Heat exchanger

1 Heat ExchangerMohamed SalahStudent
Suez University
Faculty Of Petroleum & Mining
Engineering
Prepared by/
Student/ Mohamed salah abou El_hamed
Department/ petroleum refining
Year/ third

A heat exchanger is a heat transfer device that exchanges heat between two or more
process fluids. Heat exchangers have widespread industrial and domestic applications.
Extensive technical literature is available on heat exchangers, but it is widely scattered
throughout the technical journals, industrial bulletins, codes and standards.
• A heat exchanger is used to exchange heat between two fluids of different
temperatures, which are separated by a solid wall.
• Heat exchangers are ubiquitous to energy conversion and utilization. They
encompass a wide range of flow configurations.
• Applications in heating and air conditioning, power production, waste heat
recovery, chemical processing, food processing, sterilization in bio-processes.
• Heat exchangers are classified according to flow arrangement and type of
construction.
What is A heat exchanger?
They are devices specifically designed for the efficient transfer of heat from one fluid to
another fluid over a solid surface.

Types of heat exchangers
1. Double pipe heat exchanger
Double pipe heat exchangers are the simplest exchangers used in industries. On one
hand, these heat exchangers are cheap for both design and maintenance, making them
a good choice for small industries. But on the other hand, low efficiency of them beside
high space occupied for such exchangers in large scales, has led modern industries to
use more efficient heat exchanger like shell and tube or other ones.
But yet, since double pipe heat exchangers are simple, they are used to teach heat
exchanger design basic to students and as the basic rules for modern and normal heat
exchangers are the same,
students can understand the
design techniques much easier.
To start the design of a double
pipe heat exchanger, the first
step is to calculate the heat duty
of the heat exchanger.
2. Shell and tube heat exchanger
Shell and tube heat exchangers consist of a series of tubes. One set of these tubes
contains the fluid that must be either heated or cooled. The second fluid runs over the
tubes that are being heated or cooled so that it can either provide the heat or absorb
the heat required. A set of tubes is called the tube bundle and can be made up of
several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers
are typically used for high-pressure applications (with pressures greater than 30 bar and
temperatures greater than 260 °C. This is because the shell and tube heat exchangers
are robust due to their shape.

3. Plate heat exchanger
Another type of heat exchanger is the plate heat exchanger. One is composed of
multiple, thin, slightly separated plates that have very large surface areas and fluid
flow passages for heat transfer. This stacked-plate arrangement can be more
effective, in a given space, than the shell and tube heat exchanger.
Advances in gasket and brazing technology
have made the plate-type heat exchanger
increasingly practical. In HVAC applications,
large heat exchangers of this type are called
plate-and-frame; when used in open loops,
these heat exchangers are normally of the
gasket type to allow periodic disassembly,
cleaning, and inspection. There are many
types of permanently bonded plate heat
exchangers, such as dip-brazed, vacuum-
brazed, and welded plate varieties, and they
are often specified for closed-loop
applications such as refrigeration. Plate heat exchangers also differ in the types of
plates that are used, and in the configurations of those plates. Some plates may be
stamped with "chevron", dimpled, or other patterns, where others may have
machined fins and/or grooves.
CLASSIFICATIONOF HEAT EXCHANGERS
In general, industrial heat exchangers have been classified according to
(1) construction,
( 2 )transfer processes,
(3) degrees of surface compactness,
(4) flow arrangements,
( 5 )pass arrangements,
(6) phase of the process fluids, and
(7) heat-transfer mechanisms. These classifications
1. Classification According to Transfer Process
These classifications are:
1. Indirect contact type-direct transfer type, storage type, fluidized bed.
2. Direct contact type-cooling towers.

