# N°4 – Air change rates training – level 2 to 3

Air change rate is the indication of the number of times the volume of air in a room or building is replaced for ventilation, heating or cooling purposes.

If an area of 250 [m3] is subjected to an air change rate of 3, this means that the volume of air is replaced three times per hour. Consequently the air flow rate which is supplied (and extracted) is 3 x 250 = 750 [m³/h].

If the air introduced into the area is just for ventilation purposes we refer to a “ventilation rate”, or an “air replacement rate”.

The letter symbol representing the air change rate is the ancient Greek letter: Ʈ (TAU)

#### Question

A room of 4 x 10 x 3 [m], is subject to an air replacement rate of 3 [V/h].
What is the air flow rate for supplied air in [m³/h] ?

360 [m³/h]

Explanation:
V = 4 x 10 x 3 = 120 [m3]
Ʈ = 3 [V/h]
With the air volume of the room being replaced three times per hour, the supplied air flow rate is therefore :
120 x 3 = 360 [m³/h]

#### Question

In a room of 6 x 12 x 2.5 [m] the supplied air flow rate is 300 [m³/h]
What is the air change rate of the room?

Ʈ = 1.66 [V/h]

Explanation:
V = 6 x 12 x 2.5 = 180 [m3]
The supplied air flow rate is 300 [m³/h]
By dividing the supplied air flow rate by the volume of the room we obtain an air change rate as follows:
Ʈ = 300 / 180 = 1.66 [V/h]

Air change rates vary according to whether the air is used for only ventilation, heating or air conditioning purposes:

#### Question

In an office of 6 x 12 x 2.5 [m], where 6 people work, a ventilation air flow is supplied at a rate of 20 [m3/h/p] (20 [m3] per hour per person).
What is the air replacement rate?

Ʈ = 0.7 [V/h]

Explanation:
V = 6 x 12 x 2.5 = 180 [m3]
Total air flow rate: 20 [m3/h] x 6 = 120 [m³/h]
Ʈ = 120/180 = 0.7 [V/h]

#### Question

Air enters at a speed of 1.5 [m/s] through a kitchen air vent with an area of 8 x 12 [cm].
The volume of the kitchen is 20 [m3].
What is the air replacement rate?

Ʈ = 2.59 [V/h]

Explanation:
qv = 0.08 x 0.12 x 1.5 = 0.0144 [m3/s] = 51.84 [m³/h]
Ʈ = 51.84 / 20 = 2.59 [V/h]

#### Question

Air enters at a speed of 2 [m/s] through the air vent of a technical service room, with a surface area of 3 x 5 [dm].
The volume of the room is 400 [m3].
What is the air replacement rate?

Ʈ = 2.7 [V/h]

Explanation:
qv = 0.3 x 0.5 x 2 = 0.3 [m3/s] or 1,080 [m3/h]
Ʈ = 1,080/400 = 2.7 [V/h]