HEAT TRANSFER OPERATIONS

LECTURE L2
TYPES OF HEAT EXCHANGERS

BY: HUSSEIN AYOUB

 PRINCIPLES OF HEAT EXCHANGERS
The process of heat exchange between two fluids is often required in many engineering
applications, namely in power industries, air-conditioning and chemical process industries.
The device used to execute this purpose is known as heat exchanger. The process of heat
transfer between two fluids can be accomplished either by direct contact between the fluids or
by indirect contact between them by a separating solid wall. The examples of direct contact
type heat exchangers are cooling towers, jet condensers, etc. where water is used in the form
of an atomized spray which comes in direct contact with a gas or vapor for heat transfer.
However, we will discuss only the indirect contact type heat exchangers in this course.

CLASSIFICATIONS OF HEAT EXCHANGERS

Various types of heat exchangers are made in practice which differ from one another in
geometrical configuration, construction, flow arrangement, and heat transfer mechanism. In
indirect contact type heat exchanger, as mentioned earlier, heat is transferred from one fluid to
another through a solid wall usually by the mode of conduction and convection. This type of
exchanger is classified depending upon the geometrical configuration and flow arrangement.
The simplest one is known as shell-and-tube exchanger.
 1- Direct contact heat exchanger
1- What Is A Cooling Tower?
A cooling tower is a specialized heat exchanger in which air and water are brought into direct contact with
each other in order to reduce the water’s temperature. As this occurs, a small volume of water is
evaporated, reducing the temperature of the water being circulated through the tower.
2- What is Jet Condenser
Jet condenser is a mixing type condenser where exhaust steam is condensed mix up with
cooling water. In a jet condenser, high power is required for condensation. Design of jet
condenser is simple. But after condensation, cooling water cannot be used to boiler as it is not
free from salt and other impurities.
 2- Indirect Contact heat Transfer
The principle of operation is simple enough: Two fluids of different temperatures are brought into close
contact, but they are not mixing with each other.
•One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat
between the two fluids.

Heat Exchangers - Types

Different heat transfer applications require different types of
hardware and different configurations of heat transfer
equipment
Heat exchangers come in many different types:
1. Double pipe
2. Spiral

3. Finned

4. plate type

5. Shell and tube (most common in chemical process

Double pipe heat exchanger
Shell and tube heat exchanger

Plate Heat exchanger

Spiral Heat Exchanger Finned tube Heat Exchanger Coiled Heat Exchanger
 BASIC CRITERIAS FOR THE SELECTION OF
HEAT EXCHANGERS

✓ Process specifications
✓ Service conditions of the plant environment, resistance
to corrosion by the process
✓ Maintenance, permission to cleaning and replacement of
components
✓ Cost- Effectiveness

✓ Site requirements, lifting, servicing,capabilities

Shell-and-Tube Heat Exchanger
The simplest form of this type consists of two concentric tubes. While one fluid flows
through the inner tube, the other one flows through the annulus. When the two fluids
move in the same direction, the arrangement is known as parallel flow and when they
move in the opposite direction, the arrangement is known as counterflow arrangement
Why shell-and-tube?
S h ell a n d t u b e h ea t ex ch ang ers accounted for 85% of new exchangers supplied to oil-
refining, chemical, petrochemical and power companies.

Shell and tube exchangers are the most commonly used heat exchangers in process plants today.
The reasons for this are that shell and tube heat exchangers can operate on a wide range of
operating temperature and pressure and It has well established procedure and availability of
codes and standard for design and fabrication. - The shell is basically a large cylinder,
which is normally manufactured from rolled plate.
- The tubes are usually thin-walled metal tubing, specifically designed and
manufactured for heat exchangers.
Can be designed for almost any duty with a very wide range of
temperatures and pressures
Can be built in many materials
Many suppliers
Repair can be by non-specialists
Design methods and mechanical codes have been
established from many years of experience
 Shell and tube heat exchanger

• Main Parts
• I .Tubes
Connections
«
• 2.Shell Tubesheet a a

• 3.Baffles
• 4.Tube Sheets
• 5.Head Mounting
G askets
Head
• 6.Tube Bundle
SHELL AND TUBE HEAT
EXCHANGERS Types

