Very Ghetto Hot Box Design (for OAC)

[chrismp]

As I mentioned in a recent discussion with Jan (OAC), I went to take some pictures of the hotbox we built using salvaged convection oven parts.
We don't use it to heat a press, but the design could be easily adapted. It's purpose in our shop is to dry our cores to a certain level and to cure polyurethane resin sidewalls/tipspacers (these applications require 70-110°C).

Basically what we did was build two wooden boxes out of particle board. One a bit bigger than the other. Then we put the smaller one inside the bigger one and added fire retardant insulation. Same goes for the lid of the box.
The smaller box is lined with some inflammable drywall sheets to make things safer (the sidewalls require 16h cure time).
The electronics are salvaged from an old convection oven we got for free. What we needed were the motor for the fan, the fan itself and a couple of heating elements.
We mounted the motor outside the box to keep it cool and extended the shaft so we could place the fan inside the inner box. Around the fan we placed the ring heating element.
Then we added a channel built out of drywall material to optimize airflow. That channel is covered with some steel plates.
That's it.

The box in all it's glory:



Fan motor with tube around it to help with cooling:



Fan motor again:



A look inside with the steel plates covering the air channel (the fan is behind that grill in the back):



Air channel opened:



Fan/heater detail view:



Second "backup" heating element:



The box reaches 100°C within about half an hour with the heater set at around 1200w. After that we can turn the power way down cause it only needs about 500w to keep the temp at that level (after 16h even less).
To control the wattage we use the same dimmers that we use for our heat blankets (Kemo M028N).

[MontuckyMadman]

dude. ghetto is a glorified term. Super fugging dangerous is what I would call it.
You seem like you know what you are doing, but if it were me I would be scared.
Just my opinion.
How bout a breakdown on that homemade heating blanket with pics you promised?

[Brazen]

haha, that thing is a great idea, it's a little scary what with the delicious crispy browning of the edges though. Had you thought about using kiln board? I'm not sure of the name...it's high in calcium silicate and pretty good fire-resistance qualities as I recall. Don't hurt yourself with that thing

[chrismp]

dude. ghetto is a glorified term. Super fugging dangerous is what I would call it.
You seem like you know what you are doing, but if it were me I would be scared.
Just my opinion.
How bout a breakdown on that homemade heating blanket with pics you promised?

it's not that bad actually. we have some safety features like a switch that kills all the heater power if the fan stops. then there's a remote smoke alarm right above it.

the photos of the heating blankets will be posted sometime after sunday.

@Brazen: the crispy browning was just some tests we did with a propane torch trying to get the stuff to burn it's just the paper cover that got burnt, but that was it.
if there's a fire in there it couldn't burn for long since it doesn't get any oxygen in there. that box is pretty much airtight.
we even put a small fire in there to test it...opened the box after ten minutes and the fire was out.

the biggest risk is a cable fire that could burn the outer boxes. but that risk is pretty low as well since we used really thick teflon insulated wires for the heater.

PS: this box is up and running without a single problem for about half a year (=about eight full 16h cycles).

[a.badner]

looks cool... -ish, but the heating component does look like it can bring the particle board to a nice blaze with contact. but i would not worry about that.

just a few questions, ( for my own heat box), how well does this keep heat, insulation wise) and do you think you can use a different heating component ( like a ceramic heating fan ) ?

[chrismp]

looks cool... -ish, but the heating component does look like it can bring the particle board to a nice blaze with contact. but i would not worry about that.

just a few questions, ( for my own heat box), how well does this keep heat, insulation wise) and do you think you can use a different heating component ( like a ceramic heating fan ) ?

the king of ghetto chimes in (sry, couldn't resist)

luckily the heating elements are separated from the particle board by half an inch of fire resistant drywall i guess they could burn that thing down within minutes if the two touched.

i don't have any exact numbers, but with only the larger ring heater running we start at about 1200watts which gets the box to 100°C within half an hour and after that we can turn down the power pretty much every hour or so.
after about 10-12h we're usually at about 200-300watts and that's usually what it takes to keep the temp stable at 100°C.

i don't know what kind of ceramic heater you want to use, but make damn sure it doesn't have any plastic parts!!!
we burnt a heater with a plastic fan in that box! caught fire after about 10min and all the plastic parts were completely melted.
that's why we used oven parts since they're all metal! and i suggest you do the same.
old ovens can be had for free at most scrapyards

[a.badner]

haha king of ghetto. im not going to deny it for a second.

if i were to build a hotbox, i would,make it for extremely hot temperatures. i just need something to keep my press atleast at room temperature.

im pressing at this moment in -18 degrees. and there is a heat blanket in the press from walmart. but that only keeps the press at -10 degrees.

definite problem area for living in canada

[chrismp]

well i guess that wouldn't be such a problem then.
i think with most plastic parts you're good up to at least 40°C.
keep in mind though that a standard room heater can get pretty hot too even if the temps inside your box aren't that high.
as i said we had to learn the hard way...the thermometer on the other side of the box only read 60-70°C when smoke started to emerge.

insulation is key imho. the better you seal and insulate your box the less power you'll need to make it cozy in there. proper insulation can get heavy though it's kinda hard to lift the lid of our box without help.

maybe you should try building a heat blanket like we're doing right now...a box to heat a press that big can get expensive and really hard to move around.
photos and a description of our blanket should be up by monday evening. costs are approx 100 bucks for two blankets.

[a.badner]

its not so much the problem of the box being heavy, i worried (.. if i were to make a heat box) about the masses inside the box and cold comming from the ground.

every one here has seen my press and where it is situated, lets just say no matter what insulations, lots of heat is going to escape trough the concrete floor.

also, since i have ribs running through and over my press that is a big surface area to heat from -18, and i do have doubts that it will.

furthermore, my press is 7 1/2 ' x 1' x 3'. thats 22 1/2 cubic feet of metal, air and concrete to keep heated.

at this point, im not sure making a heat blanket would be expensive.