Electric heater for hot air engine?

[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. 

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?? 

[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. 

[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.

[vtsteam]

I wonder if there's some kind of light bulb that might work. I remember some small halogen bulbs seemed to put out a whole lot of heat. Focused on the displacer cylinder from an inch away or so maybe?

[vtsteam]

Don't know much about these:

https://www.amazon.com/Simple-Deluxe-Ceramic-Emitter-Reptile/dp/B0CWGCCQWV

It does say not to turn it on and off frequently.

How frequently? 

[awemawson]

Nichrome wire as bought will be fairly easy to bend but once used will become very brittle. As a child my source of  nichrome wire for experimentation was the heater bars from electric fires and unreeling an old ones was a challenge. I was once given a brand new one still in its cardboard tube and it was a joy to work with.

[vtsteam]

Thanks Andrew. 

I guess I need to have a ballpark idea of gauge needed.

I'd like to keep the voltage low on something I that I build for safety sake. 12 volts would be ideal, but that would require over ten amps (I believe -- not actually sure, but as a guess 120 watts might be minimum needed?). My variable DC power supply is a 10 amp supply, it does however go to 30V, so I guess I have the supply for it.

I'm not sure how nichrome wire compares with equivalent gauge stainless for resistance per unit length (when hot).

I did feel in my tests that I'd have liked even thicker ss wire, for fewer turns -- after all I only need to heat about an inch of length on the displacer cylinder. Lots of turns means closer spacing, which is harder to make.

I was thinking that I do have other stainless wire besides just the fishing leader. I have aircraft grade twist lock wire, and some other miscellaneous types. They probably all coil better as well. Might give samples of those a go. The power supply has both constant current and voltage modes, and the watt meter and IR temp probe makes it possible to test a short length and then extrapolate to longer.

Also, just for giggles I did send for those Herp warmer bulbs linked above. They are 2 for $9 for the 3" dia 150 watt versions, and I have a ceramic base incandescent light fixture.

[vtsteam]

Tested a 24" (610 mm) coiled length of .032" (.8mm) aircraft safety wire T302/304 stainless steel, and it glowed dull orange in air in daylight at 10 amps and 16 volts. That's just above the 150 watt target figure. It's also easy to coil.

[vtsteam]

I've found that plaster of Paris and sand mix has worked well as a furnace lining for my aluminum melting propane furnace in the past. Plaster of Paris is a good heat insulator because it's porous, and the sand provides structural strength against cracking that pure plaster of Paris normally lacks.

One way to make a heater might be to wrap the wire around a waxed wood mandrel of say 1.5" (38mm) dia. Set that in a larger form and pour the mix over. Remove the mandrel when cured. If the form was rectangular and the mandrel located at the proper distance from the edges, it could be stood on edge and be the proper height for the displacer cylinder.

If the plaster of Paris does not prove to be heat resistant enough, I do have fire clay, and also Satanite as alternatives, though they are much less insulative.

[tom osselton]

I’m wondering if you could use a glow plug for a deisel

[vtsteam]

I don't know. Tom. To be honest, I've never actually seen one. Just glow plugs for small model airplane engines. I think they were platinum, and not meant for continuous current.

[tom osselton]

I realize they aren’t for continuous use but they could be pulsed using a arduino or a thermocouple.

[Bluechip]

Low voltage immersion (?) heaters are freely readily available ... like this:
https://www.ebay.co.uk/itm/325462059145?chn=ps&_ul=GB&mkevt=1&mkcid=28&google_free_listing_action=view_item&gQT=1
Whether it has any relevance only you can decide.
 
Dave

[shipto]

what about a 3d printer hotend heater?

[vtsteam]

Dave, I didn't realize there were low voltage versions of those. Thanks! I had one of the wall plug versions of those in college. If I remember correctly they warned that they would burn out if not immersed. Nice form for the purpose, though. A coil of about the right size.

I will also need to insulate around the heater, so kind of the opposite of the need for cooling on those immersion heaters. Thanks, though, I appreciate all suggestions! 

[vtsteam]

what about a 3d printer hotend heater?

Dwayne, I don't know. I don't have one. I do kind of remember a hot air engine that was heated by I think some kind of soldering heater coil. It was in an old Model Engineer article -- back in the day when Dennis Chaddock was judging the Hot Air Engine Competition. I don't know if I'm remembering that correctly. I think the heater was used to prove out longevity of that modeler's engine. He ran it continuously, until deciding his electric bills were suffering!

 

[shipto]

not sure what wattage they go up to but I was thinking you could run a couple in a aluminium housing that clamps to the end of the stainless with some insulation so you dont waste the heat.

[vtsteam]

Well Dwayne, if I had some I'd know better. I think I'm going to move forward on the homemade coil and plaster bedded idea.

I did try the reptile heater, and that was a dismal failure. I tried letting it heat the displacer for 20 minutes and finally gave up.

 

That thing put out so little heat, I could actually hold my hand two inches in front of it, and it just felt warm. This despite all the manufacturer's dire warnings about minimum distances, and letting the bulb cool for an hour before handling it. It didn't even warm up the back of the firebox -- 1/8" steel plate a couple inches away, after 20 minutes.

Oh well. Maybe I can use it to keep greensand from freezing overnight in this early spring, in case I want to do some casting........ which I kinda do. .....been awhile....

[vtsteam]

Well, off on an experiment using the previously tested stainless steel aircraft safety wire, and a plaster of Paris housing.

To begin with, I found a piece of 1" PVC water pipe which had an OD of close to 1-1/2" (38mm) and cut off about a 3 inch (76 mm) length. I wrapped it with the safety wire and taped the coils in place. Then I crimped a couple wires onto the ends using some copper tubing and a wire crimper. The wires were high temp type salvaged from a dead oil filled electric radiator.

 

[vtsteam]

Then I cut a small cardboard box down to the rough dimensions I wanted, taped it back together and lined it with kitchen plastic wrap as a mold for the plaster.

I painted  Vaseline on the coil form as a release agent.

 

[vtsteam]

I poked a hole through the top of the water pipe with a hot length of piano wire, heated over the kitchen stove. I used that to suspend the heating coil over the mold. I had to coil the wires up to one side to get the coil to stand vertically. The through wire was located from one end of the mold, at the center height of No. 83's displacer cylinder.

Oh, forgot to mention earlier, the bottom of the water pipe was covered with tape, so the plaster couldn't rise up inside.

A piece of masking tape was used to tie the sides of the mold box together to prevent bulging at the top.

 

[vtsteam]

Plaster of Paris was combined with water, the lumps squeezed out by hand, and then I added about one third by volume of fine sand. this was mixed again, and I poured the mold.

I found that even with the weight of the heater and wires, it still wanted to float up sideways, pivoting on the wire. So I spooned sand into the pipe, which provided enough weight to keep it vertical.

Now to wait......

 

[vtsteam]

I let the plaster cure overnight before unmolding it.

I figured that removing the core pipe might give me some problems because of the tape and I had thought of several clever ways to get it out of there. But giving it a simple twist with pliers back and forth a tiny amount, and pulling slowly freed it quite easily.

The coil of wire stayed in place. All in all a satisfying little job. Will it work to run the engine? No idea!