Reboiler critical heat flux

Critical heat flux in kettle and horizontal thermosiphon reboilers... 

Jensen and Tang also give relationships for in-line bundles. Application of Eqs. 15.195-15.202 requires a knowledge of local quality within the bundle. If the mass flux is known, then this can be obtained very simply from a heat balance, but if the mass flux is unknown and has to be calculated (as in kettle reboilers), then recourse must be had to methodologies of the type described by Brisbane et al. [213] and Whalley and Butterworth [214]. As in the case of heat transfer coefficient, simple methods have been developed for prediction of critical heat flux in tube bundles by Palen and coworkers (Palen [215], Palen and Small [224]), who relate the single tube critical heat flux (calculated by the correlations given previously on p. 15.63-15.65) by a simple bundle correction factor O/, as follows ... 

It is important to check that the design, and operating, heat flux is well below the critical flux. Several correlations are available for predicting the critical flux. That given by Zuber et al. (1961) has been found to give satisfactory predictions for use in reboiler and vaporiser design. In SI units, Zuber s equation can be written as ... 

The critical flux in forced-convection boiling is difficult to predict. Kern (1950) recommends that for commercial reboiler designs the heat flux should not exceed 63,000 W/m2 (20,000 Btu/ft2h) for organics and 95,000 W/m2 (30,000 Btu/ft2h) for water and dilute aqueous solutions. These values are now generally considered to be too pessimistic. 

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. 

---