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.