# N°4 – Three damper (mixing) plenums training (VTC to A level)

While the unit handles the introduction of fresh air, recirculation and the discharge of return air, its plenum has 3 dampers.

• Fresh air damper
• Recirculated air damper
• Exhaust air damper

#### Question

Show with arrows the direction of air distribution in the system below when operating on 100% fresh air.
Indicate the position of the dampers (open O or closed C) in the AHU.

#### Question

Show with arrows the direction of air distribution in the system below when operating on 100% recirculated air.
Indicate the position of the dampers (open O or closed C) in the AHU.

Note in the diagrams below:

– That the dampers closed in “100% fresh air” are open in “100% recirculated air”.

– That the dampers open in “100% fresh air” are closed in “100% recirculated air”.

The dampers operate in opposition.

« 100% fresh air » mode:

« 100% recirculated air » mode:

Note on the above diagrams:

– That when the recirculated air damper opens, the fresh air and exhaust air damper close (and vice versa).

– That the dampers operate in opposition.

Keep these remarks in mind when positioning and studying damper movements in the following exercise.

#### Question

The AHU below delivers 10,000 [m3/h] (350,000 ft3/h) into the room, of which
5,000 [m3/h] (175,000 ft3>/h)) of fresh air.
– Indicate the flows circulating in all the ducts.
– What in [m3/h] (ft3/h)) is the flow of recirculated air?
– If the initial situation was that of 100% recirculation (preceding exercise), what movements have been made by the plenum dampers?
What is their position now?

Recirculated air flow = blower flow – fresh air flow
= 5,000 [m3/h] (175,000 ft3/h)In the initial situation (100% recirculation), the fresh air and exhaust air dampers were closed and the recirculated air damper was fully open.

The change to the present situation led to:
– « Half » opening of the fresh air and exhaust air dampers.
– « Half » closing of the recirculated air damper.

The fresh air and exhaust air dampers have a function opposed to that of the recirculated air damper. They function in opposition.