Thermal Design 1
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Transcript of Thermal Design 1
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Thermal design
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• flow rates of both streams.• inlet and outlet temperatures of• both streams.• operating pressure • allowable pressure drop• fouling resistance• physical properties• heat duty• type of heat exchanger.• line sizes• preferred tube size• maximum shell diameter• materials of construction• special considerations
Important parameters for thermal design
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Process (thermal) design procedure
Shell and tube heat exchanger is designed by trial and error calculations.
Kern method
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• Step #1 Obtain the required thermophysical properties of hot and cold fluids thermal conductivity heat capacity, thermal expansion viscosity • Step #2 Perform energy balance and find out the heat duty (Q)
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• Step #3 Assume a reasonable value of overall heat transfer coefficient (Uo,assm)
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• Step #4 Decide tentative number of shell and tube passes. Determine the LMTD and the correction factor FT
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Continued
Determine LMTD• Assumptions1) U is constant over entire length
of path2) Cp and flow rates are also
constant.3) No phase changes in the system4) Heat losses are negligible
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continued
• Determination of correction factor
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• Step #5 Calculate heat transfer area (A) required
• Step #6 Calculate the number of tubes Calculate tube side fluid velocity
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• If u <1 m/s, fix np so that,
Step#7 Determine the equivalent diameter of shell.
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Step 8 Decide type of shell and tube exchanger (fixed tubesheet, U-tube etc.).• A triangular (or rotated triangular) pattern
will accommodate more tubes than a square (or rotated square) pattern
• For services that require mechanical cleaning on the shell side, square patterns must be used
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Baffle spacing and baffle cut
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• Step #9 Assign fluid to shell side or tube side
• Step #10 Determine the tube side and shell side film heat transfer coefficient and calculate the overall heat transfer coefficient
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Shell side heat transfer co-efficient (ho):
Overall heat transfer coefficient U
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• Step #11
go the next step # 12. Otherwise go to step #5,calculate heat transfer area (A) required using calculated U and repeat the calculations starting from step #5.
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• Step #12 Calculate % overdesign
Typical value of 10% or less is acceptable.
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Step 13
• Step #13 Calculate the tube-side pressure drop
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continued
• Shell side pressure drop
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Performance analysis
Ratio of quantity of heat removed form a fluid to the maximum which might have been removed