Maximum heat flux

The maximum heat flux may be predicted by the Kutateladse-Zuber [Trans. Am. Soc. Mech. Eng., 80, 711 (1958)] relationship, using consistent units ... 

Heat-flux data obtained from calorimeters present in the fire-affected area revealed maximum heat fluxes of 160-300 kW/m. Figure 5.1 shows the calorimeter positions, the final contours of the flash fire, and heat-flux data from calorimeters positioned near or in the flames. No data are available on flame propagation during the vapor-bum tests. 

Figure 10-102A. Maximum heat flux (or burnout). (Used by permission Cichelli, M. T. and Bonilla, C. E. Transactions. AlChE., V. 41, No. 6, 1945. American Institute of Chemical Engineers. All rights reserved.)...

This is limited by the maximum heat flux of approximately 12,000-25,000 Btu/(hr)(fP). 

Recommended limiting maximum heat flux for the tube density coefficient ... 

K = empirically determined constant used as 176 in the range of tj) for bundles q ,3 = maximum heat flux, Btu/ (hr) (fP) i)( = maximum flux physical property factor,... 

The original Zuber equation for maximum heat flux as modified by Palen ... 

Figure 10-103A. Maximum heat flux boiling outside horizontal tubes kettle and internal reboilers. When using the estimate from this curve, a safety factor of 0.7 also should be used. (Used by permission Palen, J. W., and Small, W. M. Hydrocarbon Processing, V. 43, No. 11, 1964. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

In tube bundles, if the disengaging space between the bundle and the kettle is small and insufficient to allow the vapor bubbles to break-free of the liquid and thus tend to blanket the upper tubes with gas, heat transfer will be restricted. For best design the superficial vapor velocity should be in the range of 0.6-1.0 ft/sec to prevent the bubbles from blanketing the tube through the bundle and thereby preventing liquid contact with the tubes. When the maximum heat flux is approached, this condition can occur, so the 1.0 ft/sec vapor velocity is recommended. 

The maximum heat flux recommended by Zuber and confirmed by Palen and Taborek ... 

Use maximum heat flux Q/A = 12,000 Btu/hr/fk. Note that Table 10-26 indicates this value is quite safe. You could use a higher allowable flux. Many designs are operating based upon this conservative value. 

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. 

Figure 30 shows the main characteristics of the effect of pressure. It can be seen that a maximum heat flux occurs generally below 500 psia. The most interesting feature of Fig. 30, however, is a secondary maximum in the region 1500-2000 psia which tends to occur with moderate Ljd ratios, large inlet subcooling, and high mass velocity. 

Similar results have been obtained by Bonilla and Perry 79>, Insinger and Bliss 801, and others for a number of organic liquids such as benzene, alcohols, acetone, and carbon tetrachloride. The data in Table 9.9 for liquids boiling at atmospheric pressure show that tile maximum heat flux is much smaller with organic liquids than with water and the temperature difference at this condition is rather higher. In practice the critical value of AT may be exceeded. Sauer et al.m] found that the overall transfer coefficient U for boiling ethyl acetate with steam at 377 kN/m2 was only 14 per cent of that when the steam pressure was reduced to 115 kN/m2. 

Figure 9.56. Effect of pressure on the maximum heat flux in nucleate boiling...

The heat transfer coefficient of boiling flow through a horizontal rectangular channel with low aspect ratio (0.02-0.1) was studied by Lee and Lee (2001b). The mass flux in these experiments ranged from 50 to 200 kg/m s, maximum heat flux was 15 kW/m, and the quality ranged from 0.15 to 0.75, which corresponds to annular flow. The experimental data showed that under conditions of the given experiment, forced convection plays a dominant role. 

The maximum heat flux achievable with nucleate boiling is known as the critical heat flux. In a system where the surface temperature is not self-limiting, such as a nuclear reactor fuel element, operation above the critical flux will result in a rapid increase in the surface temperature, and in the extreme situation the surface will melt. This phenomenon is known as burn-out . The heating media used for process plant are normally self-limiting for example, with steam the surface temperature can never exceed the saturation temperature. Care must be taken in the design of electrically heated vaporisers to ensure that the critical flux can never be exceeded. 

