flat duct transition - poor aspect ratio - estimates on static loss??

[BronYrAur]

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?

 

[walkes]

No idea, but you may get some static regain when it goes back to the original size.

 

[MechEngNCPE]

Check the ashrae fundamentals for your C value and velocity pressure, do the calc.

 

[lma1]

The first trnasition is obviously an irregular one. The duct is expanding on the horizontal dimension and at the same time contracting on the vertical dimension. Area- or velocity-wise this may be considered as an unsymmetrical expanding transition fitting. This system might have to be installed that way due to physical constraints or for other practical considerations, it loss is definitely not easy or impossible to estimate!

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.

 

[berkshire]

BronYrAur
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

 

[BronYrAur]

It was done to make room for some piping. Fortunately, I'm not the engineer of record, but I wanted to be able to alert them that the static drop may be a problem. I was hoping to quantify the drop.