Vertical thermosiphon

Horizontal Calandria (vertical, thermosiphon) Coil evaporators 200-400 150-500 200-400 where ... 

For horizontal thermosiphon/natural units the boiling fluid is almost always on the shell side, with the heating medium in the tubes. In the vertical units the reboiling of the fluid is in the tubes. For kettle units, the boiling is in the shell. ColUns suggests a rule of thumb that if the viscosity of the reboUer is less than 0.5 centipoise (cp), the vertical thermosiphon should be considered, but when the viscosity is more than 0.5 cp, the horizontal reboUer is probably more economical. 

Figures 10-96A-E illustrate horizontal and vertical thermosiphon reboiler flow arrangements.

Vertical thermosiphon Capable of very high heat transfer rates. Compact simple piping required. Low residence time in heated zone. Not easily fouled. Good controllability. Maintenance and cleaning can be awkward. Additional column skirt required. Equivalent to theoretical plate only at high recycle. 

Figure 10-97A compares horizontal and vertical units in the same hydrocarbon boiling service at low pressures and shows that the horizontal units are more favorable in the same service than vertical units and even more so when the mean temperature difference is low. Figure 10-97B compares horizontal and vertical thermosiphon units with kettle reboilers when boiling the same hydrocarbon mixture also see Fair, Jacobs, and Rubin. ... 

Figure 10-97B. Heat transfer data of reboilers boiling a hydrocarbon mixture in horizontal and vertical thermosiphon reboilers compared to a kettle reboiler. (Used by permission Yilmaz, S. B. Chemical Engineering Progress, V. 83, No. 11, p. 64, 1987. American Institute of Chemical Engineers. All rights reserved.)...

Figure 10-105. Nozzle connections for vertical thermosiphon reboilers.

Inlet and oudet nozzles for boiling-side fluid a. Vertical thermosiphon units ... 

The vertical thermosiphon reboiler is a popular unit for heating distillation column bottoms. However, it is indeed surprising how so many units have been installed with so little data available. This indicates that a lot of guessing, usually on the very conservative side, has created many uneconomical units. No well-defined understanding of the performance of these units exists. Kern s recommended procedure has been found to be quite conservative on plant scale units yet it has undoubtedly been the basis for more designs than any other single approach. For some systems at and below atmospheric pressure operation, Kern s procedure gives inconsistent results. The problem is in the evaluation of the two-phase gas-liquid pressure drop under these conditions. 

This method for vertical thermosiphon reboilers is based on semi-empirical correlations of experimental data and is stated to predict heat transfer coefficients 30 percent, which is about the same range of accuracy for most boiling coefficient data. The advantage of this method is that it has had significant design experience in the industry to support it. It is also adaptable to other types of reboilers used in the industry. See Figures 10-110 and 10-111. 

Physical properties at expected operating temperatures. See Figure 10-112 for temperature-pressure effects in vertical thermosiphon reboilers. 

The article of Fair and Klip presents a detailed analysis of the necessary design features and equations for horizontal kettle reboilers, horizontal thermosiphon reboilers, and vertical thermosiphon reboilers. Other useful references on reboilers are 185,186,188,190,192,194,195,196,197, and 201. 

Example 10-21. Vertical Thermosiphon ReboUer, Kern s Method ... 

Equivalent Feet of Inlet and Outlet Piping Vertical Thermosiphon Reboiler... 

Vertical Thermosiphon Reboiler Comparison Effect of Pressure Drop and Pipe Size on Selection... 

Figure 10-122. Overall AT at maximum flux vertical thermosiphon reboilers. (Used by permission Hajek, J. D. Private communication. Deceased.)...

General Guides for Vertical Thermosiphon Reboilers Design... 

Example 10-22. Hajek s Method—Vertical Thermosiphon ReboUer... 

Frequendy used tubes in many vertical thermosiphons are 8 ft long, with 12 ft or 14 ft being maximums. The popular size is 1 V4 in. O.D., with 1 in. being used sometimes. Sizes up to 2 in. O.D. are certainly acceptable, depending on the design criteria. Short tubes of 4 and 6 ft are used for special applications, including vacuum conditions. 

Johnson, A. 1., Circulation Rates and Over-All Temperature Driving Forces in a Vertical Thermosiphon Reboiler, presented at AlChE Heat Transfer Sym., Louisville, KY, Mar. 20 (1955). 

Maximum heat fluxes are lower than in kettle reboilers. Because of boiling point elevations imposed by static head, vertical thermosiphons are not suitable for low temperature difference services. 

Shell side vertical thermosiphons sometimes are applied when the heating medium cannot be placed on the shell side. 

Pressure Drop on the Tube Side of a Vertical Thermosiphon Reboiler 193... 

SubcooUng heat load is transferred at the same coefficient as latent heat load in kettle reboilers, using the saturation temperature in the mean temperature difference. For horizontal and vertical thermosiphons, a separate calculation is required for the sensible heat transfer area, using appropriate sensible heat transfer coefficients and the liquid temperature- profile for the mean temperature difference. 

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