Hydraulic Diameter and stove sizing

[roberto]

Howdy,

I came across this concept when trying to figure out how large of a system you can build for a 6" pipe.  The hydraulic diameter.  This gives you the cylindrical equivalent pipe size for things like square chimneys.

It is defined as
D_H = 4A/P 
where A is the cross sectional area and P is the wetted perimeter of the cross-section.

This says a 6" diameter pipe can carry as much fluid as a 6" square pipe.  6" = 4*36/24

en.wikipedia.org/wiki/Hydraulic_diameter

Anyone know if this is the same for gas as for liquid?  If it is, you should be able to size your brickwork up to 6"x6" for a 6" pipe.

Roberto G.

[Deleted]

Apr 18, 2014 20:09:35 GMT -8 roberto said:

Anyone know if this is the same for gas as for liquid?

In fluid dynamics there are two things that may differ for liquids and gases.
A liquid may not occupy the whole cross sectional area.
Gaseous fluids may become compressible at mach numbers above 0.3.

The hydraulic diameter handles the first case.
For gases the wetted perimeter is always the full perimeter.

[peterberg]

Apr 18, 2014 20:09:35 GMT -8 roberto said:

Anyone know if this is the same for gas as for liquid?  If it is, you should be able to size your brickwork up to 6"x6" for a 6" pipe.

Yes, it is applicable to gas as far as I know. An old formula to compare the square and rectangle chimney stacks to a round stack is as follows: D= 2x length x width/(length + width). Try this with a square and you'll see the comparable diameter is the same as one side of the square.