Air velosity

[rrewis]

This is probably a "duh" question, but I can't focus on it. I am pushing 24,000 cubic feet of air through a 48" diameter duct. Will someone please give me the formula for estimating the speed in miles per hour? Air to duct friction loss does not have to be considered because it is not neccesary the answer be that exact.
Any help will be greatly appreciated,
Russell

 

[IRstuff]

24000 cubic feet PER MINUTE? PER SECOND?

Just divide the volume by the area

TTFN

 

[STYMIEDPIPER]

"speed in miles per hour"
????
The flow rate of the 24000 cubic feet that you identified is most likely CFM (cubic feet per minute)
AS IRStuff had mentioned : divide CFM by area = velocity (ft per minute)

 

[rrewis]

I'm gonna show just how dumb about HVAC I am. 1 foot of 48" dia. duct is about 13 cubic feet. 24,000 cubic feet per minute of air flow divided by 13 equals 1846 feet per minute or 110,800 miles per hour. What am I not understanding here?

 

[IRstuff]

You're converting something incorrectly.

24,000 cfm / 12.55 ft^2 = 1910 ft/min = 21.7 mph


1910 ft ~ 2/5 mi

2/5 mi * 60 min/hr ~ 24 mph

TTFN

 

[rrewis]

Okay, that's better. I warned you I was dumb. Thanks a bunch for everyone's help. Great forum.

Russell

 

[marcoh]

just convert everything to SI units, much easier to convert numbers then!