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Transcript of Process Design of Heat Exchangers.pdf
7/25/2019 Process Design of Heat Exchangers.pdf
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(Shell & Tube HEs)
Unit III
Niteen R Yeole
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Most widely used – chemical industry
Parts- standardized in size
Design – TEMA standards
IS: 4503 – Equivalent Indian Std
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Std sizes of shell, tubes, tierods etc
Max allowable baffle spacing, minimum tubesheet thickness, baffle thickness, no. of tierods
etc. For mechanical design and fabrication, in
addition to TEMA std, ASME Code Section VIIIDiv. I is used.
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Class R- severe duties in hydrocarbon industry
Class B – chemical process industries notinvolving severe duties
Class C – commercial and in less importantprocess applications
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HTRI: Heat Transfer Research Institute, USA
HTFS : Heat Transfer Fluid Flow Services, UK
BJAC, USA
HEI: Heat Exchange Institute, USA
For ht without phase change-design methodsused by these softwares are based on Tinker’s
flow model.
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Costliest part
Size range (TEMA) – 6 in to 60 in
Std pipes available upto 24 in
Above 24 in – to be fabricated by rolling a plate
Fixed tube sheet STHE – gap between shell &bundle is minimum (10 to 20 mm)
Floating head HE – gap is max (90-100 mm)
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1. Based on actual tube sheet layout drawing
6 Parameters to be fixed
OD of tube
No of tube side pases
Tube pitch
Tube arrangement
No of tubes
Type of HE – fixed tube, U tube or floatinghead
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2 Use of standard tables
Max no of tubes – shell (6 in – 120 in), tube sidepasses, tube pitch, arrangement of tubes.
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3 Use of approximate equations
Shell dia or tube bundle dia can be found out Db = d0 (Nt/k1)
1/n1
where Db = Tube bundle dia, mm
d0 = Tube OD, mm Nt = Total no. of tubes
k1 & n1 are constants
Above equation is used for fixed tube sheettype and floating head shell and tube heatexchangers.
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Single pass shell is used in the most of thecases.
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Direct the fluid stream across the tubes
Increase the fluid velocity
Increase the turbulence
Increase the rate of ht
Indirectly support the tubes and therebyreduce the vibration in tubes
Most commonly used type-segmental baffle
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Size range – 0.25” -2.5”
For std tubes size ie equal to tube OD
Tube thickness – Birmingham Wire Gauge
(BWG) As BWG increases thickness decreases.
For no phase change HEs & condensers, ¾ in
(19.05 mm) OD tube is widely used. For Reboiler, 1 in (25.4 mm) OD is common.
Std lengths-6 ft, 8 ft, 12 ft, 16 ft & 6m.
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Tube side passes are provided to decrease thetube side flow area and to increase tube sidefluid velocity thereby to improve the tube side
htc at the expense of pressure drop.
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1. Calculation of heat duty
2. Selection of Cooling medium or heatingmedium
3. Fluid Allocation4. Establishment of energy balance or heat duty
balance/ finding of the mass flow rate of
heating medium or cooling medium required5. Calculation of Mean temperature difference
6. Estimation of overall heat transfer coefficient
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7. Finding shell diameter
8. Calculation of tube side htc
9. Calculation of tube side pressure drop
10. Calculation of shell side htc
11. Calculation of shell side pressure drop
12. Calculation of the overall htc13. Calculation of heat transfer area
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More versatile than fixed tube and U-tubeexchangers.
Suitable for high temperature differentials
Easier to clean and can be used for foulingliquids.
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Triangular and rotated square patterns givehigher heat transfer rates but at the expense ofa higher pressure drop than the square pattern.
A square or rotated square arrangement , isused for heavily fouling fluids where it isnecessary to mechanically clean the outside ofthe tubes.
Recommended tube pitch (distance betweentube centres) is 1.25 times the tube OD
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No of passes is selected to give the requiredtube side design velocity.
Exchangers are built from 1-16 tube passes.
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m/n
m-no of shell passes
n-no of tube passes
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Baffle cut specifies the dimensions.
It is expressed as a percentage of the baffle discdiameter.
Baffle cuts from 15 to 45 % are used. Optimum cut- 20 to 25 %
Spacing- 0.2 to 1.0 times shell diameter
Optimum spacing- 0.3 to 0.5 times shelldiameter
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Gas oil at 2000C is to be cooled to 400C. The oil
flow rate is 50000 kg/hr. The cooling water is
available at 250C and the temperature rise is
limited to 300C . The pressure drop allowance
on shell side is 100 kN/m2. Calculate the shell
side heat transfer coefficient as well as thepressure drop. Neglect the viscosity correction
on shell side.
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Data:
1] Properties of Gas Oil at 1200C
Specific heat capacity (Cp) = 2.28 KJ/(Kg 0C)Thermal conductivity (k) = 0.125 W/(m0C)
Dynamic Viscocity (µ) = 0.17 mN/(m2 s)
Density = 850 kg/m3
2] Tube Dimensions & Thermal conductivity
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2] Tube Dimensions & Thermal conductivity
Outside diameter (d0) = 20 mm
Inside diameter (di) = 16 mmLength = 4 m
Thermal conductivity = 50 W/(m0C)
Pitch = Triangular (Pt = 1.25 d0 )
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3] Baffle details
Type = Segmental
Cut = 25%
Spacing = 0.5* Shell inside diameter
4] Exchanger Type
1-4, Split-ring floating head