1. Indirect Contact Heat Exchangers
In an indirect contact type heat exchanger, the fluid streams remain separate, and
the heat transfer takes place continuously through a dividing impervious wall. This
type of heat exchanger can be further classified into the direct transfer type,
storage type, and fluidized bed exchangers. Direct transfer type is dealt with next
whereas the storage type and the fluidized.
2. Direct Transfer Type Exchangers
In this type, there is a continuous flow of the heat from the hot fluid to the cold
fluid through a separating wall. There is no direct mixing of the fluids because each
fluid flows in separate fluid passages. There are no moving parts. This type of
exchanger is designated as a recuperator
Some examples of direct transfer type heat exchangers are tubular exchangers,
plate heat exchangers, and extended surface exchangers. Recuperators are further
subclassified as prime surface exchangers, which do not employ fins or extended
surfaces on the prime surface. Plain tubular exchangers, shell and tube exchangers
with plain tubes, and plate heat exchangers are examples of prime surface
exchangers
Direct Contact Type Heat Exchangers In direct contact type heat exchangers, the
two fluids are not separated by a wall; owing to the absence of a wall, closer
temperature approaches are attained. Very often, in the direct contact type, the
process of heat transfer is also accompanied by mass transfer. The cooling towers
and scrubbers are examples of a direct contact type heat exchanger.
2. Classification According to Flow Arrangement
The basic flow arrangements of the fluids in a heat exchanger are
1. Parallel flow
2. Counter flow
3.Cross flow
The choice of a particular flow arrangement is dependent upon the required
exchanger effectiveness,fluid flow paths, packaging envelope, allowable thermal
stresses, temperature levels,and other design criteria. These basic flow arrangements
are discussed next.
1. Parallel Flow Exchanger
In this type, both the fluid streams enter at the same end, flow parallel to each other
in the same direction, and leave at the other end.Fluid temperature variations,
idealized as one-dimensional. This arrangement has the lowest exchanger
effectiveness among the single-pass exchangers for the same flow rates, capacity rate
(mass x specific heat) ratio, and surface area.

Moreover, the existence of large temperature differences at the inlet end may induce
high thermal stresses in the exchanger wall at inlet. Although this flow arrangement is
not used widely, it is preferred for the following reasons ;
1. Since this flow pattern produces a more uniform longitudinal tube wall
temperature distribution and not as high or as low a tube wall temperature as
in a counterflow arrangement, it is sometimes preferred with temperature in
excess of 1100°C (2000°F).
2. It is preferred when there is a possibility that the temperature of the warmer
fluid may reach its freezing point.
3. It provides early initiation of nucleate boiling for boiling applications.
4. For a balanced exchanger (i.e., heat capacity rate ratio C* = l ) , the desired
exchanger effectiveness is low and is to be maintained approximately constant
over a range of NTU values.
5 . The application allows
piping only suited to
parallel flow.
2. Countefflow Exchanger
In this type, as shown in, the two fluids flow parallel to each other but in
opposite directions, and its temperature distribution may be idealized as one-
dimensional.
Ideally, this is the most efficient of all flow arrangements for single-pass
arrangements under the same parameters. Since the temperature difference
across the exchanger wall at a given cross section is the lowest, it produces
minimum thermal stresses in the wall for equivalent performance compared to
other flow arrangements.
In certain type of heat exchangers, counterflow arrangement cannot be
achieved easily, due
to manufacturing
difficulties associated
with the separation of
the fluids at each end,
and the design of inlet
and outlet header
design is complex and
difficult .

3. Crossflow Exchanger
In this type, the two fluids flow normal to each other. Important types of
flow arrangement combinations for a single-pass crossflow exchanger include:
1. Both fluids unmixed
2. One fluid unmixed and the other fluid mixed
3. Both fluids mixed
A fluid stream is considered “unmixed” when it passes through individual flow
passage without any fluid mixing between adjacent flow passages. Mixing
implies that a thermal averaging process takes place at each cross section
across the full width of the flow passage.
tube-fin exchanger with flat (continuous) fins and a plate-fin exchanger
wherein the two fluids flow in separate passages (e.g., wavy fin, plain
continuous rectangular or triangular flow passages) represent the unmixed-
unmixed case. A crossflow tubular exchanger with bare tubes on the outside
would be treated as the unmixed-mixed case, that is, unmixed on the tube
sideand mixed on the outside.
The both fluids mixed case is practically a less important case, and represents a
limiting case of some multipass shell and tube exchangers (TEMA E and J shell).
For the unmixed-unmixed case, fluid temperature variations are idealized as
two-dimensional only for the inlet and outlet sections, and this is shown in Fig.
20. The thermal effectiveness for the crossflow exchanger falls in between
those of the parallel flow and counterflow arrangements. This is the most
common flow arrangement used for extended surface heat
exchangers, because it greatly simplifies the header design. If the desired heat
exchanger effectiveness is generally more than 80%, the size penalty for
crossflow may become excessive. In such a case, a counterflow unit ispreferred.
In shell and tube exchangers, crossflow arrangement is used in the TEMA X shell
having a single tube pass.

