Stainless Steel vs. Plastic Tubing heat loss calculations

[natem84]

Will plastic tubing retain heat better than stainless steel? I assume that answer is a yes (thermal conductivity of plastic tubing is ~ .17 W/m K vs. 16 W/m K). If I want to compare how stainless steel will lose heat vs. plastic tubing on a 5 ft line (there are various line lengths but 5 ft. is the max length we'll have) where do I incorporate the different k-values? I will be insulating the line with Armacell NH foam tube insulation (k = .045 W/m K).

I have used the relationship of qin-qout=qstor to find my outlet temperature (where qstor = mdot *cp*(tin-tout)) using k=.045 but I don't see where I would place the k value for the tubing.

I work for a cosmetic lab and it is critical that the tube temperature not fluctuate more than a degree or two over that 5 ft. run. If so, the product begins to separate/solidify/discolor.

The advantage of plastic over steel is cost/flexibility/maintenance. We have to disassemble this line and clean it on a regular basis (weekly). The advantage of SS is that we could place a nozzle heater in-line to overcome the heat loss to the environment.
• Ambient temp is usually 18-20 C
• Fluid temp max is 80 C
• Volumetric flow rate ~ .000095 m^3/s - .000238 m^3/s

Thanks in advance

 

[FeX32]

The conduction coefficient would come into play when you are analyzing the flow of energy from the inside of the pipe through the pipe. (difference in temp. from the inside area to outside area of pipe) I would picture the problem as an energy balance between the inside fluid through the pipe and also onto the surrounding air. Don't think of it as a single convection problem considering only the outside temp. of the pipe as this can cause some misleading results.
I suggest using a resistance method. (R_conv_in, R_pipe, R_conv_out)



Fe

 

[desertfox]

Hi natem84

Have a look at the site below the first link gets you to the main door the second gets you to heat conduction across a tube wall,you don't need the Cp when dealing with this problem.

desertfox

http://www.freestudy.co.uk/home.htm

http://www.freestudy.co.uk/heat and combustion/outcome1t1.pdf

 

[prex]

The tube wall and the tube insulation are on the same path for losing heat (as is BTW the release by convection to ambient air), and the heat resistances add up, as noted above.
Now the total resistance of the insulation will presumably be way much larger than that of the wall (lower conductivity, higher thickness), so the contribution of k in the tube wall will be negligible. A lower conductivity may be of help in the transition regions (e.g. joints in the insulation), but for an insulation correctly installed the effect should also be negligible. However, as you disassemble the line frequently, the insulation could show up some defects with time.
My gut feeling is that you can choose the material for the tube on the basis of the other criteria, the heat loss won't change appreciably, even with your tight specs.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

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[chicopee]

You are telling us that a cosmetic lab will allow the use of plastic?

 

[natem84]

We use only FDA approved high-temperature plastic.

 

[natem84]

Thanks everyone for your assitance. I have done a few more calculations and confirmed what prex assumed. The difference between the two pipes is minor.