OT: Toaster oven problem

[snip]

What? Did you buy a Microsoft Toaster ?:-) ...Jim Thompson

-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Why is the oven too hot???? Stated another way, "what problem are you trying to solve?"

If it's a burn problem, I solved that by limiting access. I put a wire shelf above the oven and use it to store glass bakeware that doesn't mind the heat. I can't touch the sides or top or set anything on it.

Insulation knows only about relative temperatures. Insulation restricts heat FLOW.

If you just want it cooler, the design process is straightforward. Decide the temperature you want the outside to be. At that temperature, the cooling effect is likely to be almost all convection. Given the air temp and surface temp, you can look up the power dissipation you can get via convection. If it sits on a counter with restricted air flow, the situation can be MUCH worse than the open-air convection equations show. If you blow a fan on it, it can be MUCH better. Now you know the power per unit surface area. All that comes from inside...so given the heat flux and the inside/outside temps, you can calculate the R-value of the insulation needed to get to the desired outside temperature.

My gut tells me you can't get there from here with any practical setup. I'd guess it hasn't been done because people buy less bulky units without concern for heat emitted. They want a multi-purpose device that's used more for toasting than for ovening. They want lowest possible cost.

Best you can do is paint the outside with CONDUCTIVE black paint and blow a fan on it. Be aware that cooling the outside this way causes the oven power to increase and puts more heat into the environment. Most of the power in a toaster oven does nothing to cook food. It's lost to the environment. Just to use some round numbers, If you don't change the insulation... Say the inside is 400F and the outside is 200F. Blowing air on the outside to cool it to 0F will double the power required to cook your food, double the load on your air conditioner and use power for the fan. (I said round numbers, they scale to practical values)

Make the inside as shiny as possible, like line it with aluminum foil. You won't get below 400F, but you can keep the inside surface temp below the temperature of the heating element that's radiating directly on it.

My limited experience is that a toaster oven is a lot more toaster than oven. If you're running yours for hours at a time, you might consider an appliance better suited to the task.

Are we having fun yet?

You could try using the oven as it is intended to be used - for no more than 20-30 minutes at a time.

But being as you wont like that go back to the extra sheet of galv and use a fan to blow the 'outside of the inside' with cool air.

--
We have failed to address the fundamental truth that endless growth is  
impossible in a finite world.

Don't i wish...then i could send it to Billy Boy and have him put his hand on top when it is running at 200C.

My problem is that the surface is too hot, one can get severely burned if one touches it for more than a second. I have been testing it on a large flat counter and nothing near it. The inside is a black ceramic (i think) coating, and the outside is fairly shiny (almost chromium) steel. The added plates i mentioned are 16 gauge galvanized steel, not quite as shiny but not doggie either. Since the added plates (one exception) divide spaces in half, i could add fiberglass back in, but would that help and would the innermost space be better to use or vise-versa? The exception is the back side of the oven; i would have to do a 100 percent field-stripping to get to the back space, assuming there is one. I added a 16 gauge galvanized steel plate inside about 1/16 away from the inner wall; now one sees a shiny surface instead of a dull black surface. The "task" is to evenly heat a volume as in a test oven; the fan it has helps here. Only the top two quartz lie heaters AND the bottom two quartz line heaters work in the Toaster mode; all other modes the bottom two sit unused.

• Will be easy and cheep to try that Al foil trick...thankz fer the suggestion.

• Not an option, as device testing can last hours, as well as temperature stabilization can take at least a hour.

Hmm..not that much of a mechanic,but that sounds like a good trick to at least mitigate heating at the outside. I was thinking something easier (Baer-ly possible by me) - - make an outer case so that nothing can touch the hot surfaces, and give at least 0.5 inch space for unforced airflow (adding a fan would be easier also).

One solution is to make it so you can't touch it then.

That is bad. If cosmetic appearance doesn't matter almost any colour of heat proof paint will help bring the temperature down a fair amount.

You want it to behave as a black body radiator at longwave infrared. Almost any colout of paint would do - black spray stove pipe enamel as good as anything.

The alternative is to embed the entire thing in a heavily insulated block box of fireproof material. The 2" thick dense rock wool block sold for insulating new cavity walls would be ideal. Then your oven will warm up faster and use a lot less power to maintain an even temperature.

You may need to check it has no thermoplastic parts on the outside...

Won't make any difference with foil inside. You need to make the exterior surfaces not shiny metal to reduce the surface temperature. But that will also increase your thermal losses so you will use more power. Surrounding the thing with more external insulation is the best bet.

Regards, Martin Brown

On 2/8/2012 10:53 PM, Robert Baer wrote: snip

At the risk of stating the obvious, be careful with aluminum foil. On the bottom, gravity is your friend. On the top, it's gonna be non-trivial to keep the foil from falling down on the element. That's a shock and fire risk. You MUST make sure that nothing can come loose. And you'll have to send it in to UL laboratories for certification.

