RE: Friction Rate Calculation Woes
I’m learning more and more about ducting and system design every day, but one of the drawbacks is that I’m trying to teach myself out of Manual D and J by myself. One thing I’ve gotten hung up on is calculating friction rates.
For practice, I’m designing a system based off this matchup. Here’s the data I have so far:
0.8″ WC total external static pressure
Device Pressure Losses:
-0.30″ DX Coil (wet)
-0.16″ media filter (clean)
-0.03″ x 8 supply outlets = 0.24″
-0.03″ x 4 return outlets = -0.12″
-0.03″ x 8 balancing dampers = -0.24″
Total with dampers = -1.06″ WC
Total without dampers = -0.82″ WC
Here’s where I get lost: I’m already at a negative available static pressure before I even add in total equivalent length for ducting.
What am I doing wrong? Should I not be multiplying 0.03″ pressure loss for every termination? I’m using the example of 12 drops between the supply and return which seems pretty conservative for a residential home (I’ve seen many more on a single system.) Even without the balancing dampers on the supplies, I’m over my total. What gives?
Here is a response from Jack Rise – Author of the companion to manual D
Manual D teaches a design methodology called Equal Friction. The procedure is to find the supply outlet with the greatest resistance to air flow and the return inlet with the greatest resistance to air flow. A line is then drawn between the inlet and the outlet to create the “critical path”. So, there is only one supply register, only one return grille and only one volume damper.
That friction rate is applied to every duct run, both supply and return. The assumption is that if you can get air from the return inlet with the greatest resistance to the outlet with the greatest resistance to air flow, then all the other inlets and outlets are cake.
In the field, the damper in the critical path will be wide open. The others will have to be adjusted.
Hope this helps some,