Question

Water flows on the inside of a steel pipe with an ID of 2.5 cm. The wall thickness is 2 mm, and the convection coefficient on the inside is 500 W/m2 · ◦C. The convection coefficient on the outside is 12 W/m2 · ◦C. Calculate the overall heat-transfer coefficient. What is the main determining factor for U

Answer 1

Answer:

U = 11.67 W/m² °C

Explanation:

Inner diameter, d = 2.5 cm = 0.025 m

Thickness of the wall, t = 2 mm = 0.002 m

thus, the outer diameter, D = d + 2t = 0.025 + 2 x 0.002 = 0.029 m

$h_i =500 W/m^2.^oC$

$h_0 =12 W/m^2.^oC$

Now, based on outside convection heat transfer

we have

$\frac{1}{UA_o}=\frac{1}{h_iA_i}+\frac{1}{h_oA_o}$

on rearranging, we get

$\frac{1}{U}=\frac{1}{h_i}\frac{A_o}{A_i}+\frac{1}{h_o}$

where, $A_i\ \textup{and}\ A_o$ are the perimeter respective to inner and and outer diameter

on substituting the values, we get

$\frac{1}{U}=\frac{1}{500}\frac{0.029^2}{0.025}+\frac{1}{12}$

or

U = 11.67 W/m² °C