Paul,

I'm missing something in duct design shape. Referencing Table 12-1, on
page 29 of your book, and Fig 12-11, on page 30, I'm trying to come up
with a duct shape the resembles Kuchemann and Weber's trumpet shaped
diffuser.

Lets assume I have a core with a vertical height of 13" and a width of
12". I'd like to have a 40% intake. 13" x 12" = 156 sq ins x 40% = 62.4
sq in intake area.

Using the table, the last number in the 0.40 column is .400. That
yields a 5.2 ins. high intake (13" x .4 = 5.2"). And, a intake width of
4.8 ins. (12" x .4 = 4.8").

So, according to the table the intake of my trumpet shaped ducting
should be 5.2" x 4.8", I think.

5.2" x 4.8" = 24.96 sq ins., however a 40% intake requires 62.4 sq ins.

I can't seem to make a trumpet diffuser with the numbers in the chart
that fits the required 40% intake sq ins.

Where have I messed up,

S

Let me run the numbers. Core size 12" by 13" say 12.5" average or
half that is 6.25".

So x  (the calculated length of the duct) would be 3 times
that or 18.75 inches long.

Half the height of the opening is .4 times 6.25" (which is half
the height of the exchanger)  or 2.5 inches.
Total height of the opening is then 5 inches.

The key is y/Yb is half the height of the opening. You must
double it to get the full height.

Is this understandable? It would look something like this.

The length might be a problem for you and that is why I favor
the Kays & London wedge. An overall more compact way of doing
it.

Paul Lamar