External Pipe Temperature 3

[GordonRWatson]

Does anyone know how to calculate the external pipe surface temperature of a pipe assuming the following conditions? More specifically, we want to know if the pipe will be cold enough to condense moisture from the air, but I will be happy to just find out how to do the problem.

1. Schedule 40 polypropylene pipe.
2. Water flowing through it assuming turbulent flow.
3. Water temperature inside of the pipe = 60 degrees F.
4. Air temperature outside of the pipe = 75F dry bulb and 70 degrees wet bulb.
5. Pipe has no insulation.
6. Pipe diameter = 2"

 

[imok2]

The dew point at 75*f DB and 70*f WB is 67.8*f so if you assume fora slight loss through the pipe you should get condensation. I refer you to this web site:

http://www.connel.net/freeware/psychart.shtml

 

[MintJulep]

Simple one-dimensional heat transfer problem. This page appears to provide a reasonable summary, and even a value of thermal conductivity for polypropylene.

It is probably safe to assume that the ID temperature is equal to the water temperature.

You will have condensastion if the OD surface temperature is equal to or lower than the dew point of your air.

 

[KenRad]

The key is to find the thermal conductivity for the polypropylene pipe. The dewpoint under your 75/70 conditions is about 68F, so if the exterior pipe surface is 68F or colder, you will have condensation.

This is an easy problem if you have metal pipe - the surface temperature would be about 60F and it would definitely sweat. But the plastic pipe has some thermal resistance, so the surface temperature will be a little higher than the water temperature. I believe that the polypro is fairly similar to PVC, and the 0.16" thick pipe wall will not provide enough to keep the pipe from sweating under your design conditions.

You will probably only need a thin layer, maybe 0.5 inch, of insulation with a vapor barrier to prevent the condensation. Go to

http://www.pipeinsulation.org

and download their free "3E Plus" software, and you can calculate the results of various insulation thicknesses and materials.

---KenRad

 

[MintJulep]

How about I provide a link to the page I referred to in my earlier post?

http://www.segerfrojd.se/ppvsmetal.htm