2. 1
INTRODUCTION
A heat exchanger is process equipment used for transferring heat from
one fluid to another fluid through a separating wall. Usually heat exchangers
are classified according to the functions for which they are employed.
The most widely used heat exchanger is the Shell & Tube heat
exchanger. It consists of parallel tubes enclosed in a shell. One of the fluid
flows through the shell & the other flows through the tubes. The one, which
flows through the shell side, is called as shell side fluid & the one flowing
through the tubes is called as tube side fluid.
" When none of the fluid condenses or evaporates, the unit is called as
Heat Exchanger." In this only the sensible heat transfers from the one fluid
to another.
Degradation is an inevitable process for every heat exchanger, but
affects some to great extent, depending upon the duties they are called upon
to perform. Some heat exchangers never achieve their design objective.
Their degradation stems from inadequate design or improper execution or
poor workmanship. Others achieve their design objective but then
deteriorates progressively in performance as time wears on.
Deterioration may be due to fouling, where there is acceleration of
deposits that increase the thermal resistance to heat transfer. This diminishes
the heat transfer while simultaneously increasing the compressor and the
pump work input because of the partial blockage of fluid conduit. Fouling
1

3. may be overcome by cleaning, with the potential for the restoration of the
heat exchanger to its original performance.
Corrosion is another principle source of heat exchanger degradation.
Corrosion of heat exchanger structural material arises from variety of
mechanisms and progressively weakens the element to the point where the
failure by the rupture or leakage occur is eminent. The corrosion products
will likely occupy a large volume, partially blocking the flow conduits &
increasing the input pump work or inhibiting the mass flow rate of the flow.
In heat exchanger the fluid flow do not follow the idealized path
anticipated from the elementary conditions. This departure from ideality can
be very significant indeed. As much as 50% of the fluid can behave
differently from what is expected. Maldistribution of the flow is the word
often used to describe unequal flow distribution in several parallel flow
paths found in heat exchanger. The maldistribution of the fluid flow is
reduced generally by improving the baffle arrangement & proper designing
& placement of the inlet & the outlet nozzle.
The measures to combat or repair degradation of performance are
discussed ahead.
2

4. 2
TYPES OF HEAT EXCHANGER
2.1 BASIC CLASSIFICATION (1)
2.1.1 Regenerative type
These heat exchangers have a single set of flow channels through a
relatively solid massive solid matrix. The hot and the cold fluid pass through
the matrix alternately. When the hot fluid is passing (called the ‘Hot Blow’)
heat is transferred form the fluid to heat the matrix. Later when the cold fluid
passes through (called the ‘Cold Blow’), heat is transferred from the matrix
to the matrix and the fluid cools. For moderate temperature applications this
heat exchanger is used because they may be made low in cost & the plastic
honey comb or any finely divided material as the regenerative matrix.
2.1.2 Recuperative type
Recuperative Heat Exchanger
Plate Heat Exchanger Tubular Heat Exchanger
Spiral Plate - Fin Single - Pipe Double pipe Shell & Tube
Plate - Coil Plate - Frame Cluster Pipe Fin Tube
Fig. 1
It is equipped with separate flow conduits for each fluid. The fluid
flows simultaneously through the heat exchanger in separate paths & heat is
transferred from hot to the cold fluid across the walls of the flow section.
3

5. 2.2 CLASSIFICATION BASED ON TYPE OF FLUID FLOW (3)
2.2.1 Liquid/Liquid
This is by far the most common application of tubular exchangers.
Typically, cooling water on one side is used to cool a hot effluent stream.
Both the fluids are pumped through the exchanger so that the principal mode
of heat transfer is forced convective heat transfer. The relatively high density
of liquid results in very high rates of heat transfer. So there is very little
incentive in conventional situations to use fins or other devices to enhance
the heat transfer.
2.2.2 Liquid/Gas
It is usually used for air-cooling of hot liquid effluent. The liquid is
pumped through the tubes with very high rates of convective heat transfer.
The air in cross flow over the tubes may be in forced or free convective
mode. Heat transfer coefficients on the airside are low compared with those
on the liquid side. Fins are usually added on the outsides (air side) of the
tubes to compensate.
2.2.3 Gas/Gas
This type of heat exchanger is found in the exhaust gas /air preheating
recuperators of gas turbine systems, steel furnaces & cryogenic gas
liquification systems. In many cases one gas is compressed, so the density is
high, while the other is at the low pressure with a low density. Normally the
high-density fluid flows inside the tubes. Internal and external fins are
provided to enhance the heat transfer.
4

6. 2.3 CLASSIFICATION BY FLOW ARRANGEMENTS (3)
The flow arrangement helps to determine the overall effectiveness, the
cost & the highest achievable temperature in the heated stream. The latter
affect most often dictates the choice of flow arrangement. The fig.2 indicates
the temperature profile for heating & heated stream, respectively. If the
waste heat stream is to be cooled below the load stream exit, a counter flow
heat exchanger must be used.
Fig. 2
Thin
Cold Fluid
Thout Hot Fluid
∆Τ Seperating Surface
Cold Fluid
Tcout Co - Current Flow
Tcin
Surface Area A
Thin
Cold Fluid
Tcout Hot Fluid
Thout Cold Fluid
∆Τ
Counter - Current Flow
Tcin
Surface Area A
Thin
Thin
Tcout
Tcout Tcin
Thout
Tcin
Thout
Cross Flow
5

7. 2.4 TUBULAR HEAT EXCHANGER CLASSIFICATION (1)
2.4.1 Clustered pipe heat exchanger
L-P Stream
It is the development of single
pipe heat exchanger. Two or more H-P Stream
tubes are joined by thermally
conducting medium. So that the heat is Solder
transferred between the fluids flowing Fig. 3
in the tubes. Sometimes a cluster of tubes is arranged around a central core
tube. High – density fluid passes through the core tube. The return stream of
the low-density fluid passes through the multiple tubes arranged around the
core tube. The construction is favored in small cryogenic counter flow Heat
exchanger.
2.4.2 Double pipe heat exchanger
It consists of central tube contained within a larger tube. It is
relatively cheap, flexible & hence used in smaller units. It is customary to
operate with high pressure and high pressure, high temperature, high density
or corrosive fluid in small inner tube, with less demanding fluid on outer
tube.
Hot
Fluid
Cold Fluid
Fig. 4
6

8. 2.4.3 Shell & Tube heat exchanger
To increase the capacity or reduce the required length, more than one
internal tube is incorporated within the outer tube enclosure. But the most
common form of multi tubular heat exchanger is the one shown in fig. 5.
This one is widely used for liquid/liquid heat transfer. The best-known
standards for the tubular heat exchanger are the TEMA – Standards of the
Tubular Exchanger Manufacturing Associations, which include the basic
nomenclature & classification scheme for Shell & Tube.
Heated Fluid Cooled Fluid
Cold Fluid Hot Fluid
Fig. 5
7

9. 2.5 PLATE HEAT EXCHANGER CLASSIFICATION (6)
2.5.1 Plate & Frame
It consists of a series of rectangular, parallel plates held firmly
together between substantial head frames. The plates have corner ports & are
sealed by gaskets around the ports & along the plate edges. Corrugated
plates provide high degree of turbulence even at low flow rates. In this
exchanger, hot fluid passes between alternate pairs of plates, transferring
heat to cold fluid in the adjacent spaces. The plates are readily separated for
cleaning and heat transfer area can be increased by simply adding more
plates. Plate heat exchangers are relatively effective with viscous fluids with
viscosities up to about 30 kg/m.sec (300 poise)
2.5.2 Spiral Plate
A spiral plate heat exchanger can be considered as plate heat
exchanger into which plates are formed into a spiral. The fluids flow
between the channels formed between the plates. The spiral heat exchangers
are compact units.
For a given duty the pressure drop over a spiral heat exchanger will
usually be lower than that for the equivalent shell and tube heat exchanger.
Spiral heat exchanger give true counter current flow and can be used where
the temperature correction factor for a shell and tube heat exchanger would
be too low. Because they are easily cleaned and turbulence in channels is
high, spiral heat exchanger can be used for very dirty process fluids and
slurries.
8

10. 2.6 SPECIAL PURPOSE HEAT EXCHANGER (6)
2.6.1 Scraped surface heat exchanger
Spring Clip Inner pipe
"J" Spring
Scraper Blade
Fig. 6
Shell and tube heat exchanger is basically a double pipe heat
exchanger with fairly large central tube, 100 to 300 mm in diameter,
jacketed with steam or cooling liquid. The scrapping mechanically rotating
shaft provided with one or more longitudinal scrapping blades is
incorporated in inner pipe to scrape the inside surface. The process fluid
(viscous liquid) flows at low velocity through the inside pipe and cooling or
heating medium flows through the annular space created between two
concentric pipes. The rotating scrapper continuously scrapes the surface thus
preventing localized heating and facilitating rapid heat transfer.
Liquid-solid suspensions, viscous aqueous and organic solution and
food products, such as organic juice concentration are often heated or cooled
in such type of exchanger. It is widely used in paraffin wax plants.
9

