Heat Exchanger

PAMANTASAN NG LUNGSOD NG MAYNILA University of the City of Manila

College of Engineering and Technology

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Introduction to HEAT EXCHANGERS

Heat exchangers are devices used to transfer heat energy from one fluid to

another. Typical heat exchangers experienced by us in our daily lives include

condensers and evaporators used in air conditioning units and refrigerators. Boilers and

condensers in thermal power plants are examples of large industrial heat exchangers.

There are heat exchangers in our automobiles in the form of radiators and oil coolers. In

other applications, the objective may be to recover or reject heat, or sterilize,

pasteurize, fractionate, distill, concentrate, crystallize, or control a process fluid. Heat

exchangers are abundant in chemical and process industries.

Although heat flows from hot fluid to cold fluid by thermal conduction through

the separating wall (except in direct-contact types), heat exchangers are basically

heat convection equipment, since it is the convective transfer what governs its

performance. Convection within a heat exchanger is always forced, and may be with

or without phase change of one or both fluids.

There is a wide variety of heat exchangers for diverse kinds of uses, hence the

construction also would differ widely. However, in spite of the variety, most heat

exchangers can be classified into some common types based on some fundamental

design concepts.



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In a few heat exchangers, the fluids exchanging heat are in direct contact. In

most heat exchangers, heat transfer between fluids takes place through a separating

wall or into and out of a wall in a transient manner. In many heat exchangers, the fluids

are separated by a heat transfer surface, and ideally they do not mix or leak. Such

exchangers are referred to as direct transfer type, or simply recuperators. In contrast,

exchangers in which there is intermittent heat exchange between the hot and cold

fluids—via thermal energy storage and release through the exchanger surface or

matrix— are referred to as indirect transfer type, or simply regenerators. Here, heat from

the hot fluid is intermittently stored in a thermal storage medium before it is transferred

to the cold fluid. To accomplish this, the hot fluid is brought into contact with the heat

storage medium, then the fluid is displaced with the cold fluid, which absorbs the heat.

Such exchangers usually have fluid leakage from one fluid stream to the other, due to

pressure differences and matrix rotation/valve switching.

Fouling

Material deposits on the surfaces of the heat exchanger tubes may add more

thermal resistances to heat transfer. Such deposits, which are detrimental to the heat

exchange process, are known as fouling. Fouling can be caused by a variety of reasons

and may significantly affect heat exchanger performance.

Fouling can be caused by the following sources:

Scaling is the most common form of fouling and is associated with inverse

solubility salts. Examples of such salts are CaCO3, CaSO4, Ca3(PO4)2, CaSiO3,

Ca(OH)2, Mg(OH)2, MgSiO3, Na2SO4, LiSO4, and Li2CO3.

Corrosion fouling is caused by chemical reaction of some fluid constituents with

the heat exchanger tube material.

Chemical reaction fouling involves chemical reactions in the process stream

which results in deposition of material on the heat exchanger tubes. This

commonly occurs in food processing industries.

Freezing fouling is occurs when a portion of the hot stream is cooled to near the

freezing point for one of its components. This commonly occurs in refineries

where paraffin frequently solidifies from petroleum products at various stages in

the refining process. , obstructing both flow and heat transfer.



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Biological fouling is common where untreated water from natural resources such

as rivers and lakes is used as a coolant. Biological micro-organisms such as algae

or other microbes can grow inside the heat exchanger and hinder heat transfer.

Particulate fouling results from the presence of microscale sized particles in

solution. When such particles accumulate on a heat exchanger surface they

sometimes fuse and harden. Like scale these deposits are difficult to remove.

Basic Heat Exchanger Flow Arrangements

Counter-Flow

In a counter flow

arrangement, the two streams enter

at opposite ends of the heat

exchanger and flow in parallel but

opposite directions.

This exchanger design is most

efficient when comparing heat

transfer rate per unit area. The

efficiency of a counter flow heat

exchanger is due to the fact that the average temperature (difference temperature)

between the two fluids over the length of the heat exchanger is maximized. Therefore,

the log mean temperature for a counter flow heat

exchanger is larger than that of parallel and cross

flow heat exchanger. And since it has a high log

mean temperature, then it means that it requires

smallest required heat exchanger area. This would

normally be expected to result in smaller, less

expensive equipment for a given application.

Counter flow is more appropriate for

maximum energy recovery. In a number of industrial applications there will be

considerable energy available within a hot waste steam which may be recovered

before the steam is discharged. This is done by recovering energy into a fresh cold

stream.



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Parallel Flow

In parallel flow both the hot and cold

streams enter the heat exchanger at the same

end and travel to the opposite end in parallel

streams. Parallel flow results in rapid initial rates

of heat exchange near the entrance, but heat

transfer rates rapidly decrease as the

temperatures of the two streams approach

one another. This leads to higher exergy loss

during heat exchange. Exergy is the maximum

useful work possible during a process that brings the system into equilibrium.

