HVAC Systems Encyclopedia

A comprehensive encyclopedia of heating, ventilation, and air conditioning systems

Fan Selection & Performance for HVAC Engineers

Fan Selection & Performance for HVAC Engineers

Proper fan selection matches airflow and pressure requirements at maximum efficiency. Understanding fan laws and system curves enables VFD sizing and troubleshooting.

Fan Laws

At constant speed and size:

Law 1 (Flow proportional to speed):

$$\frac{CFM_2}{CFM_1} = \frac{RPM_2}{RPM_1}$$

Law 2 (Pressure proportional to speed squared):

$$\frac{SP_2}{SP_1} = \left(\frac{RPM_2}{RPM_1}\right)^2$$

Law 3 (Power proportional to speed cubed):

$$\frac{hp_2}{hp_1} = \left(\frac{RPM_2}{RPM_1}\right)^3$$

Significance: Reducing speed by 20% cuts power by 49%

Fan Types

Centrifugal Fans

Forward-curved (FC):

  • Efficiency: 50-65%
  • Applications: Residential furnaces, low-pressure
  • Characteristics: Compact, low cost, overloading power curve

Backward-inclined (BI):

  • Efficiency: 70-80%
  • Applications: Commercial AHUs, return fans
  • Characteristics: Non-overloading power curve, efficient

Airfoil:

  • Efficiency: 80-85%
  • Applications: Large systems, energy-critical
  • Characteristics: Highest efficiency, most expensive

Axial Fans

Propeller:

  • Efficiency: 40-60%
  • Applications: Exhaust fans, cooling towers
  • Characteristics: High flow, low pressure

Tube-axial:

  • Efficiency: 55-70%
  • Applications: Duct-mounted exhaust

Vane-axial:

  • Efficiency: 70-85%
  • Applications: High-capacity, moderate pressure

System Curve

Parabolic relationship:

$$SP = K \times CFM^2$$

Where $K$ depends on duct system resistance

Operating point: Intersection of fan curve and system curve

Fan Sizing

Total Pressure Rise:

$$TP = SP_{discharge} - SP_{suction} + P_{v,discharge}$$

Power:

$$hp = \frac{CFM \times TP}{6,356 \times \eta_{total}}$$

Where $\eta_{total}$ includes fan, drive, and motor efficiencies

Safety factor: 10-15% on pressure (do NOT apply to flow)

Variable Frequency Drives (VFDs)

Energy savings at 60% speed:

$$Power_{60%} = 0.6^3 = 0.216 = 21.6%$$ of full speed power

Savings = 78% power reduction

Applications:

  • VAV systems (modulate fan to maintain duct static pressure)
  • CO₂-based demand control ventilation
  • Seasonal airflow variation

VFD efficiency: 95-97% at full load, 92-95% at 50% load

Practical Applications

  1. System design: Select fan at 85-95% of peak efficiency
  2. VFD control: Maintain duct static pressure at 2/3 point from fan
  3. Troubleshooting: If airflow low, check speed, dampers, filters

Related Technical Guides:

References:

  • ASHRAE Handbook of HVAC Systems and Equipment, Chapter 21: Fans
  • AMCA Publication 201: Fans and Systems
  • Air Movement and Control Association (AMCA) Standards