Temp drop through stainless steel tubing

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kingmathers

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I am trying to figure out if the temperature of the air from an air handling unit will be low enough for the gage once it travels through the tubing.

The gage is only rated for 120F and the AHU will be running at MAX 200F (though it should never reach more than 170F).

So basically I want to find L to make sure I have enough tubing for the temperature to dissipate from 200F to 120F.

Q/t = 2*pi*k*(Ti-To)L / ln(ro/ri)

k = 112 BTU-in/hr-ft²-°F

Ti = 200F

To = 75F

ro = .125"

ri = .09"

Q/t = 267637L BTU/hr

How would I calculate L? Am I on the right track here?

 
Found this online:

Q/t = 1.1*CFM*dT

dT = 200-120 = 80F

CFM = 500

Q/t = 267637*L BTU/hr (L=length of tubing)

Solving for L I'm getting about 2" of tubing. Doesn't sound right to me. Any help?

 
You would have to assume stagnant air to the gage if you are tapping onto a line with flow since the gage doesn't need much flow to it to "read" the pressure. I would approach this problem by treating the tube running to the gage as a cooling fin. Conduction with still air on the outside of the tube(ambient air temp) would be conservative for ensuring the temp is low enough when it reaches the gage.

 
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