Total Pressure Drop vs Equal Friction Duct Design

[dazataz]

All,
I dunno, I understand the newer Equal Friction method but dislike having to rely on introducing friction on the shortest runs with dampers/fittings for balancing.
I've slammed together a quick xls file and xlsx to look over what happens when I do what I understand was a "modified" way of doing Equal Friction.:
1) Review for longest Loop
2) Size that loop within FR and velocity limits
3) Obtain total PD of this Index (Longest Loop)
4) Match all other Branches to have nearly the same loss.

Playing around with the numbers in the spreadsheet, it's easy to see that going up or down just 1 size can louse up the Velocity and PD's quite a bit.

So, given the complexity of doing the analysis, and assuming most of you do not own fancy programs (we can't afford the $ or time to learn and won't hire a Mechanical Engineer for the one-off job we see every-so-often). Do you prefer the newer Equal Friction Method? From a designer's perspective, I prefer what I came up with in the spreadsheet (and we will still install dampers on each branch) - does this ring a bell with you all?

Disclosure, I am NO expect by a longshot ( : AND the data in the attached spreadsheet is basic but was obtained from various ACCA and other published texts.

BTW, is the newest Manual D much different than the 2007 version we have?
Thanks for looking!

[dougheret0]

The latest ASHRAE Fundamentals Handbook, 2009, continues to list equal friction as a viable duct design method, along with static regain and "T-method". In using equal friction, the recommendation is to size the ducts for each branch by equal friction, then to repeat the calculations until the pressures at each branch junction are the same for all paths beginning at that junction. For most low-pressure jobs, this is an excersize in futility, since diameters and rectangular dimensions will have to be rounded up to the nearest nominal size, there will be variations in field and factory fabrications, and branch lengths will change in actual application. All this defeats the junction balance idea. I therefore recommend simply picking the obviously highest loss path, and calculating sizes based on that for a predetermined friction rate (delta P per 100'). That will give the external static pressure needed to size the fan, and the ducts for the other branches can be sized the same way.

At my web site you will find for free download an excel spread sheet with macros to calculate duct sizes by equal friction and external static pressure loss for the critical path. www.nettally.com/doughert

The program allows close sizing to the desired friction rate, gives sizes rounded to nominal, and allows imput of a limited number of fixture coefficients. It is mainly applicable to small commercial jobs where air flow is less than 10,000 cfm.

As for static regain, it will result in larger duct sizes, and may reduce the dampering of shorter or nearer branch runs. But don't leave out the balance dampers, for the reasons noted above.

I am unfamiliar with "T-method" but a cursory review indicates that it would not be applicable to small projects.