# Heat transfer from a tube into a solid

1. Oct 8, 2009

### cgw

What is the effect of different tube material (conductivity) on the heat transfer from water in an infinite tube to a surrounding infinite solid?

The actual practical application is transfer of heat from hot water (~180F) in 3/4" tubes into sand for heat storage (and then the transfer back). What is the difference between using type L copper tubbing and PEX tubing.

2. Oct 8, 2009

### CFDFEAGURU

So you are trying to design a heat exchanger with the hot fluid being water and the cold material around the tubes being sand?

To answer your question about tube material from a pure heat transfer view point, the obvious answer is the material with the higher thermal conductivity will transfer the most heat.

But you need to consider effects such as, what is the maximum flow velocity you can have in the tubes before erossion becomes a problem. For copper the usually velocity range for water is about 4 - 8 ft/s depending on the quality of the water. You don't want to have to shutdown to repair/replace tubes every week.

You will also want to look at the effects of how tightly you pack the sand. Loosley packed sand vs tightly packed sand. That can effect your tube material and wall thickness. Since, for this design, you will want to use thin walled tubes so as to minimize wall losses and at the same have sufficient strength against buckling/collapse from the sand.

Thanks
Matt

3. Oct 8, 2009

### cgw

The only question is the difference between PEX and copper pipe. All else would remain equal. So the question is (I think) what effect would the difference in conductivity have on the over all resistance between the deep sand and the water (per foot).

4. Oct 8, 2009

### CFDFEAGURU

Allowing a higher heat flux with a more conductive material to enter the sand, the deeper the heat can penetrate. This can be modeled quite easily to obtain a ball park value with some assumptions placed on the sand packing density.

See "Introduction to Heat Transfer" by Incropera and Dewitt.

Thanks
Matt

5. Oct 9, 2009

### ank_gl

Effect on what? heat flux?, outlet temperature?

Pipe material with higher conductivity will have lower outlet temperature. About how much the temperature will go down at the outlet will depend upon the relative conductivites of sand & pipe material.

And as matt said, practical difficulties will kick in sooner or lator.