How reasonable is it to reuse a heating coil from a hair dryer?

[Deeg]

I've got a project careening in my head to build a heated magnetic stirrer. How easy (or hard) would it be to use the heating coil from an old hair dryer?

My current idea is to build a cube from soft refractory bricks and embed the coil in the top. The motor for the stirrer will be inside the bricks, protected from the heat. A raspberry pi will control the speed of the motor and the temperature of the coil via PWM. What do I need to consider to carry this out?

 

[KeepItSimpleStupid]

We used cartridge heaters **broken link removed** for the most part.

They were generally pressed into place, so an over/under reemer was required. You can easily melt aluminum.

The hair dryer does have thermal protection in the form of a temperature fuse. They don't like their air supply being cut off.

 

[MikeMl]

The bricks will heat up. The motor will be overheated. You need a layer of the tiles used on the space shuttle.

 

[ericgibbs]

I've got a project careening in my head to build a heated magnetic stirrer. How easy (or hard) would it be to use the heating coil from an old hair dryer?

You could find that an old used heating element has become quite brittle over its life and will not be easy to reform.

 

[alec_t]

The hair-drier element would need a safety cover, since the wire is exposed.
What temperature are you trying to achieve?

 

[Deeg]

Thanks for the responses!

We used cartridge heaters **broken link removed** for the most part.

The hair dryer does have thermal protection in the form of a temperature fuse. They don't like their air supply being cut off.

That won't work for what I have in mind because that can't be arranged in a circle. The magnet for the stirrer needs to be accessible inside it.

Can I circumvent the temperature fuse?

The bricks will heat up. The motor will be overheated. You need a layer of the tiles used on the space shuttle.

The bricks aren't quite as good as the space shuttle tiles but they are used to insulate kilns that reach 2k+ degrees and I won't be anywhere near that. Still, if you know where I can pick up surplus SSTs I'll gladly use them.

The hair-drier element would need a safety cover, since the wire is exposed.
What temperature are you trying to achieve?

I assume the safety problem is having the coil with mains power being exposed? If so, could I use a lower-voltage DC power source instead? I can't think of anything that I could use that would work as a safety cover but also let the heat escape. I'd be quite happy with a heating coil that reached 400-500 degrees.

 

[MrAl]

Hi,

I've actually used one of these, but i used it for a dummy load to test something else so it did not get the full voltage it normally gets when used as a hair dryer.

The thing is though that these get very very hot, red hot in fact. And they usually have a fan installed that keep air flowing through the coils and that keeps the temperature down while also forcing the hot air to flow outward where it can be useful. So one way to use it would be to force air through it as it normally would be used, and just direct the hot air to whatever has to be heated. That way you dont have to try to 'overcome' any safety devices built in to prevent fire.
If the moving air is objectionable (in some heating apps you cant have that) then you should just keep the operating voltage lower than the normal operating voltage. That will reduce the temperature but will still prevent fire.
The coils are made out of NiChrome wire and that can take a lot of abuse, but it wont last forever if it is heated to a higher temperature than normal.

Just to note, they do make immersion heaters made to be put into liquids to heat them up. They MUST be under the liquid though or they could burn up too. Not sure how much heating power you need either, 100 watts, 200 watts, 1000 watts, etc.

 

[Deeg]

I've done some more research. Heating coils are cheaper than I thought and it looks like Kanthal might be what I want. It forms a non-conductive coating when it gets heated. Here's an example. Do I have to use AC current with these? Do you guys have any thoughts about using Kanthal?

 

[jpanhalt]

That won't work for what I have in mind because that can't be arranged in a circle. The magnet for the stirrer needs to be accessible inside it.

Not sure why the heater needs to be in a circle. Just use a flat plate heater, like from a toaster, under an aluminum or ceramic plate, electrically isolate where needed, and stir through it from below. Here is a typical design:

The temperature range you are aiming for (400 to 500°C) also seems a bit high to me, based on my experience in chemistry. What are you making?

Edit: Just checked some specs for heaters and some do claim to reach 400 to 500°C. Most organic reactions will require much less. Those temperatures are in the range of molten salt baths used for heat treating and similar purposes. Lead melts at 327°C.

John

 

[alec_t]

Do I have to use AC current with these?

AC or DC will work.

 

[Deeg]

Perhaps a stupid question: I got a length of Kanthal and found a 19v, 6 amp power supply from an old laptop. Will anything bad happen if I hook up the output of the PS directly to the Kanthal coil? Is it ok to let it sit like that for a while to see how hot the coil gets?

 

[jpanhalt]

Kanthal is a brand name of a company that makes several different alloys for heating elements (**broken link removed** ).

You need to know the electrical resistance of the piece you want to test. From Ohm's Law, you can calculate the current that would flow with 19V across it.. You also need to know whether the current rating of the power supply you have is for continuous or intermittent use. If the calculated current (cold) is less than 6A, then your experiment is probably OK, assuming the power supply has a continuous current rating. Of the common alloys produced by Kanthal, resistance increases with temperature (see datasheets on the Kanthal site), so current will drop somewhat as the element heats up.

John

 

[Deeg]

Ah, ok. Looking at the product further I see that it's made from Kathal A-1, datasheet here. I'll measure the resistance this weekend. How can I determine if the PS is rated for continuous or intermittent use? Is there a label I can look for?

In general, is there a problem with using a PS that produces fewer amps than the resistance wire can use? My understanding (probably wrong) is that the PS will pump out whatever amps it can and no more without creating a dangerous situation. With normal electronics this might be a problem because the components need a certain amperage to work properly but a resistance wire doesn't care, right? I.e. it won't get as hot but fewer amps isn't a problem.

 

[jpanhalt]

In general, is there a problem with using a PS that produces fewer amps than the resistance wire can use?

Yes, that is the problem. If the wire resistance is too low, it will draw too many amps and overload the power supply. When the power supply's rated current is more than the wire draws, then there is no problem.

If you overload the PS, it may trip an internal protection device or simply get hot. It may very well have built-in short protection. At 19V, your wire must have a resistance of >3.17 ohms to avoid drawing too much current. Once it is hot, it will draw less current. So, if you are very close to that value, it might be worth a careful trial.

That supply is only 114W (max), so in any case, you may not have enough heat for what you are planning to do.

John

 

[Deeg]

Thanks John, I appreciate your help. If my wire has < 3.17 ohm resistance I assume I need to get a power resistor, correct? The calc for power dissipation is V^2 / R. Is V the voltage drop or the entire voltage (e.g. 19v in my case)? I'm assuming the former because otherwise power dissipation is ~180W for a 2 ohm resistor which is way more than my PS can produce.

This project is, to some degree, a demonstration of what can be done so I'm not overly concerned about the top temperature. It would be great if it could reach 400F but even 200F would help with proof-of-concept. I'm trying to do this fairly cheap (thus the original idea of using an old hair dryer) and I have the 19v/6a PS laying around. I also have a variable AC transformer I could try using but that ramps up the danger level. I am open to any and all suggestions.