Page 320 - Corrosion Engineering Principles and Practice
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                      recirculating system versus a once-through system are maximized at
                      about 4 to 6 cycles of concentration. Below this range, treatment costs
                      become prohibitive. At high cycles (e.g., 8 to 10), the additional water
                      savings  are  not  commensurate  with  the  increased  difficulty  of
                      effective treatment. If the blowdown is shut off entirely, there is still
                      an  effective  upper  limit  of  concentration  dictated  by  water  losses
                      from drift or windage. The normal upper limits might be about 20 to
                      22 cycles of concentration for a mechanical-draft tower.
                         The advantages of water savings affected by the cooling tower also
                      impose  certain  inherent  disadvantages.  The  water  becomes  air
                      saturated,  ensuring  its  full  corrosion  potential,  its  natural  alkalinity
                      tends to increase and aggravate scaling tendencies. The air scrubbing
                      action can contaminate the water with airborne materials, notably dust
                      fines, which form silt in the tower basin, and spores of slime, algae, and
                      fungi that can reproduce in the warm nutrient water of the system.

                      8.4.3  Heat Exchangers
                      Heat is removed from exothermic processes, hot gases, and liquids,
                      and  to  control  operating  temperatures  through  heat  exchangers
                      cooled with water. Shell and tube heat exchangers consist of a bundle
                      of tubes connected to tube sheets which are then installed into a shell
                      (Fig. 8.14). Tube bundles can be parallel with once-through flow from






























                                                 (a)

                      FIGURE 8.14  Shell and tube heat exchangers being dismantled (a) and close-up
                      of heat exchangers head (b). (Courtesy of Kingston Technical Software).
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