v. Decide type of exchanger i.e. fixed tube sheet or U- shell (use U-tube reboiler for large temperature difference), tube size (diameter, length, tube pitch), layout, effective tube length. A tube pitch of between 1.5 to 2 times the tubes OD should be used to avoid vapor blanketing.
vi. Calculate exchanger area (
) and number of tubes ( 
). The number of tubes should be calculated based on the effective tube length for U-tube reboilers. The effective tube length is less than physical tube length due to U-bend.
vii. Calculate the heat flux= 
[Btu/(h.ft2 )]. This value should be less than the maximum heat flux of 20000 Btu/(h)ft2) for forced circulation reboilers vaporizing organics and 30000 Btu/(h)ft2) for both forced or natural circulation reboilers vaporizing aqueous solution. Otherwise, go to step # v, repeat the calculation until within the allowable limits.
viii. Check for allowable vapor velocity (uv) ([3] page 749):
The maximum vapor velocity uv(m/s) at the liquid surface should be less than that given by the expression below to avoid too much entrainment.
 |
(1.18) |
where, ρl = liquid density and , ρl = vapor density
If this criteria is not satisfied, go to step # v and revise the calculation.
ix. Pressure drop calculation
Tube side pressure drop (hot fluid): The pressure drop calculation of the hot fluid can be carried out as already presented.
Shell side pressure drop (vaporizing liquid) : There will be negligible hydrostatic head for the flow of liquid from the column to reboilers (low circulation velocity) if the liquid level above the tube bundle is not too high. Therefore, shell side pressure drop may be considered negligible.
x. Calculate over surface and over design
xi. Go for mechanical design
Design problem:
Gasoline (65° API gravity) flow rate of 60,000 lb/h with a small boiling range at 400°F is to be vaporized to form 37,050 lb/h vapor at an operating pressure of 200 psig. Use gas oil (30°API gravity) in the temperature range from 600 to 500°F at 120 psig operating pressure as the heating medium. A tube side pressure drop of 10 psi is allowable. Design a suitable Kettle reboiler to serve the purpose.
References
[1]. Indian Standard (IS: 4503-1967): Specification for Shell and Tube Type Heat Exchangers, BIS 2007, New Delhi.
[2] . R. K. Sinnott, Coulson & Richardson's Chemical Engineering: Chemical Engineering Design (volume 6), Butterworth-Heinemann, 3 rd ed. 1999.
[3]. D. Q. Kern, Process Heat Transfer, McGraw-Hill Book Company, Int. ed. 1965.
[4] Dutta B.K. ‘Heat Transfer-Principles and Applications', PHI Pvt. Ltd., New Delhi, 1 st ed. 2006.
[5] James R. Couper; W. Roy Penney, James R. Fair, Stanley M. Walas, Chemical Process Equipment: selection and design, Elsevier Inc., 2 nd ed. 2005.