What is Plenum Box & How to Size a Plenum Box for AHU.

What is a Plenum Box?

A plenum is a distribution box attached to the supply outlet and often return inlet of the air handling unit. And the purpose of plenum box is to distribute the cooled or heated air to several spaces. It is also used to balance the air and attenuate the sound caused by air movement in the duct.

Plenum box sizing for air handling unit (AHU)

To size a plenum box we need to look at the famous equation of Continuity, which states that 

If any liquid is flowing in streamline flow in a pipe of non-uniform cross-section area, then rate of flow of liquid across any cross-section remains constant.

or we can also define it as;

Flow Rate of a fluid flowing through a pipe or channel is equal to Velocity of the fluid and Cross Sectional Area of the pipe or channel.

It can be written as;

                                        Q = V x A --------- (1)

Where;

Q = flow rate (Cubic feet per minute) >> CFM
V = Velocity (feet per minute) >> ft/min

A = Area of plenum box (Square feet) >> ft2

As per the standards velocity for Air handling unit (AHU) is 800 ft/min, and let say our calculated heat load tonnage is = 8 TR.

As rule of thumb 1TR needs 400 CFM

So, 8 TR needs = 8 x 400 = 3200 CFM

As per equation 1:

• A = Q/V
• A = 3200/800
• A = 4 ft2

Area of Plenum box = W x H --------- (2)

According to site condition we have to assume height of plenum box and
try to find width

Assume that H = 1.6 ft

So, using values in equation 2:

Width of plenum box

W = Area of plenum box / height of plenum box = A/H

W = 4/1.6

Depth of plenum can be calculated by using following equation:

D = 2.5 x d --------- (3)

Where d = diameter of fan blower of AHU

Let’s say d = 12”

Using equation 3:

D = 2.5 x 12

D = 30” = 2.5 ft

Plenum box sizing for Fan Coil Unit (FCU):

As per ASHRAE standard required velocity for fan coil unit (FCU) is 500 ft/min. Same procedure as discussed above except velocity i-e 500 ft/min

We introduced to you the calculation steps for sizing a plenum box for both AHU and FCU according to SMACNA. Supply air plenum sizing mainly depends on the air §ow and the air velocity.