T-method duct optimization with Excel

[bading]

I wrote a EXCEL macro for the calculation of the systems duct with equal friction and T-method.
You can compare the results of the two methods.

T-method duct optimization with Excel

Thank's
Dario

 

[EnergyProfessional]

this made me re-read the T-method as I had ignored that for long. But isn't that only for CAV systems and not really applicable for VAV systems, which are the majority?
You almost would have to combine that with some sort of energy simulation to get the actual flow and pressure profile over a year.
Here a discussion on VAV optimization.
another weakness of this method (and other methods) is they assume duct cost per ft². the problem is a round and rectangular and oval duct probably have different cost per ft² and fittings (efficient, non-efficient) are also not included. In addition larger duct cost doesn't increase linearly. Labor is the same for installing 8"and 10"duct. and if the entire jobsite has 8" and 10" duct installing one single 6" duct actually may be more expensive.

One optimization I try is to make all branches overall pressure drop nearly equal. Meaning branches near the fan a tad smaller, further branches a bit larger as they determine the fan pressure. In VAV systems you also can use slightly larger friction in main trunk since most of the time diversity will reduce that flow. Like make the far branches 0.08", the near branches 0.1" and the trunk near the fan 0.3" pressuredrop/100ft or something like that. This is probably hard to model.... I also try to have fewer transitions in a trunk. Some designs have a transition after every single branch takeoff... ignoring the fact transitions cost money and friction instead of having a transition every 2-3 branch take-offs.

Revit is a nice tool to optimize duct sizes since you can color-code the friction in each section and you have the actual layout. It is an art of some sort.

 

[bading]

Actually in VAV systems the air flow in the network is constantly changing, and this requires a mathematical model definitely complex.
Also the cost of the network of channels will change based on many factors.
The Excel spreadsheet not replace commercial software; it is a fast help for the dimensioning of simple networks and allows you to see the results of the two methods (equal friction and t-method).
Nothing can replace the experience of the designer!.
Thank's
Dario