Hi there, I am dealing with a pipe that is surrounded by air and trying to calculate the temperature drop. The inlet temperature is 1273K and the ambient air is 293K.(adsbygoogle = window.adsbygoogle || []).push({});

In my quest to do so I have calculated the overall heat transfer coefficient to be around 10 W/m2K, which I believe for a gas hot fluid/gas cold fluid situation like I have here is a fairly reasonable number. However, the problem arises when I try to calculate the temperature drop along the pipe.

The heat content of the stream at the inlet is Qin=m*cp*Tin= 113W, which is quite low as the mass flowrate is 10^-5 kg/s magnitude.

However, I thought I could calculate the temperature drop by doing Qout = Qin-Qloss, where Qloss = U*A*logmeanT. The area is 2*pi*r*l = 0.063m2 and logmeanT=579K. This gives a Qloss of 365W, which is greater than the inlet stream!

How can the Qloss be greater than Qin. This obviously gives a negative outlet temperature which cannot be true. Can you suggest what I am doing wrong?

Thanks,

David

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# Heat loss from pipe

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