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# Useful Formulas

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#### 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

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