Calculate Thickness of Oak Layer to Limit Heat Loss to 740kJ/hr in Door

[Tonyuguccioni]

The information I am given is : a door has two steel layers both are .47 mm thick, the door itself is 725 mm by 1800mm. The question asks, how thick of a layer of wood (oak) would have to be put in the door to limit the heat loss to 740kJ per hour? Temp inside is 18C and outside is -20C

All work is done with only three significant digits.

I have been working with the equation ;
H=area(temp1-temp2)/((thickness1/k1)+(thickness2/k2))
Where; k is the thermal conductivity of the material
X is the unknown

I have run threw the equation several times but every time I either can't isolate the variable or end up with a wrong answer.

0.205kJ/sec= ((1.30m^2)(38C))/((9.40*10^-5m/43W/mC)+(Xm/0.17W/mC))

Is this the right equation and/or have I made a simple error with conversions ?

Please don't post full answer I want to find it but tips or suggestions would be awesome !

 

[Orodruin]

9.40*10^-5m

$2\cdot 0.47\ {\rm mm} = 9.4\cdot 10^{-4}\ \rm m$ ... Wolfram alpha is your conversion friend: http://www.wolframalpha.com/input/?i=2*0.47+mm+in+m ;)

 

[Tonyuguccioni]

thanks but it is still escaping me i feel like i am just one step away from getting it... very frustrating but it i will prevail ! :P

 

[Orodruin]

Did you try simply solving for X in your equation? (Note that it is usually easier not to make mistakes if you solve the algebraic expression first and insert numbers when this is done)

 

[Felix Larrivee]

whats the next step? and what is the equation you started with was it H= -kA (dt/dx)