Plenum box sizing

Plenum box:

A plenum in heating, ventilation, and air conditioning (HVAC) systems is a box that connects to the HVAC system. It has a critical function and that is to bring in, distribute, and remove air. Each HVAC system typically uses two plenum boxes: a supply plenum and a return plenum.

Plenum box with air terminal

Plenum box sizing

Q = Flow rate (CFM = cubic feet per minute)

V = Velocity (ft/min = feet per minute)

A = Area of plenum box (ft2 = square feet)

W = Width of plenum box

H = Height of plenum box

D = Depth of plenum box

d = Diameter of fan (AHU/FCU)

Q = V*A   ------------------------------------- (i)

A = W*H  ------------------------------------- (ii)

D = W*d - ------------------------------------- (iii)

                         

Note: As per ASHRAE standard required velocity

1. Air Handling Unit (AHU) is 800 ft/min
2. Fan Coil Unit (FCU) is 500 ft/min 
3. As rule of thumb 1 TR = 400 CFM  

Plenum box sizing for air handling unit (AHU)

Example: 01

Let's say according to our calculate heat load tonnage is = 8 TR

So, 8 TR neets = 8*400 = 3200 CFM

Area of plenum box

As per equation (i)

A = Q/V ----------------------------- (ft2 = square feet)

A = 3200/800 ft2

Area of plenum box = 4 ft2

Height/Width of plenum box

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

let's say  Height = 1.6 ft

As per equation (ii)

W = A/H ----------------------------- (ft = feet)

W = 4/1.6 ft

Width of plenum box = 2.5 ft

Depth of plenum box

let's say Diameter of fan AHU = 12"

As per equation (iii)

D = W*d ----------------------------- (" = Inche)

D = 2.5*12 ft

D = 30"

Depth of plenum box = 2.5 ft

8 TR AHU plenum box size

Area of plenum box = 4 ft2

Height of plenum box = 1.6 ft

Width of plenum box = 2.5 f

Depth of plenum box = 2.5 ft