This is the air volume required to extract or supply air into a room during a given time. Several methods can be used to calculate the air flow (m3/h). :
1- After calculating the volume of the room, multiply volume by the number of Air Changes per Hour ( ACH ):
COMMERCIAL PREMISES | ACH |
Assembly rooms | 4 - 8 |
Bakeries | 20 - 30 |
Banks | 3 - 4 |
Cafe & Coffee bars | 10 - 12 |
Canteens | 5 - 10 |
Cinemas & Theatres | 5 - 8 |
Conference rooms | 8 - 12 |
Dans halls | 6 - 8 |
Entrance halls | 3 - 5 |
Garages | 6 - 8 |
Gymnasium | 6 - 12 |
Hair dressing salons | 10 - 15 |
Hospital sterilizing wards | 4 - 6 |
Commercial kitchens | 15 - 30 |
Laboratories | 8 - 12 |
Launderettes | 15 - 30 |
Lavatories | 8 - 15 |
Libraries | 3 - 5 |
Offices | 4 - 8 |
Photo & X-ray darkrooms | 10 - 15 |
Recording studies | 10 - 12 |
Restaurants | 6 - 10 |
School rooms | 2 - 4 |
INDUSTRIAL PREMISES | ACH |
Boiler rooms | 20 - 30 |
Dye works | 10 - 15 |
Electroplasting shops | 10 - 15 |
Engine rooms | 20 - 30 |
Factories & Workshops | 3 - 6 |
Foundries | 30 - 60 |
Laundries | 30 - 60 |
Paint shops | 30 - 60 |
Stores & Warehouses | 3 - 6 |
Welding shops | 15 - 30 |
These values must not take the place of any regulation requirements and can be modified for particular applications.
2- Depending on the ventilation rates per person:
- 20 – 25 m3/h per person for normal activity
- 30 – 35 m3/h per person if smoking is allowed
- 45 – m3/h per person in case of light physical work
- 60 m3/h per person in case of greater activity
These values are minimum airflows.
3- Air velocity required for capturing or transporting particular matter.
CAPTURE VELOCITY
Cooker hoods
Domestic kitchen from 0.15 to 0.20 m/s
Commercial kitchen from 0.20 to 0.25 m/s
Evaporating tanks from 0.25 to 0.50 m/s Degreasing from 0.25 to 0.50 m/s Welding, dipping from 0.50 to 1.00 m/s Galvanizing from 0.50 to 1.00 m/s Spray booths from 0.70 to 1.00 m/s Grinding down, rectification from 2.50 to 10.00 m/s
TRANSPORT VELOCITY
Dust 9 m/s
Flour 13 m/s
Saw dust 15 m/s
Metallic dust 15 m/s
Wood chips 18 m/s
Lead dust 20 - 25 m/s
To Calculate the required air volume (m3/s) it is necessary to multiply the air velocity by the duct cross sectional area.
Choice of fan
The fan selection must take into account some parameters such as:
1) Type of premises
Industrial
Commercial
Domestic
2) Nature of the air and its characteristics:
Clean air
Air + dust or grease
Special fluid (gases)
3) Nature of the installation:
Supply and extract
Wall fan, roof fan; ducted system
Position of the air entries
Particular conditions
4) Requirements of air flow and pressure drop
5) Acceptable sound level:
In the room
Externally
6) Type of mains supply:
single phase, three phase
voltage
frequency
Other parameters may be considered for the selection such as:
Dimensions
Speed controllable motors
Mounting accessories
The fan laws
These laws enable you to change the aerodynamic parameters:
Airflow (Q)
Pressure drop (∆p)
Power absorbed by the impeller (P)
Sound level (LP)
As a function of:
The speed (RPM) (N)
The impeller diameter (D)
And that, for fans geometrically similar.
1) For a given impeller diameter, changing the speed from N1 to N2 :
Q2 = Q1 × (N2/N1) ∆p2 = ∆p1 × (N2/N1)^2 P2 = p1 × (N2/N1)^3 Lp2 = Lp1 + 50 log(N2/N1) |
2) For a given speed, Changing the impeller diameter from D1 to D2 :
Q2 = Q1 × (D2/D1)^3 ∆p2 = ∆p1 × (D2/D1)^2 P2 = p1 × (D2/D1)^5 Lp2 = Lp1 + 70 log(D2/D1) |
Conversion Tables
Volume flow:
1 CFM = 1.699 m3/h
= 0.472 I/s
1 m3/h = 0.589 CFM
= 0.278 I/s
1 I/s = 2.119 CFM
= 3.6 m3/h
Velocity:
1 ft/min = 0.0051 m/s
1 m/s = 196 ft/min
Volume:
ft^3 = 0.00283 m3
m^3 = 35.31 ft3
Pressure:
1 WG = 249 pa
= 25.4 mmWG
1 pa = 0.004 WG
1 mmWG = 0.0393 WG