FIXED TUBE HEAT

EXCHANGERS

U-TUBE HEAT
EXCHANGERS

FLOATING HEAD HEAT

EXCHANGERS
 1- Fixed Tube Sheet Heat Exchanger
In this type of exchanger, the tube sheet is
welded to the shell. This leads to a simple and
economical structure, and the cleaning of tube
bores can be performed mechanically or
chemically. However, the outer surface of the
tubes is inaccessible other than chemical
cleaning. Rear headers are usually of L, M, and
N types.
Fixed tubesheet. A fixed-tube sheet heat exchanger (Figure 2) has straight
tubes that are secured at both ends to tube sheets welded to the shell. The
construction may have removable channel covers (e.g., AEL), bonnet-type
channel covers (e.g., BEM), or integral tube sheets (e.g., NEN). The principal
advantage of the fixed tube sheet construction is its low cost because
of its simple construction. In fact, the fixed tube sheet is the least expensive
construction type, as long as no expansion joint is required.
Other advantages are that the tubes can be cleaned mechanically after removal
of the channel cover or bonnet, and that leakage of the shell side fluid is
minimized since there are no flanged joints. A disadvantage of this design is
that since the bundle is fixed to the shell and cannot be removed, the outsides
of the tubes cannot be cleaned mechanically. Thus, its application is
limited to clean services on the shell side. However, if a satisfactory chemical
cleaning program can be employed, fixed-tube sheet construction
may be selected for fouling services on the shell side.
FIXED TUBE HEAT EXCHANGERS

• They are the most economical type design.

• They have very popular version as the heads can be
removed to clean the inside of the tubes.
• Cleaning the outside surface of the tubes is impossible
as these are inside the fixed part.
• Chemical cleaning can be used.
FIXED TUBE HEAT EXCHANGERS
have straight tubes that are secured at both
ends to tube sheets welded to the shell.

Bonnet Bonnet
(Stationary Stationary Support Stationary (St ationa ry
Head) Tubesheet Bracket Tubesheet Head)

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f - • •- ·
I
c:,
L
,
' •. .
L #
r ,

• le]
\ V ~

Baffles Tie Rods

and Spacers
2- U -TUBE HEAT EXCHANGERS
U-tube. As the name implies, the tubes of a U-tube heat exchanger
(Figure 3) are bent in the shape of a U. There is only one tube sheet in a U tube
heat exchanger. However, the lower cost for the single tube sheet is
offset by the additional costs incurred for the bending of the tubes and the
somewhat larger shell diameter (due to the minimum U-bend radius), making the
cost of a U-tube heat exchanger comparable to that of a fixed tube sheet
exchanger. The advantage of a U-tube heat exchanger is that because 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 disadvantage of the U-tube construction is that the insides of the
tubes cannot be cleaned effectively, since the U-bends would require flexible-
end drill shafts for cleaning. Thus, U-tube heat exchangers should not be used
for services with a dirty fluid inside tubes.
 U-TUBE HEAT EXCHANGERS
• Examples : reboilers, evaporators and Kettle type.
• They have enlarged shell sections for vapor-liquid
separation.

Shell Tubes Baffles

 3 FLOATING HEAD HEAT EXCHANGERS

Floating head. The floating-head heat exchanger is the most versatile type of
STHE, and also the costliest. In this design, one tube sheet is fixed relative to
the shell, and the other is free to “float” within the shell.

This permits free expansion of the tube bundle, as well as cleaning of both the
insides and outsides of the tubes. Thus, floating-head SHTEs can be used for
services where both the shell side and the tube side fluids are dirty — making this
the standard construction type used in dirty services, such as in petroleum
refineries.
There are various types of floating- head construction. The two most common are
the pull-through with backing device (TEMA S) and pull through (TEMA T)
designs.
-

• A floating head is excellent for applications where the

difference in temperature between the hot and cold fluid causes
unacceptable stresses in the axial direction of the shell and tubes.
• The floating head can move, so it provides the
possibility to expand in the axial direction.
• Design allows for bundle to be removed for inspection,
cleaning or maintenance.
FLOATING HEAD HEAT EXCHANGER

one tube is free to float within the shell and the other is fixed
relative to the shell.

Pass Stationary Tie Rods Floating Shell

Partition Tubesheet Shell and Spacers Tubesheet Cover

. ,
up po5
Stationary-Head Saddles Floating-Head
Channel Baffles Cover
 SHELL AND TUBE HEAT EXCHANGERS STANDARDS
1- Tubular Exchanger Manufacturers Association (TEMA)
A universally accepted code for shell and tube exchangers is TEMA (1988), which although
designed to supplement ASME VIII, can be used in conjunction with other pressure vessel codes.
TEMA has classified Shell and Tube Heat Exchanger Types that describes them based on
the Front-End Stationary Head type, Shell type and Rear End Head types.

2- API 660, Shell-and-Tube Heat Exchangers, is a standard developed and released by the
American Petroleum Institute (API) that covers specific requirements for the mechanical design,
material selection, fabrication, inspection, testing, and shipping of shell-and-tube heat exchangers
for the petroleum and petrochemical .

3- British standard (BS) for shell and tube heat exchanger

Scope: Design, construction, inspection, testing of cylindrical shell and plain tube heat exchangers.
4- ASME Section VIII Div. 1 and TEMA Codes are the most widely used standards for the
mechanical design of shell and tube type Heat Exchangers.

