Gallery, Projects and General > How do I??

Electric heater for hot air engine?

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vtsteam:
As I test ideas out on my hot air engine, No 83, I'd like a consistent heat source so I can actually compare output of changes made. So far I've used a can of Sterno, but it isn't very consistent. Less heat as the can burns down, etc.

What needs to be heated is the end of the stainless steel displacer tube -- heated area = approximately 1" long by 1" diameter (25mm x 25mm).

Temp of the cylinder wall should be some number in the region of say 250F - 450F (120C - 230C) -- not a critical number as long as it stays consistent. An electric heat source would be ideal for consistent heating.

I need something I can make myself from readily available materials or buy at low cost.

What's available:
1.)House voltage is US standard, 120 V.

2.) I have on hand various refractory materials that I've used in the past in my melting furnace, including fire clays and refractory mix, insulative blanket, and plaster of Paris.

3.) I also have stainless steel fishing leader of various fine diameters, which I've used as heated wire for cutting wing panels out of foam. That's generally performed at 12 V with a small power supply -- I think I was using .011" wire for that, but I don't think it lasts long. Definitely not used at red heat.

4.) I have various computer power supplies, transformers, wall warts, and a 10 amp variable DC power supply.

5.) However if a cheap ready made or salvage solution suggests itself to anyone -- I'm definitely open to that route.

awemawson:
Maybe a few dozen turns of nichrome or similar resistance wire wound over a suitable electrically insulating tape (perhaps Kapton) with a thermal jacket of some sort over it all? Driver at the crudest a simple DC supply or going up in complexity a variable voltage source.

vtsteam:
Thanks Andrew.  :beer:

Unfortunately I don't have either Kapton tape or nichrome wire on hand.

Looks like Kapton is available on Amazon at reasonable price for the amount I would need. How hot does it allow the wire to get? Also nichrome wire is available there at not too bad a price either. But what gauge to get?

While casting about on Amazon looking up "heating coil" some small induction heaters showed up. Examples:

https://www.amazon.com/HURRISE-5V-12V-Induction-Heating-Frequency/dp/B09SV8N52X
https://www.amazon.com/Voltage-Induction-Heating-Module-Flyback/dp/B0BL767R3Q

But possible problems: some bad reviews, and how would a moving aluminum displacer with steel pushrod inside a stainless steel cylinder be affected by the alternating field? Also how big is "small", etc.

As a guess, induction may not be a great idea. Still it's cheap and easy to fit a coil around the end of the displacer cyl. But Probably straight resistance heat is best, right??  :zap:

vtsteam:
Hey, look what I just found!

 

It's a liner for a pouring cup for metal casting that I once made out of fireclay and sand. I packed it into a section of steel tubing, but since it sat for a couple years it shrank and released from it.

I was just hunting through scrap for something else I needed, and spied it sitting there loose in its tube.  :med:

vtsteam:
So just as a ballpark test to discover what I could, I created coils of the stainless steel fishing leader samples I have, using about 30" (76 cm) of wire in each and fitted them into the fireclay tube. I chose that length to give roughly 8 turns inside the tube. Then I hooked up to a variable DC power supply, and a kill-a-watt meter, and checked the internal temperatures with a non-contact thermometer.

Making the coils was very difficult because the stainless leader material is extremely resistant to bending. It's basically a spring version of stainless steel. I couldn't get it to form a coil around a mandrel larger than 1/2" (12mm) diameter. So I resorted to just bending it by hand roughly to fit into the tube. A series of many small kinks to yield the ID. The springiness held it against the inside of the tube..

If I substitute nichrome wire, presumably it will be more bendable on a mandrel close to the ID of the tube (about 1.3", 33mm) --- I hope!

It looks from these experiments like I need about 150 watts  to get the temperature in the range I wanted in that tube.

The heaviest leader material I have is .021" dia (.53 mm) -- and that was the best performer, and it starts to glow at about 5.5 amps.

That's all the info I have today.

Ideas and suggestions welcome.

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