Piping Heat Loss

[kryanl]

Hello Everyone, I have a piping problem that is two-fold:

Situation: A pipe, roughly 100’ long, in 30-degree F (ambient winter) weather. The inlet water flow and temperature (approx. 100F) are known. I am trying to figure out the water outlet temperature for two cases:

1. The water is constantly flowing through the pipe, therefore the pipe’s temperature is near that of the water.
2. The pipe is initially empty, and assumed that it’s temperature has reached equilibrium with ambient.

I am having a problem with this because I do not know what the film coefficients are, nor do I know how to calculate (or for that matter, even estimate them). All my old textbooks refer to similar problems, except for those, there is only one unknown, either the coefficient or the outlet temperature. I have two unknowns. Can anyone help me figure this out? I appreciate it.

Kayla

 

[jte]

Kayla-

For the air to pipe convection coefficient you can use 2 BTU/(hr*ft^2*°F)for still air and perhaps 10 for windy conditions. Rain and snow - all bets are off. Consider radiation to be included in the convection coefficient.

For water to pipe - this will not usually govern (the air to pipe convection will be the bottleneck in the heat transfer). Depending on flow conditions this can vary widely. Say anywhere from 50 to 1000.

What I'd suggest is that you determine an upper limit for the water temperature by using 2 for air and 50 for water. Then determine the lower limit using 10 for air and 1000 for water. If you only get a 5°F difference in results you might be done. If you play around with the numbers a bit you'll find that the outlet temperature will vary only a few degrees even if you vary the water convection coefficient from 500 to 1000.

jt