5- AD 2000 Merkblatt
AD 2000 Merkblatt was developed for pressure vessel design and manufacture. The certification
includes assessment of technical documents, assessment of test reports, assessment of testing
procedures, check of the reliability of working procedures and personnel appraisal of materials use.
Baffles: Baffles are used to increase the fluid velocity
by diverting the flow across the tube bundle to obtain
higher transfer co-efficient. The distance between
adjacent baffles is called baffle-spacing. The baffle
spacing of 0.2 to 1 times of the inside shell diameter is
commonly used. Baffles are held in positioned by
means of baffle spacers. Closer baffle spacing gives
greater transfer co-efficient by inducing higher
turbulence. The pressure drop is more with closer
baffle spacing.
Baffle Type and Geometry
Baffles support the tubes for structural rigidity, thus prevent tube
vibration and bending.
They also divert the flow across the tube bundle to obtain a higher heat
transfer coefficient.
Baffles can be transverse or longitudinal Transverse baffles are plate
type or rod type.

Baffles types
• Single and double segmental most common‫ﺯ‬
• Baffle spacing is critical (optimum between 0.4 and 0.6 of the
shell diameter.
• Triple and no-tubes-in-window segmental baffles for low
pressure drop applications
and
In case of cut-segmental baffle, a segment (called baffle cut)
is removed to form the baffle expressed as a percentage of
the baffle diameter. Baffle cuts from 15 to 45% are normally
used. A baffle cut of 20 to 25% provide a good heat-transfer
with the reasonable pressure drop. The % cut for segmental
baffle refers to the cut away height from its diameter.
Shell and Tube Heat Exchanger

Doughnut and Disc Type Baffles

Shell and Tube Heat Exchanger

Shell Inlet

I Shell Outlet

Double Segmental Transverse Baffles

Tube Layout
• Angle between the tubes
• 30° results in greatest tube density, most common
• P,/d, is between 1.25 and 1.50
• Maximum number of tubes that can be accommodated within a shell
under specified conditions given in Table

Flow Flow

30° «

Flow
Flow

45°
Tubes and Tube Passes
• A large number of tube passes are used to increase fluid velocity and heat
transfer coefficient, and to minimize fouling
• Tube wall thickness is standardized in terms of the Birmingham Wire
Gauge (BWG) of the tube
• Small tube diameters for larger area/volume ratios, but limited for in-tube
cleaning
• Larger tube diameters suitable for condensers and boilers
• Fins used on the outside of tubes when low heat transfer coefficient fluid
is present on the shell-side
• Longer tubes ~ fewer tubes, fewer holes drilled, smaller shell diameter,
lower cost. However limitations due to several factors result in 1/5 - 1/15
shell-diameter-to-tube-length ratio
Tube pitch
Tube pitch is defined as the shortest distance between two adjacent
tubes. For a triangular pattern, TEMA specifies a minimum tube
pitch of 1.25 times the tube O.D. Thus, a 25- mm tube pitch is
usually employed for 20-mm O.D. tubes. For square patterns,
TEMA additionally recommends a minimum cleaning lane of 1/4 in.
(or 6 mm) between adjacent tubes. Thus, the minimum tube pitch
for square patterns is either 1.25 times the tube O.D. or the tube
O.D. plus 6 mm, whichever is larger. For example, 20-mm tubes
should be laid on a 26-mm (20 mm +6 mm) square pitch, but 25-
mm tubes should be laid on a 31.25-mm (25 mm  1.25) square
pitch.
Fouling Considerations: Most of the process fluids in the
exchanger foul the heat transfer surface. The material deposited
reduces the effective heat transfer rate due to relatively low
thermal conductivity. Therefore, net heat transfer with clean
surface should be higher to compensate the reduction in
performance during operation. Fouling of exchanger increases
the cost of (i) construction due to oversizing, (ii) additional
energy due to poor exchanger performance and (iii) cleaning to
remove deposited materials. A spare exchanger may be
considered in design for uninterrupted services to allow
cleaning of exchanger. The effect of fouling is considered in
heat exchanger design by including the tube side and shell side
fouling resistances. Typical values for the fouling coefficients
and resistances are summarized in Table below.
 Tubing vs Piping
Though both the words tube and pipe are often used
interchangeably, largely because both are hollow shaped, there
are important distinctions between the two when determining
welded vs. seamless tubing needs. Tubes are measured by the
outside diameter (OD) and wall thickness. A pipe, on the other
hand, is measure by its inside diameter (ID). In terms of
functionality, tubing is generally used in structural and aesthetic
applications whereas piping is used for transporting fluids,
liquids, and gases.
he schedule number definition (SCH) is the thickness of the walls of a pipe. The value itself has no
dimensions or units so it's represented by a number alone. Engineers measure the ratio of design pressure to
allowable stress of pipes to estimate schedule numbers.
How to Calculate a Pipe Schedule?
Schedule Numbers for pipe size/wall thickness combinations are calculated
(approximated) to get a uniform relationship equal to 1000 times the
P/S (P=Design Pressure and S=Allowable Stress) expression contained in the
modified Barlow formula for pipe wall thickness. The pipe schedule is
abbreviated as SCH. So,
SCH=1000*(P/S)