Thanks for the offer - it may be a bit hard to model well and there =

are some variable variables :-) - but to give you an idea.

There are two identical ducts in series with an essentially lossless =

section in between. .

Per duct:

Duct is a very wide and shallow rectangle (a slot in cross section )

- Duct length is (only) 400mm
- Duct width is 350mm.
- Air flow is about 10l / second

Desired pressure is such that a fan rated in the 5 to 10 Watt range =

will maintain the given airflow with the smallest possible duct height =

(and therefore cross section).
I'd guesstimate that 5000 to 1000 Pa range would be OK. (about 1/3 to =

2/3 psi)

In fact just about everything is potentially variable but that's a =

typical configuration. I think that a bit of playing is going to teach =

me much. An added factor to make it all more fun is that the air =

temperature drops or rises by around 20C across the length of each =

short duct :-).

Odds are that's far too variable a set of variables to expect any =

useful results but you are most welcome to comment if appropriate.

My friend Ken Mardle provided this reference

        http://www.engineeringtoolbox.com/darcy-weisbach-equation-d_646.html

which looks like it will be highly useful once I take the time to come =

to grips with its nuances.



        Russell



I have a paper source (the in Germany well-known Recknagel/Sprenger
"Taschenbuch f=FCr Heizung+Klimatechnik"). It contains a few empirical
diagrams, so for certain defined situations, I could look up the =

values.
This may then enable you to extend it further, as long as only =

parameters
change that have a known relationship (length, for example).

Gerhard

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