Kern recommends that the heat flux in thermosyphon reboilers, based on the total heat-transfer area, should not exceed 37,900 W/m2 (12,000 Btu/ft2h). For horizontal thermosyphon reboilers, Collins recommends a maximum flux ranging from 47,300 W/m2 for 20-mm tubes to 56,800 W/m2 for 25-mm tubes (15,000 to 18,000 Btu/ft2h). These rule of thumb values are now thought to be too conservative see Skellence el al. (1968) and Furzer (1990). Correlations for determining the maximum heat flux for vertical thermosyphons are given by Lee et al. (1956) and Palen et al. (1974) and for horizontal thermosyphons by Yilmaz (1987). 

The maximum heat flux for stable nucleate boiling will, however, be less for a tube bundle than for a single tube. Palen and Small (1964) suggest modifying the Zuber equation for single tubes (equation 12.64) with a tube density factor. This approach was supported by Palen et al. (1972). 

Furzer, I. A. (1990) Ind. Eng. Chem. Res. 29, 1396. Vertical thermosyphon reboilers. Maximum heat flux and separation efficiency. 

If the vapor stream velocity exceeds this value, vapor cannot easily get away and thus a partial vapor blanketing (film boiling) may occur. This result is used to predict the maximum heat flux by relating the heat flux to the vapor velocity (see Sec. 2.4.4). 

Clinch, J. M., andH. B. Karplus, 1964, An Analytical Study of the Propagation of Pressure Waves in Liquid Hydrogen-Vapor Mixtures, Report IITRI-N-6054-6, IIT Research Inst., NASA-CR-54015. (3) Cobb, C. B., and E. L. Park, Jr., 1969, Nucleate Boiling—A Maximum Heat Flux Correlation for Corresponding States Liquids, Chem. Eng. Prog. Symp. Ser. 65(92) 188—193. (4)... 

Dhir, V. K., 1992, Some Observations from Maximum Heat Flux Data Obtained on Surfaces Having Different Degrees of Wettability, in Pool and External Flow Boiling, V. K. Dhir and A. E. Bergles, Eds., pp. 185-192, ASME, New York. (2)... 

Macbeth (M5) has recently written a detailed review on the subject of burn-out. The review contains a number of correlations for predicting the maximum heat flux before burn-out occurs. These correlations include a dependence upon the tube geometry, the fluid being heated, the liquid velocity, and numerous other properties, as well as the method of heating. Sil-vestri (S6) has reviewed the fluid mechanics and heat transfer of two-phase annular dispersed flows with particular emphasis on the critical heat flux that leads to burn-out. Silvestri has stated that phenomena responsible for burn-out, due to the formation of a vapor film between the wall and the liquid, are believed to be substantially different from phenomena causing burn-out due to the formation of dry spots that produce the liquid-deficient heat transfer region. It is known that the value of the liquid holdup at which dry spots first appear is dependent on the heat flux qmi. The correlations presented by Silvestri and Macbeth (S6, M5) can be used to estimate the burn-out conditions. 

The maximum heat flux that will have to be maintained in order to operate isothermally will be that generated at the start of the reaction. 

The single tube values for hb have been correlated by equation 14.2, which applies to the true nucleate boiling regime and takes no account of the factors which eventually lead to the maximum heat flux being approached. As discussed in Volume 1, Chapter 9, equations for maximum flux, often a limiting factor in evaporation processes, have been tested by Palen and Taborek , though the simplified equation of Zuber is recommended. This... 

A test of Eq. (21), or (20), is shown in Fig. 14. The same maximum heat-flux data of Cichelli and Bonilla (C2) for three organic liquids which produced three separate lines by Rohsenow s correlation give a single line on the Forster-Zuber plot. The data of Kazakova (Kl) for water boiling at pressures varying from 10 to 80% of the critical pressure also fit the line nicely. 

Inasmuch as the critical AT corresponds to a maximum heat flux, this AT will permit a maximum duty for an evaporator, reboiler, or other boiling equipment. In practice, industrial equipment is designed to operate at slightly less than the critical A T. This gives a performance which is somewhat less than the optimum, but it provides insurance against exceeding the critical AT. 

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