Friday, September 02, 2011

Heat Exchanger - 3

Fouling in Heat Exchanger 
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General :



  • Deposition of extraneous material on Heat transfer (HT) area.
  • Resistance to flow
  • increases Pressure drop in order to maintain the flow rate.
  • Must be considered during designing & ease of cleaning must be permitted.
  • More fouling fluid allocated to tube side & allow access for cleaning
  • keep fluid velocity constant
  • 5 ft/s for tube side & 3 ft/s for shell side for fouling fluid


Fouling Types:


Fig. 1. Types of Fouling
fig. 2. Corrosion Fouling

fig. 3. Chemical Fouling
                    

fig. 4. Crystallization Fouling

fig. 5. Biological Fouling







Fouling Prevention Parameters:


Fig.6. Fouling Prevention parameters


Fouling Cost:


fig. 7. Costs in Fouling

Removal:

Chemical Cleaning:
  • Chlorination or intermittent shock chlorination to kill organisms and avoid biological fouling
  •  Also used to remove deposited calcium salts
Mechanical Cleaning:
  • Scraper or rotary brushing used to clean shell side.
  • For tubes, sponge rubber continuously sent through them & collected in mesh basket at other end & ball pump injects it again in water flow
  • ATB system:
Fig.8. Tube Brushing












  • uses nylon bristles brush to remove foulants
  • back & forth movement of brush.
  • Online cleaning, no need to shutdown the unit.
  • no further cleaning is required.
  • Installation is time consuming & costly ( due to special valves & control unit to reverse the flow )
  • Reduces maintenance cost & shutdown loss.
Brain Map:


Fig. 9. Pre-designing questions to be asked 


Designing Velocities:


fig. 10. Water Velocities in case of different materials.

                                                          For Other Liquids:














Cleaning Time:


Time dependency of fouling factor:


fig. 11. Time dependency
  • Fouling always increases with time either linear or asymptotically.
  • With increase in time, fouling resistance increases & reaches a constant value. ( this constant resistance value can be compensated while designing ) called as " Induction period. "
  • After Induction Period it increases linearly with time.
Optimum Cleaning Time:

fig. 12. Optimum time at minimum total cost
  • It is that time at which the total cost (energy cost + cleaning cost) is minimum. 
  • Since fouling increases linearly with time, hence Energy cost also increases linearly with time.
  • And with time cleaning cost reduces.
  • Addition of the two cost values gives a curve (green color), whose minimum value givesthe optimum time for cleaning.


4 comments:

  1. Nice compilation... very good resource for people who left school long time back...

    ReplyDelete
  2. Hi all,

    Fouling may significantly impact the thermal and mechanical performance of heat exchangers. It is a dynamic phenomenon which changes with time. Thanks for sharing it......

    Sea Water Evaporator

    ReplyDelete
  3. There is also the self cleaning heat exchanger:
    http://www.rccostello.com/klaren.html

    ReplyDelete
  4. short bt sweet..!!!like it

    ReplyDelete

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