In heating very viscous fluids, parallel

flow provides for rapid initial heating and

consequent decrease in fluid viscosity and

reduction in pumping requirement. In

applications where moderation of tube wall

temperatures is required, parallel flow results

in cooler walls. This is especially beneficial in

cases where the tubes are sensitive to

fouling effects which are aggravated by

high temperature.

Cross-Flow

Crossflow heat exchangers are intermediate in

efficiency between countercurrent flow and

parallel flow exchangers. In these units, the

streams flow at right angles to each other. Cross

Flow Heat Exchangers are one of the most

common types of heat exchanger used in

countless applications such as engine radiators,

air heaters, refrigeration evaporators and

condensers, super-heaters and economisers.



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PLATE AND FRAME HEAT EXCHANGER (PHE)

A plate heat exchanger (PHE) is a heat exchanger that uses multiple layers of

corrugated metal plates, with two media running in alternate layers, transferring heat

from one media to another. The plates are either smooth or have some form of

corrugation, and they are either flat or wound in an exchanger.

The importance of the plate heat exchanger can be seen through the various

structural advantages it has to offer. This has a more advantage over a conventional

heat exchanger in that the fluids are exposed to a much larger area surface area

because the fluids spread out over the plates. This facilities the transfer of heat, and

greatly increases the speed of the temperature change. The heat transfer coefficients

obtained are significantly higher than the other heat exchangers for comparable fluid

conditions, which lead to a much smaller thermal size. Because of their high heat

transfer coefficients and true-counter flow arrangement, PHEs are able to operate

under very close approach temperature conditions which results in up to 90% heat

recovery.



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Generally, this cannot accommodate very high pressures, temperatures, or

pressure and temperature differences. This type of exchanger is ideal for applications

where the fluids have relatively low viscosity with no particles. Also they are an ideal

choice where there is a close approach between product outlet temperature and

service inlet temperature.

MAJOR APPLICATIONS

Plate heat exchangers were introduced in 1923 for milk pasteurization

applications and now find major applications in liquid–liquid (viscosities up to 10 Pa.s)

heat transfer duties. They are most common in the dairy, juice, beverage, alcoholic

drink, general food processing, and pharmaceutical industries, where their ease of

cleaning and the thermal control required for sterilization/pasteurization make them

ideal.

They are also used in the synthetic rubber industry, paper mills, and in the process

heaters, coolers, and closed-circuit cooling systems of large petrochemical and power

plants. Here heat rejection to seawater or brackish water is common in many

applications, and titanium plates are then used. Plate heat exchangers are not well

suited for lower-density gas-to-gas applications. They are used for condensation or

evaporation of non-low-vapor densities. Lower vapor densities limit evaporation to

lower outlet vapor fractions. Specially designed plates are now available for

condensing as well as evaporation of high-density vapors such as ammonia, propylene,

and other common refrigerants, as well as for combined evaporation/condensation

duties, also at fairly low vapor densities.



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IMPORTANT PARTS/ BASIC CONSTRUCTION OF PLATE AND FRAME HEAT EXCHANGER

The plate heat exchanger consists of

a pack of corrugated metal plates

with portholes/ flowports for the

passage of the two fluids between

with which heat transfer will take

place. The plate pack is assembled

between a fix frame plate and a

movable pressure plate and

compressed by tightening bolts.

The plates are fitted with a gasket which seals the interpolate channel and directs the

fluids into alternate channels. The arrangement of the gaskets (field and ring gaskets)

results in through flow in single channels, so that the primary and secondary media are

Figure 2: Plates with Gaskets Around the Port

Figure 1: Plate and Frame Heat Exchanger



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in counter-current flow. The media cannot be mixed because of the gasket design. The

number of plates is determined by the flow rate, physical properties of the fluids,

pressure drop and temperature program. The plate corrugations promote fluid

turbulence and support the plates against differential pressure. This turbulence, in

association with the ratio of the volume of the media to the size of heat exchanger,

gives an effective heat transfer coefficient.

The plate and the pressure plate are suspended from an upper carrying bar and

located by a lower guiding bar, both of which are fixed to a support column, to ensure

proper alignment. The carrying bars are longer than the compressed stack, so that

when the movable end cover is removed, plates may be slid along the support bars for

inspection and cleaning.

MECHANISM/ DIRECTION OF FLOW

Typically, plate and frame heat

exchangers are used for liquid-liquid

exchange at low to medium pressures.

The heated liquid flows in one

direction in one set of plates and the

cool liquid flows in the opposite

direction in the other set. The heat

radiating from the hot set will be

absorbed by the cool liquid in the

second set. As the energy leaves the

hot media it cools off, and as it enters

the cold media it warms it.

ADVANTAGES AND LIMITATIONS

Some advantages of plate heat exchangers are as follows:

They can easily be taken apart into their individual components for cleaning,

inspection, and maintenance.