11. 2.6.2 Finned tube Heat exchanger
When the heat transfer coefficient of one of the process fluids is very low
as compared to the other, the overall HTC becomes approximately equal to
the lower coefficient. This reduces the capacity per unit area of heat transfer
surface, making it necessary to provide very large heat transfer area. Such
situations often arise in,
1. heating of viscous liquids.
2. heating of air or gas stream by condensing steam.
Air or gas side HTC is very low in comparison of film coefficient on the
condensing side. In such cases it is possible to increase the heat transfer by
increasing / extending the surface area on the side with limiting coefficient
(air, gas or viscous liquid side) with the help of fins.
The heat transfer area is substantially increased by attaching the metal
pieces. "The metal pieces employed to extend or increase the heat transfer
surface are known as fins". The fins are most commonly employed on
outside of the tubes. According to the flow of the gas, longitudinal and
transverse fins are used.
2.6.3 Graphite Block heat exchanger
Generally heat exchangers are made from various metals and alloys,
suitable to process streams. But corrosive liquids like H2SO4, HCl etc.
require the use of exotic metals as titanium, tantalum, zirconium and others.
In such cases, graphite heat exchangers are well suited for handling
corrosive fluids. Graphite is inert towards most corrosive fluids and has very
high thermal conductivity. Graphite being very soft, these exchangers are
made in cubic or cylindrical blocks.
10

12. 2.6.4 Jackets and cooling coils in vessels
In chemical industries a number of reactions are carried out in agitated
vessel. In such cases, addition or removal of heat is conveniently done by
heat transfer surface, surface that can be in the form of jacket fitted outside
the vessel or the helical coil fitted to inside.
Jackets as well as helical coils are used for heating or cooling purpose
depending upon the situation.
Helical Vessel
Coil
Jacket
Baffle
Agitator
Fig. 7
11

13. 3
IMPORTANCE OF HEAT EXCHANGER
3.1 INTRODUCTION
Heat recuperators or heat exchangers as they are called so, are pieces
of equipment, which can abstract sensible heat from one stream of flowing
fluid and supply it to another stream. They are essential features of all
production process in chemical industry. Because of importance of
improving heat recovery, consequent on the very rise in prime energy costs.
Heat exchangers are becoming increasingly important in the heating &
ventilating field as well.
3.2 MAIN USES OF HEAT RECUPERATORS (4)
1. To extract useful heat from the waste hot liquid & gases. The heat is
transferred to secondary fluids, which can then be used for either
space heating or for the supply of preheated water to the boiler.
2. For normal heat transfer from the stream heaters or flues to circulating
air, in order to raise this air to the required working temperature.
3. For normal operating of air-conditioning equipment, in which, the
heat is being abstracted from room air by refrigeration fluid or by
chilled air.
4. For heat recovery from exhaust air, flue gases & other sensible heat
source.
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14. 3.3 UNIT OPERATIONS (3)
3.3.1 Exhaust – Gas stream
Recuperation is the most promising candidate for heat recovery from
high temperature exhaust gas streams. As shown in fig.8 the hot gases will
be cooled by the incoming combustion air which will be supplied to the
same furnace. Because of the temperature of the gases leaving the furnace,
the heat exchanger being selected is the radiation recuperator. This is the
concentric tube heat exchanger, which replaces the present stack.
The incoming combustion air is needed to cool the base of the
recuperator & thus the parallel flow occurs. In figure, temperature profile
sketch is drawn. It is seen that, in parallel flow heat exchanger, heat recovery
ceases when the two streams approach a common exit temperature.
Cooled
Exhaust Gas
Heated Furnace Tc Th
Air
Height of Chimney
as
st G
Coo
au
ling
Exh
Gas
Cool Furnace
Air
Th Temperature Tc
Hot
Exhaust Gas
Fig. 8
13

15. 3.3.2 Boiler economizer
An economizer is constructed as a bundle of finned tubes, installed in
boiler breaching. Boiler feed water flows through the tube to be heated by
the exhaust gases. The extent of the heat recovery in the economizer may be
limited by the lowest allowable exhaust gas temperature in the exhaust stack.
The exhaust gases may contain water vapor both from the combustion
air & from the combustions of hydrogen that is contained in the fuel. If the
exhaust gases are cooled below the dew point of the water vapor,
condensation will occur & may cause damage to the structural material.
300°F
Flue
Exhaust 220°F
Feed water
Finned tube from deaerator
Economiser
Boiler
Exhaust
500°F
Water
Tube Boler
Fig. 9
14

16. 4
CORROSION IN HEAT EXCHANGER (1)
4.1 INTRODUCTION
Corrosion is defined as "The degradation of a material because of
reaction with environment".
It is the part of the cycle of growth and decay that is natural order of
things. Corrosion is principal cause of failure for engineering systems. The
annual cost of corrosion runs grater than costs of floods, and earthquakes.
4.2 UNIFORM OR GENERAL CORROSION
Uniform or general corrosion is the most common form of corrosion.
It is characterized by chemical or electrochemical reactions that proceed
uniformly over the entire exposed surface or a substantial portion of that
surface. The metal becomes progressively thinner and eventually fails
because of the stress produced on it.
This type of corrosion is easy to handle. The rate of decomposition
can be can be determined by comparatively simple immersion test of the
specimen in the fluid. The life of the equipment can therefore be predicted
and extended to the degree required by the addition of corrosion allowance
to the metal wall thickness to sustain the pressure or the other stress loading
applied.
15

17. Prevention of Uniform general corrosion
Uniform corrosion can be prevented or reduced by the selection of
appropriate materials (including internal coatings), the addition of corrosion
inhibitors to the fluid, treatment of fluids to remove corrosive elements and
the use of the sacrificial cathodic protection or impressed electrical
potentials. Other forms of corrosion are difficult to predict. They tend to be
localized and concentrated with the consequent premature or unexpected
failure.
4.3 GALVANIC OR TWO METAL
Copper Zinc
CORROSION
When two dissimilar metals are Electrolyte
immersed in a corrosive or electrically
conductive solution, a voltage will
become established between them. It
the metals are then connected by
electrically conducting path, a small
current will pass continuously
Fig. 10
between them. The principle is shown
in fig.10. Corrosion of less corrosion resistant metal is accelerated and that
of the more resistant metal is decreased, as compared with their behavior
when they are not coupled electrically. The less resistant metal is described
as 'Anodic' and the more resistant metal as ‘Cathodic’. Usually corrosion of
the cathode is virtually eliminated.
The combination of dissimilar metals and a corrosive or electrically
conductive medium constitutes a galvanic cell. The various metals and
16

18. alloys, along with other materials of interest can be arranged in order of
decreasing corrosion resistance as shown in table.
The noble metals leading the list are cathodic and the least subject to
corrosion. Those at the bottom are anodic and most subjected to attack. The
combination of metal from the upper half of the table with any other further
down the table will establish a galvanic cell with the potential to accelerate
the rate of corrosion of the anode, lower in the table, while decreasing the
corrosion rate of the cathode. The effect increase for the metals that is
further apart in table. Magnesium will rapidly corrode in seawater in
conjunction with a titanium cathode, but less rapidly in combination with
aluminum or zinc.
Prevention of Galvanic Corrosion
Use a single material or a combination of materials that are close in the
galvanic series.
1. Avoid the use of small ratio of anode area to the cathode area. Use
equal areas or large ratio of anode to cathode area.
2. Electrically insulate dissimilar metals where possible. This
recommendation is shown in fig.11
Insulating Sleeve
Insulating
Washer
Nut
Bolt
Pipe Valve
Fig. 11
17

19. 3. Local failure of the protective coating, particularly at the anode can
result in small anode to cathode area, marked by accelerated galvanic
corrosion. Maintain all coatings in good condition, especially at the
anode.
4. Avoid the use of riveted or threaded joints in favor of welded or
brazed joints.
5. Install a sacrificial anode lower in the galvanic series than both the
materials involved in the process equipment.
4.4 CREVICE CORROSION
It is charachterised by the intense local
corrosion in the crevices and other shielded
areas on the metal surfaces exposed to stagnant
Crevice
corrosive liquids. It can occur where any
undistributed liquid film exists, such as at a
small hole, gasket - flange interface, lap joints,
surface deposits, and the crevice under bolt and Crevice
Fig. 12
rivet heads. Relative to heat exchangers, it is
important to note that nonmetallic deposits (fouling) of sand, or crystalline
solids may act as a shield and create the necessary stagnant condition the
essence of crevice corrosion.
The mechanism of crevice corrosion is associated with the depletion
of the oxygen in the stagnant liquid pool, which results in the corrosion of
the metal walls adjacent to the crevice. This type of corrosion occurs with
many fluids but is particularly intense with those containing chlorides. The
nature of electrochemical process is such that the corrosion attack is
18