components of heat exchanger
Several thermal design features must be considered when designing the tubes in the
shell and tube heat exchangers:
Tube diameter: Using a small tube diameter makes the heat exchanger both
economical and compact. However, it is more likely for the heat exchanger to foul
up faster and the small size makes mechanical cleaning of the fouling difficult. To
prevail over the fouling and cleaning problems, larger tube diameters can be used.
Thus to determine the tube diameter, the available space, cost and the fouling
nature of the fluids must be considered.
Tube thickness: The thickness of the wall of the tubes is usually
determined to ensure:
1.1.There is enough room for corrosion
1.2.That flow-induced vibration has resistance
1.3.Axial strength
1.4.Availability of spare parts
1.5.Hoop strength (to withstand internal tube pressure)
1.6.Buckling strength (to withstand overpressure in the shell)
Tube length: heat exchangers are usually cheaper when they have a smaller
shell diameter and a long tube length. Thus, typically there is an aim to make the
heat exchanger as long as physically possible whilst not exceeding production
capabilities. However, there are many limitations for this, including space available
at the installation site and the need to ensure tubes are available in lengths that are
twice the required length (so they can be withdrawn and replaced). Also, long, thin
tubes are difficult to take out and replace.
Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch
(i.e., the centre-centre distance of adjoining tubes) is not less than 1.25 times the
tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter,
which leads to a more expensive heat exchanger.
Tube corrugation: this type of tubes, mainly used for the inner tubes, increases
the turbulence of the fluids and the effect is very important in the heat transfer
giving a better performance.
Tube Layout: refers to how tubes are positioned within the shell. There are four
main types of tube layout, which are, triangular (30°), rotated triangular (60°),
square (90°) and rotated square (45°). The triangular patterns are employed to give
greater heat transfer as they force the fluid to flow in a more turbulent fashion

around the piping. Square patterns are employed where high fouling is experienced
and cleaning is more regular
Baffle Design: baffles are used in shell and tube heat exchangers to direct fluid
across the tube bundle. They run perpendicularly to the shell and hold the bundle,
preventing the tubes from sagging over a long length. They can also prevent the
tubes from vibrating. The most common type of baffle is the segmental baffle. The
semicircular segmental baffles are oriented at 180 degrees to the adjacent baffles
forcing the fluid to flow upward and downwards between the tube bundle. Baffle
spacing is of large thermodynamic concern when designing shell and tube heat
exchangers. Baffles must be spaced with consideration for the conversion of
pressure drop and heat transfer. For thermo economic optimization it is suggested
that the baffles be spaced no closer than 20% of the shell’s inner diameter.
Having baffles spaced too closely causes a greater pressure drop because of flow
redirection. Consequently having the baffles spaced too far apart means that there
may be cooler spots in the corners between baffles. It is also important to ensure
the baffles are spaced close enough that the tubes do not sag. The other main type
of baffle is the disc and donut baffle, which consists of two concentric baffles. An
outer, wider baffle looks like a donut, whilst the inner baffle is shaped like a disk.
This type of baffle forces the fluid to pass around each side of the disk then through
the donut baffle generating a different type of fluid flow
Fixed tube liquid-cooled heat exchangers especially suitable for marine and harsh
applications can be assembled with brass shells, copper tubes, brass baffles, and
forged brass integral end hubs.

