I am curious how correct is it to use equivalent lengths for the fittings?
If you wanna be precise, equivalent length would depend on a duct size and velocity, so you cant use the same value for different diameter ducts. The difference in actual pressure drop would be huge.
Also I couldn’t find any good data for register boots. Pressure drop will obviously depend on an aspect ratio as well as ratio of a rectangular to round transition. How can they suggest to use the same equivalent length?
And what’s the deal with no pressure loss data for an angle with only outside corner rounded? This is the mostly used fitting!
I have a feeling that manual D is just a step up from a rule of thumb but nothing close to the real world and a real engineering.