flat duct transition - poor aspect ratio - estimates on static loss??
flat duct transition - poor aspect ratio - estimates on static loss??
(OP)
I have a low pressure 16"x8" duct that transitions to a 42"x4". The length of the transition is approx 18". The 42"x4" duct only runs for 24", and then there is another transition (18"long) back to the original 16"x8" There is an 8"x4" tap on the 42"x4" piece serving a 100 CFM diffuser. The remaining 16x8 downstream of the transition delivers a total of 500 CFM. See attached.
Any idea how much static is being lost in this fitting?
Any idea how much static is being lost in this fitting?





RE: flat duct transition - poor aspect ratio - estimates on static loss??
RE: flat duct transition - poor aspect ratio - estimates on static loss??
RE: flat duct transition - poor aspect ratio - estimates on static loss??
The designer should strive to design and arrange duct fittings according to some fitting standards or database so the system loss can be estimated. These are ASHRAE Duct Fitting Database (2009), ASHRAE Fundamentals Handbooks, SMACNA HVAC Systems Duct Design or other technical handbooks or texts.
Notice too the AR (Aspect Ratio) of the transitioned piece is 42:4 or 10.5:1. This is considered bad design. The designer should strive to achieve an AR of ≤ 3:1. In addition to increased surface area, insulation and weight of metal, friction rate and operating cost increase as AR increases.
RE: flat duct transition - poor aspect ratio - estimates on static loss??
That type of fitting would drive me nuts in the dust collection industry, because of the velocity loss in the middle of the transition letting the entrained dust drop out.
I am presuming you are having to do this to get around or under an obstruction. I think the only thing that might give you some relief would be to make the transitions longer if you have the room to do so.
In most cases like this you are stuck with what you have got.
Figure you are going to get velocity and static loss in the center of both transitions. If it bothers you, put a diamond or lozenge splitter in the center of the transition to keep the velocity up.
B.E.
The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor
RE: flat duct transition - poor aspect ratio - estimates on static loss??