Materials of heat exchanger
1. Tubes
Heat exchangers with shell diameters of 10 inches to more than 100 are typically
manufactured to industry standards. Commonly, 0.625 to 1.5" tubing used in
exchangers is made from low carbon steel, Admiralty, copper, copper-nickel,
stainless steel, Hastelloy, Inconel, or titanium.
Tubes can be drawn and thus seamless, or welded. High quality electro resistance
welded tubes display good grain structure at the weld joints. Extruded tubes with
fins and interior rifling are sometimes specified for certain heat transfer
applications. Often, surface enhancements are added to increase the available
surface or aid in fluid turbulence, thereby increasing the operative heat transfer
rate. Finned tubes are recommended when the shell-side fluid have a considerably
lower heat transfer coefficient than the tube-side fluid. Note, the diameter of the
finned tube is slightly smaller than the un-finned areas thus allowing the tubes to be
installed easily through the baffles and tube supports during assembly while
minimizing fluid bypass.
A U-tube design finds itself in applications when the thermal difference between
the fluid flows would otherwise result in excessive thermal expansion of the tubes.
Typical U-tube bundles contain less tube surface area as traditional straight tube
bundles due to the bended end radius, on the curved ends and thus cannot be
cleaned easily. Furthermore, the interior tubes on a U-tube design are difficult to
replace and often requiring the removal of additional tubes on the outer layer;
typical solutions to this are to simply plug the failed tubes.
2. Tube Sheets
Tube sheets usually constructed from a round, flattened sheet of metal. Holes for
the tube ends are teen drilled for the tube ends in a pattern relative to each other.
Tube sheets are typically manufactured from the same material as tubes, and
attached with a pneumatic or hydraulic pressure roller to the tube sheet. At this
point, tube holes can both be drilled and reamed, or they are machined grooves
(this significantly increases tube joint strength).

The tube sheet comes in contact with both fluids in the exchanger, therefore it
must be constructed of corrosion resistant materials or allowances appropriate for
the fluids and velocities. A layer of alloy metal bonded to the surface of a low
carbon steel tube sheet would provide an effective corrosion resistance without the
expense of manufacturing from a solid alloy.
The tube-hole pattern, often called ‘pitch’, varies the distance between tubes as
well as the angle relative to each other allowing the pressure drop and fluid
velocities to be manipulated in order to provide max turbulence and tube surface
contact for effective heat transfer.
Tube and tube sheet materials are joined with weld-able metals, and often further
strengthened by applying strength or seal weld to the joint. Typically in a strength
weld, a tube is recessed slightly inside the tube hole or slightly beyond the tube
sheet whereas the weld adds metal to the resulting edge. Seal welds are specified
when intermixing of tube liquids is needed, this is accomplished whereas the tube is
level with the tube sheet surface. The weld fuses the two materials together, adding
no metal in the process. When it becomes critical to avoid the intermixing of fluid, a
second tube sheet is designed in. In this case, the outer tube sheet becomes the
outside the shell path, and the inner tube sheet is vented to atmosphere, so that a
fluid leak can be detected easily effectively eliminating any chance of cross
contamination.
3. Shell Assembly
The shell is constructed either from pipe or rolled plate metal. For economic
reasons, steel is the most commonly used material, and when applications
involving extreme temperatures and corrosion resistance, others metals or
alloys are specified. Using off-the-shelf pope reduces manufacturing costs and
lead time to deliver to the end customer. A consistent inner shell diameter or
‘roundness’ is need to minimize the baffle spacing on the outside edge,
excessive space reduces performance as the fluid tends to channel and
bypasses the core. Roundness is increased typically by using a mandrel and
expanding the shell around it, or by double rolling the shell after welding the
longitudinal seam. In some cases, although extreme, the shell is cast and then
bored.
When fluid velocity at the nozzle is high, an ‘impingement’ plate is specified to
distribute fluid evenly in the tubes, thereby preventing fluid-induced erosion,
vibration and cavitation.

Impingement plates effectively eliminate the need to configure a full tube
bundle, which would otherwise provide less available surface. An impingement
plate can also be installed above the shell thereby allowing a full tube count
and therefore maximizing shell space .
4. Bonnets and End Channels
Bonnets / end channels regulate the flow of fluid in the tube-side circuit, they are
typically fabricated or cast. They are mounted against the tube sheet with a bolt
and gasket assembly; many designs include a ‘machine grooved’ channel in the
tube sheet sealing the joint.
If one or more passes are intended, the head may include pass ribs that direct flow
through the tube bundle (figure C). Pass ribs are aligned on either end to provide
effective fluid velocities through an equal number of tubes at a time ensuring a
constant, even fluid velocity and pressure drop throughout the bundle.
Impingement plate distributing the fluid to the tubes preventing fluid-induced
erosion, vibration and cavitation
Figure C. Heads contain pass ribs that direct flow on the tube-side fluid for one or
more passes across the tube bundle.

