Sizing Duct by Equal Friction Method?
Sizing Duct by Equal Friction Method?
(OP)
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?
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?
RE: Sizing Duct by Equal Friction Method?
Static pressure in ducts is only 1/2 of the necessary information.
RE: Sizing Duct by Equal Friction Method?
RE: Sizing Duct by Equal Friction Method?
RE: Sizing Duct by Equal Friction Method?
RE: Sizing Duct by Equal Friction Method?
RE: Sizing Duct by Equal Friction Method?
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?
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.