Heat rise in a closed system

[aqe1]

I have 10 electrical components that carries a total of 350 watts inside a sealed .075" thk. steel box of 24"Hx30"Wx8"Deep. What would be the inside temp inside the box before heat starting to dissipate outside the box through convection with outside ambient temp of 50 degrees Centigrade.

 

[thermcool]

Heat will start dissipating as soon as the bulk temperature of the inside water will exceed 50 C.

Zeroth Law of Thermodynamic

 

[25362]

You are speaking of unsteady state heat transfer. If it is a sealed box full of liquid, as thermcool insinuates, and depending on the original liquid nature and temperature, thermal expansion of the liquid may rupture the box before any heat starts to dissipate.

If it is air, because of its low thermal conductivity, the electrical elements, being by definition constant heat flux system components, may heat up markedly and be damaged as a result, until the inner steel wall reaches a temperature just above 50^oC by conduction and radiation (assuming no air convection currents inside the box). The tabulated thermal conductivity for air being about 0.024 W/(m*^oC).

I hope you can estimate the final steel box wall temperature when reaching steady state or equilibrium heat transfer considering external convection and radiation as well.

 

[chicopee]

>509.6 deg Rankine

 

[25362]

To chicopee, why have you selected about 50^oF ?

 

[chicopee]

To 25362, greater than 50 deg F because I assume there is fuid inside the box and delta T is the driving force in this case(I am also assuming the box initial temp is 50 deg F). If there is no fluid and just a pure vacuum than heat transfer from the heating element will be by radiation. If the inside of the box is in direct contact with the heating elements that again 50 def F and heat transfer will be by conduction.

 

[25362]

But the outside air was given at 50^oC !

 

[chicopee]

To 25362- right so my reply deals with heat transfer when a temperature gradient is established and that is when the inside of the box is > 50 deg F --or am I overlooking something about the original question?.

 

[IRstuff]

The OP specified 50ºC. 50ºF is only 10ºC. If the inside of the box is at 50ºF, then heat will flow IN, not OUT.

TTFN