Shell and tube configurations with up to (4) passes are the most common, however
specialty designs do allow 20 or more crossings. The tube sheet configuration in a
multi-pass shell and tube design must have provisions for the pass ribs, requiring
either removal of tubes to allow a low cost straight pass rib or alternately a pass rib
with curves around the tubes adding cost to the manufacture process. When a full
bundle count is needed for the thermal requirement, machine pass ribs usually
prevent the need to ‘upsize’ to the next larger shell diameter.
The material used in the cast bonnets / heads used in smaller diameters (ie 15” or
less) are typically, poured from iron, steel, bronze, Hastelloy, nickel plated, or
stainless steel. Pipe connections are normally NPT, others including SAE, tri-clamp,
ASME flanged, BSPP, and others types are available.
5. Baffles
Baffles function in two ways, during assembly they function as tube guides, in
operation they prevent vibration from flow induced eddies, last but most
importantly they direct shell-side fluids across the bundle increasing velocity and
turbulence effectively increasing the rate of heat transfer.
All baffles must have diameter slightly smaller than the shell in order to fit,
however tolerances must be tight enough to avoid a performance loss as a result of
fluid bypass around the baffles. This is where the concept of ‘shell roundness’ is of
up most importance in sealing off the otherwise would be bypass around the
baffle.
Baffles are usually stamped / punched, or machined drilled; such configurations
vary based on size and application. Material selection must be compatible with the
shell side fluid to avoid failure as a result of corrosion. It is not uncommon for some
punched baffle designs to include a lip around the tube hole to provide more
surfaces against the tube to reduce wear on the adjoining parts. Tube holes must
be precisely manufactured to allow easy assembly and possible field tube
replacement, all the while minimizing fluid flow through the hole and against the
tube wall.
In typical liquid applications, baffles occupy between 20-30% of the shell diameter;
whereas in a gas application with a necessary lower pressure drop, baffles with 40-
45% of shell diameter are used (figure D). Baffle placement requires an overlap at
one or more tubes in a row to provide adequate tube support.

Additionally baffles are spaced evenly throughout the shell to aid in reducing
pressure drop and even fluid velocity.
Impingement plate distributing the fluid to the tubes preventing fluid-induced
erosion, vibration and cavitation
Figure C. Heads contain pass ribs that direct flow on the tube-side fluid for one or
more passes across the tube bundle.
In a 'single-segmental’ configuration, baffles move fluid or gas across the full tube
count. When high velocity gases are present, this configuration would result in
excessive pressure loss thus calling fourth a ‘double-segmental’ layout. In a
‘double-segmental’ arrangement, structural effectiveness is retained, yet allowing
gas to flow in a straighter overall direction. While this configuration takes full
advantage of the full available tube surface, a reduction in heat transfer
performance should be expected.

troubleshooting of heat exchanger
It is stressful when exchangers go online and don't perform as they should. But not
all scary things go "bump in the night." Heat exchangers that go onstream and don't
perform are also scary. This is especially true if there is a lack of thermal wells,
pressure gages and flowmeters. In addition, you are told that many hundreds of
thousands of dollars a day are being lost due to decreased production, so the
problem has to be found immediately. In many cases, there has to be a shutdown.
Here is helpful information on finding a cure, including:
• What information to collect and what to look for
• The importance of calculated pressure drops and how they help analyze the
problem
• Two-phase flow emphasizing low heat transfer due to stratified flow
• Actual case histories of design and fabrication errors to help with the diagnosis.
The concentration is on thermal problems; problems due to vibration and exchanger
leaks are not discussed.
INFORMATION COLLECTION
Besides the obvious process information of flow, temperatures and pressure drops,
you win probably need the manufacturer's heat exchanger drawings. Hopefuny, you
will not have to run heat exchanger tests. But if you do, there are procedures in the
literature.1, 2
Using the collected process information, make a full thermal design computer run.
The printout will have much more information than a standard specification sheet.
Check the printout with the following in mind:
1. Are there any error messages about the physical properties used?
2. Are there error messages for the input data?
3. Check the section that analyzes the design for comments. This is a section of the
program that acts as expert system software.
4. Was the correct heat-transfer type specified on input?
5. Have any warnings been ignored in the heat exchanger's design?
6. Was the advice on bundle-sealing devices followed?
7. Ifthe problem is freezing or heat damage, could the temperatures in the clean
condition be the problem?
In some cases, it is helpful to measure temperatures on the exchanger's exterior. This
can identify unvented gas, stratified flow or fluid bypassing. Ifthese temperatures are
not too hot or cold, you can check the shell by feeling with your hands.

