The figure below shows a cross section through an insulated heating pipe which is made from steel (k = 45 W/mK) with an inner radius of 150 mm and outer radius of 155 mm. The pipe is coated with 100 mm thickness of insulation having thermal conductivity of k = 0.06 W/mK. Air at inlet temperature Ti = 60°C flow through the pipe has a value of convective coefficient hi = 35 W/m2K. The outside surface of the pipe is surrounded by air which is at 15°C and convective heat transfer coefficient on this surface has a value of ho = 10 W/m2K. Find the critical insulation radius?

The figure below shows a cross section through an insulated heating pi

1.	The figure below shows a cross section through an insulated heating pipe which is made from steel (k = 45 W/mK) with an inner radius of 150 mm and outer radius of 155 mm. The pipe is coated with 100 mm thickness of insulation having thermal conductivity of k = 0.06 W/mK. Air at inlet temperature Ti = 60°C flow through the pipe has a value of convective coefficient hi = 35 W/m2K. The outside surface of the pipe is surrounded by air which is at 15°C and convective heat transfer coefficient on this surface has a value of ho = 10 W/m2K. Find the critical insulation radius?
A.	14.00 mm
B.	3.00 mm
C.	20.00 mm
D.	6.0 mm
Answer» E.