| name | centrifugal-fan |
| description | Centrifugal fan design — fan laws, pressure rise, specific speed, impeller types (forward/backward/radial), performance curves, system resistance, motor sizing, noise. |
| metadata | {"priority":7,"promptSignals":{"phrases":["centrifugal fan","centrifugal blower","fan design","fan laws","specific speed fan","impeller design","fan performance"],"minScore":3}} |
Centrifugal Fan Design — Complete Skill
Fundamental Relations
Euler turbomachine equation:
ΔP_ideal = ρ (U₂ C_θ2 - U₁ C_θ1)
U₂ = tip speed = ω r₂; U₁ = inlet speed
C_θ = tangential velocity component
For no pre-swirl (axial inlet): C_θ1 = 0
ΔP_ideal = ρ U₂ C_θ2 [Pa]
Power: P = ṁ (U₂ C_θ2 - U₁ C_θ1) = ρ Q U₂ C_θ2 [W]
Fan Laws (Affinity Laws)
For geometrically similar fans operating in same system:
Speed scaling (at constant size):
Q ∝ N; ΔP ∝ N²; P ∝ N³
Size scaling (at constant speed):
Q ∝ D³; ΔP ∝ D²; P ∝ D⁵
Correction for different density:
ΔP ∝ ρ; P ∝ ρ (at same speed and size)
Specific Speed
N_s = N √Q / ΔP^(3/4) [dimensionless or in mixed units]
Dimensionless form: N_s = ω √Q / (ΔP/ρ)^(3/4)
Impeller type selection by N_s:
| N_s (US, rpm, cfm, inH₂O) | N_s (SI approx) | Type |
|---|
| < 1000 | < 0.1 | Backward-curved (high press, low Q) |
| 1000–2500 | 0.1–0.5 | Backward inclined or radial |
| 2500–4000 | 0.5–1.0 | Forward-curved; mixed flow |
| > 4000 | > 1.0 | Axial flow preferred |
Impeller Types
Backward-curved (BCF): Most efficient (η = 0.75–0.85); non-overloading power curve; quiet
Most common for HVAC systems; stable operation
Backward-inclined (BI): Similar to BCF; flat blades; simpler manufacturing; slightly lower η
Radial (paddle) blades: Low efficiency (η = 0.55–0.65); handles dirty/abrasive gas; self-cleaning
Used in dust-laden applications
Forward-curved (FC): Small, compact, high flow; runs at lower speed for given Q
Overloading power characteristic (motor can overload); requires careful motor selection
Used in residential HVAC, small blowers
Performance Curve (Fan Curve)
ΔP vs. Q: parabolic for radial; various shapes for BCF, FC
Operating point: intersection of fan curve and system curve
Stall region: avoid operating left of peak pressure (backward-curved)
System curve: ΔP_system = k × Q² (parabola through origin)
k = system resistance constant from ductwork, filters, coils
Impeller Sizing
Tip diameter D₂:
U₂ = √(ΔP / (ρ ψ)) where ψ = pressure coefficient (0.4–0.7 for BCF)
D₂ = 2 U₂ / ω
Flow coefficient φ:
φ = Q / (π/4 × D₂² × U₂) (typically 0.05–0.2)
Inlet diameter D₁: D₁ ≈ 0.5–0.7 D₂ for BCF
Number of blades: 7–16 typical for BCF; fewer for FC
Efficiency
Total-to-total efficiency:
η_tt = Q ΔP_tt / P_shaft
BCF: η = 0.75–0.85 (peak); 0.65–0.75 (operating point)
FC: η = 0.60–0.70
Radial: η = 0.50–0.65
Motor sizing:
P_motor = P_shaft / η_motor × service factor (1.10–1.25)
Noise
Fan noise sources:
- Blade pass frequency (BPF): f = N × z_blades [Hz]
- Turbulence noise (broadband)
- System noise (ductwork resonance)
Noise reduction:
More blades at lower speed → lower BPF amplitude
Backward curved < forward curved (noise)
Inlet guide vanes, careful duct design
Specific sound power level (L_w):
L_w = K_w + 10 log Q + 20 log ΔP [dB re 10⁻¹² W]
K_w: BCF ≈ 56; axial ≈ 65; FC ≈ 62 (typical values from AMCA 301)
Motor and Drive
Direct drive: simpler; speed fixed by motor (speed change via VFD)
Belt drive: adjustable speed ratio; loss 1–3%
VFD (Variable Frequency Drive): best energy efficiency for variable flow systems
Energy savings: P ∝ N³ → 50% speed → 12.5% power
Standards
AMCA 210: Air performance testing
AMCA 300: Reverse-acting controls
AMCA 301: Sound power methods
ISO 5801: Industrial fans — performance testing
Output
Provide: D₂ [mm], D₁ [mm], N [rpm], U₂ [m/s], Q [m³/s], ΔP [Pa], η [%], P_shaft [kW], P_motor [kW], BPF [Hz], L_w [dB], impeller type recommendation.
