Boiling departure from

FIG. 27-40 Elffect of departure from nucleate boiling (DNB) on tube-metal temperature. 

In typical cause-and-effect mode, where chlorides penetrate the deposit or where a localized overconcentration of hydroxyl ions occurs, the magnetite film is disrupted and particular forms of very damaging corrosion occurs. In addition, where localized heat flux exceeds design limits within a boiler and may be accompanied by departure from nucleate boiling (DNB) conditions, overheating and metal failure may also occur. 

Steam blanketing results in a departure from nucleate boiling (DNB) and typically dryout (localized total evaporation) conditions. It also may result in the formation of secondary (silicate-based) deposits that cannot be rinsed or resolublized, overheating problems, and eventual boiler tube or furnace shell rupture. 

Concentration may also result from waterline evaporation. In higher pressure WT power boilers, it may develop as a result of high-firing cycle operations, which may lead to departure from nucleate boiling (DNB). 

Swenson, H. S., Carver, J. R., and Kakarala, C. R., The influence of axial heat flux distribution on the departure from nucleate boiling in a water-cooled tube, ASME Paper No. 62-WA-297 (1962). 

The following phenomena pertaining to bubble departure from a heated surface are discussed in this section bubble size at departure, departure frequency, boiling sound, and heat transfer effects by departing bubbles. 

Cermak, J. O., R. F. Farman, L. S. Tong, J. E. Casterline, S. Kokolis, and B. Matzner, 1970, The Departure from Nucleate Boiling in Rod Bundles during Pressure Blowdown, Trans. ASME, Ser. C, J. Heat Transfer 92(4) 621-627. (4)... 

Swenson, H. S., J. R. Carver, and C. R. Kakarala, 1962a, The Influence of Axial Heat Flux Distribution on the Departure from Nucleate Boiling in a Water Cooled Tube, ASME Paper 62-WA-297, Winter Annual Meeting, ASME, New York. (5)... 

Tong, L. S., 1967a, Prediction of Departure from Nucleate Boiling for an Axially Non-uniform Heat Flux Distribution, J. Nuclear Energy 21. 241-248. (3)... 

Tong, L. S., 1967b, Heat Transfer in Water-Cooled Nuclear Reactors, Nuclear Eng. Design (5 301. (3) Tong, L. S., 1968a, An Evaluation of the Departure from Nucleate Boiling in Bundles of Reactor Fuel Rods, Nuclear Sci. Eng. 33 7-15. (5)... 

Weisman, J., A. H. Wenzel, L. S. Tong, D. Fitzsimmons, W. Thorne, and J. Batch, 1968, Experimental Determination of the Departure from Nucleate Boiling in Large Rod Bundles at High Pressures, AIChE Chem. Eng. Prog. Symp. Ser. 64(82) 114-125. (5)... 

The rate of heat transfer from the tubes to the fluid depends primarily on turbulence and the magnitude of the heat flux itself. Turbulence is a function of mass velocity of the fluid and tube roughness. Turbulence has been achieved by designing for high mass v ocities, which ensure that nucleate boiling takes place at the inside surface of the tube. If sufficient turbulence is not provided, departure from nucleate boiling (DNB) occurs. DNB is the production of a film of steam on the tube surface that impedes heat transfer and results in tube overheating and possible failure. This phenomenon is illustrated in Fig. 27-40. 

The principal physical properties are shown in Table XL. It is to be observed that there are close resemblances within the triads, although there is considerable departure from the order of atomic weights. Among the notable facts are to be observed the high melting points and boiling points and the high density... 

An essential basis for the study of boiling heat transfer is the thermodynamics of multiphase systems. Here, it is normal practice to consider systems at thermodynamic equilibrium, in which the temperature of the system is uniform. Of course, as we will see, departures from such thermodynamic equilibrium are important in many instances. In what follows, the thermodynamic equilibrium of a single-component material is first considered. In many applications of boiling (particularly in the process and petroleum industries), multicomponent mixtures (for example, mixtures of hydrocarbons or refrigerants) are important, and the subject of multicomponent equilibrium is dealt with in the final part of this section. 

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