1. MODULE 4
HEAT EXCHANGERS ANALYSIS

2. • Heat Exchangers are the devices that transfer
heat from one fluid to another.
• They are used in the industry for three reasons:
1. They are necessary to the function of some
plant systems.
2. They are used to make certain systems operate
more efficiently and
3. They help to ensure safety and protect
equipment and personnel

3. Factors that affect heat transfer
The four factors are as follows:
1. The type and thickness of the material through which
heat is transferred
2. The kinds and amounts of fluids flowing through the
heat exchanger
3. The difference in temperature between the two fluids
in the heat exchanger
4. The presence of any contamination in the fluids or in
the surfaces that transfer heat in the heat exchanger
• The direct transfer of heat is affected by factor 2 and 3
The indirect transfer is affected by all the four factors

4. • The first factor affecting the heat transfer is the materials used. The
heat moves faster through thinner tube wall, the tube must also be
able to resist erosion.
• The second factor affecting the heat exchanger is the type and
amount of fluids flowing through it. The greater the capacity in both
volumes and flow of a heat exchanger, the more heat can be
transferred from one fluid to another fluid.
• The third factor is the temperature difference.
The greater the difference in temperature between one substance
and another, the faster the heat will flow between them.
• The fourth factor is contamination. Contaminant can also reduce
the efficiency of heat exchanger, oxygen and other gases present in
steam can block the flow of heat from the water.

5. TYPES OF HEAT EXCHANGERS
• Double Pipe
• Shell Tube
• Kettle
• Air coolers
• Plate type
• Calandria type
• Condensers

6. FEATURES OF DOUBLE PIPE HEAT
EXCHANGERS
• The outer pipe is called the shell
• The inner pipe is called the tube
• The space between the shell and the tube is
called the annulus
• Double pipe exchangers are usually used for
low flow rates and high temperatures. Double
pipe H.E is also called as a pipe within a pipe
exchanger.

10. • The temperature variations of the fluids in parallel and
counter flow are shown in fig
• Temperatures are plotted against length or area of heat
exchanger surface.
• The inlet end, where length or area is zero is regarded as
being the end where the hotter of the two fluids enters.
• The fluids are regarded as being hot or cold for
convenience, and th is a temperature of the hot fluid, tc a
temperature of cold fluid. Suffixes 1 and 2 are used for inlet
and outlet of individual streams, and Ɵi is the temperature
difference between fluids at the inlet end and Ɵo the
difference at the outlet end of the exchanger.

16. • The primary purpose of a heat exchanger is to
achieve the required transfer rate using the
smallest possible transfer area and fluid
pressure drop.
• The heat transfer requirement ,Q, can be
expressed in three ways.

17. • Ɵm is a mean temperature difference between
the fluids and UA and UL are mean co-efficients ,
in kW/m2K and kW/(mK) or equivalent units,
applicable over the entire area A or length L of
the exchanger.
• The mean temperature difference Ɵm for both
parallel flow and counter flow given by.
Ɵo and Ɵi are defined in fig2
Problems:

18. THE FEATURES OF SHELL AND TUBE
HEAT EXCHANGERS
• It has an outer casing called the shell which
contains one fluid .This is called the shell side
fluid.
• The fluid flowing through the tubes is termed as
the tube side fluid.
• The amount of heat exchange can be increased
by increasing the velocity of the shell –side
fluid.This is done by means of baffles,which are
metal discs installed inside the shell.

19. • The efficieny of a shell and the tube exchanger
can be increased by increasing the velocity of
the tube-side fluid. This is done by changing
the direction of the flow, thus increasing the
flow path inside the exchanger.
• Exchangers in which the tube-side flow is
more than one direction are called multipass
exchangers

20. • The type of exchanger can be identified by the
position of the inlet and outlet pipes.In a two-
pass exchanger which will cause the tubes to
expand.
• As the temperature increases, the force will
also increase until either of the following
occur:
1. The tube bends
2. The tube breaks free of its fixing

21. • To prevent this happening in a heat exchanger
which is designed for large temperature
differences a floating head can be used.
• One end of tube is fixed,the other end is free
to move or float.The floasting head can be
removed to enable the tubes to be cleaned

22. TUBE BUNDLE ARRANGEMENTS IN
SHELL-AND –TUBE HEAT EXCHANGERS
• Tube Bundles-Consist of three main
parts:tubes,tube sheets, and baffles.
• Triangular Pattern-This gives the most tubes for a
given size but cleaning the outside of the tubes
can only be done by chemical means
• Square Pitch- This type is the easiest to clean the
outside of the tubes.
• Diagonal Pitch-is a similar to square pitch but
being rotated through 45o. This layout contains
the least number of tubes.

23. • Increasing the number of passes on the tube
or shell side increases the velocity or flow of
the fluid which in turn increases the heat
transfer rate.

24. BAFFLE ARRANGEMENTS
• The three types of transverse baffles used to
increase velocity in the shell side are:
1. Orifice baffles
2. Segmental baffles
3. Disk and Doughnut baffles
With a bored shell clearance and consequent
leakage increases because of corrosion.
With unbored shell clearance may be considerably
large because of greater tolerances.

25. KETTLE TYPE HEAT EXCHANGER
• Kettle type boiler or reboiler as it is called when
connected to a distillation column .
• A horizontal shell contains a relatively small tube
bundle, two pass on the tube side, with u-tube and
tube sheet.
• The tube bundle is submerged in a pool of boiling
liquid, the depth of which is set by the height of an
overflow weir.
• Feed is admitted to the liquid pool from the bottom.
Vapor escapes from the top of shell ,any unvaporized
liquid spills over the weir and is withdrawn from the
bottom of the shell.

26. AIR COOLERS
• Air coolers are usually used when cooling
water is expensive or where the temperature
to which the fluid is to be cooled is high and
air can provide adequate cooling.The purpose
of the fin is to increase the surface area of the
tubes pulls air across the tubes and is known
as an induced draft fan, and a fan mounted
below the tubes pushes air across the tubes
and is known as a forced draft fan.

27. • To control the outlet temperature, the air flow
through the finned elements is varied, either by
varying the pitch of the fan blade or by opening
and closing louvers.
• The fan speed is set and determined by the tip
speed .all fans have vibration switches and in case
of severe vibration, the motor is stopped
automatically .during normal operation regularly
check the header boxes for leaking plugs.check
the fan blades for rotation and completeness.

28. PLATE TYPE HEAT EXCHANGER
• Can be used for multiple duties,several
different fluids can flow through different
parts of the exchanger and can be kept
separate from one another.
• Are very effective with viscous fluids.

29. CALANDRIAS
• Vertical shell and tube units are also known as
THERMOSIPHON REBOILER.
• Are used for distillation and evaporation
operations.

30. A portable boiler

31. Diagramof a fire-tube boiler

32. Diagram of a water-tube boiler