Film coefficients boiling liquids

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

Film coefficients for the boiling of liquids other than water have been investigated. Coulson and McNelly [Trans. In.st. Chem. Eng., 34, 247 (1956)] derived the following relation, which also correlated the data of Badger and coworkers [Chem. Metall. Eng., 46, 640 (1939) Chem. Eng., 61(2), 183 (1954) and Trans. Am. Jnst. Chem. Eng., 33, 392 (1937) 35, 17 (1939) 36, 759 (1940)] on water ... 

With a number of heat sensitive liquids it is necessary to work at low temperatures, and this is effected by boiling under a vacuum, as indeed is the case in the last unit of a multieffect system. Operation under a vacuum increases the temperature difference between the steam and boiling liquid as shown in Table 14.1 and therefore tends to increase the heat flux. At the same time, the reduced boiling point usually results in a more viscous material and a lower film heat transfer coefficient. 

Cryder, D. S. and Gilliland, E. R. Ind. Eng. Chem. 24 (1932) 1382-7. Heat transmission from metal surfaces to boiling liquids. I. Effect of physical properties of boiling liquid on liquid film coefficient. 

Cryder. D. S.. and A. C. Finalbargo Heat Transmission from Metal Surfaces to Boiling Liquids Effect of Temperature of the Liquid on Film Coefficient. Trans. AK hE. vol. 33. p. 346, I937. 

Boiling liquids outside horizontal tubes Film boiling (above critical Atj) l n Tkv Pv(pL PtOffXc D. l1,, Nucleate boiling (below critical At,) Value of h depends on At/, type of surface, and materials involved (critical At/ for water as temperature drop from heating surface to liquid is approximately 45°F). Bromley t For saturated liquids on submerged surfaces film coefficient h,0 is for conduction through the vapor, no radiation effect is included. 

Film coefficients for the boiling of liquids other than water... 

In boiling water at 1 atm pressure outside a stainless steel tube with a surface temperature of 410°F, the heat-transfer coefficient h in the absence of radiation is 32 Btu/h-ft -°F. If the emissivity of the stainless steel is 0.8, would radiation significantly augment the rate of boiling (that is, by more than 5 percent) Assume that the vapor film is transparent to radiation and the boiling liquid is opaque. 

As shown in Chap. 11 [Eq. (11.35)], the overall resistance to heat transfer between the steam and the boiling liquid is the sum of five individual resistances the steam-film resistance the two scale resistances, inside and outside the tubes the tube-wall resistance and the resistance from the boiling liquid. The overall coefficient is the reciprocal of the overall resistance. In most evaporators the fouling factor of the condensing steam and the resistance of the tube wall are very small, and they are usually neglected in evaporator calculations. In an agitated-film evaporator the tube wall is fairly thick, so that its resistance may be a significant part of the total. 

OVERALL COEFFICIENTS. Because of the difficulty of measuring the high individual film coefficients in an evaporator, experimental results are usually expressed in terms of overall coefficients. These are based on the net temperature drop corrected for boiling-point elevation. The overall coefficient, of course, is influenced by the same factors influencing individual coefficients but if one resistance (say, that of the liquid film) is controlling, large changes in the other resistances have almost no effect on the overall coefficient. 

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