20. localized within the stagnant or shielded area while the surrounding surfaces
over which the fluid moves suffer little or no damage.
Some time is required between the initial establishment to the
conditions for the crevice corrosion and the occurrence of the visible
damage, which is called the incubation period.
Prevention of Crevice Corrosion
1. Use welded butt joints instead of bolted or riveted joints. Good welds
with deep penetration are required to avoid porosity and crevices on
the inside if the joint is welded on one side only.
2. Eliminate crevices by continuous welding by solder or brazing filling
and by caulking.
3. Design to eliminate the sharp corners, crevices and the stagnant areas
and complete drainage.
4. Clean at regular intervals.
5. Eliminate the solids suspended in the fluids, if possible.
6. Weld tubes to the tube sheet, instead of rolling.
4.5 PITTING CORROSION
Pitting corrosion is the phenomenon
whereby an extremely localized attack results
in the formation of the holes in the metal
surface that eventually perforates the walls. It
is shown in the fig.13. The holes or pits are of
various sizes and may be isolated or grouped
Fig. 13
very closely.
19

21. The mechanism of pitting is very close to crevice corrosion. Pits
usually grow in the direction of gravitational action i.e. downward form
horizontal surfaces. They sometimes develop on vertical surfaces, but only
in very exceptional cases do pits grow upward form the bottoms of
horizontal surfaces.
As with crevice corrosion an incubation period is required before
pitting corrosion starts; thereafter, it continues at an accelerated rate. Further
more once below the surface, the pits tend to spread out, undermining the
surface as shown in figure. This particularly is unfortunate for the small
surface pits can easily become obscured by the corrosion products or other
sediments and the deposits. Failure as leak resulting from the complete
perforation of the metal wall therefore occurs suddenly and unexpectedly.
Most pitting corrosion arises from the action of the chloride or
chlorine containing ions. The process of establishing a pit site is unstable
and is interrupted by any movement of the fluid over the surface. Thus,
pitting corrosion is rarely found in metal surface over which fluids move
constantly. Even in these few cases it can be reduced if the fluid velocity is
increased. Often a heat exchanger pump or a tube carrying a corrosive fluid
shows no sign of pitting corrosion when in service but rapidly deteriorates if
the plant is shutdown and the fluid not drained from the system.
Stainless steel alloys are particularly susceptible to pitting corrosion
attack. Carbon steel is more resistant to pitting than stainless steel.
Prevention of pitting corrosion
The principal measure is to use material that is known to be resistant
to pitting. These include:
20

22. Titanium, Hastelloy C or Chloriment 20, Type 316 stainless steel, Type 304
stainless steel (Pits badly in chloride solution).
4.6 EROSION CORROSION
Erosion corrosion is the
Water Flow
term used to describe corrosion Corrosion
Corrosion Film Original metal
pits
surface
that is accelerated as a result of
increase in the relative motion
between the corrosive fluid and
Fig. 14
the metal wall. The process is
usually a combination of chemical or electrochemical decomposition and
mechanical wear action. Erosion corrosion therefore differs from most other
forms of corrosion, where the rate of attack is highest under stagnant or low-
velocity conditions.
Erosion corrosion can be recognized by the appearance of the
grooves, gullies, and waves in the directional pattern, similar to sand
formations on the shorelines. Fig.14 is a sketch of the erosion corrosion
corrosion pattern on a condenser tube wall. Failure by erosion corrosion can
occur in a relative short time (a matter of weeks or months). It often comes
as a surprise, following satisfactorily tests for the corrosion susceptibility of
the specimen submerged in the corrosive fluid under static condition.
Metals that depend for their corrosion resistance on the formation of a
protective surface film are particularly susceptible to attack by the erosion
corrosion. Aluminum and stainless steel are in this category. The protective
film is eroded by mechanical scrubbing, exposing the soft core to chemical
or electrochemical attack in addition to the continued mechanical wear.
21

23. Many fluids that are not normally considered aggressive corrosion
agents can promote erosion corrosion. High velocity gases and vapors at
high temperature may oxidize a metal and then physically strip off the
otherwise protective scale.
Many erosion corrosion failures in heat exchanger, occurs in the tube
side, particularly at the tube inlet; the process is frequently called inlet-tube
corrosion. It arises essentially from the highly turbulent flow ensuing as a
consequence of the sudden change in the section as the fluid leaves the inlet
bonnet and enters the reduced flow section of the tubes. An increase in the
rate of erosion corrosion as the velocity increases. For many materials there
appears to be a critical value, above which the rate of attack increases.
Prevention of Erosion corrosion
1. Use materials with superior resistance to erosion corrosion.
2. Design for minimal erosion corrosion.
3. Change the environment.
4. Use protective coating.
5. Provide cathodic protection.
4.7 STRESS CORROSION
Stress corrosion is the name given to the process whereby the cracks
appear in the metals subject simultaneously to a tensile stress and specific
corrosive media. The metal is generally not subjected to appreciable uniform
corrosion attack but is penetrated by fine cracks that progress by expanding
over more of the surface and proceeding further into the wall. The cracks
may or may not be branched. They may proceed along the grain boundaries
22

24. only or may be transgranular and advance with no preference to follow the
grain boundaries.
Stress corrosion cracks develop in specific metal-fluid combination
when the stress level is above a minimum level that depends on the
temperature, alloy structure, and environment. In some materials minimum
stress levels for crack formation are as low as 10% of the yield stress. In
other cases the critical value may be as high as 70%.
For stress corrosion cracks to initiate, the stress must be tensile in
character and exceed the critical level referred to above. They are induced
from any source, including residual welding stress. Stress corrosion often
occurs in lightly loaded structures that are not stress relived after fabrication.
Not all metal fluids are susceptible to cracking. Stainless steels crack
with fluids containing chloride but not with ammoniacal fluids, whereas
brasses crack in ammonia but not in chlorides.
It is likely that stress corrosion cracks are initiated at a corrosion pit or
other surface regularity. The base of the pit acts as a stress raiser so the local
stress concentration is very high. Once a crack is started, the stress at the tip
of the crack is very high and the fosters continuing development of the
crack. As the crack penetrates further into the metal, the remaining wall
section assumes the whole load. The general stress level is therefore raised
and is further magnified at the tip of the crack, so the rate of propagation is
accelerated. Eventually the metal fails suddenly and catastrophically when
the stress in the remaining metal exceeds the ultimate.
Prevention of Stress Corrosion
1. Lower the stress level below the critical threshold level by reducing
the fluid pressure or increasing he wall thickness.
23

25. 2. Relieve the stress by annealing.
3. Change the metal alloy to one that is less subjected to stress corrosion
cracking in the given environment.
4. Modify the corrosion fluid by process treatment or by adding
corrosion inhibitors, such as phosphates.
4.8 HYDROGEN DAMAGE
Hydrogen damage is a term applied to the variety of consequences
followed by exposure of metal to hydrogen. Hydrogen may exist in the
mono atomic form (H) or the diatomic form (H2). Atomic hydrogen can
diffuse through many metals. Molecular hydrogen cannot do this, nor can
any other chemical species. There are various source of atomic hydrogen,
including high temperature atmospheres, corrosion and electrochemical
process. Corrosion and cathodic protection, electroplating, and electrolysis,
all produce hydrogen ions, which reduce to atomic hydrogen molecules.
Some substances (sulfide ions, phosphorus and arsenic compounds) inhibit
the reduction of hydrogen ions, leading to a concentration of atomic
hydrogen on the metal surfaces. The hydrogen damages are of four distinct
types.
4.8.1 Hydrogen Blistering
The production of hydrogen ions will, in some way, result in the
aggregation of hydrogen ions, atomic hydrogen and molecular hydrogen on
the metal surface of a heat exchanger. Some of the atomic hydrogen will
diffuse into and through the metal before reducing to molecular hydrogen on
the outer surface.
24

