plates 9 16 MAINTENANCE 11 DOCUMENTS REFERRED TO IN
THIS PROCESS ENGINEERING GUIDE 12
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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.
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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.
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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.
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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.
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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.
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8 FOULING RESISTANCES AND LIMITING TEMPERATURES Recommended
fouling resistances for treated cooling waters are given in
GBHE-PEG-HEA-501. Systems with good water treatment should in
general not have surface temperatures in excess of 70C. Bulk
temperatures should normally be kept to lower values, typically 60C
to prevent crystallization. On some plants that have reasonable
water treatment, 60C is the preferred maximum surface temperature,
with bulk temperatures limited to no more than 50C, 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.
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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 (
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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 " NS.
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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 U-tubes 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).
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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.
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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 50-60C. Pollution from the cooling tower
plume usually limits the return water temperature to the range 30-
35C, 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.
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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
21-23C) 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 10C 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.
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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).
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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 " NS if the
contractor is specifically asked to do so.
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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.
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