Electronic – Heating a box (as a thermodynamics dumthe)

heatresistors

I expect to build a dehydrator and am knowledgeable enough for most of it (fan, timer, power supply and so on), bar the most important part: the heating element.

If you're not familiar with electric dehydrators: you usually seek a 30-70°C (most of the time ~40°C) temperature control for a 5-12h drying timespan.

Problem is, I have absolutely no clue how to pick a heating element. The box will be made of wood (not sure about the door, will be either wood or glass) and its dimensions should be something like 30x30x50cm. The box is partially open so air can pass through (with the help of a fan).

Is there a fairly simple way to guess which amount of heating power I need? To pick a heating resistor which will "roughly" do the job at expected temperature?

Best Answer

Basically a thermodynamics problem with thermal conductivity, area, thickness, ΔT

q=​d​​KA(T​.hot​​−T.​cold​​)​​ [watts]

Where:
q = Conduction heat transfer (W)
K = Materials thermal conductivity (W/mK)
A = Cross sectional area (m²)
T​.Hot​​ = Higher temperature (°C)
T.Cold​​ = Colder temperature (°C)
d = Material thickness (m)

  1. Convert your area 30x30x50cm = 6,600 cm² to 0.66 m²
  2. Look up K for wood and choose d
  3. Compute Q heat in watts.

But if this is too slow, adding forced air evaporates faster Then the air flow volume and rate has to be pre-heated and forced thru the box at the desired temperature.

This is more complicated by the efficiency at which air temp rises thru the heat per unit volume of air. A radiator has high efficiency, a circular tube is lower coupling but allows higher flow rates.

I might suggest a 50W power resistor (s) mounted to a CPU heatsink ( tap and screw or clamp) from say an ATX PSU using 12V, using a CPU Fan with a variable speed control and same for heat control and temperature sensing to design a servo loop for the heat and fan speed. LM317's can also be mounted on heatsink for each so no heat is lost here.

schematic

simulate this circuit – Schematic created using CircuitLab

A Cheap and Dirty solution for fan control might be to use 5V on a 12V fan to reduce flow rate since evaporation rate is all day, as long as it is a good fan that starts at 4V.

For compact units choose an old Pentium heatsink. ( free at most repair stores)

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