26. The atomic hydrogen diffusing through the metal will enter any voids
in the metal. Some will then reduce to molecular hydrogen, which cannot
permeate the metal wall. The equilibrium pressure for atomic pressure for
the atomic and the molecular hydrogen is several hundred thousand
atmospheres so the one way accumulative process continues, giving rise to
very high pressures - far exceeding the yield stress of the material. The
growth appears as "Blisters" on the wall of the heat exchanger.
4.8.2 Hydrogen Embrittlement
It arises from the source as blistering - the penetration of apparently
solid metal by atomic hydrogen. In some metals the hydrogen reacts to form
brittle hydride compounds. In others the mechanism of embrittlement is not
known. Alloys are most susceptible to cracking from hydrogen
embrittlement at their highest strength levels. The tendency to embrittlement
increases with the hydrogen concentration in the metal.
4.8.3 Decarbonisation and Hydrogen attack
It is associated with metals exposed to high temperature gas streams
containing hydrogen and variety of other gases. Decarbonisation is the
removal of carbon from a steel alloy on exposure to hydrogen at high
temperatures. It results in reduction of tensile strength and increase in
ductility and creep rate. Hydrogen attack is the interaction of metals or an
alloy constituent with hydrogen at high temperature.
25

27. Prevention of Hydrogen Damage
1. Use of void free steels.
2. Use of metallic, inorganic and organic coatings and the liners in steel
vessels. The liner must be impervious to hydrogen penetration and
resistant to other media in the vessel. Carbon steel clad with nickel is
sometimes used. Rubber, plastic and brick liners are also used.
3. Addition of inhibitors to reduce corrosion and the rate of hydrogen -
ion production. These are economically feasible in closed circulating
systems.
4. Fluid treatment to remove hydrogen – generating compounds such as
sulphides, cyanides and phosphorous containing ions.
5. Use of low hydrogen welding rods and the maintenance of dry
conditions during welding operations. Water and water vapor sources
are major sources of hydrogen.
26

28. 5
MALDISTRIBUTION OF FLUID FLOW
5.1 INTRODUCTION
The fluid flows do not follow the idealized paths anticipated from the
elementary considerations. These departures form ideality can be very
significant indeed. As much as 50% of the fluid can behave differently from
what is expected, based on the simplistic model. The maldistribution of flow
is a term often used to describe unequal flow distribution in the several
parallel flow paths found in most heat exchangers.
5.2 THE TINKER DIAGRAM (1)
Flow on the shell side of the shell and tube heat exchanger, was
classified by Tinker, into a number of separate streams, as represented
diagrammatically in fig.15, 16. The A stream represents flows that occur in
the clearance between the baffles tube holes and the tubes. Flow is due to
pressure drop between the upstream and the downstream sides of the baffle.
The B stream is the true cross flow stream, passing through the tube bundles
and performing the real function of the shell-side fluid.
The C stream bypasses the tube bundle and flows in the annulus
between the shell and the tube bundle. This is highly ineffective use of the
fluid. If the tube bundle shell clearance is greater than the tube pitch, it is
advisable to include a sealing device to inhibit bypass flow. The sealing
devices can be stripes, rods or dummy tubes, as shown in fig.16.
27

29. The F stream includes other bypass streams that arise when the tube
partitions of the multipass tube bundles are arranged parallel to the direction
of the main cross flow stream. The D stream is leak flow that occurs in the
clearance space between the edge of the baffle and the shell. This represents
direct loss of fluid, for it serves no useful heat-transfer function.
D
A
C
A
B
B
A
Fig. 15 Tinker diagram
Bypass stealing strips Dummy rods or tubes
Fig. 16 Seal for by – pass flow
Note : For more information on “Maldistribution of Fluid Flow” refer TEMA (Tubular Exchanger Manufacturers Asso.)
28

30. 5.3 PARALLEL - PATH FLOW (1)
Flow paths in the tube side of shell and tube heat exchanger cannot be
made absolutely identical and fluid flows are incredibly sensitive to
apparently trivial differences between one path and another. When the
number of parallel paths is limited to two or three and the paths are highly
restricted, the difference in channel mass flow rates may be as high as 90
percent. The flow is then function of some power of the principal flow
resistance parameter e.g. the third power of the width of a slit or the square
of the cross-section area of a flow aperture.
Tube distortion in bending or the squashing resulting from improper
handling fabrications, can contribute appreciably to flow maldistribution as
shown in fig.17. A difference in the mass rate of flow through the tube
carries the implication that the flow velocity is significantly different. The
heat transfer rate depends on the fluid velocity and the tube wall and the
fluid temperatures depend on the heat transfer. Low mass flow and fluid
velocity in some tubes may give rise to high fluid and wall temperatures
with accelerated corrosion and fouling deposition rates. The fouling deposits
and products of corrosion exacerbate the difference in flow resistance
between one tube and the other and further diminish the mass flow in tubes
already starved of fluid. The process is a cancer feeding on itself.
Alternative solutions to heat transfer problem are also explored.
Special heat exchangers are shown in the fig.18. The flow channel are of
variable geometry designs to incorporate a compensatory feedback
mechanism, acting to adjust the duct geometry to ensure uniform distribution
of flow in various channels. The miniature high performance heat exchanger
was designed to achieve huge NTU of 200 (The NTU of most of industrial
exchanger is less than 5). Even with great attention to manufacturing detail,
29

31. the early high performance heat exchangers were unable to exceed an NTU
of 33. With the compensation feed back geometry, values of 167 were
achieved.
Tube deformation increases flow resistance.
Tube subject to erosion corrosion at the
site of deformation
Fig. 17
Cold Flow
Hot Flow
Fig. 18
30

32. 5.4 STAGNANT AREAS (1)
Disappointing heat exchanger thermal performance often arises from
the creation of stagnant areas in the fluid – flow circuits. In stagnant or semi
stagnant areas the fluid velocities are, by definitions, zero or negligibly low.
The consequences are often very serious. The obvious effect is that with low
fluid velocity area for heat transfer is not effectively utilized. Less obvious
but of greater importance is the fact that corrosion and fouling processes are
highly accelerated under stagnant conditions. Sediments in slurries aggregate
in the low velocity areas. Surface temperature in the low velocity areas may
be appreciably higher than the mean design condition, which further
accelerates the chemical reactions exacerbating the corrosion and fouling
processes.
A common location of semi stagnant fluid zones in shell and tube heat
exchanger is the region on the shell side between the tube sheet and the inlet
and outlet nozzles (fig.19). It is necessary to establish the centerlines of the
inlet and outlet nozzles some distance from the tube sheets so as to
accommodate the nozzle flanges and to provide sufficient shell strength in
the high stress areas near the tube sheets. The existence of some low velocity
regions on the shell side near the ends of the tubes is then virtually
inescapable but is frequently overlooked by inexperienced thermal
designers. They fail to add extension to the calculated tube length to
compensate for the “dead area”.
Baffle design and placement are the principal means by which to
ensure adequate fluid velocities on the shell side and a well – regulated,
dispersed flow. Even good designs can be hopelessly compromised if they
are improperly or inadequately executed. Excess clearance of the baffles in
the shell will certainly facilitate loading the tube bundle in the shell during
31

33. the construction. However, that clearance will lead to substantial bypassing
of the fluid at the periphery of the baffle, so that little of the fluid actually
traverses the tube bundle. Excessive clearance of the tube holes will greatly
facilitate construction, but again will result in a proportion of the fluid not
passing through the tube bundle as intended.
In figure upper diagram shows the tube bundle correctly installed. In
lower diagram the bundle has been reversed. It is immediately clear that the
compartments between the tube sheets and the first and last baffles are
completely stagnant and virtually useless for heat transfer. The effectiveness
of the tube bundle is reduced by as much as 40 percent.
(a)
Stagnant areas
(b)
fig. 19 (a) correct (b) incorrect placement of the tube bundle in shell and tube heat exchanger
32

34. 6
FOULING
6.1 INTRODUCTION (2)
Most process application involve fluids that form some type of
adhering film or scale on to the surface onto the inside or outside of the tube
wall separating the two systems. These deposits may vary in nature (brittle,
gummy), texture thickness, thermal conductivity, ease of removal etc.
Although there are deposits on the clean tube or the bundle, the design
practice is to attempt to compensate for the reduction in heat transfer
through these deposits by considering them as resistance to heat transfer.
These resistances or fouling factors have not been accurately determined for
many fluids and metal combinations. Yet the general practice is to “throw
in” a fouling factor. This can be disastrous to an otherwise good technical
evaluation of the expected performance of the unit. Actually considerable
attention has to be given to such value as the temperature range, which
affects the deposits, the metal surface (steel copper, nickel) as it affects the
adherence of the deposit and the fluid velocity as it flows over the deposit or
else moves the material at such a velocity to reduce the scaling or fouling.
The percentage effect of the fouling factor on the effective overall
heat transfer coefficient is considerable more on units with the normally high
value of the clean unfouled coefficient than for one of low value. For
example an unit with clean overall HTC of 400 when corrected for 0.003 the
total ends up with effective coefficient of 180, but a unit with clean
33

