Heat Loss from Water Pipe 1

[Netherby]

Can anyone help?

I need to calculate the heat loss from a buried water pipe. I know the input water temperature, length of pipe and flow rate, the pipe dimensions and burial depth. I also know the thermal resistivity of the pipe wall and surrounding soil.

What I am looking for is a formula that will give me the Watts lost per metre of pipe.

Thanks

 

[desertfox]

Hi Netherby

This is not a very simple calculation however my approach would be to assume that the temperature throughout the length of the pipe that is buried is constant ie that the heat loss is purely radial through the soil, you can then use :-

Q= k1A1(T1-T2)/X1

Q= k2A2(T2-T3)/X2

where Q = heat transfer watts/metre

k1&k2 = thermal conductivity of pipe & soil
respetively

A1,A2 = area of pipe and soil respectively

T1,T2 = temperature across wall of pipe

T2,T3 = temperature across outer wall of pipe to
surface of soil

X1,X2 = thickness of pipe and soil respectively

For the area of soil I would consider the depth at whih the pipe is buried to be radius of a cylinderical outer wall of a lagging material

regards

desertfox

 

[desertfox]

Hi Netherby

I should of added you will also need to know surrounding temperature of the soil otherwise there are to many unknowns.

regards

desertfox

 

[magicme]

The exit temperature of the water (assuming constant soil temperature) would be ...

T(m,0) = T(amb) - [T(amb) - T(m,i)] * exp [ - U * A / mdot / Cp ]
where
T(m,0) is the fluid exit temp
T(amb) is the external ambient temp (soil in this case)
T(m,i) is the fluid inlet temp
mdot is the mass flow rate (kg / s)
A is the total internal pipe surface area ( 2 * pi * r(inner) * L )
U is the overall heat transfer coefficient from the flowing water, through the wall into the constant soil temperature on the other side. (This includes convection and conduction terms.)

The overall heat loss would be....

Q = mdot * Cp * [ T(m,0) - T(m,i) ]

These equations can be found in text by Incroprera, Cjengel, etc etc etc. for a constant external ambient temperature.

You can disregard external convection from the pipe, but will have to calculate the internal convection coefficient between the water and the pipe, in calculating the value of U.

regards,

magicme

 

[quark]

thread378-102246