Blower Inlet Transition Sizing

[machine85]

I am retrofitting a replacement blower to an existing ring dryer to accommodate some increased airflow requirements. I have calculated the system static and flow requirements and have selected a blower that matches. (38" WC at 18,000 CFM) The question I have is regarding the inlet flange (suction side) on the blower which is 30" ID, vs the existing duct size which is 20". I wasn't sure if there were any issues with fabricating a transition piece to match these dimensions. The manufacture has recommended a 10X diameter length of the transition to minimize any system effects, and otherwise says would not be an issue with airflow or otherwise.

Other fans I considered met the same requirements but had the fan inlet ranging at 21" - 23" ID, but these were significantly more costly, so I would like to use the other unit with the larger inlet.

Just thought I would get a sanity check from anyone with more experience than I on this. Thanks.

 

[racookpe1978]

OK. Logic check here: How do you propose to get the increased airflow through a suction duct only 20 inch dia into a new fan requiring a 30 inch dia suction duct?

I do not doubt that there are 20 x 30 transition pieces -> Those are only sheet metal/flange elements. But I don't see how you have addressed air flow area through that small a suction. It will screw up your efficiency in the fan at minimum.

 

[chicopee]

The attachment will tell all that you need to know about the length of the transition. Now some literature will allow the angle to be as high as 17.5degrees. If you need to know the friction loss, the formula is as follows: FL= Kc*V2^2/ 2*Gc whereby V2(ft/sec) is the outlet velocity of the 30"dia, Gc=32.2 lbm-ft/lbf-sec^2. FL unit will be in lbf-ft/lbm so you'll need to know the density and mass rate of the air to get FL in ft; other unit such as psi, you'll need the appropriate conversion factor.
Kc will vary; for angles between 3.5 and 17.5 degrees, Kc= .011*angle^1.22; for angles between 2.5 and 3.5, Kc=.13; any angle less than 2.5 degrees consider straight run.