Heat transfer in pipe? 1

[StructureMan44]

Please bear with me, I have a structural background. If we are heating one end of an 18"Ø schedule 160 pipe to 225°F is there an equation that states the length of pipe until the heat dissipates and the pipe is back at the ambient temperature of 70°F? I imagine along the length of pipe it'll fall quickly to ambient temperature, maybe 6in away it is 200°F, 1ft at 150°F, 2ft at 100°F. Is there an equation that approximates this behavior and tells the length needed to return to ambient?

 

[bimr]

There is a thermodynamic equation. The temperature does not drop that quickly because the heat transfer to air is not very efficient. A process engineer should be able to answer your query based on the operating conditions.

 

[BigInch]

I'd guess your BBQ skills aren't really up to par. You can have pretty long tongs and still get burnt. Steel pipe will draw off the heat input into it and transfer it along the pipe length quickly. In fact to reach 225F at one section and keep it there for any length of time can be difficult, especially if the adjacent pipe is initially cool or cold. It will sap off a lot of heat from your source as the temperature of the whole pipe raises. Copper pipe is twice as fast as steel at doing that. If you need to keep it cooler close to where you are heating it, better arrange a blower, or even a water jacket. Ever notice how the whole engine block in your car heats up quickly when there's no water in the radiator.

 

[LittleInch]

StructureMan,

As said above, what you are describing is essentially a sort of heat exchanger and maybe the question would be best in the heat transfer and thermodynamics forum. However the heat transfer rate will be affected by what is happening internally in the pipe, any coatings or other influence on the outer part of the pipe and crucially what is the fluid which is taking the heat away? - air or water? What velocity is present? If "natural convection" does the heat go up and disappear or does it re-circulate?

Is the pipe confined?

Lots of questions which will significantly affect the outcome. It is quite difficult to see how this would work and as BI sas, the amount of heat loss along the pipe could be significant - will this affect the heat input available to maintain a fixed temperature?

If you figure these out or assume some values, then yes there are calculations and equations to allow you to create a temperature profile in steady state, but unless you've got a lot of air movement, your initial guess for a pipe that thick might be an order of magnitude out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

There is some dim possibility of finding a closed form solution, but it's unlikely. You will probably need to discretize the problem and solve for the solution iteratively.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[BigInch]

Are you cooking dry pipe, or is there something inside it and you're really doing a hot tap or something?

 

[IRstuff]

Please refrain from double posting. I suggest that you red flag this one and go with the one you just started in the Heat Transfer forum

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[georgeverghese]

This problem is solved in Chemical Engg by Coulson and Richardson,3rd edn., Vol 1, chapter on heat transfer. If you can accept the approx solution where the heat transfer at the far tip of this pipe is ignored, and we assume no convection currents within the pipe (ie. the natural convection current is only by flow of air over the external surface), then the solution to this is given by

θ/θ₁=cosh[m(L-x)]/cosh(mL)

where θ = (Temp on pipe surface at position x) - bulk air temp; θ₁ = (Temp on pipe at x=0) - bulk air temp - units in deg K

m = [h.b / (k.A)]^0.5, where h is the sum of convective and radiative heat transfer coeff, averaged out over the length of the pipe (w/m2/degK), b = external perimeter or circumference of this pipe in metres, k=thermal conductivity of the pipe metal, w/m/degK, A=cross sectional area of the pipe metal wall, m2

L = length of pipe where θ approaches zero, which theoretically is ∞
x = length of pipe in axial direction

Values for h can be found in Perry's Chem Engg Handbook, 7th edn., page 5-14, table 5-2.

Play around with trial and error values for L and see what values give reasonable approaches to this value.

If you have any specific clarification requests, write back on this same thread.