Sizeing heating element power for home-made low-temp oven

[ChristofferB]

Hi!

I'm on a quest to design a low-cost gas chromatograph and have reached the temperature controlled oven.

I want to design a small low-ish temperature oven (ideally room->300 degrees, room->150degrees is acceptable).


Unfortunately, My days at engineering college ended before getting to thermal design, so i'm not really sure how to spec a heating element for something like this.



The only set requirement is that it can heat the air in the box fairly quickly. maybe within 20 minutes.

The oven I want to build is going to be 20x20x10 cm, in aluminium sheet (1.5mm), then a standoff-mounted inner layer of sheets 1.5-2cm further in, making the air volume inside 17x17x8.5 cm. (lid will be insulated on the outside I think). The gap between the sheets will be packed with mineral wool.

And a heating element will be mounted on a raised plate in the center of the box, and a fan will be mounted in the side to keep the entire insides of the box at same temp.

Here's how i'd approach it, but i'm not sure how good an approximation it is:
-> assume the inner aluminum box is an isolated system.
->calculate heat capacity of aluminum and air on the inside of the mineral wool per degree Q=(m_Alc_Al+m_airc_air)DELTAt
->P=E/T ; calculate degrees per second for the system.

I kinda feel there should be a ballpark-er way of doing this. I mean, I guess the design-wise easiest would be to grossly oversize the heating element and just let the PID controller run it at very low effect if needed.

What wattage element would you guys chose?

Thanks in advance, here's a VERY crude ms paint drawing of what i'm trying to do.

[sokoloff]

At 300° C, you will have significant losses to the environment around the outer box. At the very least, you should model those losses (to ensure the outer box won't be too hot, but also to ensure that you have enough wattage on the element to achieve the temps and temp stability you desire).

[langwadt]

stick a heater of know wattage inside, say a light bulb. let i reach steady state, you now know have the watt per degree (inside-outside) losses

how much more you need depends on how fast and how much you need to heat, aluminium is roughly 1J per gram per Kelvin

[ChristofferB]

I get just measuring steady state temp. for a random heater, but that relation is probably not very linear right? I guess I could try a range of heaters or just the same heater PWM'd at different duty cycles.

thanks for the replies!

[james_s]

Just going by the size and temperature range I would say 1500W-3000W would probably work. You might try harvesting the elements from a cheap toaster oven, or you could use tungsten halogen lamps.

[JS]

The easiest way is to heat the thing with some random element and do the math from there. Use whatever heater you have or can get cheaply that will heat the oven a few tens of degree, or buy a set to further expand later. You know the power of the heater, measure it while heating to be sure. Turn the thing on, capture the temperature profile. Whith this you can get the thermal capacity of the oven and the thermal losses. You could find the final temperature, let's say ambient is at 20°C, you've reached 120°C. Final temp with 83°C 10min into the cycle with a 500W heater fully on for an hour.

The thermal resistance will be θtot=100°C/500W=0.2°C/W and your time constant will be those 10 minutes,
τ = θ×Ct
Ct= 600s / 0.2°C/W = 3kWs/°C

Do the math back to find the the required heater for your specs, the minimum heater to reach 300°C is pretty straight forward, for 300°C / 0.2°C/W=1500W

To find the required heater to reach that temp in a short time, let's say 20 min, you need to use the time constant, for this case you need about a 15% increase in power to be there in time with the heater fully on till it gets there. As your time restrictions aren't too tight that number might be good, if you want to control the profile you should go higher in power and let the controller modulate the heat. Also, account for mains variations, ambient temp, etc which could let you without enough heat.

Now, my numbers seems quite a bit off, the thermal resistance is that of a good heatsink rather than a well insulated oven, so I would expect the power requirements to be much lower. 1kW heaters are easy enough to get, if you can fit one inside might work for you. In the other hand, 10min is not a long time, the one you describe might be higher. Thermal capacity is easier to estimate as the mass of the air and the inner layer of Al should make for most of it. Thermal resistance of an insulated box depends on way too many factors to estimate it good enough fast and dirty, might be some calculators out there for ovens, but as you have your physical design already in mind build it, measure it and then get the heater you need.

JS

[ChristofferB]

wow, that's very informative! thanks!

Yeah, maybe get some toaster heaters, or a small kitchen immersion heater as element and then model after that. Maybe it'd be easier to do what all the cool kids do, and just add some SSR's to an actual toaster oven...

[langwadt]

wow, that's very informative! thanks!

Yeah, maybe get some toaster heaters, or a small kitchen immersion heater as element and then model after that. Maybe it'd be easier to do what all the cool kids do, and just add some SSR's to an actual toaster oven...

An immersion heater is probably going to selfdestruct quickly if not in water

[mzzj]

1kW is going to be plenty for 2 liter oven.

For comparison electric kitchen ovens are about 70 liters of volume, have about 3kW of power and really lousy glass door and yet reach ~300C temperature.
YMMV but based on some measurements I did with kitchen ovens they run at less than 50% duty cycle even at 300c setting once they reach the temperature.  (models with pyrolytic cleaning can get up to 500 celsius during burn-off)

[JS]

1kW is going to be plenty for 2 liter oven.

For comparison electric kitchen ovens are about 70 liters of volume, have about 3kW of power and really lousy glass door and yet reach ~300C temperature.
YMMV but based on some measurements I did with kitchen ovens they run at less than 50% duty cycle even at 300c setting once they reach the temperature.  (models with pyrolytic cleaning can get up to 500 celsius during burn-off)

Do you know how fast they heat up?

JS

[ChristofferB]

A commercial oven usually heats up to whatever temperature within 20 min/30min, i'd say. Never timed it though.

[langwadt]

1kW is going to be plenty for 2 liter oven.

For comparison electric kitchen ovens are about 70 liters of volume, have about 3kW of power and really lousy glass door and yet reach ~300C temperature.
YMMV but based on some measurements I did with kitchen ovens they run at less than 50% duty cycle even at 300c setting once they reach the temperature.  (models with pyrolytic cleaning can get up to 500 celsius during burn-off)

Do you know how fast they heat up?

JS

some of the newer ovens with "fast heating" claims 180'C in 5 minutes

[rhb]

I think the answers have already been given, but in piece meal fashion.

Build the oven enclosure. Use as much insulation as possible.  Depending upon temperature range consider multiple layers.  Air entrained refractory and glass or rock wool on the outside layer.  For fast heatup, minimize the heated mass and use a large heating element.

Place a heating element in the oven (e.g. stove top element)

Supply various power levels (e.g. with a variac) and measure inside and outside steady state temperatures at different power levels.

If you like I can scan the relevant pages from "The Conduction of Heat in Solids" which is the classic treatise on the topic.  The difficulty of getting accurate values for the thermal conductivity make the experimental method more practical