Page 327 - Mechanical Engineers' Handbook (Volume 4)
P. 327

316   Heat Exchangers, Vaporizers, Condensers

                          27, 31, and 32. Therefore, the design heat flux must be limited to a practical maximum
                          value. For many years the limit used by industry was in the range of 10,000–20,000 Btu/
                                                                  2
                             2
                          hr ft for hydrocarbons and about 30,000 Btu/hr ft for water. These rules of thumb are still
                          considered reasonable at moderate pressures, although the limits, especially for water, are
                          considerably conservative for good designs. However, at both very high and very low pres-
                          sures the maximum heat fluxes can be severely decreased. Also, the maximum heat fluxes
                          must be a function of geometry to be realistic. Empirical equations are presented in Ref. 25;
                          the equations give much more accurate estimates over wide ranges of pressure and reboiler
                          geometry.
                             (a) For kettle (HS/K) and horizontal thermosiphon (HS/X, G, H)
                                                    803P          1     0.9
                                                              0.35
                                                           P
                                                                      P
                                               q                                              (53)
                                                max      c  P c       P c   b
                                                    3.1
                                                         DL
                                                           b
                                                 b
                                                         A o                                  (54)
                          In the limit, for     1.0, let     1.0. For     0.1, consider larger tube pitch or vapor
                                        b          b           b
                                     25
                          relief channels. Design heat flux should be limited to less than 0.7 q  .
                                                                                 max
                             (b) For vertical thermosiphon (VT/E)
                                                16,080     0.35 P         1
                                                                      0.25
                                                         2
                                                                   P
                                                                              P
                                                        D
                                           q max         i     0.61                           (55)
                                                               c
                                                        L          P c        P c
                             (c) For tubeside forced circulation boiling
                             Recent research by Heat Transfer Research, Inc. has provided new proprietary data. For
                          horizontal flow the phenomena are very complicated, involving preferential vapor flow at
                                                                                  62
                          the top of the tube. The best published predictive methods are by Katto and Kattan et al. 63
                             In addition to the preceding check, the vertical tubeside thermosiphon should be checked
                                                                      28
                          to insure against mist flow (dryout). The method by Fair was further confirmed in Ref. 33
                          for hydrocarbons. For water, extensive data and empirical correlations are available as de-
                          scribed by Collier. 27  To determine the flow regime by these methods it is necessary to
                          determine the flow rate, as described, for example, in Ref. 28. However, for preliminary
                          specification, it may be assumed that the exit vapor weight fraction will be limited to less
                          than 0.35 for hydrocarbons and less than 0.10 for aqueous solutions and that under these
                          conditions dryout is unlikely.
                             For some applications, such as LNG vaporization, it is required to fully vaporize and
                          superheat the gas. For tubeside vaporization, this is very difficult due to droplet entrainment.
                          Twisted tape inserts, such as supplied by Brown Fin Tube, solve the problem.
           3.4  Air-Cooled Heat Exchangers
                          Detailed rating of air-cooled heat exchangers requires selection of numerous geometrical
                          parameters, such as tube type, number of tube rows, length, width, number and size of fans,
                          etc., all of which involve economic and experience considerations beyond the scope of this
                          chapter. Air-cooled heat exchangers are still designed primarily by the manufacturers using
                          proprietary methods. However, recommendations for initial specifications and rating are
                                                                                           34
                          given by Paikert and by Mueller. A preliminary rating method proposed by Brown is also
                                      2
                                                    3
                          sometimes used for first estimates owing to its simplicity.
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