Heat Transfer through a Pipe Wall

[subtechy]

Ok, here's the deal......I was never top notch with thermodynamics at Uni but have now been faced with a heat transfer problem.

We have a piece of tubing (slick joint) within a BOP stack that we are flowing oil through. The BOP rams are restricted to 180degF and we therefore have to ensure that we do not exceed the temperature rating of the rams. To do this, we have to restrict the flow rate at surface. We use WellCat at the moment to do this, but we cannot accurately model the Ram across the tubing (i.e. giving an insulating effect essentially). We do not know the heat transfer coefficient for the oil, we do know the temperature of the fluid inside the tubing, we know the temperature of the annulus fluid (40degF external to the ram and which is static), we can use a generic figure for thermal conductivity of the pipe and rubber. My struggle is with the heat transfer coefficients to use.

Can anyone help with this.

 

[BigInch]

Heat transfer coefficients.

In fluids, heat transfer is by convection and in solids heat transfer is by conduction.

For each heat transfer across a solid, you will need to know the heat transfer coefficient of that solid and the temperature on each side,

Heat transfer in a liquid or fluid is by convection, which depends on the density differences created by temperature changes between fluid layers, packages or particles. If the entire cross section of fluid flow is at the same temperature, you don't have to consider heat being transferred from one fluid layer to another, so the calculation would be much simpler, (you don't have to know the heat conductivity of the fluid), but still not quite as simple as one would like. Reason being is because the heat transfer from the fluid to its container also depends on how that fluid is contacting the container wall. Lots of convective action by the fluid against the container wall, means more heat transfer, whereas slow laminar flow along the wall usually means a lot less heat transfer is occuring. So for fluid to solid heat transfer you must evaluate the heat transfer coefficient across the interface. Then across the solid, then into the other fluid on the opposite side, etc. In which case you will need the density, viscosity, Reynold's number, Prandtl, Grashoff, Nusselt numbers, and a few others to find out which convection regime you're in, then calculate the heat transfer coefficient across the interface. You can try this wikilink to the Dittus-Boelter equation,

http://en.wikipedia.org/wiki/Nusselt_number

but in so many words, I don't see any way around cracking that thermo book left over from college and probably buy another one too. I'd suggest you get a hold of J.P. Holman's book "Heat Transfer".

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/