Calculating heater requirement for an enclosure - Page 1
Forum section: electronics / projects

author: Mr.B
author_rank: Unknown
timestamp: April 18, 2020, 05:40:11 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: April 18, 2020, 06:45:31 am
content: 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.

author: beanflying
author_rank: Super Contributor
timestamp: April 18, 2020, 08:03:56 am
content: 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.

author: Mr.B
author_rank: Unknown
timestamp: April 19, 2020, 12:21:20 am
content: 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.

author: wizard69
author_rank: Super Contributor
timestamp: April 19, 2020, 01:53:47 pm
content: 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.

author: Mr.B
author_rank: Unknown
timestamp: April 19, 2020, 08:32:06 pm
content: The problem will be the restrictions of the enclosure.It is a bog standard wireless bullet camera - made in china.This one to be precise:https://www.aliexpress.com/item/32882187122.html?spm=a2g0s.9042311.0.0.22b14c4dGS2njfI was incorrect in my original post about the enclosure being polycarbonate.The cameras arrived this morning and they are cast aluminium - a big heatsink unfortunately.The insides consist of 3 boards separated by quite lengthy standoffs.I can replace the standoffs with much shorter ones, giving me enough room for a 62mm x 76mm custom board.My custom board will have the automotive filtering, buck converter, MCU and heater.It will not be practical to get the heater resistors closer to the front window.I should be able to add some 2mm thick self adhesive foam round the inside of the case.Thank you for your help.

author: Ian.M
author_rank: Super Contributor
timestamp: April 19, 2020, 10:44:53 pm
content: That's nasty. I think most of us thought you already had the enclosures and were going to mount a camera board inside them. Getting enough heat to the front window will almost certainly cook the boards at the back if you insulate the interior of the case. You need to insulate the exterior so you can use the case to conduct the heat forward, and probably need TO-220 package resistors, with heat spreaders shaped to be a very close fit to the case interior for good thermal contact, slid into place with a smear of heatsink compound.

author: Mr.B
author_rank: Unknown
timestamp: April 20, 2020, 12:29:36 am
content: Thanks Ian,Yes, I am sorry my OP was a bit misleading.I might have to revisit the enclosure and only use the guts of the original camera with my supply/heater board.I do like your recommendation of insulating the exterior and using the die cast aluminium to my advantage.I will explore that avenue.