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Heat Exchangers
Heat Exchangers 1
How is the heat transfer?
Mechanism of Convection
Applications .
Mean fluid Velocity and Boundary and their effect on the rate of heat transfer.
Fundamental equation of heat transfer
Logarithmic-mean temperature difference.
Heat transfer Coefficients.
Heat flux and Nusselt correlation
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Heat can transfer between the surface of a solid conductor and the surrounding medium whenever temperature gradient exists.
Conduction Convection Natural convection Forced Convection
Natural and forced Convection Natural convection occurs whenever heat flows
between a solid and fluid, or between fluid layers.
As a result of heat exchange
Change in density of effective fluid layers taken place, which causes upward flow of heated fluid.
If this motion is associated with heat transfer mechanism only, then it is called Natural Convection
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Forced Convection
If this motion is associated by mechanical means such as pumps, gravity or fans, the movement of the fluid is enforced.
And in this case, we then speak of Forced convection.
A device whose primary purpose is the transfer of energy between two fluids is named a Heat Exchanger.
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A heat exchanger is used to exchange heat between two fluids
of different temperatures, which are separated by a solid wall.
Heat exchangers are used to carry out energy conversion and
utilization. They utlize a wide range of flow configurations.
Applications in heating and air conditioning, power
production, waste heat recovery, chemical processing, food
processing, sterilization in bio-processes.
Heat exchangers are classified according to flow arrangement
and type of construction.
Heat Exchangers
prevent car engine
overheating and
increase efficiency
Heat exchangers are
used in Industry for
heat transfer
Heat
exchangers are
used in AC and
furnaces
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The closed-type exchanger is the most popular one.
One example of this type is the Double pipe exchanger.
In this type, the hot and cold fluid streams do not come
into direct contact with each other. They are separated by
a tube wall or flat plate.
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Heat Exchangers Chee 318 11
The baffle heat exchanger design (Phillips Petroleum Co.)
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Tube Bundles
Tube Pitch
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Baffles are used to establish a cross-flow and to induce turbulent mixing of the shell-side fluid, both of which enhance convection.
The number of tube and shell passes may be varied
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One Shell Pass and One Tube Pass
One Shell Pass,
Two Tube Passes Two Shell Passes,
Four Tube Passes
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Heat Exchangers Chee 318 17
Heat Exchangers 18
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TEMA AES Exchanger
TEMA Designations
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Heat Exchangers Chee 318 23
Heat Exchangers 24
Finned - Both Fluids
Unmixed
Finned - Both Fluids
Unmixed
Unfinned - One Fluid Mixed
the Other Unmixed
Unfinned - One Fluid Mixed
the Other Unmixed
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Widely used to achieve large heat rates per unit volume, particularly when one or both fluids is a gas.
Characterized by large heat transfer surface areas per unit volume (>700 m2/m3), small flow passages, and laminar flow.
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Heat Exchangers Chee 318 26
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Heat Exchangers Chee 318 27
Baffles
How do baffles help? Where are they installed and which
fluid is directly affected? Common practice is to cut away a
segment having a height equal to one-fourth the inside
diameter of the shell. Such baffles are called 25 percent
baffles.
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Baffle Arrangement
Tubes
Standard tube lengths are 8,
12, 16 and 20 ft.
Tubes are drawn to definite
wall thickness in terms of
BWG and true outside
diameter (OD), and they
are available in all common
metals.
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•Heat Exchanger (HEX) Rating
Checking the existing design for compatibility with the user requirements (outlet temperature, heat load etc.)
given: flow rates, inlet temperatures, allowable pressure drop; thus HT area and passage dimensions
find: heat transfer rate, fluid outlet temperatures, actual pressure drop
•HEX Sizing
Thermal and pressure drop considerations, maintenance scheduling with fouling consideration.
given: inlet and outlet temperatures, flow rates, pressure drop
find: dimensions -type and size of HEX
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Assumptions for Basic Design Equations for Sizing
steady-state, steady flow
no heat generation in the HEX
negligible ΔPE, ΔKE
adiabatic processes
no phase change (later)
constant specific heats and other physical properties.
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LMTD Method
Expression for convection heat transfer for flow of a fluid inside a tube:
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)( ,, imompconv TTcmq
• For case 3 involving constant surrounding fluid temperature:
lms TAUq )/ln( io
iolm
TT
TTT
In a two-fluid heat exchanger, consider the hot and cold fluids separately:
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)(
)(
,,,
,,,
icoccpcc
ohihhphh
TTcmq
TTcmq
lmTUAq and
Need to define U and Tlm
(11.1) (11.2)
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Parallel Flow CounterflowParallel Flow Counterflow
• - : • :
Parallel Flow CounterflowParallel Flow Counterflow
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Condenser: Hot fluid is
condensing vapor (eg. steam)
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Evaporator/boiler:
Cold fluid is evaporating liquid
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For tubular heat exchangers we must take into account the
conduction resistance in the wall and convection resistances of the
fluids at the inner and outer tube surfaces.
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oo
io
ii AhkL
DD
AhUA
1
2
)/ln(11
Parallel Flow CounterflowParallel Flow Counterflow
where inner tube surface
outer tube surface LDA
LDA
oo
ii
(11.3a)
ooii AUAUUA
111
Note that:
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Heat Exchangers
Heat exchanger surfaces are subject to fouling by fluid impurities,
rust formation, or other reactions between the fluid and the wall
material. The subsequent deposition of a film or scale on the surface
can greatly increase the resistance to heat transfer between the fluids.
An additional thermal resistance, can be introduced: The Fouling
factor, Rf.
Depends on operating temperature, fluid velocity and length of
service of heat exchanger. It is variable during heat exchanger
operation.
Typical values see Heat Transfer for Kern .
The overall heat transfer coefficient can be written:
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ooo
ofio
i
if
ii AhA
R
kL
DD
A
R
AhUA
1
2
)/ln(11"
,"
,
(11.3b)
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Heat Exchangers
Heat Exchangers
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Fins reduce the resistance to convection heat transfer, by increasing surface area.
Expression for overall heat transfer coefficient includes overall surface efficiency, or temperature effectiveness, ho, of the finned surface, which depends on the type of fin.
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hoho
hf
conductionco
cf
co
hhcc
hAA
RR
A
R
hA
AUAUUA
)(
1
)()()(
1
111
",
",
(11.3c)
where c is for cold and h for hot fluids respectively
Heat Exchangers
Example 1
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Heat Exchangers
Heat Exchangers
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Heat Exchangers
Example 2
Heat Exchangers
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Heat Exchangers