Question

A thin metallic wall may be constructed using copper (properties are given in the table). The piping is required
to have a radius r = 0.008 m and carries steam at 385 K. The wall is inside a room surrounded by air at a temperature
of 298 K. The wall is insulated with a material (properties are given in the table).

Properties	Copper	Insulation material
Thermal Conductivity (W/m per ˚C)	385	0.071
Density (kg/m3)	8940	453

i. If the external the convective heat transfer coefficient, h= 4.0 W/m2.K, what is the critical thickness of insulation?The conductive resistance due to the wall of the copper wire may be assumed to be negligible. (answer: 17.8 mm)

ii. Determine radial heat transfer per meter length of copper tube for the following cases:
a. Insulation thickness = 4 mm (answer: 20.12 W/m)
b. When the insulation thickness reaches the critical          
value r_crit  (answer: 20.10 W/m)
c. Insulation thickness = 50 mm (answer: 17.33 W/m)

Answer 1

Answer for the first question is given for CRITICAL RADIUS OF INSULATION. But the question is about CRITICAL THICKNESS OF INSULATION. so please check that.

And regarding the answers of second question.

They are nearly obtained correct.But the first two answers are given wrong in your problem.

The heat transfer at critical radius of insulation must be more than the heat transfer at 4mm thickness of insulation.

Finally,what is meant to say is follow the answers provided by me in the solution.They are correct and followed the principle.

Hope it helps.

Thank you.

answered by: anonymous