Fan Selection: Useful Formulas
Understand how to use fan formulas for industrial fan specification.
Useful Formulas
Conversion Factors
1 cfm = 1.7m³/h
1litre/s = 3.6m³/h
1m³/s = 3600m³/h
1 mmWG = 9.81Pascals
1inWG = 249Pascals
1mBar = 100Pascals
Note: 1Pascal = 1N/m³
Pressure
Velocity Pressure | 'K' factor (round galv.duct) |
Pv (Pascals) = ½ ρ v² or 0.6 v²
For air ρ ≈ 1.2kg/m³ & v = velocity (m/s) |
90° bend ≈ 0.35, 45° bend ≈ 0.18, 'T' junction ≈ 1.40 Discharge = 1.0 Straight section per metre ≈ 0.01 in Ø200 duct (consult loss chart) |
‘k’ factor depends on diameter and internal surface friction – check any existing ductwork.
General Ventilation
Situation | Storage | Library | Laboratory | Bar | Workshop | Cafe | Laundry | Kitchen | Paint Shop |
Changes/h | 2 | 3 | 5 | 7 | 8 | 10 | 12 | 25 | 50 |
Heat Extraction
Volume flow rate m³/h = (3000 x W)/Δt
W = Power Loss (kW)
Δt = Temp. Rise (K) from inlet to discharge
Heat Radiation
W = C x A x T
W = Power Loss
C = Coefficient of heat transfer (6 for steel)
A = Surface area of enclosure
T = Temp. difference in/out
Fume Cupboard Air Velocities
(Very general guide. Observe the specification)
At sash test height > 0.50m/s Sash test height = 500mm
In-duct ≈ 6m/s
At stack discharge ≈ 10m/s
Fan Laws
V ∝ N P ∝ N² K ∝ N³ P ∝ I K ∝ I
V = Air Volume Flow Rate
N = Rotational Speed
P = Pressure
K = Absorbed Power
I = Air Density