Here is our selection of useful formula's to assist in fan selection.

2. Formulae are an essential part of any
engineers toolkit. Without knowing a
specific formula it is hard to know what
product is needed. Here are some useful
formulae you made need for fans.

3. 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

4. Pressure
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
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)
Velocity Pressure ‘K’ factor (round galv.duct)
‘k’ factor depends on diameter and internal surface
friction – check any existing ductwork.

5. General Ventilation
Situation Storage Library Laboratory Bar Workshop Cafe Laundry Kitchen Paint Shop
Changes / h 2 3 5 7 8 10 12 25 50

6. Heat Extraction
Volume flow rate m³/h =
(3000 x W)/Δt
W = Power Loss (kW)
Δt = Temp. Rise (K) from
inlet to discharge

7. 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

8. 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

9. 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

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www.axair-fans.co.uk
Or alternatively Email us at Sales@axair-fans.co.uk
Or Call us on 01782 349430