How Fast Does Gas Cool

[NewMechanicalEngr]

I need to get sample gas from a Hot Oil Heater stack and would like advice determining how fast the gas cools after it leaves its source.

The 20 burner (fuel gas) heater has stack gas about 750 deg F at the elevation I need to sample. I intend to use an existing 4" nozzle and attach a 4" to 3" adapter. To the 3" flange, I will attach a blind flange with sample probe attached (it will protrude into the heater). This probe/flange assembly will have a tubing fitting on the outside to which I will connect 316SS tubing between the probe and the analyzer.

I estimate the nozzle will project aobut 12 inches from the heater wall, but keep in mind the probe will be within this region and actually extend into the heater.

I would like to get the sample down to about 300 deg F. I plan to leave some minimal amount of tubing exposed to accomplish this temperature drop, but need help figuring how much. I know the low mass of gas causes it to lose/gain heat very easily, but I am having trouble modeling this. The area around the nozzle is warm, but personnel can stand in the area with no problem.

Thanks for your time & guidance.

 

[JLSeagull]

Consider having the analyzer system manufacturer select the probe and tubing. Often a stainless steel box contains the transition from the probe to the tubing. The tubing is heat traced to avoid water condensation.