Homework Help: Energy emitted by radiation from a pipe

1. Sep 4, 2010

Karol

1. The problem statement, all variables and given/known data
A vertical steam pipe of diameter 8 cm and emissivity e=0.8 has it's surface at 950C. The surrounding air and the temperature of the room and things in it are at 270C.
What is the heat emission rate H from the pipe, for meter length, by radiation?

2. Relevant equations
Stefan's law: $R=e\sigma T^4$
$$\sigma=5.672\times 10^{-8}\left[Watt/m^2\right]$$
$$1\left[Calorie\right]=4.186\left[Joule\right]$$

3. The attempt at a solution
$$R=0.8\cdot 5.672\times10^{-8}\left(373^4-300^4\right)=511\left[Watt/m^2\right]$$
$$H=511\cdot \pi \cdot 0.08\cdot 1=128\left[Watt/m\right]$$
$$128\left[Watt\right]\div 4.186=30.6\left[Calorie/Sec/m\right]$$
The answer, according to the book, is $29\left[Calorie/Sec/m\right]$

2. Sep 4, 2010

JaWiB

95C is ~368K not 373K, though using 368K I get only 28 cal/s/m

3. Sep 4, 2010

Karol

The book uses Kelvin as Celsius+273