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Calculating heater requirement for an enclosure
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Mr.B:
Hi all,
Is there a calculation that would allow me to determine the heating requirement, in watts, for a given enclosure size?

Here is the requirement:
A polycarbonate enclosure with a clear window front, measures 120mm x 65mm x 65mm externally. Wall thickness is about 2mm.
In the enclosure is a fisheye camera, IR Leds, WiFi and a buck converter to supply the 12v to the camera. Note: This project is still in development.
The buck converter is supplied with a nasty automotive 48v DC battery forklift supply. The buck converter claims 95% efficiency.
Total power dissipation of the circuit is 8.25w with the IR Leds on and 2.9w with them off.

The camera is mounted above the driver to monitor driver behaviour.
The forklifts work a coldstore. In summer here in Nelson NZ, the forklift can transition between -30oC, 12%RH in the freezers to +28oC, 80%RH outdoors (in summer).
In doing so, everything naturally becomes covered in condensation. When the forklift returns to the freezer that condensation, if still on the devices, will freeze.

The existing forklift mounted touch screen computers we have on the forklifts are internally heated. The surface temperature of the touchscreen sits between +25oC and +28oC.
This prevents condensation forming on the screen during normal operation.

My plan is to FET switch resistors to act as heaters inside the enclosure, maintaining an internal air temperature of about +30oC.
I will control the temperature with a simple MCU and temperature sensor.
Overall current consumption of the circuit is not a consideration. It is on a 4 tonne battery forklift with a monster battery.

So the question is:
How do I calculate the maximum power dissipation in watts required to keep the window warm enough to prevent condensation?
Or, how do I calculate the power required in watts to offset the thermal loading of -30oC and maintain a window surface temperature of between +25oC and +28oC…?
Is there even a way to calculate this, or is it just a matter of trial and error?

Thank you for reading this far, and thank you in advance for any pointers.
Ian.M:
Although it can theoretically be calculated if you have enough data, it would almost certainly be quicker, easier and more accurate to fit a heating element, a dummy PCB to get a realistic internal temperature distribution, and some sort of crude power controller into one of the intended enclosures, with a LCD film thermometer stuck to the lid, taking it into the cold store and twiddling the control knob till you find the required power level to maintain the desired surface temperature.
beanflying:
Yes it can be calculated but there is a lot of variables and unknowns if you try and do it by the numbers. It would be far easier and cheaper to put in to much heating and wrap the enclosure in some insulation and drop a controller on it than to worry about the calculations IMO.

Have a glance at this video if you are not aware of them already likely overkill for your job but interesting tech.

Mr.B:
Thank you both.
I am inclined to agree that the trial and error method will be the easiest.
As beanflying suggests, too much heating, as long as it is controlled, will not be an issue.
I will rename 'trial and error' to be 'structured research' and get on with it.
Thank you again.
wizard69:
Structure research is the key here, this especially if somebody else is paying for it.

In any event I’d seriously consider insulation first. Try to pack internally  as much as possible.  Beyond that your heat generator should be close to the glass, that is the point of heat loss.   

Don’t forget safety in the case of loss of control.  A high temp switch or something similar to break power at excessive temps should’ve implemented. 
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