Heat transfer coefficients for nucleate boiling

Compare the heat-transfer coefficients for nucleate boiling of water, as shown in Fig. 9-8, with the simplified relations given in Table 9-3. 

The equation for nucleate-boiling heat transfer can be rearranged to become a function of AT, the temperature difference between the surface and the fluid. The minimum temperature difference required to effect nucleate boiling will occur when the heat-transfer coefficients for nucleate boiling and natural convection are equal. This will permit solution for the temperature difference AT. 

Here aB is the heat transfer coefficient for nucleate boiling from section 4.2.6, ac is that by forced, single phase flow, sections 3.7.4, page 338, and 3.9.3, page 384, L is the adiabatic mixing temperature of the liquid. 

U(j) = single tube overall heat transfer coefficient hj = nucleate boiling coefficient for single tube, outside Btu/hr (ft) (°F)... 

Liquid level. When liquid level falls below the top of the bundle, the unflooded tubes heat vapor rather than perform nucleate boiling. Since vapor heating has a much lower heat transfer coefficient than nucleate boiling, reboiler heat transfer suffers. Further, the tube wall temperature in the unflooded portion approaches the heating fluid temperature, which may overheat tube metal. For these reasons, it is important to maintain the desirable liquid level in the reboiler. 

Figure 7.2.16. Heat transfer coefficient for nucleate pool boiling of PEO aqueous solutions, (p = 9.8x10 Pa). Curve 1 corresponds to pure water, for curves 2-7, c = 0.01, 0.02, 0.04, 0.08, 0.16 and 1.28%, respectively. [Adapted, from S.P. Levitsky, and Z.P. Shulman, Bubbles in polymeric liquids, Technomic Publish. Co., Lancaster, 1995, with permission from Technomic Pub-lishing Co., Inc., copyright 1995]...

Estimate the nucleate-pool-boiling heat-transfer coefficient for a water-26% glycerin mixture at 1 atm in contact with a copper surface and AT, = 15°C. 

Consider the criteria requiredfor nucleate boiling. Nucleate boiling occurs when the difference between the temperature of the hot surface and the bulk fluid temperature is above a certain value. At temperature differences less than this value, heat transfer occurs as a result of natural convection. Nucleate-boiling heat-transfer coefficients for a steel tube may be calculated using the equation... 

Fouling coefficient on inside of tube Nucleate boiling-heat transfer coefficient Nucleate boiling coefficient in equation 12.67 Heat transfer coefficient outside a tube Heat transfer coefficient for cross-flow over an ideal tube bank... 

Certain heat-transfer processes, notably nucleate boiling, do not follow the proportionality described above. Nevertheless, the concept of a film heat-transfer coefficient is so convenient for practical computation that it is often used in these cases. Special care is required to ensure that the heat flux, the temperature difference, and the coefficient are consistent. 

The heat-transfer coefficient for the combined nucleate-boiling and convective vaporization processes, is related to the nucleate-boiling coefficient / nb, and the convective coefficient, by... 

Bankoff [343] suggested that heat transfer coefficients in film boiling could be substantially improved by continuously removing vapor through a porous heated surface. Subsequent experimental work [344, 345] demonstrated that coefficients could be increased by as much as 150 percent, provided that a porous block was placed on the surface to stabilize the flow of liquid toward the surface. Wayner and Kestin [346] extended this concept to nucleate boiling and found that wall superheats could be maintained at about 3 K (5.4°F) for heat fluxes over 300,000 W/m2 or 95 x 103 Btu/(h-ft2). This work was extended by Raiff and Wayner [347], The need for a porous heated surface and a flow control element appears to limit the application of suction boiling. 

Fig. 4. Nucleate boiling heat transfer coefficients for liquid Nz on horizontal platinum.

There are also correlations for the heat transfer coefficient in nucleate pool boiling that take into account the influence of process pressure. The equations give data related to a basis heat transfer coefficient ho dependant on the projjerties of the heating surface (Cw), the reduced pressure p = p/pc, and the related heat flux q/cio [53, 54]. 

At higher values of AT the bubbles form more rapidly and form more centres of nucleation. Under these conditions the bubbles exert an appreciable agitation on the liquid and the heat transfer coefficient rises rapidly. This is the most important region for boiling in industrial equipment. 

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