1. Fouling.
Ifthere is a gradual decline in heat transfer, fouling may be the culprit. Heat
exchanger software can give the available fouling as compared to design fouling.
Sometimes fouling is so severe that tubes can be plugged inside or the shellside
ligaments between the tubes can be filled. This is sometimes seen when bundles
from a refinery are sent to be repaired. Actual fouling can be much higher than the
TEMA CThbular Exchanger Manufacturer's Association) specification.
Ifyou suspect that fouling is a problem, check the exchanger's operating history. Are
there deviations from design conditions? Are there periods of operation where flows
are lower than design? Heat exchangers will foul faster at low velocities. Ifwater
fouling is a problem, have the water flows been cut back in the winter?
Ifyou determine that fouling is a problem, make a chemical analysis of the fouling
material. Knudson4 discusses different fouling control methods and types of cleaning.
Online and offline mechanical cleaning plus chemical cleaning is discussed. If the
fouling cannot be controlled, a tube electropolishing process can slow scale and other
buildup. It eliminates small ridges and pits that contribute to fouling.
2. Debris.
Check to see if there is a strainer in the piping ahead of the inlet nozzles. Ifthere is no
strainer, there may be debris in the exchanger. Itis amazing what types of debris can
be found in heat exchangers after startupsuch things as rocks, trash, wrenches,
gloves, weld rods, clothing, pencils, etc. Possibly during a work force shift change, the
first shift left something that the second shift did not see before closing the piping.
3. Excess surface problems.
In the design stage, the clean condition may not have been evaluated. Many
exchangers are designed for fouled conditions only. Most of the time this is all that
is necessary. However, in some situations the clean condition must be checked.
More exchanger oversurface means more deviations from outlet design
temperatures and a greater potential for problems.
For high-temperature applications, the outlet temperature of the heated stream
must be checked. It will be higher than the process design temperature. Ifthis
temperature is higher than what was used to select the metallurgy, there may be a
problem. Small5 relates a case where the effect of oversurface and the clean
condition was not checked. It resulted in ruptured tubing and the loss of tube fins.
Another problem with higher-than-process-design outlet temperatures is that a
liquid may degrade or lose its thermal stability. For cold applications, the outlet
temperature ofthe stream to be cooled must be checked. It can cause stream
freezing and tube plugging. It can also cause brittle tubing and tube failures.
Another case where excess surface can be a problem is in vaporizer design.
Ifvaporizing is done too well, there will be surging ofvapor leaving the exchanger.

As a young engineer, I saw an ammonia vaporizer in a nitric acid plant creating
surging vapor to the reactor. All the liquid would flash to vapor inside the kettle;
liquid feed would then surge in and flash again. Our process group determined there
was excess surface and plugged off some tubes. The kettle operation smoothed out
and reactor efficiency improved.
Excess surface problems are cured by plugging tubes in the inlet channel. There are
many different types of plugs, but metal plugs with a slight taper are most common.
Unless the temperatures are high, wooden plugs can be used in a pinch.
4. Venting.
Proper venting is a startup necessity. Improper venting usually occurs on startup and
is recognized by poor heat transfer and a high pressure drop. Exchangers operating
under a vacuum can be more ofa problem than those operating under pressure. The
vacuum will suck air into the exchanger ifit isn't perfectly sealed. Vents should be
located at the exchanger's highest points. The shellside is especially vulnerable to
pockets of air or noncondensables. Gas can get trapped at the bundle~s top or by
"ears" at the top of baffles. Ifa venting problem is suspected, talk to operations
about their startup procedures. recommends startup procedures. YokelF has a more
complete discussion ofvents, especially vertical fixed tube sheet exchangers.
5. Field mistakes.
In one instance, a heat exchanger was piped up backwards. The fluid that should
have been on the shells ide was piped to the channel side and vice versa. When both
streams are in turbulent flow, this switch may go unnoticed. In this case, fluid that
shouM have been in the shell was semiviscous. On the shellside, the fluid would
have been turbulent and given better heat transfer. When on the tubeside, the fluid
flowed in the transition region between turbulent and viscous. This gave a
noticeably lower heat transfer, although better heat transfer than calculated.

Heat exchanger

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