35. coefficient of 60, when corrected for 0.003 fouling allowance, shows an
effective coefficient of 50.5 as shown in the graph (Fig.20).
Fig. 20 Effect of fouling resistance on transfer rates (2)
34

36. F.F. U
0.286 3.5
0.25 4.5
0.182 5.5
0.125 8
After 16 Months
0.0825 12 After 6 Months
Clean
0.04 25
0.02
0.01 Gas outside tubes
500 100 50 30 20 17 15 Gas inside tubes
Flow Rate
Fig. 21 Graph for prediction of fouling and HTC as a function of velocity over a period of time (2)
The above (fig.21) working chart presents a plot of actual operating
Ua values to allow projection back to infinity and to establish the base
fouling factor after the operating elapsed time. The flow rate inside or
outside the tubes is plotted against the overall heat transfer coefficient, U.
As the value of B or the fouling factor increases with time, the
engineer can determine when the condition will approach that time when
cleaning of exchanger will be required. Gas flows are used because usually
gas film controls in a gas – liquid exchanger.
Fouling factors are suggested by TEMA in table below. These values
are predominantly for the petroleum operations, although portions of the
table are applicable to general use and to petrochemical process.
35

37. GUIDE TO FOULING RESISTANCES (2)
Fouling resistance for Industrial fluids
Oils:
Fuel oil 0.005
Quench oil 0.004
Gases and vapors:
Steam (non oil – bearing) 0.005
Compressed air 0.001
Ammonia vapor 0.001
Chlorine vapor 0.002
Coal flue gas 0.010
Liquids:
Refrigerant liquids 0.001
Ammonia liquid (oil – bearing) 0.003
Co2 liquid 0.001
Chlorine liquid 0.002
Fouling resistances for chemical processing streams
Gases and vapors:
Acid gases 0.002
Solvent vapors 0.001
Liquids:
MEA and DEA solutions 0.002
Caustics solutions 0.002
Vegetable oils 0.003
Fouling resistance for natural gasoline processing stream
Gases and vapors:
Natural gas 0.001
Overhead products 0.002
Liquids:
Rich oil 0.002
Natural gasoline 0.001
Crude and vacuum liquids:
Gasoline 0.002
Kerosene 0.003
Light gas oil 0.003
Heavy gas oil 0.005
36

38. 6.2 GENERAL CONSIDERATIONS (2)
Fig.22 shows data on some fluids showing the effects of velocity and
temperature. Also see fig.23.
The fouling factors are applied as a part of the overall HTC to both the
inside and the outside of the heat transfer surface using the factors that apply
to the appropriate material or fluid. As a rule the fouling factors are applied
without correcting for the inside diameter to outside diameter, because these
differences are not known, to any degree of accuracy. To fouling resistance
of significant magnitude, a correction is made to convert all values to the
outside surface of the tube. Sometimes only one factor is selected to
represent both sides of the transfer fouling film or scales.
In the tables the representative or typical fouling resistances are
referenced to the surface of the exchanger on which the fouling occurs - that
is, the inside or the outside tubes. Unless the specific plant/equipment data
represents fouling in question, the estimates listed in table are the reasonable
starting point. It is not wise to keep changing the estimated fouling to
achieve the specific overall HTC, U. Fouling can be generally kept to
minimum provided the proper and general cleaning of the surface takes
place.
Unless a fabricator is guaranteeing the performance of the exchanger
in a specific process service they cannot and most likely will not accept the
responsibility for the fouling effects on the heat transfer surface. Therefore,
the owner must expect to specify a value to use in the thermal design of the
equipment. This value must be determined with considerable examinations
of the fouling range, both inside and the outside of the tubes and by
determining the effects of these have on the surface area requirements. Just a
large unit may not be the proper answer.
37

39. Fig.. 22 Fouling factors as a function of time & temperature
0.03
.
.P
M
to
Oil
black
F
2°
g
Fouling Resistance - ro or ri
-3
atin
°C
Lamp
ax
0.02 - 86
ric
t
W
hal
Asp
Lub
in
d
ra f
Roa
Pa
0.01
Scale
- Boiler
CaSO4
Co ke
Cracking Coil
0.02 0.04 0.06 0.08 0.10
Thickness of layer - Inches
Fig. 23 Fouling resistance offered by various substances
38

40. 6.3 OVERALL HEAT TRANSFER COEFFICIENT ‘U’ (2)
In a heat exchanger the process of heat transfer from hot fluid to cold
fluid involves various conductive and convective process. This can be
individually represented in terms of thermal resistances. The summation of
individual resistances is the total thermal resistance and its inverse is the
overall HTC, U. That is,
1 = 1 + Ao 1 + Rfo + Ao Rfi + Rw
U ho Ai hi Ai
Where,
U = overall heat transfer coefficient based on outside area of tube wall
A = area of tube wall
h = convective heat transfer coefficient
Rf = thermal resistance due to fouling
Rw = thermal resistance due to wall conduction
and suffixes ‘i’ and ‘o’ refer to the inner and outer tubes, respectively.
It is customary in design work for the heat transfer coefficient ho and
hi to be determined from complicated relations involving the Nusselt,
Prandtl, Reynolds and Grashof numbers. Similarly, the thermal resistance is
determined from calculations involving properties and dimensions of the
material of the tube walls. Such detailed process is not involved in
determining the fouling resistance, the so called fouling factors Rf and Rfo.
The uncertainty is such that one simply includes arbitrary values of the
fouling factor selected from the sources based on the experience. The less
experience on has, the less confidence one will have in the eventual result.
39

41. 6.4 FOULING AS A FUNCTION OF TIME (1)
The assumption of constant
A
values for the internal and the external B
D
fouling factors implies that, when put E
in service, the new heat exchanger C
instantaneously deteriorates to the
fouled condition. Of course it does not
do this, but instead deteriorates
Time
progressively. Considerable time, Fig. 24
years, perhaps may elapse before it arrives at the condition where it can no
longer perform adequately and must be cleaned.
The build up of fouling resistance as a function of time may follow
various forms as indicated in fig.24. Curve A describes a process starting
with clean surfaces having zero fouling resistance, which then develops at
constant rate with time. Curve B describes a process where the fouling
resistance develops at a progressively diminishing rate. The family of curves
C, D and E all share a lengthy incubation or induction period in which there
is little or no build up of fouling resistance, followed by a rapidly increasing
build up.
There is therefore a substantial time lapse before the heat exchanger
fouling resistance approaches the design value arbitrarily selected from some
experience based source. When first put into service, the heat exchanger will
operate with a reduced thermal resistance and therefore with surplus of heat
transfer area. In many cases involving boiling, the fouling resistance is the
principal resistance. Thus, when the heat exchanger is new, the available
temperature difference may be so great as to carry the process into the film
40

42. boiling region, with the possibility of enhanced surface corrosion and
consequent accelerated development of fouling resistance.
In other cases the new heat exchanger with zero fouling resistance
may be so effective as to overcool the process stream. To compensate the
cooling water flow may be reduced, with the result that the water velocity is
decreased and the water temperature increased. Both these factors are highly
conducive to fouling on the water – side. The provision of excess allowance
for fouling or an excess heat transfer area “just to be on the safe side” does
not automatically increase the interval before cleaning is necessary; quite
likely it has the reverse effect. The excess area has the reduced flow
velocities and elevated temperatures, so the exchanger deteriorates in
performance at drastic rates.
6.5 MECHANISMS OF FOULING (1)
Various mechanisms of fouling have been recognized and can be
categorized as follows:
1. Precipitation or scaling fouling : Precipitation on hot surfaces or due
to inverse solubility.
2. Particulate or scaling fouling : Suspended particles settle on heat
transfer surface.
3. Chemical reaction fouling : Deposits formed by chemical reaction in
the fluid systems.
4. Corrosion fouling : corrosion products produced by a reaction
between fluid and the heat transfer surface and tube surface becomes
fouled.
5. Solidification fouling : Liquid and/or components in liquid solution
solidify on tube surface.
41

