Given the question:
Calculate the energy transfer rate across a 6 in wall of firebrick with a temperature difference across the wall of 50 °C. The thermal conductivity of the firebrick is $0.65\ \frac{\text{Btu}}{\text{hr ft }^\circ\text{F}}$ at the temperature of interest.
The correct answer is 369 W/m2
I used the following approach:
$${ x = 6in => 0.5ft }$$ $${ \triangle T = 50^oC => 122^oF }$$ $${ k = 0.65Btu/hr-ft-F }$$
$${ \frac{Q}{A} = \frac {k\triangle T}{x} = 158.6Btu/hr*ft^2 => 500W/m^2 }$$
But I did not calculate the correct answer. Is there a step that I am missing?