Page 321 - Corrosion Engineering Principles and Practice
P. 321

292    C h a p t e r   8                                                                                                           C o r r o s i o n   b y   W a t e r    293


































                                                 (b)
                      FIGURE 8.14  (continued)




                      one end to the other, or with U-tube in which the hot and cooled fluid
                      enter and exit from the same end of the exchanger.
                         The  selection  of  materials  for  these  applications  is  often  a
                      compromise  between  the  requirements  of  the  process  flow  and
                      the  type  of  water.  Associated  with  such  heat  exchangers  are
                      pumps, pipes, and valves to distribute the water and return it to
                      source.  The  various  metals  commonly  used  in  heat  exchangers
                      have quite different thermal conductivities (Table 8.9). However,
                      the thermal conductivity of the metal wall is only one component
                      of  the  resistance  to  heat  transfer  in  a  heat  exchanger  tube.  In  a
                      condenser  (i.e.,  where  steam  is  condensing  on  cold  tubes),  for
                      example,  the  resistance  to  heat  transfer  through  a  tube  wall  is
                      made up of five main components as illustrated in Fig. 8.15 [8].
                      The tube wall resistance is comparatively small so that changes in
                      thermal  conductivity  from  the  use  of  different  metals  in  not
                      necessarily very significant.
                         Most metals used in heat exchangers perform well in clean water,
                      that is, free of sediment, debris, fouling organisms, and pollutants.
   316   317   318   319   320   321   322   323   324   325   326