43. 6. Biological fouling : Biological organisms attach to heat transfer
surface and build a surface to prevent good fluid contact with the tube
surface.
Fouling occurs to some extent in all systems where liquids, gases and
vapors are being heated or cooled. The process may involve boiling,
condensing or heat transfer without phase change. The greatest source of
fouling, principally inverse solubility crystallization and chemical reactions
occurs on hot surfaces in heating process without phase change. Cooling
processes without phase change also results in appreciable fouling as a result
of particulate deposition, sedimentation and chemical reaction.
6.6 EFFECTS OF SURFACE MATERIAL AND STRUCTURE (1)
By the time the fouling deposit has
covered most of the surface, the material
and the finish of the wall has become
irrelevant; the primary effect is during
the incubation or the induction period.
Different materials have different
catalytic actions with various fluids and
Time
may promote or inhibit the reactive Fig. 25
process responsible for initial fouling. The figure shows typical fouling
resistance development histories during the induction period for carbon-
steel, stainless – steel and brass surfaces exposed to brackish water streams
under constant flow conditions.
Polished surfaces resist the growth of fouling deposits but are highly
susceptible to corrosive action that roughens the surface and increase the
42

44. potential crystallization sites. Improperly cleaned heat exchangers with
residual fouling deposits on the surface will degrade by fouling more readily
than those restored to the “as new” clean condition.
6.7 EFFECT OF FLUID VELOCITY
There is much evidence
suggesting fluid velocity as the most
important parameter affecting fouling.
In most cases, an increase in velocity
decreases both the rate of fouling
deposit formation and the ultimate level
Time
attained, as shown by the typical Fig. 26
development histories given by fig. Improvement tends to be at
progressively diminishing rate. Doubling the fluid velocity from a low value
may halve the fouling resistance. Doubling it again may halve the remaining
resistance. However, the second doubling requires an increase to four times
the original velocity and gains only a reduction of one quarter the original
thermal resistance.
In addition to decreasing the fouling, the higher velocity increases the
heat-transfer coefficients so that a double – barraled reduction in the size and
cost of the heat exchanger might be anticipated. With reduced fouling there
will also be a decrease in the maintenance requirements and cost. However it
must be recalled that the pressure drop is a function of the square of the fluid
velocity. Doubling the fluid velocity increases the pressure drop by four
times, increasing both the capital cost and operating cost of the pumping.
43

45. 6.8 EFFECT OF TEMPERATURE
Temperature has a pronounced
effect on fouling that can be
generalized as shown in fig. The rate
of development of fouling resistance
and the ultimate stable level both
increase as the temperature increases.
Time
Temperature refers to either or both of Fig. 27
the surface temperature and the fluid bulk temperature. The rates of
chemical and inverse crystallization including catalytic effects, are strongly
dependent on temperature, which explains the increase in fouling rate. The
rate of removal of fouling deposits is less a function of temperature than
fluid velocity. Therefore an increase in the rate of deposition with no
increase in removal will result in a higher ultimate stable level.
6.9 EFFECT OF BAFFLE & TUBE PATTERN (1)
The relative propensity to fouling and the ease with which cleaning
can be accomplished are important factors in selecting the type of exchanger
for a given application. On the shell side, baffle designs and tube
arrangements are influenced by fouling and cleaning considerations.
Because high velocity is important to minimize fouling, it is clear that the
baffle arrangement shown in fig.28(a). would lead to many stagnant areas in
the shell - side flow, with consequent high fouling. The baffle arrangement
shown in fig.28(b) has fewer stagnant areas and a longer mean flow path. If
the shell side mass flow were the same in both exchangers, the velocity in
fig. (b) would be much greater than that in fig.28(a). Of course the pressure
drop and cost of pumping increases as the square of the fluid velocity.
44

46. Tubes are generally arranged in the triangular in the triangular or
square pattern shown in fig. Triangular arrangements allow for inclusion of
the greatest number of tubes in a given shell diameter and for the strongest
tube-sheet ligaments. However they are much difficult to clean with
mechanical scrapers and brushes than square tube arrangements. Exchangers
likely to require periodic cleaning on the shell side should therefore have
square tube arrangements. Of course their may be other compelling reasons
to override this general rule, so as to increase the tube count or take
advantage of the stronger tube - sheet ligaments of triangular arrangements.
(a)
(b)
Fig. 28 Baffle designs affecting fluid velocity at the creation of stagnant areas
Square Triangular
Fig. 29 Triangular and square pitch pattern
45

47. 6.10 PRACTICAL FOULING FACTORS (2)
It is customary for the purchaser to specify the fouling resistance used
in the thermal design of the exchanger. The exposition will do little to
increase user's confidence in the value of the fouling resistance marked on
the exchanger specifications sheets; however they should have a clearer
understanding of the uncertainties prevailing in the specifications. Many
users have their own private collection of fouling factors, based on past
experience with similar equipment under equivalent conditions. These are
the most reliable data. However, the indiscriminate application of these
factors to equipment larger in size and the operating under more arduous
conditions is of questionable validity. The uncertainty increases the more
one departs from past experience.
46

48. 7
ENERGY CONSERVATION TECHNIQUES
IN HEAT EXCHANGER
7.1 INTRODUCTION
Fouling factor plays a major role in overall HTC of heat exchanger. It
decides the area required for heat transfer. The higher the value of ‘U’, lesser
will be the area required for heat transfer. This area required is directly
proportional to the energy required for pumping of the fluid and pressure
drop.
A = Q / (U . ∆Tm )
Where,
Q = Total heat transfer
U = overall heat transfer coefficient (HTC)
∆Tm = Log mean temperature difference
A = Area of heat transfer
7.2 MODE OF OPERATION (4)
It is always feasible with counter current heat exchangers to have a
heat donating fluid entering the heat exchanger, at say, 150oC and leaving
the exchanger at 80oC, while the heat receiving fluid is heated up from 40oC
to 120oC or more. This is impossible to achieve with co – current operation.
Since in counter current mode of operation the hottest inflow faces the
warmest out flow, the vale of ∆T i.e. (th – tc) throughout the heat exchanger
is constant. By and large the efficiency of such heat exchanger is directly
proportional to their length and the surface area of calendria.
47

49. Co – current operation is used,
1. When it is necessary to transfer as much as heat possible from heat
donating fluid to the heat receiving fluid.
2. When the difference in the temperature between the fluid is less.
3. When the temperature of the heat donating fluid leaving the heat
exchanger is lower than the temperature of the heat receiving fluid
leaving the heat exchanger.
7.3 FLUID FLOW CHARACTERISTICS (4)
In a stream line flow, liquid molecules flow along in a parallel fashion
& in consequence, heat transfer from the center of the fluid to the walls of
heat exchanger tubes proceed by conduction only. As table below shows,
thermal conductivness of fluids are remarkably poor compared with those of
metals.
Thermal Conductivity of Metals and Fluids (4)
Thermal Thermal
Material Conductivity Material Conductivity
W/moK at 20oC W/moK at 20oC
Aluminum 237 Water 1.967
Copper 166 Toluene 0.44
Iron 147 Petrol 0.47
Magnesium 159 Oil 0.75
Silver 427 Glycerol 0.97
Zinc 115 Air 0.025
It is therefore necessary to ensure that the fluids in heat exchangers
move turbulently i.e. in such a fashion that constant mixing occurs.
48

50. When turbulent motion occurs, one can accept that the entire body of
the fluid has the same temperature because of the turbulence. The only
conduction heat transfer needed is across the boundary layer. Turbulence can
be inducted in a fluid if the Reynolds number exceeds about 2000.
NRe = Dvρ
µ
Where,
D = Diameter of pipe containing fluid (m)
v = velocity of fluid (m/s)
ρ = Density of the fluid (Kg/m3)
µ = Viscosity of fluid (Kg/m.s)
7.4 PRESSURE DROP AND PUMPING POWER (7)
Apart from heat transfer requirements an important consideration in
heat exchange design, is the pressure drop or pumping cost. The size of the
heat exchanger can be reduced, by forcing the fluids through it at higher
velocities thereby increasing the overall heat transfer coefficient. But higher
velocities will result in larger pressure drops and corresponding larger
pumping costs. The selection of optimum pipe size also has a bearing on the
pumping cost. For a given flow rate, the smaller diameter pipe may involve
less initial (capital) cost but definitely higher pumping cost for the life of
heat exchanger.
It is known that the pressure drop of an incompressible fluids flowing
through pipes and fittings is
∆p ∝ m2
Where m is the mass flow rate.
49

