Adding Convection

[seattleuserfriendly]

It would seem to me that there might be an easy way to add convection to an existing electric oven.

Anyone come across such an idea?

[Jack A Lopez]

The word "convection," when prepended to the word "oven," really just means an oven that has a fan in it.

There is a Wikipedia article for "Convection oven." And the first sentence of this article, echoes this sentiment,

A convection oven (also known as a fan-assisted oven or simply a fan oven) is an oven that has fans to circulate air around food which gives a very even heat.

So I think your notion of, "way to add convection to an existing electric oven" is equivalent to, "way to add a fan."

The tricky part is perhaps the fan is in a place that is really hot, perhaps too hot for the usual electric motor that drives a fan.

My intuition is telling me, the easiest way to do this, is to put the blades of the fan on the end of a really loooooooong shaft, which comes into the oven, by way of a small hole drilled in the side of the oven.

That way the fan blades can be inside the oven, where the hot air to be moved is, and the motor driving the fan, can be some distance outside the oven, where the surrounding air is much cooler.

I do not know if other tricks will be needed to prevent heat from conducting through the turning shaft, but I can imagine such tricks; e.g. make the shaft hollow instead of solid, or make some sections of the shaft out of thermal insulating material; e.g. ceramic or silicone.

By the way, there are maybe some analogies to consider. I mean, examples of existing appliances, that have a part that spins, in a place that is kind of unsafe for motors or electronics.

Like, magnetic hotplates, the kind used in chemistry laboratories,

https://en.wikipedia.org/wiki/Magnetic_stirrer

use a magnet attached to a motor, on one side, and a sturdy, heat and chemical-resistant "stir bar" magnet, on the side that is immersed in hot chemical solution.

Also the motor bolted to the bottom of a microwave oven, the one that moves the rotating glass tray... somehow that motor is safe from microwave radiation. And somehow the microwaves do not leak out through the hole that motor's shaft goes through.

That might make one wonder about the size of these so-called "micro" waves, since it seems there are some kinds of holes, big enough to see, but too small for the waves to "leak" through.

[seattleuserfriendly]

Yes. I understand Convection means moving air. How does one do this so the fan/wiring does not catch fire? It would only need a small fan. But even the wires could melt/catch fire. Drilling a hole in the oven is dangerous as the heat could burn nearby objects. Also, that would mean that air might be exchanged which is not the goal.

Does anyone make a small fan (just 2-3 inches) that can be safely operated up to 400F? And what sort of wiring would be safe? The wire could be pinched between the door and insulation.

People find this non-intuitive as they think of a fan as cooling.

Let's say you put something that is room temperature in a 350 oven. Most of the oven's air is 350. But as you get closer to the item, it becomes cooler. When the air is still, the item may only be heated up to 250 or 300. A tiny fan mixes the air so that is consistently at a higher temperature. A regular oven may be set at 350, but at the beginning, it only cooks at 200....like steaming. You miss the browning Moisture is driven off more quickly which creates browning and pulls water from the food. It's is great for baking, but you have to set the temperature about 25 degrees lower.

A new convection oven costs a lot of money. If there are fans that handle the heat and wiring, it would be an inexpensive upgrade.

[Jack A Lopez]

An electric motor that can go inside the oven, without burning up? That sounds like a job for Shadrach, Meshach, and Abednego.



Seriously though, I decided to see if my favorite patent browsing site, could turn up some patents for, uh, forced convection type ovens.

https://www.freepatentsonline.com/resul ... rch=Search

I wonder if that link is going to work? The search mojo I used was,

Code: Select all

abst/oven and 
abst/convection and
abst/fan and
abst/motor

basically just looking for all those words, present in the patent's "abstract" section.

That, plus clicking the button that sorts the results in chronological order, which is handy because it sorts everything from newest to oldest.

https://www.freepatentsonline.com/search.html

What I was looking for was some pictures of how the fan, and motor, were arranged. I will try to attach a few of these I clipped. I have not looked at a huge number of these, but the examples I am going to point to (USP 3529582 and USP 5121737) show a wall with a hole through it, a shaft going through the hole, a fan on the inside (the hot side), and the motor on the outside (the cool side).





I know these examples just confirm my bias; i.e. I found what I was expecting to find. But you know, anyone who wants to, can look through these old patents to try to discover if there exist drawings of this being done some other way.

[seattleuserfriendly]

Apparently the idea has simultaneously popped into the minds of several other "I wonder if..." people. We are those with no knowledge of materials or engineering.

