Fan Selection
Proper fan selection ensures HVAC systems deliver required airflow efficiently and reliably. This systematic approach covers system analysis, fan sizing, performance matching, and specification development.
System Analysis
Airflow Requirements
Determine total system airflow based on:
Cooling Load Method: $$CFM = \frac{Q_{sensible}}{1.08 \times \Delta T}$$
Ventilation Requirements: $$CFM_{OA} = R_p \times P_z + R_a \times A_z$$ (ASHRAE 62.1)
Air Changes Method: $$CFM = \frac{Volume \times ACH}{60}$$
System Pressure Loss
Calculate total system resistance:
$$\Delta P_{total} = \Delta P_{duct} + \Delta P_{fittings} + \Delta P_{components}$$
Component Losses:
| Component | Typical Range |
|---|---|
| Filters (clean) | 0.25-0.50" w.g. |
| Filters (dirty) | 0.50-1.00" w.g. |
| Cooling coil | 0.40-1.00" w.g. |
| Heating coil | 0.20-0.40" w.g. |
| Dampers | 0.05-0.20" w.g. |
| Sound attenuator | 0.25-0.75" w.g. |
System Effect Factor
Account for non-ideal fan connections:
$$\Delta P_{actual} = \Delta P_{calculated} + \Delta P_{system\ effect}$$
System Effect Sources:
- Sharp inlet elbow
- No inlet duct (plenum inlet)
- Short discharge duct
- Elbow at discharge
- Obstructions near fan
Use AMCA 201 for system effect factors.
Fan Type Selection
Application Matching
| Application | Recommended Type | Reason |
|---|---|---|
| Central AHU | BC Centrifugal | Efficiency, reliability |
| Packaged RTU | FC Centrifugal | Compact, economical |
| Exhaust systems | Tube/Vane-Axial | High CFM, moderate pressure |
| Material handling | Radial Centrifugal | Particle tolerance |
| Condenser fans | Propeller | High CFM, very low pressure |
| Space-constrained | Mixed-flow | Compact, good pressure |
Efficiency Priority
For energy-critical applications, prioritize:
- Backward-curved airfoil (80-88% efficiency)
- Backward-inclined (75-82%)
- Vane-axial (70-82%)
- Mixed-flow (70-80%)
Special Requirements
High Temperature:
- Welded steel construction
- High-temperature bearings
- Shaft cooling provisions
Corrosive Environment:
- FRP construction
- Coated steel
- Stainless steel
Explosive Atmosphere:
- Spark-resistant construction (AMCA A, B, or C)
- Non-sparking materials
Performance Matching
Fan Curve Analysis
Steps for proper selection:
- Calculate required CFM and SP
- Add safety factor (typically 10% CFM, 20% SP)
- Plot operating point on candidate fan curves
- Verify operating point is in stable region
- Check efficiency at operating point
- Confirm power requirements
Selection Criteria
Optimal Operating Point:
- 70-80% of peak CFM (typical sweet spot)
- Near peak efficiency
- Right of surge/stall region
- Adequate pressure margin
Multiple Operating Points
For VAV or variable systems:
- Verify performance at minimum flow
- Check for surge/stall at turndown
- Consider VFD speed range
- Evaluate efficiency across range
Sizing Calculations
Fan Static Pressure
$$FSP = TP_{outlet} - TP_{inlet} - VP_{outlet}$$
For draw-through configuration: $$FSP = \Delta P_{system}$$
For blow-through: $$FSP = \Delta P_{downstream} + VP_{outlet}$$
Power Calculation
$$BHP = \frac{CFM \times SP}{6356 \times \eta_{static}}$$
Include drive losses:
- Belt drive: 95-97% efficiency
- Direct drive: 100% (fan shaft = motor shaft)
Motor sizing: $$HP_{motor} = BHP \times Service\ Factor$$ (SF typically 1.10-1.15)
Speed Selection
For belt-driven fans:
$$RPM_{fan} = RPM_{motor} \times \frac{D_{motor\ sheave}}{D_{fan\ sheave}}$$
Standard motor speeds: 1,750, 1,450, 1,150, 870 RPM (60 Hz)
Selection Documentation
Fan Schedule Information
| Item | Required Data |
|---|---|
| Tag | Unique identifier |
| Type | Centrifugal, axial, etc. |
| CFM | Design airflow |
| SP | Static pressure |
| BHP | Calculated brake horsepower |
| Motor HP | Specified motor size |
| RPM | Fan speed |
| Efficiency | At operating point |
| Sound | Sound power levels |
| Arrangement | AMCA arrangement number |
| Class | Construction class |
AMCA Ratings
Specify AMCA certifications:
- AMCA 210: Air performance
- AMCA 300: Sound rating
- AMCA 500: Damper/louver ratings
Construction Details
Arrangement (AMCA 99-0098):
- Arr. 1: SWSI, bearing on sub-base
- Arr. 3: SWSI, bearing on housing
- Arr. 4: DWDI, bearing on sub-base
- Arr. 8: Direct-drive
Class (construction strength):
- Class I: Low speed, low pressure
- Class II: Medium duty
- Class III: High speed, high pressure
- Class IV: Heavy duty
Selection Example
Given Requirements
- Airflow: 15,000 CFM
- System SP: 4.0" w.g.
- Application: Central AHU, VAV system
- Priority: Energy efficiency
Selection Process
Add Safety Factor:
- CFM: 15,000 × 1.10 = 16,500 CFM
- SP: 4.0 × 1.20 = 4.8" w.g.
Select Fan Type: BC Airfoil for efficiency
Evaluate Options:
Model RPM BHP Efficiency Fan A 1,340 16.2 81% Fan B 1,450 17.0 78% Fan C 1,180 15.8 83% Select: Fan C - highest efficiency, adequate speed margin
Motor Selection: 20 HP (next standard size above 15.8 × 1.15 = 18.2)
Verification
- Confirm operating point on curve
- Verify stability at minimum VAV flow (40-50%)
- Check sound power levels
- Confirm motor service factor adequate
Systematic fan selection ensures proper matching between system requirements and fan capabilities, delivering efficient, reliable air distribution throughout the system’s operating range.