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The heat transfer surface area can readily be changed or rearranged for a

different task or for anticipated changing loads, through the flexibility of plate

size, corrugation patterns, and pass arrangements.

High shear rates and shear stresses, secondary flow, high turbulence, and mixing

due to plate corrugation patterns reduce fouling to about 10 to 25% of that of a

shell-and-tube exchanger, and enhance heat transfer.

Because of high heat transfer coefficients, reduced fouling, the absence of

bypass and leakage streams, and pure counterflow arrangements, the surface

area required for a plate exchanger is one-half to one-third that of a shell-and

tube exchanger for a given heat duty, thus reducing the cost, overall volume,

and space requirement for the exchanger.

Leakage from one fluid to the other cannot take place unless a plate develops a

hole. Since the gasket is between the plates, any leakage from the gaskets is to

the outside of the exchanger.

The residence time (time to travel from the inlet to the outlet of the exchanger)

for different fluid particles or flow paths on a given side is approximately the

same. This parity is desirable for uniformity of heat treatment in applications such

as sterilizing, pasteurizing, and cooking.

There are no significant hot or cold spots in the exchanger that could lead to the

deterioration of heat-sensitive fluids. The volume of fluid held up in the exchanger

is small; this feature is important with expensive fluids, for faster transient response,

and for better process control.

Finally, high thermal performance can be achieved in plate exchangers. The

high degree of counterflow in PHEs makes temperature approaches of up to 18C

(28F) possible. The high thermal effectiveness (up to about 93%) facilitates

economical low-grade heat recovery. The flow-induced vibrations, noise,

thermal stresses, and entry impingement problems of shell-and-tube exchangers

do not exist for plate heat exchangers.



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Some inherent limitations of the plate heat exchangers are caused by plates and

gaskets as follows:

The plate exchanger is capable of handling up to a maximum pressure of about

3 MPa gauge (435 psig) but is usually operated below 1.0 MPa gauge (150 psig).

Gasket life is sometimes limited. Frequent gasket replacement may be needed in

some applications.

Pinhole leaks are hard to detect. For equivalent flow velocities, pressure drop in a

plate exchanger is very high compared to that of a shell-and-tube exchanger.

PHEs are not suited for high-vacuum applications. PHEs are not suitable for

erosive duties or for fluids containing fibrous materials. In certain cases,

suspensions can be handled; but to avoid clogging, the largest suspended

particle should be at most one-third the size of the average channel gap.

Viscous fluids can be handled, but extremely viscous fluids lead to flow

maldistribution problems, especially on cooling. Plate exchangers should not be

used for toxic fluids, due to potential gasket leakage.



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DOUBLE PIPE HEAT EXCHANGER

Double pipe heat exchanger is perhaps the simplest of all heat exchanger types.

A double pipe heat exchanger is just one pipe inside another larger pipe. One fluid

flows through the inside pipe and the other flows through the annulus between the two

pipes. The wall of the inner pipe is the heat transfer surface. The pipes are usually

doubled back multiple times in order to make the overall unit more compact.

DIVISION OF DOUBLE-PIPE HEAT EXCHANGERS

Double pipe exchangers are divided into two major types:

1.Single-tube

2. Multi-tube.

The Single-tube type consists of a single tube or pipe, either finned or bare, inside a

shell. Double-pipe sections permit true counter-current or true co-current flow, which

may be of particular advantage when very close temperature approaches or very long

temperature ranges are required. Multiple tube double pipe sections contain from 7 to

64 tubes, bare or longitudinally finned, within the outer pipe shell. Normally, it has only to

bare tubes are used in sections containing more than 19 tubes.



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Double-pipe units are well suited for high pressure applications because of their

relatively small diameters. This allows the use of small flanges and thin wall sections, as

compared to conventional shell and tube equipment.

MAJOR APPLICATIONS

Double pipe heat exchangers are often used in the chemical, food processing and oil

& gas industries. They have a particular advantage when close temperature

approaches are needed or in high pressure applications.

1. Pasteurization

2. Digester heating

3. Heat recovery

4. Pre-heating

5. Effluent cooling.

Double pipe heat exchangers are used when the heat transfer area is small. If we

connect them in series to increase the heat transfer area it will require much space as

well the pressure drop will be higher due to more fittings. Also, we can’t increase no of

passes for either side fluids. In addition, the double pipe heat exchanger can’t be used

for dirty fluids due to choking and cleaning is tougher.

MECHANISM/ DIRECTION OF FLOW



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In designing double pipe heat exchanger, an important factor is the type of flow

pattern in the heat exchanger. A double pipe heat exchanger will typically be either

counterflow or parallel flow. Crossflow just doesn't work for a double pipe heat

exchanger.

A primary advantage of a double pipe heat exchanger is that it can be

operated in a true counterflow pattern, which is the most efficient flow pattern. That is,

it will give the highest overall heat transfer coefficient for the double pipe heat

exchanger design.

