NFPA 96 Duct Velocity

[Mechee]

When calculating the minimum grease duct exhaust velocity in a square duct, am I to use the circular round equivalent to calculate the duct area or just multiply the length by width? For HVAC systems, I was taught to use circular equivalent, but I've seen equipment manufacturers use the straight-forward length * width approach.

Example: 1,800 cfm in a 12"x12" duct. Per NFPA, is the velocity 1,800 ft/min [1,800 cfm / 1 sq. ft] or is the velocity 1,917 ft/min [1,800 cfm / (PI* 13.12 inches^2 / 4]?

Thanks in advance.

 

[trashcanman]

I would always use the circular equivalent. The air does not know if it is used for air conditioning or for exhausting uranium dust. Always fall back on the basics.

 

[cheute79]

always use the circular area equivalent, as the molecules don't know the geometry, but be aware of issues related to the geometry. in long, straight sections, flow will be less in the corners of square or rectangular duct, and turns in square present similar issues relative to round, so if you have particulates or entrained droplets, you can expect buildup or extra abrasion/erosion in those locations.

 

[cdxx139]

I use the Trane ductalator's and match up the rectangular size to the velocity.

I recall a senior engineer telling me that the ductalator takes into effect the minimal velocities around the edges of the rectangular duct, when calculating the velocity.

I have compared your equation to this and found them to be off, especially when the cfm gets up over 5000 cfm.

I have not considered using the circular equivalent (but probably should)

knowledge is power