51. The power requirement to pump fluid in steady state is given by,
Power = v dp = (m/ρ) ∆p ~ m3
So the power requirement is proportional to the cube of the mass flow
rate of the fluid and it may be further increased by dividing it by pump (fan
or compressor) efficiency. Since the pumping cost increases tremendously
with the higher velocities, a compromise between the larger overall HTC
and corresponding velocities will have to be made.
A – overall HTC
B – Pumping Power
Above graph (fig.30) explains C – Pressure drop
D – Fouling factor
that at higher fluid velocity fouling D
C
B,
will be reduced but will require
A,
Annual Cost
higher pumping power and higher
pressure drops with increased
overall HTC. At lower fluid
Optimisation
velocities, pumping power will
reduce and reduce pressure drop, but Fluid Velocity
Fig. 30 Optimization for fluid velocity
with less overall HTC and higher
fouling factor.
Hence optimization is done where a velocity of fluid is decided which
will give economical pressure drops and heat transfer, since higher annual
cost is directly related to higher energy requirements. Hence optimization
helps in cutting the annual cost and conserving energy.
50

52. 7.5 RUBBER BALL CLEANING (5)
Fig. 31 (a)
The basic principle of cleaning with sponge rubber balls is to
frequently wipe clean the inside of the tube while the unit is in operation.
Since the balls are slightly larger in diameter than the tube, they are
compressed as they travel the length. This constant rubbing action keeps the
walls clean and virtually free from deposits. Thus suspended solids are kept
moving and not allowed to settle, while bacterial fouling is wiped quickly
away. Pits do not form as deposits are prevented. The balls are selected in
accordance with the installation, their specific gravity being nearly equal to
that of cooling media. Therefore, they distribute themselves in a
homogeneous fashion. They travel the length of the tube forced by the
pressure differential between the inlet and the outlet. The ball's surface
allows a certain amount of water to follow through the area of contact with
the wall, flushing away accumulated deposits ahead of the ball. They are
available in various degrees of resiliency, depending on requirement.
51

53. An abrasive coated ball is also
available for situations where the
cooling water tubes have already been
heavily fouled. Here the effect is
gentle souring that removes the scale
slowly but steadily, until the tube is
ready to be maintained by the normal
sponge-rubber ball. Heat - transfer
efficiency climbs steadily throughout
this treatment. Fig. 31 (b)
The balls are circulated in closed loop, including the heat exchanger
as shown in fig. At the discharge end they are caught in a screen installed
directly in the line. They are then rerouted through the collector back to the
condenser ball - injection nozzles to ensure that the balls are uniformly
distributed.
At the collector unit, the balls can be counted or checked for size. The
number required for a particular service is a function of the number of
cooling tubes. Naturally, some wear occurs so that the balls must be
eventually replaced.
These cleaning systems can be retrofitted into most existing heat
exchangers, although some modifications of piping or unit design may be
required. The slight increase in pumping resistance due to pressure drop
across the screening device is more than offsets by the reduction in fouling
resistance in the heat exchanger tubes. The most effective way to take
advantage of these systems is for its installation at the design stage. A filter
prevents solid debris from entering the water box of the heat exchanger.
52

54. Located in the cooling water inlet, it is flushed as need without shutting
down or bypassing the filter.
Examples of continuous tube cleaning
Fig. 32 Before and after use of rubber ball cleaning
A typical case is shown in the "Before and "After" graphs (Fig.32).
An instance involved stainless steel tubing, where the rubber system
maintained a cleanliness factor and a backpressure of 1.49 in. Hg. After
1,800 hr of operations, the tube cleaning system was taken out of service for
testing purposes. During a month of operations without cleaning, the heat
exchanger back – pressure climbed to 1.65 in. Hg and the cleanliness factor
dropped from 98 to 81%. When the cleaning was restarted, the original
backpressure and the cleanliness was recovered in 10 days.
After extensive testing, it was proved that the continuous system was
highly economical and produced superior performance over manual
cleaning. Continuous cleaning gives 17% better performance than manual
cleaning. Continuous cleaning and filtering systems maintain a high level of
heat exchanger efficiency. The ball cleaning scheme results in fuel saving,
fewer outages and reduction or elimination of cleaning chemicals.
53

55. 7.6 PLATE OVER TUBULAR HEAT EXCHANGER (5)
7.6.1 Introduction
The continuous search for
greater economy and efficiency
has led to the development of
many different types of heat
Fig. 33
exchanger, other than the popular
shell and tube. Some of these have been highly successful in particular fields
of application.
Briefly, a plate heat exchanger consists of number of corrugated metal
sheets provided with gaskets and corner portals (to achieve the desired flow
arrangement, each fluid passes through alternate channels). Plates are spaced
close together, with nominal gaps ranging from 2 to 5 mm. The plates are
corrugated so that the very high degree of turbulence is achieved. One of the
most widely used plates, are of the following relationship:
NNu = (0.374) NRe0.668 NPr0.333 ( µ / µw)0.15
7.6.2 Pumping cost
In the fig. it can be seen that for a given energy loss (HP / unit area),
the plate heat exchanger produces higher film coefficient than does a tubular
unit (considering the flow inside the tube).
When accessing various heat exchanger types, the question of
pumping should be considered, since these will probably represent by far the
greatest of the operating costs. Plate heat exchangers are by far the best in
this respect.
54

56. Fig. 34 Advantages of PHE over Fig. 35 Performance of plate heat
tubular heat exchanger exchanger
7.6.3 Fouling factors in plate heat exchangers
Fouling factors required in plate heat exchangers are small compared
to those commonly used in shell and tube designs for six reasons:
1. High degree of turbulence, maintain solids in suspension.
2. Heat transfer surfaces are smooth. For some types, a mirror finish may
be available.
3. No dead spaces where fluid can stagnate, as in case of shell and tube.
4. Since the plate is necessarily of a material not subject to massive
corrosion (being relatively thin), deposits of corrosion products to
which fouling can adhere are absent.
5. High film coefficients tend to lead to lower surface temperature for
the cold fluid (the cold fluid is the culprit as far as fouling is
concerned).
6. Extreme simplicity of cleaning. The small hold up volume and very
large turbulence in plate heat exchanger (plus the absence of dead
spaces) mean that the chemical cleaning methods are rapid and
effective.
55