Thanks for the site. I searched using some other terms and saw that other people have patented the idea. It reminds me of an old Sydney Harris Cartoon, where a professor is in front of a board filled with equations and someone points to a line that says, "And then a miracle happens" saying he may need more work on that step.

I guess the more practical questions:
1) Is there a small fan that can stand temperatures up to 500 F?
2) Is there some sort of wire for that fan that can also handle the heat?

[Downunder35m]

To answer you fan question: No there isn't fan you can use inside any oven.

Works a bit differently.
You only have the fan part inside the oven.
It sits on a longer drive shaft with the motor mounted OUTSIDE the oven.
So you only have hole of 5 or so millimeter.
And around this hole you usually have soft rock wool "seal".
The long drive shaft allows the motor to be mounted with an additional heat shield and OUTSIDE the insulation material of the oven.
The only heat itgets from the oven is what is tranfered through the drive shaft.
For good motors they are made from a stainless steel with a low conductivity for heat.
The motor often has an additional, smaller fan on the other side to provide cool air for the motor cooling.

You can find suitable motor for these purposes online or by salvaging them from other devices.
However be aware that although you can add a fan to some old electric oven this way if you dare, but not to an old gas fired oven.
With gas you need a controlled airflow and ventilation system adding a fan can have catastrophic results....
There is a reason why modern gas ovens with convections are so expensive copared to their electrical cousins....

[Jack A Lopez]

Regarding your question (1) about the existence of a small fan motor that can withstand 500 F surrounding temperature...

Motors have windings made of magnet wire,

https://en.wikipedia.org/wiki/Magnet_wire

and I think it is the insulation material, surrounding this wire, which determines the maximum temperature, for a motor.

It is not a subject I know much about, but I have discovered there are temperature classes for these various kinds of insulation.

Here is the Wikipedia story, regarding wire insulation temperature ratings,

https://en.wikipedia.org/wiki/Insulation_system

and here are a few other pages I found,

https://www.engineeringtoolbox.com/nema ... d_734.html

https://kaesertalksshop.com/2016/03/28/ ... e-ratings/

http://electronicstechnician.tpub.com/1 ... n-307.html

I am guessing these temperature ratings are for the windings themselves, which are necessarily hotter than the air surrounding the motor, when electric currents are moving through those windings, while the motor is running.

I dunno. My intuition is still telling me the same thing: a motor intentionally placed inside an oven... is going to get cooked.

Regarding question (2), the wires outside the motor, I do not think you need to worry about those, because those wires are short and thick, and in a cool place, compared to the long thin wires comprising the windings inside the motor.

That is to say, I expect the windings inside the motor to fail from the insulation getting cooked, before the outside connecting wires fail in a similar way.

[Downunder35m]

Problem is not windings, insulation or such things.
It is the bsic stuff, like lubrication for the bearings and expansion under temperature ruining tolerances.
Not to hard to create motor that would survive oven temps but keeping it spinning at low noise levels will be a problem.
Same for performance issues of the magnetic materials....
Hense the use of long shafts or inductive coupling to make thigs spin in very hot places.

[Orngrimm]

Keep in mind, that it doesnt need a hurricane in there... A small blower / Lets say it can even be a little "air-disturber" is more than enough to have the temperature more uniform in your oven.
As we are in a board where also ideas can be thrown at a wall to see what sticks, here is my 2 cents to this project in PURE DIY-fashion:

• Have a small non-magnetic metallic (So it doesnt melt) fan-plate on an axle (Metal).
• This fan-plate has to have a stand of some sort where you can have the fan-plate close to the glass-window of the oven
• Attach a small piece of ferromagnetic metal to each blade, as close to the window as possible. Dont take megnets for this here because the curie-point will be lower than what you need in your oven. Plain old iron-sheet for the win. See that the fan is still somewhat balanced in weight on the fan-plate.
• Now to the magic-trick: Add 2 strong magnets to a stick and attach this stick to a slowturning motor... Like 120 RPM or so...
• Place the magnet-motor close to the oven window on the outside of the oven. The magnets should "couple" with the ferromagnetic pieces on the fan-plate and spin it along as the motor turns. Magnetic coupling
• You just added temporary convection to your oven

A less DIY-metod to keep the motor save from the heat is a simple Belt-link from motor to Fan-axle. See that the room with the belt is well fentilated to keep the axle of the fan and the belt cool.

[Downunder35m]

We seem to forget that a gas oven creates convection naturally anyway