Low pressue duct air flow velocity

[ssn61]

I have an existing 28x18 low pressure duct with air flow of 4470 cfm. It is located in a school library. In order to satisfy the cooling load, I need to bump up the AHU capacity and the air flow to about 5950 cfm. I will implement the additional external static pressure into the fan selection. I know the standard is not to go above 1200 fpm in low pressure ducts, and this will be in the range of 1800 fpm. In your experience, is the noise going to be a huge issue?

 

[berkshire]

ssn61,
You need to recheck your velocities.
1800 fpm is very close to dust extraction velocities
Is this a student question?
Even with your greater volume, I do not see you going much over 1400fpm.
B.E.

"A free people ought not only be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government."
-George Washington, President of the United States----

 

[cdxx139]

No ss is right. At 28x18 duct at 5950 cfm, youll get close to 1800 fpm.

No sound consultant on board?

Your definately over the velocity, especially for a school library.

What kind of system? Is it VAV? Will that airflow only happen during peak summer load? When school is not in session?

Re check your load. Do you have to many computers? Where did you get your occupancy? from the architect or people per 1000 sf from the ventilation tables?

Can you dump 1000 cfm right as the existing duct enters the space? So your downstream airflow is reduced.

Is it too late to replace the ductwork?





knowledge is power

 

[berkshire]

cdxx139 (Mechanical)
You are right I just re-checked that and came up with over 1700 fpm. Is this a main trunk where you can get the velocity down in the droppers?
It is going to be noisy.
B.E.

 

[Drazen]

yes it is.

libraries ought to be silent places.