HEELP !! safety Tubes ( double wall) 1

[Mac001]

Hi there !!

How can I calculate with HTRI an exchanger with double wall tubes. It's practically a safety exchanger with a OD 19.05mm ( ID 14.5 mm) 19 FPI copper tube on the outside and 1 OD 14mm tube on the inside (ID ~12 mm)?

Hoe can I simulate such a tube ? (Let's say that:
in the intermediate chamber there is compress air)

Thank You in ADVANCE

 

[chicopee]

Is there any conductive material in the .25mm spacing between the tubes?

 

[IRstuff]

With such a narrow gap, that air can be treated as a purely conductive medium. There would be no convective whatsoever in the gap.

TTFN

FAQ731-376

 

[Mac001]

THank you for the help !!

By considering the air a pure conductive medium, I should consider it like a tube with a thickness of 19.05-12 =3.5 mm with a resistance due to the sum of (2.5/K(copper) + 0.25/K(air) +1/Kcopper) or more simply 0.25/K(air).

 

[IRstuff]

Are you using the thermal conductivity of STD air, or "compressed air" as stated in the OP?

TTFN

FAQ731-376

 

[Mac001]

Right question !!

Since the pressure will be at around 15 bar.
Since the conductivity is very low, this could be a relevant step in the right direction !!

Thank you very much for the tip !!

Do you know where I could find a table with these values ?

Thank you very much !!

 

[Mac001]

Oops i forgot,
the temperatura will be around 40°C

 

[Mac001]

By the way I have another issue !!

THe tube is finned with 19 FPI geometry. the conductivity value that i'm going to insert will not "interfere" with the parameters of the finns ( which are in copper). I mean it has sense simulate the air just changing the conductivity values , or it's a too rude approach ?

Thank you in advance !!

 

[IRstuff]

??? are the fins in the gap? if so, then they're in parallel, but you can treat them like parallel thermal resistances, so the net thermal resistance would be lower, if that's the case.

I don't do much with gas properties, but NIST has a database somewhere with that information:

http://www.nist.gov/srd/WebGuide/nist72/72.htm

http://srdata.nist.gov/gateway/gateway?property=standard+air

Engineering Toolbox might have something:

http://www.engineeringtoolbox.com/air-properties-d_156.html

TTFN

FAQ731-376

 

[Mac001]

Actually the tube is finned on both side internally and externally .

THe question is how it will behave externally with the new fins?

Thank you very muche for the tip for the series !!

 

[IRstuff]

for SHORT fins, they simply add to the surface area

for fins between the two tubes, you would simply determine the thermal conductivity of the fins and add to that the thermal conductivity of the air between the fins. If there are a lot of fins between the tubes, you might be able to ignore the air altogether.

TTFN

FAQ731-376

 

[Mac001]

Thank you very much for the help and the precious tips !!