Boiling liquids coefficients

In boiling liquids on a submerged surface it is found that the heat transfer coefficient depends very much on the temperature difference between the hot surface and the boiling liquid. The general relation between the temperature difference and heat transfer coefficient was first presented by Nukiyama(77) who boiled water on an electrically heated wire. The results obtained have been confirmed and extended by others, and Figure 9.52 shows the data of Farber and Scorah<78). The relationship here is complex and is best considered in stages. 

Cryder and Flnalborgo(82) boiled a number of liquids on a horizontal brass surface, both at atmospheric and at reduced pressure. Some of their results are shown in Figure 9.54, where the coefficient for the boiling liquid h is plotted against the temperature difference between the hot surface and the liquid. The points for the various liquids in Figure 9.54... 

Figure 9.54. Effect of temperature difference on the heat transfer coefficient for boiling liquids (Cryder and...

Table 9.10. Heat transfer coefficients for boiling liquids...

The coefficients of heat transfer from the inner metal surface to the boiling liquid depend upon the temperature difference are ... 

Temperature difference metal surface to boiling liquid (K) Heat transfer coefficient metal surface to boiling liquid (kW/m2 K)... 

A liquid is boiled at a temperature of 360 K using steam fed at a temperature of 380 K to a coil heater Initially the heat transfer surfaces are clean and an evaporation rate of 0.08 kg/s is obtained from each square metre of heating surface. After a period, a layer of scale of resistance 0.0003 nr K/W is deposited by the boiling liquid on the heat transfer surface. On the assumption that the coefficient on the steam side remains unaltered and that the coefficient for the boiling liquid is proportional to its temperature difference raised to the power of 2.5, calculate the new rate of boiling. 

Ivashkevitch, A. A., 1961, Critical Heat Flux and Heat Transfer Coefficient for Boiling Liquids in Forced Convection, Teploenergetika, October, Vol. 8. (5)... 

The heat transfer coefficient (h) for a vapor bubble rising through a boiling liquid is given by... 

Dengler and Addoms 8 measured heat transfer to water boiling in a 6 m tube and found that the heat flux increased steadily up the tube as the percentage of vapour increased, as shown in Figure 14.4. Where convection was predominant, the data were correlated using the ratio of the observed two-phase heat transfer coefficient (htp) to that which would be obtained had the same total mass flow been all liquid (hi) as the ordinate. As discussed in Volume 6, Chapter 12, this ratio was plotted against the reciprocal of Xtt, the parameter for two-phase turbulent flow developed by Lockhart and Martinelli(9). The liquid coefficient hL is given by ... 

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. 

The result to be expected when an immiscible liquid is added to a boiling liquid is uncertain. The addition of water to boiling butadiene has a negligible effect, whereas the addition of water to boiling styrene has a pronounced effect (B6). The unusual behavior of water-styrene mixtures is shown in Fig. 36. Note that all mixtures of the two components give poorer boiling coefficients than do either pure component. 

At this point we should remark that the installation of fins on a heat-transfer surface will not necessarily increase the heat-transfer rate. If the value of h, the convection coefficient, is large, as it is with high-velocity fluids or boiling liquids, the fin may produce a reduction in heat transfer because the conduction resistance then represents a larger impediment to the heat flow than the convection resistance. To illustrate the point, consider a stainless-steel pin fin which has k = 16 W/m °C, L = 10 cm, d = I cm and which is exposed to... 

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. 

The tubes are cooled by a boiling liquid, with an effective heat-transfer coefficient of 2.85 X 10 2 cal./cm.2 sec. deg. 

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