Also, double pipe heat exchangers can handle high pressures and temperatures

well. When they are operating in true counterflow, they can operate with a

temperature cross, that is, where the cold side outlet temperature is higher than the hot

side outlet temperature.

For example, in the diagrams, consider Fluid 1 to be the hot fluid and Fluid 2 to be the

cold fluid. Then, in the counterflow diagram, you can see that the cold side outlet

temperature, T2out, can approach the hot side entering temperature, T1in, which is higher

than the hot side outlet temperature, T2out. For the parallel flow shown, T2out can only

approach T1out; it could not be greater.

ADVANTAGES AND LIMITATIONS

Some of the advantages of the double pipe heat exchanger are:

Easy to operate.

Counter currents are obtained easily.

It can withstand high pressure and temperature.



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Modular structure.

Maintenance is easy and repairing also easy

Easily displace from one place to another if required.

It can be adjusted according to the process need.

Occupy less space.

Structure is simple and heat transmission is large.

Provides shorter deliveries than shell and tube due to standardization of design and

construction.

Many suppliers are available worldwide

Some of the limitations are:

Double pipe heat exchanger is expensive for heavy duties.

The use of two single flow areas leads to relatively low flow rates and moderate

temperature differences.

Can’t be used in handling dirty fluids.

It is difficult to readily inspect the shell side of the tubes for scaling or tube damage



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EXTENDED SURFACE HEAT EXCHANGER

An extended surface (also known as a combined conduction-convection

system or a fin) is a solid within which heat transfer by conduction is assumed to be one

dimensional, while heat is also transferred by convection (and/or radiation) from the

surface in a direction transverse to that of conduction.

Some typical fin configurations

Straight fins of (a) uniform and (b) non-uniform cross sections; (c) annular

fin, and (d) pin fin of non-uniform cross section.

Extended surfaces have fins attached to the primary surface on one side of a

two-fluid or a multi-fluid heat exchanger. Fins can be of a variety of geometry—plain,

wavy or interrupted—and can be attached to the inside, outside or to both sides of

circular, flat or oval tubes, or parting sheets. Fins are primarily used to increase the

surface area (when the heat transfer coefficient on that fluid side is relatively low) and

consequently to increase the total rate of heat transfer. In addition, enhanced fin

geometries also increase the heat transfer coefficient compared to that for a plain fin.

Fins may also be used on the high heat transfer coefficient fluid side in a heat

exchanger primarily for structural strength (for example, for high pressure water flow

through a flat tube) or to provide a thorough mixing of a highly-viscous liquid (such as

for laminar oil flow in a flat or a round tube). Fins are attached to the primary surface by

brazing, soldering, welding, adhesive bonding or mechanical expansion, or extruded or

integrally connected to tubes.



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Types of Extended Surface Heat Exchanger

1. Finned Tube Heat Exchanger

Finned Tube Heat Exchangers consist of a shell & finned tubes assembly. Fins are

used to increase the effective surface area of heat exchanger tubing. Finned tubes

are used when the heat transfer coefficient on the outside of the tubes is

appreciably lower than that on the inside; as in heat transfer from a liquid to a gas,

vapor to a gas, such as steam to air heat exchanger, thermic fluid to air heat

exchanger. When an extended surface is needed on only one fluid side (such as in

a gas to liquid exchanger) or when the operating pressure needs to be contained

on one fluid side, a finned tube heat exchanger may be selected.

Types of finned tube heat exchanger

a. Low Finned tube

The low finned tube commonly used in shell and tube exchangers

provides about 3 to 4 times as much as outside area as inside. The outside

diameter of the fins is just slightly less than that of the bare tube at the ends, so

the tube can be inserted through the tube sheet holes. The wall under the fins is

controlled to a specified thickness and the wall at the plain ends is about two

gauges heavier.

Low finned tubes are used for condensing organic vapors, which have

condensing coefficients only a third or a quarter of that of the cooling water

inside the tubes. In addition to providing additional heat transfer area, the fins



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provide drip points that facilitate the drainage of the condensate. On the other

hand, finned tubes are not used for condensing steam or high surface tension

fluids on the fin side. The high surface tension causes the liquid to hang on the

surface, largely insulating it by a static film of liquid.

b. Medium Fin tube

A medium fin height tube with 11 fins per inch is also used in shell and tube

exchangers.

Medium finned tubes are used in variety of sensible, condensing And

boiling services in shell and tube exchangers. A typical sensible heat transfer

application would be cooling a compressed gas in a compressor intercooler,

using cooling water in the tubes.

c. High fin tube

The high fin tube is used to advantage when gases are to be heated or

cooled or when a process stream is to be air-cooled. High fin tubes come in a

wide variety of fin heights, thicknesses and spacing. For corrosion protection, a

mechanically bonded liner tube may be used inside the finned tube. The liner

can be made of corrosion-resistant alloy, while the other tubes and fins are

made of high conductivity metal such as copper or aluminum to improve heat

transfer.

