A little help please..long winded but i do have a few questions!!!

[jlengineering]

Hello everyone... Ok im going to give a run down of what i have going on and why... I have been powder coating small parts out of my garage for a while now. I have just been using an old natural gas oven i picked up for free.. It has worked great for me.. well my son and i deceided we were going to tear down our quads and pc the frames. Well my little oven wasnt going to cut it so i deceided to build a bigger one. My plan was to build a 3x3x6 tall and just use the parts out of my gas oven. Well after talking to almost anyone and everyone i could, I was just about to purchase metal studs and sheet metal and go at it. then a buddy of mine called me up and suggested that i use drywall as my oven walls because of its high fire rating and it is cheap... at first i thought he was crazy but after i thought about it for a while i figured what the heck... so off i went.. So here is how it went.. I started off with a wood 2x4 frame.. i figured since wood was a bit easier to work with that it would help me out and i planned to cover every piece of wood with drywall, there would not be much of a fire issue.. the inside of the oven i doubled the 5/8 drywall giving me a 1.25 wall thickness all the way around except on the ceiling were i trippled it. i did use metal studs on the floor of the oven where the burner was going because i couldnt see attatching that to wood.. So as my finished product i ended up with a drywall oven that was heavier than hell.. I used a lot of drywall and really felt it was well insulated and over all fell really safe about firing it up... so after firing it up and working out some bugs with ventalation, i was only able to get it up to 320 deg in 30 min.. thats when i went to the computer and started reading...for days..!!! i came across this forum.. it has been a lot of help by the way..after more reading i came to the conclusion that I needed to put sheet metal on the inside of the oven, i figured it would help to get my temp up..so i lined the intire inside with 22guage sheet metal.. well it didnt.. now i couldnt get it past 280 deg.. I figured the metal was needing more energy to heat and my burner wasnt producing it fast enough... so i decieded to pull out the gas set up and convert the whole thing to an electric oven... so now after all that i have a few questions that i hope i can get help with.. and remember that there is no wood exposed, it is covered by at least 1.25 inches of drywall.. I must say though..at the ovens hotest of 320, i felt no heat rise on any of the outside walls so figure it is well insulated with all the drywall.. I have a whirlpool double electric oven that i plan to use both broil elements... they each are 3000w and according to the heat calc. i only need 3800w or so.. i will have 6000w total.. is my math correct.. will these 2 elements be plenty? One of my main questions is even though i have a metal inside..is the drywall, being so thick on the out side going to help or hender me..? It seem to work as a good insulator.. sorry for the long thread but i just wanted to be clear of what i was working with..

[Noob2PC]

I can not speak on the construction but I am using 2 broil element in my over which is 4x3x6 and they heat it up in like 8 min to 425*. I would imagine that the Drywall would be good for retaining heat but I myself would add Insulation around the oven.

[Scott Amsler]

The calculator you want to use is the time to rise calculator (It's a sticky at the top of this forum). But since I was already there, I ran the numbers for you.

You'd have 6000 watt elements, with a 25 amp current draw @ 240volts. It'd take just over 23 minutes to heat to 400 degrees if the ambient temp is 75 (it's warmer than that here)

As far as construction: I've seen oven build pics on this forum framed in wood. Not sure of the results. I think your best bet would be to go with the mineral wool insulation commonly used in other oven builds. I'm just not sure the drywall is going to do what you need to do.

[jlengineering]

Thanks guys for the feed back... I did forget to mention earlier but as I started with the drywall as my oven wall, I did insulate between the drywall and the outer wall which is also drywall... So now I have 22 gauge inner metal on top of 1.25 thick drywall then 3 inches of insulation then the outer wall of drywall which is 5/8 thick..... So between metal drywall and isolation fro inner to outer I have about a six inch thick wall...Might be overkill but I already had it up... I figured it couldn't hurt!

[dbotos]

Drywall is not really a good insulator as you probably discovered. R values are going to be down around 0.9 per inch. Contrast that to 3+ per inch for fiberglass / rockwool batts. A good oven shell should have the following attributes:

1) insulation - keeps oven from losing heat via conduction through walls. You can pretty much assume it will be exposed to cure temperatures on the side touching the inner shell.

2) capacitance - you want the metal (or whatever the inside of your oven is made of) to be able to hold some heat so you don't lose it all when you open the door and the hot air comes out. Too thin will cause more drastic door-opening effect and too thick will take a long time for the oven to heat up initially.

3) relative air-tightness - hot air wants to expand, so you obviously don't want it pressure-vessel tight, but it's just like a house where you don't want air leaks around windows and doors causing your heating/cooling dollars to be wasted.

[jlengineering]

thanks you guys for all the feedback today... i am going to try what i have with the heating elements and make adjustments from there... after i try it out i will post my results.... Thanks for doing the math for me Scott... made it easy to tell my electrican buddy what i need done...

[dbotos]

Post some pics of this beast.

[jlengineering]

A beast it is... I will get some pics up soon..

[baz]

i hope you guys building ovens out of wood know that auto ignition (lowest temp needed for self ignition ) of wood is 572 f and considering that you might need to outgas some parts around 450 F you are getting quite close to burning the place down . if your oven has a few hot spots and if your thermocouple is a few degrees off and your in big troubles !
riffle powder self ignition is just a few degrees below wood 550 f compared to 572 f, yet i am sure no one wants to build an oven with riffle powder .

[Zigeuner]

Well, I'm not the expert here, but I certainly wouldn't care for either wood or drywall in a powdercoating oven. As mentioned above, you're getting close to the auto-ignition point if you use any kind of wood in your structure. That can't be good.

As for drywall, also as mentioned above, it's not all that great an insulator and, with all of the handling and moving things in and out of the oven when using it, you're going to get dents, cracks, splits and have screws pulling through. It's only covered with paper too. In my opinion anyway, I don't think it would last.

I used metal studs with 22 ga. galvanized sheet metal inside and out on my 2' X 3' X 6' oven. The top and sides are insulated with mineral wool. In operation, I often have it at 400 deg. F. and the outside never gets above 105 deg. F.

I used the basic design shown in the link below. The builder in that link used styrofoam insulation but I was unable to locate any in my area. I was able to get all the mineral wool I needed. It took less than one bale for about $40. The whole oven cost me about $1,000 but I'm getting it back with the work that it does for me.

Happy Trails!

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