1

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

2

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.

3

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

Plate Heat Exchanger Tubular Heat Exchanger

Recuperative Heat Exchanger

Spiral

Plate - Coil

Plate - Fin

Plate - Frame

Single - Pipe

Cluster Pipe

Double pipe

Fin Tube

Shell & 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.

4

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.

5

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

∆ Τ

Thin

Tcin

Thout

Tcout

Seperating Surface

Thout

Tcout

Thin

Tcin

∆ Τ

Tcout

Thin

Thout

Tcin

Surface Area A

Surface Area A

Cold Fluid

Hot Fluid

Cold Fluid

Cold Fluid

Hot Fluid

Cold Fluid

Thin

Thout

TcinTcout

Co - Current Flow

Counter - Current Flow

Cross Flow

6

2.4 TUBULAR HEAT EXCHANGER CLASSIFICATION (1)

2.4.1 Clustered pipe heat exchanger

It is the development of single

pipe heat exchanger. Two or more

tubes are joined by thermally

conducting medium. So that the heat is

transferred between the fluids flowing

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.

Cold Fluid

Hot

Fluid

Fig. 4

Solder

H-P Stream

L-P Stream

Fig. 3

7

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.

Cold Fluid Hot Fluid

Heated Fluid Cooled Fluid

Fig. 5

8

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.

9

2.6 SPECIAL PURPOSE HEAT EXCHANGER (6)

2.6.1 Scraped surface heat exchanger

Spring Clip

"J" Spring

Scraper Blade

Inner pipe

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.

10

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.

11

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

Coil

Baffle

Agitator

Vessel

Jacket

Fig. 7

12

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.

13

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

Hot

Exhaust Gas

Cool Furnace

Air

Heated Furnace

Air

Heig

ht

of

Chim

ney

Th

Tc Th

TcTemperature

Exhau

st G

as Coolin

g G

as

Fig. 8

14

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.

Finned tube

Economiser

Boiler

Exhaust

Water

Tube Boler

500°F

300°F

Flue

Exhaust 220°F

Feed water

from deaerator

Fig. 9

15

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.

16

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

CORROSION

When two dissimilar metals are

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

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

Zinc

Electrolyte

Copper

Fig. 10

17

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

Pipe Valve

Nut

Bolt

Insulating

Washer

Insulating Sleeve

Fig. 11

18

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

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

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

Crevice

Crevice

Fig. 12

19

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

very closely. Fig. 13

20

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:

21

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

term used to describe corrosion

that is accelerated as a result of

increase in the relative motion

between the corrosive fluid and

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.

Water Flow

Corrosion Film Original metal

surface

Corrosion

pits

Fig. 14

22

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

23

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.

24

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.

25

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.

26

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.

27

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.

28

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.

A

B

C

B

A

A

D

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.)

29

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,

30

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

Cold Flow

Hot Flow

Fig. 18

Fig. 17

31

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

32

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.

Stagnant areas

(a)

(b)

fig. 19 (a) correct (b) incorrect placement of the tube bundle in shell and tube heat exchanger

33

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

34

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)

35

UF.F.

0.286

0.25

0.182

0.125

0.0825

0.04

0.01

0.02

After 16 Months

After 6 Months

Clean

3.5

4.5

5.5

8

12

25

500 100 50 30 20 17 15 Gas inside tubes

Gas outside 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.

36

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

37

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.

38

Fig.. 22 Fouling factors as a function of time & temperature

Lam

p b

lack

Lub

rica

ting

Oil

Para

fin

Wax

- 32

°F to

M.P

.

Road Asphalt - 86°C

CaSO4 - Boiler Scale

Cracking Coil Coke

Thickness of layer - Inches

Fou

ling R

esis

tance

- r

o o

r ri

0.02 0.04 0.06 0.08 0.10

0.01

0.02

0.03

Fig. 23 Fouling resistance offered by various substances

39

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,

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.

1 = 1 + Ao 1 + Rfo + Ao Rfi + Rw

U ho Ai hi Ai

40

6.4 FOULING AS A FUNCTION OF TIME (1)

The assumption of constant

values for the internal and the external

fouling factors implies that, when put

in service, the new heat exchanger

instantaneously deteriorates to the

fouled condition. Of course it does not

do this, but instead deteriorates

progressively. Considerable time,

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

A

B

C

D

E

Time

Fig. 24

41

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.

42

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

may promote or inhibit the reactive

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

Time

Fig. 25

43

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

attained, as shown by the typical

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.

Time

Fig. 26

44

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.

Temperature refers to either or both of

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.

Time

Fig. 27

45

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.

Fig. 28 Baffle designs affecting fluid velocity at the creation of stagnant areas

Square Triangular

Fig. 29 Triangular and square pitch pattern

(a)

(b)

46

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.

47

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 40

oC

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.

48

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)

Material

Thermal

Conductivity

W/moK at 20

oC

Material

Thermal

Conductivity

W/moK at 20

oC

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.

49

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.

50

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.

Above graph (fig.30) explains

that at higher fluid velocity fouling

will be reduced but will require

higher pumping power and higher

pressure drops with increased

overall HTC. At lower fluid

velocities, pumping power will

reduce and reduce pressure drop, but

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.

D

A, B

, C

An

nu

al C

ost

Fluid Velocity

Optimisation

Fig. 30 Optimization for fluid velocity

A – overall HTC

B – Pumping Power

C – Pressure drop

D – Fouling factor

51

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.

52

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.

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.

Fig. 31 (b)

53

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.

54

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

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.

Fig. 33

55

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.

Fig. 34 Advantages of PHE over

tubular heat exchanger

Fig. 35 Performance of plate heat

exchanger

56

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

57

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.

58

7.7.2 Fluidized bed heat exchanger (10)

Fig. 37(A) Self cleaning heat exchanger with

Cyclone

Fig. 37(B) Self cleaning heat exchanger with

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.

59

Table: Comparison of self cleaning heat exchanger v/s conventional heat exchanger (10)

SELF – CLEANING

HEAT EXCHANGER

CONVENTIONAL HEAT

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

60

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

61

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.

62

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

63

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.

64

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.

65

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.

66

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, 3

rd 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, 4

th 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, 6

th Ed, Pg. no. 414 – 435, 503 – 553

7. R. C. Sachdeva – Fundamentals of Engineering Heat & Mass Transfer New Age International Publication, 4

th 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