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Effect of Air Face Velocity in Air-Cooled Heat Exchangers (ACHEs)

Air face velocity (typically measured in m/s or ft/min) is a critical operational parameter that significantly impacts the thermal performance, energy consumption, and operational reliability of air-cooled heat exchangers.

1. Definition & Typical Ranges

• Face Velocity (Vₐ): Volumetric airflow rate divided by frontal face area
• Common Design Ranges:
  • Low Velocity: 1-2 m/s (200-400 ft/min)
    • Used for dirty/fouling services
  • Medium Velocity: 2-3 m/s (400-600 ft/min)
    • General industrial applications
  • High Velocity: 3-4+ m/s (600-800+ ft/min)
    • Clean air/space-constrained applications

2. Thermal Performance Impact

A. Heat Transfer Coefficient

• Higher Vₐ → Increased turbulence → Better heat transfer
  • Approximate relationship: h ∝ Vₐⁿ (where n ≈ 0.6-0.8)
  • Example: Increasing from 2 to 3 m/s may improve U by 25-35%

B. Approach Temperature

• Higher velocities reduce the approach (T_out,air – T_in,air)
• Practical limit: Minimum 5-8°C approach temperature

3. Pressure Drop & Fan Power

A. Airside ΔP Relationship

• ΔP ∝ Vₐ² (quadratic relationship)
  • 2 m/s → ΔP ≈ 0.3″ H₂O
  • 3 m/s → ΔP ≈ 0.7″ H₂O
  • 4 m/s → ΔP ≈ 1.2″ H₂O

B. Fan Power Consequences

• Power ∝ Vₐ³ (cube law relationship)
  • 25% velocity increase → ~2x power requirement
  • Example: 3 m/s vs 2 m/s → 3.375/8 = 42% more power

4. Fouling & Maintenance Impact

Velocity	Fouling Risk	Maintenance Needs
<1.5 m/s	Low	Infrequent cleaning
1.5-2.5 m/s	Moderate	Annual cleaning
>3 m/s	High	Quarterly cleaning

• High velocity effects:
  • Particle impingement on fins
  • Moisture carryover in humid climates
  • Potential fin erosion (>4 m/s with particulates)

5. Economic Optimization

*Typical life-cycle cost minimization occurs at 2-3 m/s*

B. Design Strategies

• Variable Speed Fans: Adjust velocity to match load
• Louver Controls: Regulate airflow
• Staged Banks: Parallel operation at partial loads

6. Special Considerations

A. Frozen Conditions

• <1 m/s risks condensate freezing
• 2.5 m/s prevents ice formation

B. Noise Generation

• Sound power ∝ Vₐ⁶ (50% velocity reduction → -12 dB)
• Acceptable limits:
  • Daytime: <65 dB at 1m
  • Night-time: <55 dB at 1m

C. Wind Effects

• Crosswinds >3 m/s disrupt airflow
• Wind walls recommended when V_wind/Vₐ > 0.7

7. Industry-Specific Guidelines

Industry	Recommended Vₐ	Rationale
Power Plants	2.2-2.8 m/s	Balance efficiency & fouling
Petrochemicals	1.8-2.5 m/s	Handle dirty air
HVAC	2.5-3.5 m/s	Clean air, compact size
Compressed Air	3-4 m/s	High thermal loads

8. Practical Example: Gas Compressor Cooling

• Conditions:
  • 500 kW heat load
  • Ambient 35°C
  • Moderate fouling
• Design Choices:
  • Selected Vₐ = 2.3 m/s
  • 8 FPI wavy fins
  • VFD-controlled fans
• Results:
  • 5°C approach temperature

• Annual cleaning sufficient
  • 12% energy savings vs fixed-speed 3 m/s design

Conclusion

Air face velocity represents a critical balance between:

1. Thermal performance (favours higher Vₐ)
2. Energy consumption (favours lower Vₐ)
3. Maintenance requirements (favours moderate Vₐ)

Optimal design velocities typically fall between 2-3 m/s for most industrial applications. For your specific application, would you like assistance in:

• Performing velocity optimization calculations?
• Evaluating VFD vs fixed-speed fan economics?
• Analysing fouling risks at different velocities?