One major application of high finned tubes is in air-cooled heat

exchangers. Atmospheric air, like all low pressure gases, gives very low heat

transfer coefficient at normal velocities. By contrast, the side tube fluid, usually a

fluid to be sensibly cooled or a vapor to be condensed may have a coefficient

up to 100 times higher or even more. Therefore, high finned tubes are used in

these exchangers to reduce the overall size of exchanger required. Even so,

some of these installations cover several acres.

2. Plate Fin Heat Exchanger

This type of heat exchanger uses "sandwiched" passages containing fins to

increase the effectiveness of the unit. The designs include cross flow and counter



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flow coupled with various fin configurations such as straight fins, offset fins and wavy

fins.

Plate fin heat exchangers are usually made of aluminum alloys, which provide

high heat transfer efficiency. The material enables the system to operate at a lower

temperature difference and reduce the weight of the equipment.

Corrugations (Fins)

Corrugations are also made with heat transfer enhancement devices.

a. Plain corrugation is the basic form and is used normally for low pressure

drop streams.

b. Perforated corrugation shows a slight increase in performance over plain

corrugation, but this is reduced by the loss of area due to perforation. The

main use is to permit migration of fluid across fin channels, usually in

boiling duties.

c. Serrated corrugation is made by cutting the fins every 3.2 mm and

displacing the second fin to a point half way between the preceding fins.

This gives a dramatic increase in heat transfer.

d. Herringbone corrugation is made by displacing the fins sideways every 9.5

mm to give a zigzag path. Performance is intermediate between the plain

and serrated forms. The friction factor continues to fall at high Reynolds

numbers, unlike the serrated, showing advantages at higher velocities

and pressures.

The designer can, therefore, vary fin heights, fin pitch and fin thickness

together with four standard fin types giving great versatility of design.



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Direction of Flow

Plate-fin units are normally arranged for counter flow heat exchange. Cross flow

units are used for vehicle radiators and cross counter flow is used for liquid sub coolers.

Finned tube units are arranged for cross flow heat exchange.

Advantages

Extended surfaces may exist in many situations but are commonly used as fins to

enhance heat transfer by increasing the surface area available for convection (and/or

radiation). They are particularly beneficial when the heat transfer coefficient on that

fluid side is relatively low is small, as for a gas and natural convection.

High heat transfer efficiency especially in gas treatment

Larger heat transfer area

Able to withstand high pressure

Limitations

Might cause clogging as the pathways are very narrow

Difficult to clean the pathways



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Application

Plate and fin heat exchangers are mostly used for low temperature services such

as natural gas, helium and oxygen liquefaction plants, air separation plants and

transport industries such as motor and aircraft engines.

Applications of finned tube heat exchangers include Steam air heater / steam

radiator, Thermic fluid air heater / thermic fluid radiator, Hot water air heater / hot water

radiator, Air heater for fluid bed dryers, Air heater for spray dryers, Air heater for flash

dryers and Air heater for dryers. These are manufactured using high grade carbon

steel, stainless steel, copper, brass and aluminum.

Finned tubes are also used in boiling services, especially when condensing steam

is the heating medium inside the tubes. The condensing coefficient in this case may be

2000 or 3000 btu/hr.ft2.F, so even the high heat transfer coefficients commonly

associated with nucleate boiling may be relatively small by comparison and the design

can benefit by the use of low-finned tubes.

https://en.wikipedia.org/wiki/Helium

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SHELL-AND-TUBE HEAT EXCHANGER (STHE)

The shell-and-tube heat exchanger is the most commonly used heat exchanger. It is

considered as the “workhorse” of the industrial process heat transfer (Minton, 1990). It is

widely used as a power condenser, oil cooler, preheater, and steam generator. Shell-

and-tube heat exchanger consists of many tubes mounted parallel to each other in a

tube sheet, like a bundle, and this tube bundle is enclosed by a cylindrical shell or

casing. The flow may be parallel, counter-current, or crossflow and in some cases

combinations of these flow arrangements as a result of baffling. Shell and tube designs

are relatively simple and most often designed according to the Tubular Exchanger

Manufacturer’s Association (TEMA) standards .

Figure 1: Shell-and-Tube Exchanger

Trivia: Tubular Exchanger Manufacturer’s Association (TEMA) is an

association of manufacturers of shell-and-tube heat exchangers. TEMA

has established a set of construction standards for STHE. The Standards

are regularly updated and published; the most recent edition is the

ninth, published in 2007.

http://www.thermopedia.com/content/1121/



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The Standards recognize three classes of heat exchanger construction:

Class R for the severe requirements of petroleum processing (and usually including

most large scale processing applications)

Class C for general commercial application

Class B for chemical process service

Advantages of Using Shell-and-Tube Heat Exchangers

The shell and tube heat exchanger provides a comparatively large ratio of heat

transfer area to volume and weight. It can be easily constructed from a wide range of

sizes and the shell and the tubes can be made of different materials.There is substantial

flexibility regarding materials of construction to accommodate corrosion and other

concerns. In addition to this, there are many modifications of basic configuration which

can be used to solve special problems. Extended heat transfer surfaces (fins) can be

used to enhance heat transfer. The pressures and pressure drops can be varied over a

wide range. Thermal stresses can be accommodated inexpensively. Condensation or

boiling heat transfer can be accommodated in either the tubes or the shell, and the

orientation can be horizontal or vertical. It can be reasonably easily cleaned, and those

components most subject to failure, like gaskets and tubes, can be easily replaced

because it can be dismantled.

