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

 

[ardilesd]

First problem - steady state

Tout = Ta - (Ta-Tin).exp(-U.Pi.D.L/F.Cp)

where 1/U = 1/ha + 1/hw + e/k (...plus some diameter ratios)

Hope the notation is clear enough

In fact, hw (water side coefficient) is a function of Temperature, so the process could be:

- estimate an outlet temperature
- determine average water temperature
- evaluate fluid properties at average temperature
- determine coefficient hw
- Evaluate Tout from the equation & compare to your estimate
- iterate to achieve the desired precision.

Normally iteration converges fast as variations in temperature dont have a great impact on the hw value.

Next chapter, transient states in pipes.