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Effect of Flow Velocity in Shell and Tube Heat Exchangers

 

Flow velocity plays a critical role in the heat transfer efficiency, pressure drop, fouling, and mechanical integrity of a shell and tube heat exchanger. Below is a detailed breakdown of its effects:

1. Heat Transfer Efficiency

Higher Velocity → Better Heat Transfer

• Increased turbulence reduces the thermal boundary layer, improving heat transfer.
• Reynolds number (Re) increases, shifting flow from laminar to turbulent (better mixing).
• Typical recommended velocities:
  • Shell side: 0.3 – 1.5 m/s
  • Tube side: 1 – 3 m/s

Too High Velocity:

• No significant improvement in heat transfer beyond a certain point.
• Excessive pressure drop (increases pumping cost).

2. Pressure Drop (ΔP)

ΔP ∝ Velocity² (Square Relation)

• Higher velocity = Much higher pressure drop (Darcy-Weisbach equation).
• Impact on operating costs:
  • More pump/fan power needed.
  • Risk of cavitation (if velocity exceeds limits).
  •  

Recommended Limits to Avoid Excessive ΔP:

Side	Max Velocity (m/s)
Tube Side	2 – 3 (liquids), 20 – 30 (gases)
Shell Side	1 – 1.5 (liquids), 10 – 15 (gases)

Fouling & Erosion

Low Velocity → Fouling Risk

• Particulate deposition increases (sludge, scale, biofilms).
• Stagnant zones promote corrosion.

High Velocity → Erosion Risk

• Tube/shell wall thinning (especially near bends & baffles).
• Sand, slurries, and abrasive fluids accelerate wear.

Optimal Velocity for Fouling Control:

• Tube side: 1.5 – 2.5 m/s (liquids)
• Shell side: 0.5 – 1 m/s (liquids)

4. Vibration & Mechanical Damage

• Excessive shell-side velocity can induce flow-induced vibration (FIV).
  • Causes tube fatigue, fretting wear, and failure.
  • Critical velocity depends on tube material, support spacing, and fluid density.
• Remedies:
  • Use baffle cuts (20-25%) to reduce cross-flow velocity.
  • Anti-vibration bars or helical baffles for stabilization.

5. Pumping Power & Energy Costs

• Higher velocity = More energy consumption (pump/fan power ∝ ΔP).
• Trade-off:
  • Increase velocity → Better heat transfer but higher ΔP.
  • Decrease velocity → Lower ΔP but risk of fouling & poor heat transfer.

Economic Optimization:

• Find a balance where heat transfer gains justify pumping costs.

6. Design Considerations for Optimal Velocity

Tube Side

• Liquids: 1 – 3 m/s
• Gases/Vapours: 10 – 30 m/s
• Two-phase flow: 5 – 15 m/s

Shell Side

• Liquids: 0.3 – 1.5 m/s
• Gases: 5 – 15 m/s

Baffle Spacing Impact

• Wider baffle spacing → Lower shell-side velocity (but poor heat transfer).
• Narrower baffle spacing → Higher velocity (better heat transfer but higher ΔP).

7. Summary of Effects

Velocity Impact	Too Low	Too High	Optimal Range
Heat Transfer	Poor (laminar flow)	No extra benefit	1 – 3 m/s (tube), 0.5 – 1.5 m/s (shell)

Pressure Drop	Low (good)	Very high (costly)	Balance with heat transfer
Fouling	High risk	Low risk	1.5 – 2.5 m/s (tube)
Erosion	Negligible	Severe risk	Avoid >3 m/s (abrasive fluids)
Vibration	None	Tube damage possible	Baffle design critical

8. Practical Recommendations

For liquids: 1.5 – 2.5 m/s (tube), 0.5 – 1.5 m/s (shell)
For gases: 10 – 30 m/s (tube), 5 – 15 m/s (shell)
Fouling fluids: ≥1.5 m/s (tube side) to prevent deposits.
High ΔP systems: Reduce velocity or increase tube diameter.