Duct Sizing by Hand vs. Ductulator 1

[UtilityLouie]

Hello,

I have been forced to calculate some ductwork pressure drop by hand and ran into some inconsistancies. For a given rectangular duct size on a Ductulator, the velocity doesn't match up with what you would typically calculate as an average duct velocity. ASHRAE Fundamentals even states that the average velocity in a duct is Flow over Area, but the Ductulator doesn't seem to support this.

For example:
64" x 60" duct, 100,000 CFM
The Ductulator shows a velocity over 4000 FPM. Hand calculation shows an average velocity of 3750 FPM. This velocity affects your Reynolds number, and from there the friction factor and from there the pressure drop. This velocity also affects your calculated velocity pressure.

Can anyone out there straighten this out for me?

Thanks.

 

[gmensing]

Dear Louie,

I just started in the HVAC business, but read something this week about calculations in ducts. It seems that to calculate stuff from a rectangular duct, you have to convert it into an equivalent from a round duct. I have seen tables where this rectangular-to-round equivalent is determined. The round equivalent is a bit larger in cross-sectional area than the rectangular duct actually is.

Regards,

Gert

 

[Goorah]

Howdy, Louie:

Having worked at Trane for 25 years (I am now in private HVAC practice), and since that "Tatoo" still hasn't quite come off in the shower, I felt compelled to respond! I TOO use the ductulator almost EVERY DAY!!! (maybe that's the real reason I wanted to reply? ... to make sure we're BOTH using this WONDERFUL, time-saving tool properly; and to be sure of my "faith" in it!) So I broke out my 1989 copy of the Trane "Blue Book" which explains the ductulator's use and duct sizing in general starting on page 283 thru of chapter IX.

In looking thru this section, I was reminded of some basics (one of which is that I am NOT a fluid dynamicist!): The pressure drop thru round and circular ducts of the same cross-sectional area is NOT the same. The term "equivalent" round duct (to a rectangular); or "equivalent" rectangular (to a round, does NOT mean that they have the same area (and therefore the same "average" velocity). On the ductulator, it means that all things being equal (shiny sheetmetal to shiny sheetmetal), 100 feet of straight, round, metal duct has the same pressure drop as the "equivalent" size of 100 feet of straight, rectangular duct as shown on the ductulator. There are laborious tables to do this calculation of equivalency; but the ductulator is much simpler.

Notice that on the instructions of the ductulator it says that the velocity is "The DESIRED velocity in a ROUND duct". This doesn't mean that you can use the ductulaor to find the velocity in rectangular ducts .... but you can find the velocity in an equivalent (P drop wise) round duct.

As for the velocity in the "round duct equivalent" to the 64 x 60 rectangular duct, the ductulator is quite accurate; given its limitations and weaknesses (mine is cardboard): cardboard shrinks and swells with moisture changes, the accuracy of the printing, the exact centeredness of the pivot point, the thickness of the logarithmic scale lines .... even our ability to align and read the thing! ... a lot of variables. My ductulator shows that the equivalent round duct to the 64 x 60 is "around" 67" DIAMETER. Using the cross sectional area of the 67" round to calculate velocity mathematically yields 4,082 fpm. My ductulator at 100,000 cfm shows just a line width over 4,000 fpm. With 200 cfm per line in this range of the dial, I would "read" the ductulator's answer as 4,050 fpm. This is "close enough for government work!" (less than 1% of the mathematical answer) .... Especially in HVAC ductwork with all of it's many variables .... and it gets us in the ballpark for design purposes. The final "proof in the pudding" of course, is the commissioning and balancing of the system; and this is why air handlers have adjustable sheaves ... or better yet: AFD's that use actual measured cfm to adjust the speed.

Perhaps I've rambled too much?? But I hope this helps.

Thanks for forcing me to restore my faith in my trustee, portable, no-batteries required, time-saving friend!!

 

[Tatonka]

If you look in any number of HVAC textbooks, they will give you an alternative "equivalent duct diameter" for rectangular tubing. Depending on the textbooks definition, some will give the equivalent diameter for a round duct section with the same pressure drop per unit length. It may be worth stopping by a library with some ASHRAE texts and spending a few minutes.