Heat transfer of stainless steel

[bill nye scienceguy!]

Homework Statement

a thick walled tube of stainless steel, ks=19W/m.K, with a 2cm internal diameter and a 4cm outer diameter is covered with a 3cm thick layer of insulation. If the inside and outside wall temperatures are 600C and 100C respectively and the thermal conductivity of the insulation, ki= 0.2W/m.K, determine the heat loss per metre length.

Homework Equations

The Attempt at a Solution

i drew out my diagram of the pipe, with the 'hollow' section of radius 1cm, the radius to the outside of the pipe 2cm and the radius of the outside of the insulation at radius 3.5cm. I then set about working the thermal resistances of the different materials using R=ln(r2/r1)/2pikl which i found to be 0.01466/l for the steel pipe and 0.4599/l for the insulation.

i then tried to find the heat loss using Q/l=(T1-T2)/R where R is the total resistance of the composite. my answer of 1086W/m is almost twice the given answer of 680W/m.

where have i gone wrong?

 

[bill nye scienceguy!]

I looked again at the problem and id drawn my diagram of the pipe wrong so the ratios of the radii were wrong. I've solved the problem now so everyone can disregard this thread!