57. 7.7 ADVANCES IN HEAT EXCHANGER TECHNOLOGY
7.7.1 Spiral tube heat exchanger (9)
Fig. 36 Heliflow Heat Exchanger
The Graham Heliflow is a unique type of shell and tube heat
exchanger. The tubes in the Heliflow are arranged in parallel, starting with
an inlet manifold on one end, and terminating at an outlet manifold on the
opposite end. The tube bundle is wound into a helical pattern. This coiled
construction creates a spiral flow path for the fluid inside the coil.
Each tube is in close contact with the tube above and below it. The
coiled tube bundle is fit into a two – piece casing. When the casing is
tightened, it is designed to slightly compress the tubes. Because of the tight
fit, the shell side fluid is forced to circulate in a spiral pattern, which is
created by the open spaces between the coils.
The unique arrangement of the Heliflow Heat Exchanger creates
spiral flow paths for both tubeside and shellside fluids, providing 100% true
countercurrent-flow design. The spiral pattern also promotes turbulence,
leading to increased heat transfer rates. In addition, there are no baffles or
dead spaces that lead to inefficiencies commonly found in other types of
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58. shell and tube exchangers. The net result is a Heliflow Heat Exchanger that
is up to 40% more efficient than a standard shell and tube.
Originally built for use in boiler sample cooling over 60 years ago,
there are thousands of Graham Heliflow heat exchangers being used today in
hundreds of services. Many units have been in operation for well over 40
years. The service life of a Heliflow varies with the application, but its many
features add to its reliability when compared to a shell and tube exchanger.
No gaskets are required for the tube side of the Heliflow. Aggressive
fluids are often placed tube side for this reason. No gaskets on the tube side
will minimize the chance of leakage. The spring-like coil of the Heliflow
reduces stresses caused by thermal expansion of the tube material.
Heliflow can do the job for you in a fraction of the space required by
typical straight shell and tube exchangers. With higher heat transfer
efficiencies, the surface area required is normally less than a straight shell
and tube. Smaller surface requirements, and the coiled tube design result in a
very compact unit. Access space required for maintenance or inspection is
very small compared to straight shell and tube exchangers. The only space
required for a Heliflow is to remove the casing, which allows inspection of
both the entire tube bundle and shellside of the exchanger. You can mount a
Heliflow on columns, nozzles, walls, ceilings, or in-line; certain sizes
require no support.
A Heliflow is easy to maintain. The casing of the unit can be removed
without disturbing any of the piping connections. Once the casing is
removed, the entire tube bundle is exposed for inspection. With the casing
removed, the shellside of the unit can easily be cleaned in place.
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59. 7.7.2 Fluidized bed heat exchanger (10)
Fig. 37(A) Self cleaning heat exchanger with Fig. 37(B) Self cleaning heat exchanger with
Cyclone widened outlet channel
Self-cleaning heat exchange technology applying a fluidized bed of
particles through the tubes of a vertical shell and tube exchanger was
developed in the early 1970s for sea – water desalination service. Since that
time, several generations of technological advancements have made the
modern self-cleaning heat exchanger the best solution for most severely
fouling liquids.
In the 90s, a chemical plant in the United States compared for their
severely fouling application a conventional solution versus the installation of
self – cleaning heat exchangers. The result of this comparison is also shown
in table 1.
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60. Table: Comparison of self cleaning heat exchanger v/s conventional heat exchanger (10)
SELF – CLEANING CONVENTIONAL HEAT
HEAT EXCHANGER EXCHANGER
Heat transfer surface 4,600 m2 24,000 m2
Pumping power 840 kW 2,100 kW
Number of cleanings per year 0 12
As could be expected, but also convinced by a test, plant management
decided in favor of the self-cleaning configuration. During operation, the
expectations for the self-cleaning heat exchangers were fully met and even
better than that: After 26 months of continuous operation, the self-cleaning
heat exchangers still have not been cleaned.
This striking example of the self-cleaning heat exchange technology
and a large number of improvements and new developments have
substantially increased the potential applications, which can benefit from
this unique self-cleaning heat exchange technology. These improvements
and developments leading to new and very interesting applications will be
discussed in the next paragraphs.
Principles of Operation
The principle of operation with respect to the original configuration of
the self-cleaning heat exchanger employing an external down comer is
shown in figure 1. The fouling liquid is fed upward through a vertical shell
and tube exchanger that has specially designed inlet and outlet channels.
Solid particles are also fed at the inlet where an internal flow distribution
system provides a uniform distribution of the liquid and suspended particles
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61. throughout the internal surface of the bundle. The particles are carried
through the tubes by the upward flow of liquid where they impart a mild
scraping effect on the wall of the heat exchange tubes, thereby removing any
deposit at an early stage of formation. These particles can be cut metal wire,
glass or ceramic balls with diameters varying from 1 to 4 mm. At the top,
within the separator, connected to the outlet channel, the particles disengage
from the liquid and are returned to the inlet channel through a downcomer
and the cycle is repeated. Figure 2 shows an improved configuration. Now,
the particles disengage from the liquid in a widened outlet channel and, then,
are again returned to the inlet channel through an external downcomer and
are recirculated continuously. For both configurations, the process liquid fed
to the exchanger is divided into a main flow and a control flow that sweeps
the cleaning particles into the exchanger. By varying the control flow, it is
now possible to control the amount of particles in the tubes. This provides a
control of aggressiveness of the cleaning mechanism. It allows the particle
circulation to be either continuous or intermittent.
7.7.3 Helixchanger heat exchanger (11)
Heat exchanger fouling has been very costly for the industry both in
terms of capital costs of heat exchanger banks as well as operation and
maintenance costs associated with it. The HELIXCHANGER heat
exchanger, when applied in typically fouling services, has proven to be very
effective in reducing the fouling rates significantly. Three to four times
longer run-lengths are achieved between bundle cleaning operations. Proper
attention is required in designing the heat exchangers placed at the hot end
of crude oil pre-heat operations where temperatures and velocity thresholds
are highly dependent on heat exchanger geometry. The helical baffle design
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62. offers great flexibility in selecting the optimum helix angles to maintain the
desired flow velocities and temperature profiles to keep the conditions below
the “fouling threshold”.
In a Helixchanger heat exchanger, the quadrant shaped baffle plates
are arranged at an angle to the tube axis in a sequential pattern, creating a
helical flow path through the tube bundle. Baffle plates act as guide vanes
rather than forming a flow channel as in conventionally baffled heat
exchangers. Uniformly higher flow velocities achieved in a Helixchanger
heat exchanger offer enhanced convective heat transfer coefficients. Helical
baffles address the thermodynamics of shell – side flow by reducing the flow
dispersion primarily responsible for reducing heat exchanger effectiveness.
Least dispersion (high Peclet numbers) achieved with the helical baffle
arrangements approach that of a plug flow condition resulting in high
thermal effectiveness of the heat exchanger.
In a Helixchanger heat exchanger, the conventional segmental baffle
plates are replaced by quadrant shaped baffles positioned at an angle to the
tube axis creating a uniform velocity helical flow through the tube bundle.
Near plug flow conditions are achieved in a Helixchanger heat exchanger
with little back-flow and eddies. Exchanger run lengths are increased by two
to three times those achieved using the conventionally baffled shell and tube
heat exchangers. Heat exchanger performance is maintained at a higher level
for longer periods of time with consequent savings in total life cycle costs
(TLCC) of owning and operating Helixchanger heat exchanger banks.
Feedback on operating units, are presented to illustrate the improved
performance and economics achieved by employing the Helixchanger heat
exchangers.
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63. Helixchanger heat exchangers have demonstrated significant
improvements in the fouling behavior of heat exchangers in operation. In a
Helixchanger heat exchanger, the quadrant shaped shellside baffle plates are
arranged at an angle to the tube axis creating a helical flow pattern on the
shellside. Uniform velocities and near plug flow conditions achieved in a
Helixchanger heat exchanger, provide low fouling characteristics, ordering
longer heat exchanger run-lengths between scheduled cleaning of tube
bundles.
Fig. 38
Fig. 39 HTC using helical baffles of various angles
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64. Fig. 40 Performance of segmental bundles
Fig. 41 Performance of Helix bundles
Although it may be observed from the graphs that the HELIX bundles
show marginal improvement in the drop in overall heat transfer coefficient
with time in the initial stages, it has since achieved and sustained an
asymptotic level of performance much higher than the performance level
achieved in the earlier segmental bundles. The HELIX bundles are
reportedly expected to achieve more than three years of continuous
operation, thus increasing the run-length by three times.
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65. Earlier segmental bundles required two to three times cleaning in this
time period. The HELIX bundles have achieved significantly enhanced heat
transfer performance and have sustained this performance for a long period
of time. Three to four times longer run-length has already been achieved
with these bundles.
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66. 8
CONCLUSION
In this seminar various heat exchanger types, along with their
applications have been given. Various types of trouble – shooting and non –
ideal behavior of heat exchanger, along with its causes and prevention have
been discussed in this seminar.
It is generally seen that even though shell and tube heat exchanger
gives less heat transfer for a particular pressure drop than in plate or spiral
tube heat exchanger, but still is widely used in Chemical Process Industries,
due to its rugged construction and various design and trouble - shooting data
available to the designers, which is not the case for other type of heat
exchangers, even if they are having better efficiency.
From energy aspect, proper cleaning of heat exchangers and regular
maintenance to reduce fouling and if possible to avoid corrosion, is needed.
Lesser the fouling, which is the main cause for lower heat transfer in the heat
exchanger, lesser will be the wastage of energy, and higher will be the
efficiency of heat exchanger.
Upcoming technologies like the fluidized bed heat exchanger, spiral
tube heat exchanger and helical shaped baffles, although not heavily used in
industry but in near future, where energy resources will become scares and
need of highly efficient heat exchangers will be the need of hour, more
advanced, complex and compact heat exchangers like mentioned above will
be in demand, which helps in reducing the fouling or in some cases
eliminates fouling.
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67. 9
BIBLIOGRAPHY
1. G. Walker – Industrial Heat Exchanger
McGraw Hill, 2002, Pg. no. 45 – 75, 213 – 271
2. Ernest E. Ludwig – Applied Process Design
Gulf Professional Publication, 3rd Ed, Pg. no. 79 – 90
3. W. C. Turner – Energy Management Handbook
Printice Hall, 2003, Pg. no. 207 – 215
4. G. D. Rai – Non Conventional Energy Sources
Khanna Publishers, 4th Ed, Pg. no. 851 – 858
5. Richard Greene – Process Energy Conservation
McGraw Hill, Pg. no. 156 – 162, 281 – 284
6. Coulson and Richardson’s – Chemical Engineering
Butterworth Heinman, Vol. 1, 6th Ed, Pg. no. 414 – 435, 503 – 553
7. R. C. Sachdeva – Fundamentals of Engineering Heat & Mass Transfer
New Age International Publication, 4th reprint 1996, Pg no. 520 – 523
8. “Heliflow Heat Exchangers” – Chemical Processing (Journal)
Putman Media, January – 2004
9. Heliflow Heat Exchangers – Introduction & applications
http://www.graham-mfg.com/heat
10. Dick G. Klaren – “Improvements and New Developments in Self-
Cleaning Heat Transfer Leading to New Applications”
http://services.bepress.com/eci/heatexchanger/39
11. Bashir I. Master, Krishnan S. Chunangad – “Fouling Mitigation using
Helixchanger Heat Exchanger”
http://services.bepress.com/eci/heatexchanger/43
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