Proper Plenum Planning Problem
Proper Plenum Planning Problem
(OP)
Sorry - I could not resist the alliterative impulse.
I have a project that will be moving 30,000 cfm of air and the process unit requires that the air be slowed to an approximate velocity of 300 fpm. Temperature is ambient.
If the ducting carring the air stream is 4 ft diameter, the velocity of the air will be 2381 fpm.
If the ducting carrying the air stream is 5 ft diameter, the velocity of the air will be 1500 fpm.
My question is can someone point me to a reference text to find the equations to calculate needed volume of space to slow the air down to the target velocity? And if there is a simple rule of thumb that will help too!
Many thanks and happy holidays!
I have a project that will be moving 30,000 cfm of air and the process unit requires that the air be slowed to an approximate velocity of 300 fpm. Temperature is ambient.
If the ducting carring the air stream is 4 ft diameter, the velocity of the air will be 2381 fpm.
If the ducting carrying the air stream is 5 ft diameter, the velocity of the air will be 1500 fpm.
My question is can someone point me to a reference text to find the equations to calculate needed volume of space to slow the air down to the target velocity? And if there is a simple rule of thumb that will help too!
Many thanks and happy holidays!





RE: Proper Plenum Planning Problem
Q/v= A
For a circular duct A=pi.D2/4.
In this particular case, Q=30,000 cfm; v=300 fpm. Taking pi=3.14159, the diameter D is found to be:
{(30,000)(4)/[(300)(3.14159)]}0.5 = 11.3 ft.
Season greetings also to you.
RE: Proper Plenum Planning Problem
As you have noted - the face area of the process unit that will be accepting this air flow is ~ 11 ft2.
I need a plenum between the ducting and the inlet that will lower the air velocity to that of the process unit. So my design problem is how do I size this plenum? Can the area of the ducting to the entry into the process unit increase from 4ft2 to 1l ft2 in a 2 ft length? or is some other length required? 3, 4 or maybe 5 ft? The air velocity must be slowed uniformly - so that the air enters the process unit at a very uniform 300 fpm. The target is no more an a 10% variation from 300 fpm at any single point of the 11 ft2 entry area.
RE: Proper Plenum Planning Problem
If I'm not wrong the Re at the slowest velocity of 300 fpm will exceed 105 at NTP. For a smooth duct the velocity profile in turbulent flow would show approximately that the ratio of the velocity v, at any point, to the maximum velocity vmax, would depend on the position (radius r) of the point in the cross section of radius ro as follows:
v/vmax = [(ro-r)/ro]1/7
Thus at a point where, say, r = 0.75 ro (i.e., nearing the wall) the velocity would be 82% of vmax, the deviation being larger than the maximum of 10% requested.
Some sort of flow smoothening fixtures may be necessary to fulfill the target velocity profile. Good luck.
RE: Proper Plenum Planning Problem
The air will be carried via either a 4 ft or 5 ft round duct.
Then the air will pass a plenum - the design problem at hand - before it enters the process unit that is square. I did answer your post above a bit to hastily.
The square unit will be 10 ft by 10 ft - 100 ft2, so the velocity through the face area of the unit will be ~ 300 fpm.
I need to get from the round duct - to the square unit - with relatively uniform air flow across the entire 100 ft2 area.
RE: Proper Plenum Planning Problem
Good Luck
RE: Proper Plenum Planning Problem
RE: Proper Plenum Planning Problem
Peter.