Film boiling

For film boiling, Bromleys [Chem. Eng. Prog., 46, 221 (1950)] correlation may be used ... 

Figure 10-101. Maximum AT values occur at the indicated threshold of film boiling, a typical example using 100% ethyl alcohol from a clean surface. (Used by permission Cichelli, M. T. and Bonilla, C. F. Transactions. AlChE, V. 41, No. 6, 1945. American Institute of Chemical Engineers. All rights reserved.)...

Palen and Taborek proposed as the best choice for circular tube bundles (as compared to square) the following film boiling coefficient after analyzing available data (their... 

For the entire bundle, the film boiling coefficient (assumes all tubes boiling, no liquid head effect so that some tubes are not boiling) ... 

In reboiler design, film boiling should be avoided. However, such rules-of-thumb as 10-12,000 Btu/(hr) (fti) maximum heat flux are frequently quite conservative. 

Film boiling should be avoided however, nucleate boiling often can be found at heat flux values greater than the rule-of-thumb values of 10-12,000 Btu/hr-fT. These are often conservative values. See Figure 10-119 from Fair. ... 

The temperature difference between the exiting vapor-liquid mixture and the inlet shell-side steam or hot fluid should not exceed 75-82°F, primarily due to fouling problems and possible conversion in the tube to inefficient film boiling in the upper section of the tubes. 

Normally the designer does not try to establish film boiling condidons for the vaporizers or reboilers. However, for systems set by other controlling processing condidons, these film condidons may be imposed. In such cases, they should be recognized and handled accordingly. The principles of design for the equipment are the same as other such equipment, and only the actual value of the coefficient affected needs special attendon. 

Nucleate boiling is boiling at the tube surfece at a temperature difference between outside tube surface temperature and the fluid body, less than the critical temperature difference. At and beyond the critical temperature difference, metastable and film boiling take place. These produce lower transfer coefficients as the temperature difference increases. 

Banchero,J. T, G. E. Barker, and R. H. Boll, Stable Film Boiling of Liquid Oxygen Outside Single Horizontal Tubes and Wires, Chem. Eng. Prog. Sym. Series, No. 17, Heat Transfer— St. Louis, V. 51,p. 21, (1955). 

Shah, M. M., A General Predictive Technique for Heat Transfer During Saturated Film Boiling in Tubes, Heat Trans. Eng, V. 2, Oct.-Dec. (1980) p. 51. 

Oily surfaces induce film boiling and the stabilization of steam bubble on the film surface, and overheating results. Oily surfaces also cause other problems, including ... 

Film boiling at heat transfer areas due to nonwettable metal surfaces... 

Oil, fat, grease, and hydrocarbon contamination results in Film boiling Coke deposition Contaminant binding Sludging and gunk balls... 

The equation given by Bromley (1950) can be used to estimate the heat-transfer coefficient for film boiling on tubes. Heat transfer in the film-boiling region will be controlled by conduction through the film of vapour, and Bromley s equation is similar to the Nusselt equation for condensation, where conduction is occurring through the film of condensate. 

Film Boiling and Heat Transfer in Liquid-Deficient Regions 274... 

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