Major Parts of a Shell-and-Tube Heat Exchanger

The shell-and-tube exchanger consists of four major parts:

Front Header - This is where the fluid enters the tube side of the exchanger. It is

sometimes referred to as the Stationary Header.

Shell - This contains the tube bundle.

Tube bundle - This comprises of the tubes, tube sheets, and baffles, etc. to hold the

bundle together.



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Rear Header - This is where the tube side fluid leaves the exchanger or where it is

returned to the front header in exchangers with multiple tube side passes.

Figure 2: Major Parts of a Shell-and-Tube Heat Exchanger

Parts of a Tube Bundle:

Tubes - Tubing that is generally used in TEMA sizes is made from low carbon steel,

copper, Admiralty, Copper-Nickel, stainless steel, Hastalloy, Inconel, titanium and a

few others. It is common to use tubing from 5/8” to 1-1/2” in these designs. Tubes

are generally drawn and seamless, but welded tubes with superior grain structure at

the weld are also common. The two types of tubes are straight and U-tubes.

Tube Sheet - This is usually made from a round flat piece of metal with holes drilled

for the tube ends in a precise location and pattern relative to one another. Tubes

are attached to the tube sheet by pneumatic or hydraulic pressure or by roller

expansion. Tube holes are typically drilled and then reamed and can be machined

with one or more grooves.



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(a) (b)

Figure 3: (a) photo of a tube sheet (b) diagram of tube sheet and tube assembly

Baffle - This support the tubes during assembly and operation and help prevent

vibration from flow induced eddies and direct the shell side fluid back and forth

across the tube bundle to provide effective velocity and heat transfer rates. There

are a number of different baffle types, which support the tubes and promote flow

across the tubes.

Single Segmental (this is the most common)

Double Segmental (this is used to obtain a lower shell side velocity and pressure

drop)

Disc and Doughnut

Figure 4: Types of Baffles



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Tie Rods and Spacers - These hold the baffle assembly together and maintain the

selected baffle spacing. The tie rods are secured at one end to the tube sheet

and at the other end to the last baffle. They hold the baffle assembly together. The

spacers are placed over the tie rods between each baffle to maintain the selected

baffle pitch. The minimum number of tie rod and spacers depends on the diameter

of the shell and the size of the tie rod and spacers.

Figure 5: Outer and Inner Parts of STHE

Common Types of Shell-and-Tube Heat Exchanger

U-Tube heat exchanger



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It is bent in the shape of U and has only one tube sheet. The shell diameter is larger

due to the minimum U-bend radius. The advantage of this type is that one end is free,

the bundle can expand or contract in response to stress differentials. In addition, the

outsides of the tubes can be cleaned, as the tube bundle can be removed. The

drawbacks of this construction is that individual tubes can be difficult of expensive to

replace, especially for interior tubes. Also, the tube interior cannot be effectively

cleaned in the U-bends. Erosion damage is also frequently seen in the U-bends in high

tube side velocity applications. In large diameter shells, the long length of unsupported

tube in the U-bends of outer tubes can lead to vibration induced damage. Steam

generator used in pressurized water reactor in nuclear power plants typically have U-

tubes.

Single-Pass shell-and-tube heat exchanger

The fluid flowing through the tubes enters a header or channel where it is distributed

through the tubes in parallel flow and leaves the unit through another header in one

pass. While the other fluid enters the other end of the shell and flows counter-flow

across the outside of the tubes. Cross-baffles are used so that the fluid is forced to flow

perpendicular across the tube bank rather than parallel with it. Surface condensers in

power plants are often singe-pass heat exchangers.



Heat Exchanger | 27

Multi-Pass shell-and-tube heat exchanger

In a two-pass heat exchanger, the fluid enters and exit on the same side of the

heat exchanger. There are many different types or designs of shell-and-tube heat

exchangers to meet various process requirements. STHEs can provide steady heat

transfer by utilizing multiple passes of one or both fluids.

TEMA Designation for Shell-and-Tube Heat Exchangers

The popularity of shell and tube exchangers has resulted in a standard

nomenclature being developed for their designation and use by the Tubular Exchanger

Manufactures Association (TEMA). This nomenclature is defined in terms letters and

diagrams. The first letter describes the front header type, the second letter the shell type

and the third letter the rear header.



