Page 326 - Mechanical Engineers' Handbook (Volume 4)
P. 326

3 Rating Methods  315

                                                                   2
                                                        h   [h 2 nb    h ] 0.5                 (46b)
                                                         b
                                                                   cb
                              Equation (46a) includes a nucleate boiling suppression factor,  , that originally was
                           correlated by Chen. 60
                              Equation (46b) is a simple asymptotic proration that was found to work well by Steiner
                           and Taborek. 61
                              The convective boiling coefficient h cb  depends on the liquid-phase convective heat-
                           transfer coefficient h , according to the same relationship, Eq. (29), given for shear-controlled
                                           l
                           condensation. For all reboiler types, except forced flow, the flow velocities required to cal-
                           culate h depend on complex pressure balances for which computers are necessary for prac-
                                 l
                           tical solution. Therefore, the convective component is sometimes approximated as a
                           multiplier to the nucleate boiling component for quick estimations, 25  as in the following
                           equation:
                                                         h   hF  b                              (47)
                                                              nb
                                                          b
                                                              h    h
                                                         F    nb   cb                           (48)
                                                          b
                                                                h nb
                           where F is approximated as follows:
                                 b
                              For tubeside reboilers (VT/E thermosiphon)
                                                           F   1.5                              (49)
                                                            b
                              For shellside reboilers (HS/X, G, H, K)
                                                           F   2.0                              (50)
                                                            b
                           Equations (49) and (50) are intended to give conservative results for first approximations.
                           For more detailed calculations see Refs. 28–30.
                              The nucleate boiling heat-transfer coefficient (h ) is dependent not only on physical
                                                                     nb
                           properties, but also on the temperature profile at the wall and the microscopic topography
                           of the surface. For a practical design, many simplifications must be made, and the approx-
                           imate nature of the resulting coefficients should be recognized. A reasonable design value
                                                            25
                           is given by the following simple equation :
                                                  h nb    0.025FP  0.69 0.70 (P/P ) 0.17        (51)
                                                                 q
                                                                        c
                                                               c
                                                             c
                           The term F is a correction for the effect of mixture composition on the boiling heat-transfer
                                    c
                           coefficient. The heat-transfer coefficient for boiling mixtures is lower than that of any of the
                           pure components if boiled alone, as summarized in Ref. 27. This effect can be explained in
                           terms of the change in temperature profile at the wall caused by the composition gradient at
                           the wall, as illustrated in Ref. 31. Since the liquid-phase diffusional methods necessary to
                           predict this effect theoretically are still under development and require data not usually
                           available to the designer, an empirical relationship in terms of mixture boiling range (BR)
                           is recommended in Ref. 25:
                                                                        ]
                                                   F   [1   0.018q 0.15 BR 0.75  1              (52)
                                                    c
                           (BR   difference between dew-point and bubble-point temperatures,  F.)
                           Maximum Heat Flux
                           Above a certain heat flux, the boiling heat-transfer coefficient can decrease severely, owing
                           to vapor blanketing, or the boiling process can become very unstable, as described in Refs.
   321   322   323   324   325   326   327   328   329   330   331