Page 242 - Chemical Process Equipment - Selection and Design
P. 242

212  HEAT TRANSFER  AND  HEAT  EXCHANGERS
                The heat is transferred by direct radiation and convection and also   tubes because the higher convection transfer counteracts the lesser
                by reflection from refractory walls lining the chamber.   radiation due to lack of  refractory wall backing. Accordingly, shield
                   Three  zones are identified in  a  typical heater  such as  that  of   tubes are never finned.
                Figure 8.19(a). In the radiant zone,  heat  transfer is predominantly   The usual temperature  of  flue gas  entering the  shield section
                (about 90%) by radiation. The convection zone is “out of  sight” of   is  1300-1650°F  and  should  be  200-300°F  above  the  process
                the burners; although some transfer occurs by radiation because the   temperature at this point. The proportions of heat transferred in the
                temperature  still is  high  enough,  most  of  the  transfer  here  is  by   radiant  and convection zones can be  regulated by  recirculation of
                convection.  The  application  of  extended  surfaces  permits   hot flue gases into the radiant zone, as sketched on Figure 8.19(b).
                attainment  of  heat  fluxes per  unit  of  bare  surface comparable to   Such  an  operation  is  desirable  in  the  thermal  cracking  of
                those  in the  radiant  zone.  Shield section is the  name  given to the   hydrocarbons,  for  instance,  to  maintain  a  proper  temperature
                first two rows or so leading into the convection section. On balance   profile; a  negative gradient  may  cause condensation of  polymeric
                these tubes receive approximately the same heat flux as the radiant   products  that  make  coke  on  the  tubes.  Multiple chambers  as  in


                                          Stack






                                  000


                                  tubes

                                                  Radiant
                                                  section







                                    (a)
                              A

                          5T#CK











                                               r REFRACTORY








                      c
                    /////      ///’/////A   r‘   b-                           7’    ////////////////

                                   (C)                                                     (d)
                Figure 8.19. Some types of  process fired  heaters  (See  also Fig.  17.16 for  a  radiation  panel heater).  (a)  Radiant,  shield, and  convection
                sections of  a box-type heater. (b) Heater with a split convection section for preheating before and soaking after the radiant section (Lobo and
                Evans,  1939). (c) Vertical radiant tubes in a cylindrical shell. (d) Two radiant chambers with a common convection section.
   237   238   239   240   241   242   243   244   245   246   247