My lawyer insists that I say that you should NEVER, EVER modify an appliance for any reason. So, forget what I said. It's a thought experiment to illustrate the problem. Don't try to implement it.

That is a good point. Any modifications are done at your own risk, and = in=20 case of fire or injury, it may void the manufacturer's liability as well = as=20 your homeowners insurance. But you *might* be able to put the toaster = oven=20 in an enclosure which could be insulated well enough to contain the heat =

(increasing its efficiency), while keeping its surface at a lower=20 temperature which should be touch-safe. Actually, if the original unit = can=20 be touched for as long as one second before causing an actual burn (more =

than a

= it=20 may be considered safe enough unless there are children or invalids=20 involved.

Consider class A metal chimneys. They handle 500 degree smoke from a=20 woodstove and their surface is generally safe to touch, although you = don't=20 want to let your finger-linger very long. The "Metalbestos" type I have = is=20 made of two concentric tubes of stainless steel, insulated with about =

3/4"=20 of something that is similar to asbestos, sealed into interlocking = lengths=20 of 1, 2, and 3 feet. There is a also a "triple-wall" type that has an = inner=20 liner of stainless steel, then an intermediate cylinder of aluminum=20 (probably for heat conduction), and an outer wall of galvanized steel. = It=20 must have some sort of spacer to maintain the air gap without conducting = too=20 much of the heat. So that is an idea.

And you might also be able to wrap it in fiberglass with an outer layer = of=20 Mylar, like an oven bag. A metal outer shell will still get hot and will =

have enough thermal mass to burn skin before the heat is transferred and = the=20 temperature drops to a safe or comfortable level. But thin plastic, even =

though it may still reach 300F or so, will quickly dissipate its heat = into=20 your skin with only a brief sensation of high temperature.

Paul=20

Of all that i have seen and heard, your idea to embed the entire thing in a heavily insulated block box of fireproof material is the best. Thanks for mentioning 2" thick dense rock wool block sold for insulating new cavity walls. Check; i know that the oven will warm up faster and use a lot less power to maintain an even temperature, as i did something like that to a breadmaker.

No problem to make aluminum foil stick; GE Silicone II is close to ideal. Do not have to send anything to UL labs; waste of many thousands of dollars and a year or so. One can build almost anything one damn well pleases even if patents are current and device is the latest and greatest money-maker...IF it is for yourself and you do not sell it. And your reference to a lawyer is rather incomplete to say the least; modify previous sentence from "build" to "modify" and add "if there are any problems after the modification, YOU MUST accept 200 percent of the responsibility".

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Wire "hot" to the case?

-- Cheers, James Arthur

My gut tells me that's not gonna be feasible. If the outside of the enclosure is colder, the outer surface of the oven is gonna heat up to approach the inner surface temperature. So, there's risk that the feet, knobs, anything plastic might deteriorate/melt faster than you'd like. And it's gonna be hard to get the door open if you remove the plastic handle.

I don't have experience with newer kitchen ovens, but I've never seen a standard oven with a door that could be safely touched while it's operating.

My solution of putting some wire-type shelving around mine so I can't easily touch the outside or put something on top of the oven seemed, to me, to be the best compromise I could do.

Well, i ordered some automotive heat shielding mats; cheap enough to be worth a try. BTW, the fiberglass insulation i added (inserted between the inner heated area and the outer case) does wonders. Outside only slightly warms up after hours of 204C/400F use, and power use rather low.

Insulating might make internal wiring fry. The wiring need some cooling thus walls are thin. The back might be ok. I got fiberglass in back in my unit. The part that needs cooling is the crimped on heater connections made from steel and the attached copper. I fixed my wiring twice.

Greg

Encase it in wire mesh so that fingers cannot touch it.

Sylvia.

g r

en is gonna heat up to approach the inner surface temperature.

True, but let's take a WAG. If the toaster's 15x8x6 inches, and, to be conservative we only count the top surfaces, that's 216 in^2 of surface area available for dissipation. The sarcophagus will be bigger still, but let's ignore that too.

If he can limit heat loss to, say 100W, the outer skin only has to dissipate 1/2W/in^2, half the old 1W/in^2 rule.

To achieve a skin temp of 50C (25C rise) with a cooking temp of 200C, he needs an overall thermal resistance from oven chamber to outside skin to air, of about 150C/100W, or about 1.5C/W. Part of that's already built into the oven.

I don't have much of an intuitive feel for this, but it seems doable. I'd have to measure. Heat-shielding just the exposed, touchable surfaces might be a better compromise--that'll save the plastic bits, inquiring fingers, and a lot of energy too.

-- Cheers, James Arthur

The toaster oven comes with black enamel coating inside, and the right end that has wiring and a fan for the oven is convection air cooled (stays cool).

This chicken is mechanically challenged, so the result would cause anyone to squawk.