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  • Andrew George

Required air volume calculation

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:

  1. Airflow (Q)

  2. Pressure drop (∆p)

  3. Power absorbed by the impeller (P)

  4. Sound level (LP)

As a function of:

  1. The speed (RPM) (N)

  2. 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

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