Double Wall Pipe Pressure Loss 1

[STYMIEDPIPER]

Can anyone suggest how to calculate the pressure loss thru the outer cooling fluid of double wall pipe. The outer fluid is utilized to cool the process fluid in the inner pipe. The outer pipe is eqully spaced around the inner pipe. Hence, the outer fluid has two (2) surfaces of contact.
Can I take the free area between the two pipes and find an equivalent nominal pipe size & then double the length of pipe to account for the second surface of contact.
This sounds like a logical consevative approach.
I would appreciate any comments/suggestions.
Thanks

 

[Montemayor]

STYMIED:

The way I've always done it and the way I was taught in text books is using the Darcy Equation (found in Crane's Tech Paper #410) with a "wetted diameter" in the equation. This is the way I've also seen and had it calculated by others in process design.

If you are familiar with the Darcy equation, it should be no big deal. If you are trying to "calculate" by the use of tables, then you won't get any credible results. What method(s) are you using?

 

[STYMIEDPIPER]

Thanks
I just was not sure about the diameter to use in Darcy.
Please verify that the internal pie size diameter is the equivalent of the free area between the pipes.
Crane does explain that the hydraulic radius is approximately one half the width of the passage when the open width is relatively small when compared to length.
Thanks for your input

 

[Latexman]

Stymied,

I hope you understand that Crane is referring to the cross-sectional shape and parameters of the *annular opening* only, and not to the length of pipe you will use.

The width they speak of would be Ri of the outside pipe – Ro of the inside pipe. The length they speak of would be the circumference of the opening, or Pi x [(Di of the outside pipe + Do of the inside pipe)/2].

I suggest you calculate D = 4 x R_H as they define it.

Also, be sure you use D only for the Reynolds number and in combination with the frictional loss parameters (K = fL/D). Use an “equivalent diameter” of actual flow area, like d_e = (4A/Pi)^1/2 for the velocity terms. This means you will need an understanding of the derivation of the simplified equations they provide.


Good luck,
Latexman

 

[STYMIEDPIPER]

Latexman
Thanks for your input.
Apparently I was mistaken about the length (pipe length versus circumference length)
Very appreciative for the support