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Sizing Duct by Equal Friction Method?

Sizing Duct by Equal Friction Method?

Sizing Duct by Equal Friction Method?


I am trying to learn how to correctly size duct in a system. From what I understand I need to find the E.S.P. of the fan I use, and account for all pressure drops in the system, such as (coils, filters, dampers, grilles, etc.) and then use this calculation to find the design pressure loss rate(friction loss rate).

Pressure loss rate = [(External Static Pressure - Pressure Drops) * 100]/ Equivalent length of duct

Once I get this pressure loss rate I can use that to size the main duct run and critical branch. By adding dampers on the non critical branches, I can balance the other locations.
I just started working as a project engineer, and my company used the rule of thumb (.08 friction loss in w.c./100 ft) to size all duct systems before I showed up. I know this isn't necessarily acceptable, so any resources/advice is greatly appreciated.

On another note, does anyone know where to find a database with the pressure drop info for filters, coils, dampers, duct fittings, etc?

RE: Sizing Duct by Equal Friction Method?

On supply (or better to say downstream of fans) I use static regain as it will give some more efficient design compared to equal friction. In VAV systems some people also use equal friction and use 0.3" in the beginning of trunk (with diversity it will be less) and 0.05" in branches...But either method gives you a pressure drop. You should review ASHRAE Fundamentals and the SMACNA duct design manual.

I personally use Revit for the design and to determine pressure. But you also can use ductulators etc.

Read some here and also the listed resources.

Coil and other device-internal pressure drops typically are provided by manufacturer. Also diffusers come with data (I built functions into Revit families to automatically calculate those) Read data and their assumptions very carefully! For devices with fan (AHU etc.) You get an external static pressure that already accounts for the internal drops. Your external ductwork needs to be less than that. You kind of design duct first, and select fan afterwards.

RE: Sizing Duct by Equal Friction Method?

Not sure what you mean by "find the ESP of the fan I use" you would have to calculate the ESP of the system so that you can select your fan on TSP, which is not the same as TP and could make a difference depending on your system/fan configuration. The 0.1 rule of thumb works well for constant volume systems depending on construction/energy costs, for VAV systems you are better off sizing at around 0.3 on the main branches and then reducing to 0.1. Like most HVAC engineering, a bit of an art and a science.

RE: Sizing Duct by Equal Friction Method?

A basic place to start with duct air friction is that Total Pressure = Static Pressure + Velocity Pressure.

Static pressure in ducts is only 1/2 of the necessary information.

RE: Sizing Duct by Equal Friction Method?

We use 0.10 in wg/100 ft. for low velocity ductwork supply and 0.08 in wg/100 ft for exhaust or return ductwork. Get slide chart ductulator from your local Carrier Rep. Trane also makes one but I prefer the one by Carrier. See if your office have a copy of SMACNA HVAC Systems Duct Design or get one. This also have fitting and typical equipment losses. You should confirm the equipment loss you use matches the actual equipment submission. Add about 15% loss on ductwork and fitting but not on equipment. Typically filter pressure drop allowance is the mean pressure drop which is the average of the clean and dirty pressure drop.

RE: Sizing Duct by Equal Friction Method?

Thanks for the help guys. I've fixed my mistakes, and learned a couple things. I found an E-Ductalator, which is an excel template from the book "HVAC Simplified". This book is actually recommended on the ASHRAE website. It breaks down the critical duct path, by section, fittings, diffusers, return duct, grills, and lastly what filter is in the system. If you can buy this book I recommend it, as its helped me a lot.

RE: Sizing Duct by Equal Friction Method?

To the OP. Is the author "Stephen Kavanaugh? Thanks.

RE: Sizing Duct by Equal Friction Method?

Yes Sir

RE: Sizing Duct by Equal Friction Method?

this topic has always bugged me because some firms think "we size at 0.08 so we are progressive/efficient blah blah"

q=1.08xcfmxdt, airflow is one of the three variables.

of the three i would argue the load is the most conservative because it increases equipment and duct size.

dt is the next most conservative. again this has more impact on equipment.

the least conservative becomes friction. a firm using 2.5 watt/sqft internal is more than justified using more aggressive friction rates.

you certainly cannot discuss friction without reviewing what the other 2 variables are.

a 150 sqft office

firm a 240 cfm 8" duct
firm b 170 cfm 8" duct
firm c 120 cfm 6" duct
firm d 240 cfm 10" duct

RE: Sizing Duct by Equal Friction Method?

MP, careful with the filters. When sizing a unit, you size the ESP, external static pressure. The filters are usually internal static pressure. ESP + ISP = TSP. ISP is internal to the unit (RTU, AHU...) and while you can get an idea, every manufacturer has a different number due to unit construction.

I design the duct system using several different variables, then go back and determine the path of most resistance to calculate the ESP, then add a little safety factor on. Utilizing SMACNA for correction factors, although ASHRAE fundamentals includes correction factors along with the equations needed.

Airflow required is determined under a totally different method.

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