Heat Exchanger | 28

Table 1. Shell and tube geometric terminology

Fixed Tube Sheet

Exchangers U-tube Exchangers Floating Head Exchangers

AEL

AEM

AEN

BEL

BEM

BEN

AEU

CEU

DEU

AES

BES

Essentially there are three main combinations:

A. Fixed Tubesheet Exchanger (L, M, and N Type Rear Headers)

In a fixed tube sheet exchanger, the tube sheet is welded to the shell. This results in

a simple and economical construction and the tube bores can be cleaned

mechanically or chemically. However, the outside surfaces of the tubes are

inaccessible except to chemical cleaning. If large temperature differences exist

between the shell and tube materials, it may be necessary to incorporate an expansion

bellows in the shell, to eliminate excessive stresses caused by expansion. Such bellows

are often a source of weakness and failure in operation. In circumstances where the

consequences of failure are particularly grave U-Tube or Floating Header units are

normally used. This is the cheapest of all removable bundle designs, but is generally

slightly more expensive than a fixed tube sheet design at low pressures.

B. U-Tube Exchangers

In a U-Tube exchanger any of the front header types may be used and the rear

header is normally a M-Type. The U-tubes permit unlimited thermal expansion, the tube

bundle can be removed for cleaning and small bundle to shell clearances can be

achieved. However, since internal cleaning of the tubes by mechanical means is

difficult, it is normal only to use this type where the tube side fluids are clean.



Heat Exchanger | 29

C. Floating Head Exchanger (P, S, T and W Type Rear Headers)

In this type of exchanger the tube sheet at the Rear Header end is not welded to

the shell but allowed to move or float. The tube sheet at the Front Header (tube side

fluid inlet end) is of a larger diameter than the shell and is sealed in a similar manner to

that used in the fixed tube sheet design. The tube sheet at the rear header end of the

shell is of slightly smaller diameter than the shell, allowing the bundle to be pulled

through the shell. The use of a floating head means that thermal expansion can be

allowed for and the tube bundle can be removed for cleaning. There are several rear

header types that can be used but the S-Type Rear Head is the most popular. A floating

head exchanger is suitable for the rigorous duties associated with high temperatures

and pressures but is more expensive (typically of order of 25% for carbon steel

construction) than the equivalent fixed tube sheet exchanger.

Considering each header and shell type in turn:

A-Type front header: This type of header is easy to repair and replace. It also gives

access to the tubes for cleaning or repair without having to disturb the pipe work. It

does however have two seals (one between the tube sheet and header and the other

between the header and the end plate). This increases the risk of leakage and the cost

of the header over a B-Type Front Header.

B-Type front header: This is the cheapest type of front header. It also is more suitable

than the A-Type Front Header for high pressure duties because the header has only one

seal. A disadvantage is that to gain access to the tubes requires disturbance to the

pipe work in order to remove the header.

C-Type front header: This type of header is for high pressure applications (>100 bar). It

does allow access to the tube without disturbing the pipe work but is difficult to repair

and replace because the tube bundle is an integral part of the header.

D-Type front header: This is the most expensive type of front header. It is for very high

pressures (> 150 bar). It does allow access to the tubes without disturbing the pipe work

but is difficult to repair and replace because the tube bundle is an integral part of the

header.

N-Type front header: The advantage of this type of header is that the tubes can be

accessed without disturbing the pipe work and it is cheaper than an A-Type Front

Header. However, they are difficult to maintain and replace as the header and tube

sheet are an integral part of the shell.



Heat Exchanger | 30

Y-Type front header: Strictly speaking this is not a TEMA designated type but is generally

recognized. It can be used as a front or rear header and is used when the exchanger is

to be used in a pipe line. It is cheaper than other types of headers as it reduces piping

costs. It is mainly used with single tube pass units although with suitable partitioning any

odd number of passes can be allowed.

E-Type shell: This is most commonly used shell type, suitable for most duties and

applications. Other shell types only tend to be used for special duties or applications.

F-Type shell: This is generally used when pure counter-current flow is required in a two

tube side pass unit. This is achieved by having two shells side passes—the two passes

being separated by a longitudinal baffle. The main problem with this type of unit is

thermal and hydraulic leakage across this longitudinal baffle unless special precautions

are taken.

G-Type shell: This is used for horizontal thermosiphon reboilers and applications where

the shell side pressure drop needs to be kept small. This is achieved by splitting the shell

side flow.

H-Type shell: This is used for similar applications to G-Type Shell but tends to be used

when larger units are required.

J-Type shell: This tends to be used when the maximum allowable pressure drop is

exceeded in an E-Type Shell even when double segmental baffles are used. It is also

used when tube vibration is a problem. The divided flow on the shell side reduces the

flow velocities over the tubes and hence reduces the pressure drop and the likelihood

of tube vibration. When there are two inlet nozzles and one outlet nozzle this is

sometimes referred to as an I-Type Shell.

