Question

3. Write down the total thermal resistance for a double-pipe heat exchanger. Show how to convert from total resistance to an overall heat-transfer coefficient. Explain how flow regime may affect overall heat transfer rate.

Answer 1

Answer: please check image for detailed calculation and explanation.

At the boundary between the pipes, there is discontinuity due to different regime; this gap can insulate heat flow causing a fluctuation in heat flow from the inner pipe to the outer pipe.

Answer 2

Answer:

$\frac{1}{UA} =\frac{1}{A_{0}h_{0} } +\frac{Rf_{0} }{A_{0} } +\frac{1}{2k\pi L} ln(\frac{di,0}{di,i} )+\frac{Rf_{i} }{A_{i} }$

Explanation:

Global heat transfer determines the heat that is transferred from the inner tube to the outer tube. In the double tube heat exchanger, this global heat transfer coefficient is equal to:

$\frac{1}{UA} =\frac{1}{A_{0}h_{0} } +\frac{Rf_{0} }{A_{0} } +\frac{1}{2k\pi L} ln(\frac{di,0}{di,i} )+\frac{Rf_{i} }{A_{i} }$

where subscript i refers to the inner surface and subscript 0 refers to the outer surface