special slides - heat exchangers and airfin coolers
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2/27/2015
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Department of Chemical Engineering
College of Engineering
University of the Philippines Diliman
Second Semester, AY 2014-2015
ChE 142: Chemical
Engineering Plant Design
Detailed Design of
Static Equipment (HEx and AFC)
Introduction Heat Exchanger Types Design Calculations P&ID Representation Cost Estimation
Outline of Lecture
Devore et al recommend the
following heat exchanger types:
Spiral heat exchanger if area is less than 2 m2.
Double-pipe heat exchanger if area is between 2 and 50 m2.
Shell-and-tube heat exchanger if area is greater than 50 m2.
Heat Exchanger Types Heat Exchanger TypesSpiral Heat Exchanger
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Heat Exchanger TypesDouble Pipe Heat Exchanger
Heat Exchanger TypesShell and Tube Heat Exchanger
Shell and tube heat exchangers are
broadly classified into two: removable
and non-removable tube bundle.
Of the two classifications, the cheapest
options are the U-tube heat exchanger
and the fixed tubesheet heat exchanger
respectively.
Heat Exchanger TypesShell and Tube Heat Exchanger
Heat Exchanger TypesShell and Tube Heat Exchanger
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Heat Exchanger TypesShell and Tube Heat Exchanger
Heat Exchanger TypesShell and Tube Heat Exchanger
Heat Exchanger TypesShell and Tube Heat Exchanger
Heat Exchanger TypesShell and Tube Heat Exchanger
Cheaper for the same heat transfer
area, but cant clean shell side.
Costlier for the same heat transfer
area, but can clean shell side.
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Heat Exchanger TypesShell and Tube Heat Exchanger
Heat Exchanger TypesShell and Tube Heat Exchanger
Design Calculations
Design and draw the P&ID representation of
a shell-and-tube heat exchanger with the
following information:
Design CalculationsProblem Statement
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Fluid Placement
Place the fluid on the tube side if it is:
(arranged in order of priority)
1. Corrosive
2. Cooling water
3. More fouling
4. Less viscous
5. More pressurized
6. Hotter
Design CalculationsInitial Specifications
Fluid Placement
Fouling factors (refer to Table 3.3)
Crude oil = 0.004-0.005 hr-ft2-F/Btu
Kerosene = 0.001-0.003 hr-ft2-F/Btu
Place the crude oil and kerosene in tube
side and shell side respectively.
Design CalculationsInitial Specifications
Shell and Head Type
What shell and tube heat exchanger type
is suitable for this service, BEU or BEM?
Design CalculationsInitial Specifications
Tube and Tubing Layout
The following guidelines are
observed in the selection of tube
dimensions and layout:
14 BWG tubes with 1 triangular pitch for straight tubes
1 14 BWG tubes with 1 square pitch for U-tubes
Design CalculationsInitial Specifications
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Tube and Tubing Layout
What are the advantages and disadvantages
of triangular pitch over square pitch?
Design CalculationsInitial Specifications
Tube and Tubing Layout
The preferred straight tube lengths are 16 ft
and 20 ft. For the same heat transfer area,
which is more economical?
Design CalculationsInitial Specifications
Longer but thinner
heat exchanger?
Shorter but fatter
heat exchanger?
Baffle Dimensions
The baffle spacing is recommended to
be between 20% and 100% of the
shell diameter. The default is 20%.
Design CalculationsInitial Specifications
Baffle Dimensions
The baffle cut is recommended to be
between 15% and 45%. The default is 20%.
Design CalculationsInitial Specifications
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Allowable pressure drop for shell and tube
exchangers and air coolers in pumped liquid
service may be considered as follows:
Design CalculationsMaximum Pressure Drop
Viscosity (cP)Allowable Pressure Drop (psi)
Shell Side Tube Side
Less than 1.0 5.0 10
1.0 to 5.0 7.5 10
5.0 to 15.0 10 15
15.0 to 25.0 15 20
25.0 to 50.0 15 25
Allowable pressure drop for shell and tube
exchangers and air coolers in condensing
service may be considered as follows:
Pressure (psig) Allowable Pressure Drop (psi)
Up to 50 2.5 per shell
50 and above 5.0 per shell
Design CalculationsMaximum Pressure Drop
The initial specifications are as follows: Kerosene at shell, crude oil at tube
o Kerosene = 0.003 hr-ft2-F/Btu
o Crude oil = 0.005 hr-ft2-F/Btu
Heat exchanger type BEU 1 14 BWG tubes with 1 square pitch
o Outer diameter (Do) = 1.000 in
o Inner diameter (Di) = 0.834 in
o Tube pitch (PT) = 1.250 in
o Clearance (C) = 0.250 in
Tube length of 20 ft Baffle spacing of 20% of shell diameter Baffle cut of 20% Maximum shell-side P = 5.0 psi Maximum tube-side P = 10.0 psi
Design CalculationsInitial Specifications
The initial specifications are as follows: Kerosene at shell, crude oil at tube
o Kerosene = 5.248 x 10-4 m2-K/W
o Crude oil = 8.806 x 10-4 m2-K/W
Heat exchanger type BEU 1 14 BWG tubes with 1 square pitch
o Outer diameter (Do) = 0.02540 m
o Inner diameter (Di) = 0.02118 m
o Tube pitch (PT) = 0.03175 m
o Clearance (C) = 0.00635 m
Tube length of 6.096 m Baffle spacing of 20% of shell diameter Baffle cut of 20% Maximum shell-side P = 34.46 kPa Maximum tube-side P = 68.93 kPa
Design CalculationsInitial Specifications
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Assume that only one shell pass will suffice.