K-Type shell: This is used only for reboilers to provide a large disengagement space in

order to minimize shell side liquid carry over. Alternatively a K-Type Shell may be used as

a chiller. In this case the main process is to cool the tube side fluid by boiling a fluid on

the shell side.

X-Type shell: This is used if the maximum shell side pressure drop is exceeded by all other

shell and baffle type combinations. The main applications are shell side condensers and

gas coolers.

L-Type rear header: This type of header is for use with fixed tube sheets only, since the

tube sheet is welded to the shell and access to the outside of the tubes is not possible.

The main advantages of this type of header are that access can be gained to the

inside of the tubes without having to remove any pipework and the bundle to shell

clearances are small. The main disadvantage is that a bellows or an expansion roll are



Heat Exchanger | 31

required to allow for large thermal expansions and this limits the permitted operating

temperature and pressure.

M-Type rear header: This type of header is similar to the L-Type Rear Header but it is

slightly cheaper. However, the header has to be removed to gain access to the inside

of the tubes. Again, special measures have to be taken to cope with large thermal

expansions and this limits the permitted operating temperature and pressure.

N-Type rear header: The advantage of this type of header is that the tubes can be

accessed without disturbing the pipe work. However, they are difficult to maintain and

replace since the header and tube sheet are an integral part of the shell.

P-Type rear header: This is an outside packed floating rear header. It is, in theory, a low

cost floating head design which allows access to the inside of the tubes for cleaning

and also allows the bundle to be removed for cleaning. The main problems with this

type of header are large bundle to shell clearances required in order to pull the bundle,

limited to low pressure nonhazardous fluids, because it is possible for the shell side fluid

to leak via the packing rings, and only small thermal expansions are permitted. In

practice it is not a low cost design, because the shell has to be rolled to small

tolerances for the packing to be effective.

S-Type rear header: This is a floating rear header with backing device. It is the most

expensive of the floating head types but does allow the bundle to be removed and

unlimited thermal expansion is possible. It also has smaller shell to bundle clearances

than the other floating head types. However, it is difficult to dismantle for bundle pulling

and the shell diameter and bundle to shell clearances are larger than for fixed head

type exchangers.

T-Type rear header: This is a pull through floating head. It is cheaper and easier to

remove the bundle than with the S-Type Rear Header, but still allows for unlimited

thermal expansion. It does, however, have the largest bundle to shell clearance of all

the floating head types and is more expensive than fixed header and U-tube types.

U-tube: This is the cheapest of all removable bundle designs, but is generally slightly

more expensive than a fixed tube sheet design at low pressures. However, it permits

unlimited thermal expansion, allows the bundle to be removed to clean the outside of

the tubes, has the tightest bundle to shell clearances and is the simplest design. A

disadvantage of the U-tube design is that it cannot normally have pure counter flow

unless an F-Type Shell is used. Also, U-tube designs are limited to even numbers of tube

passes.

W-Type rear header: This is a packed floating tube sheet with lantern ring. It is the

cheapest of the floating head designs, allows for unlimited thermal expansion and



Heat Exchanger | 32

allows the tube bundle to be removed for cleaning. The main problems with this type of

head are: the large bundle to shell clearances required to pull the bundle and; the

limitation to low pressure nonhazardous fluids (because it is possible for both the fluids to

leak via the packing rings). It is also possible for the shell and tube side fluids to become

mixed if leakage occurs.

Applications of Shell-and-Tube Heat Exchangers

Shell and tube heat exchangers represent the most widely used vehicle for the

transfer of heat in industrial process applications. They are frequently selected for such

duties as:

Process liquid or gas cooling

Process refrigerant vapor or steam condensing

Process liquid, steam or refrigerant evaporation

Process heat removal and preheating of feed water

Thermal energy conservation efforts, heat recovery

Compressor, turbine and engine cooling, oil and jacket water

Hydraulic and lube oil cooling



Heat Exchanger | 33

TEMA Designations for Shell-and-Tube Heat Exchangers



Heat Exchanger | 34

References

Basic Construction of Shell and Tube Heat Exchangers. Retrieved July 2, 2015 from

http://local.alfalaval.com/en-us/key-technologies/heat-transfer/shell-and-tube-heat-

exchangers/process-industrial/Documents/TEMA%20basics%20of%20construction%20-

%2007.10.pdf

Bell, K.J. (2011). TEMA Standards. Retrieved June 30, 2015, from


Brogan, R.J. (2011). Shell and Tube Heat Exchangers. Retrieved June 30, 2015, from


Heat Exchangers. Retrieved July 2, 2015, from

http://web.pdx.edu/~yongkang/main/class/Heat%20Exchangers.pdf

Kuppan, T. (2000). Heat Exchanger Design Handbook. New York: Marcel Dekker Inc.

http://local.alfalaval.com/en-us/key-technologies/heat-transfer/shell-and-tube-heat-exchangers/process-industrial/Documents/TEMA%20basics%20of%20construction%20-%2007.10.pdf