Calculate the LMTD correction factor (F).
Design CalculationsNumber of Shell Passes
Assume that only one shell pass will suffice.
Calculate the LMTD correction factor (F).
Design CalculationsNumber of Shell Passes
If calculated F is less than 0.80, set the
number of shell passes to two.
Design CalculationsNumber of Shell Passes
Calculate the area using the equation
Q = UAFTlm
Estimate the overall heat transfer
coefficient using the individual heat
transfer coefficients (should have been
done during ChE 141).
Design CalculationsEstimated Area
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The heat exchanger area is equal to the
surface area of each tube multiplied by
the number of tubes:
A = nt x (DoL)
Design CalculationsMinimum Number of Tubes
The tube-side fluid loses pressure as it
expands at the inlet nozzle, flows inside the
tubes, and contracts at the outlet nozzle.
Assume that nozzle losses are negligible.
How can the maximum number of tube
passes be calculated using the equation
above?
Design CalculationsMaximum Number of Tube Passes
Given the heat exchanger type and the tube-
side details, a shell diameter can be selected:
Design CalculationsShell Diameter
Given the selected tube length and
the tube count on the selected shell
diameter, calculate the area available
for heat exchange:
A = nt x (DoL)
Calculate the required overall heat
transfer coefficient:
Ureq = Q/AFTlm
Design CalculationsRequired Overall Heat Transfer Coefficient
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The tube-side heat transfer coefficient is
calculated using the Seider-Tate and Hausen
equations. Viscosity correction is neglected:
Design CalculationsInside Heat Transfer Coefficient
The shell-side heat transfer coefficient is
calculated using the following correlation.
Viscosity correction is neglected:
Design CalculationsOutside Heat Transfer Coefficient
Calculate the overall heat transfer coefficient
using the equation below. Metal resistance is
neglected:
The magnitude of oversize is based on the
required overall heat transfer coefficient.
Design CalculationsOverall Heat Transfer Coefficient
The tube-side fluid loses pressure as it
expands at the inlet nozzle, flows inside the
tubes, and contracts at the outlet nozzle.
Assume that nozzle losses are negligible.
Design CalculationsTube-Side Pressure Drop
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Design CalculationsShell-Side Pressure Drop
The shell-side fluid loses pressure as it
expands at the inlet nozzle, flows outside the
tubes, and contracts at the outlet nozzle.
Assume that nozzle losses are negligible.
What adjustments need to be done?
Design CalculationsDesign Assessment
Case
Is overall HTC
greater than
required?
Is tube-side
P less than
maximum?
Is shell-side
P less than
maximum?
1 YES YES YES
2 YES YES NO
3 YES NO YES
4 NO YES YES
P&ID Representation P&ID Representation
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P&ID Representation P&ID Representation
P&ID Representation Cost Estimation
Towler and Sinnott (2008) expressed the
January 2006 purchased cost of heat
exchangers as a function of area.
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Air Fin Coolers
Because of low heat
transfer coefficient on
the air side, the tubes
are finned to increase
the area available for
heat transfer.
Air Fin Coolers
Air fin coolers are second only to shell-
and-tube heat exchangers in frequency
of occurrence in chemical and
petroleum processing operations.
Assuming no process restrictions,
when is air cooling economically
advantageous over water cooling?
Air Fin Coolers Air Fin CoolersForced Draft Air Fin Coolers
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Air Fin CoolersInduced Draft Air Fin Coolers
Air Fin CoolersForced vs. Induced Draft Air Fin Coolers
Differentiate the two configurations in
terms of:
1. Accessibility of tubes and fan parts
2. Fan power consumption for the same
mass flowrate of air
3. Area for the same air fin cooler duty
Air Fin CoolersForced vs. Induced Draft Air Fin Coolers
Air Fin CoolersForced vs. Induced Draft Air Fin Coolers
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Air Fin CoolersForced vs. Induced Draft Air Fin Coolers
For inlet process fluids above 350F,
use forced draft configuration.
Air Fin CoolersTube Dimensions
Tube lengths are typically from 6 ft to 50 ft,
with 40 ft commonly used. Tubes are
typically stacked from three to eight rows,
with six rows commonly used.
Air Fin CoolersTube Dimensions
Air Fin CoolersBay Dimensions
Bay widths are typically from 4 ft to 30 ft, with
14 ft commonly used. Axial-flow fans with four
or six blades and diameters of 6 ft to 18 ft are
typically employed.
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Department of Chemical Engineering
College of Engineering
University of the Philippines Diliman
Second Semester, AY 2014-2015
ChE 142: Chemical
Engineering Plant Design
Detailed Design of
Static Equipment (HEx and AFC)
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