Heat Exchanger Design

[derwood]

I thought I would start a thread to discuss what I have learned about heat exchangers. As some of you know I have been working on a thermal lag engine for quite a while. Hours of testing has revealed to me what makes a better heat exchanger. a couple of days ago I decided to raise the compression on my engine to see just how high it would go. I cut 4 inches off of the heat tube and the compression was a lot higher. After the engine got up to operating temp it ran about 50 rpm faster but the power increase was much more than I thought possible. It ran great for about five minutes and died. I removed the heat tube and a handful of slag powder fell out. also when I cut the end off of the heat tube a big ball of slag rolled out. I had put regular steel wool in the exchanger and it completely burned up. I went back to the ss wool and when I got the engine running it was much weaker and would only run for a bout ten seconds at a time. The heat exchanger could not keep up with the demand of the higher compressed air. I know most of us always use ss for heat exchanger material due to its ability to withstand high temps but really it's about the last thing you would want to use for conducting heat. The performance of the regular steel wool was a lot better but unfortunately it can't withstand the high heat. The ss dumps its heat into the compressed air much slower. the regular steel dumps heat much faster and charges back up for the next compression stroke. I will be trying copper wool as soon as I can find a place to get it. It should out perform the regular steel wool. I would like to see some people who are building the bigger stirling cycle engines try using mild steel hot caps. it would have to be a little thicker but it would probably produce more power. A copper hot cap machined with interior fins would probably increase power even more. A good heat break between the hot and cold side would be very important. I have found that abrasive cutting blades make excellent heat breaks, you can even stack them with red silicone between each layer. The heat exchanger seems to be the most important part of any hot air engine and the better it conducts heat the more power you will get. More compression is the key to more power but it requires more heat.

[Ian S C]

Was the stainless steel wool about the same grade as the plain steel wool? Maybe you will have to experiment with finer, and coarser wool.
With heat exchangers, the cold side is just as important as the hot side, for all the heat you get into the motor, you have to get it out again. That's what I'm working on at the moment, I'm building one, and rebuilding another radiator to see which one works the best on my free piston GAMMA motor that I'v just got going. I'm not worrying too much about the hot end for now, it's just mild steel, but will later be made of stainless steel.
Ian S C

[derwood]

You might be right about that. The stainless is a scouring pad grade and I have been using that throughout all of my testing. The strands are thicker than the regular steel wool. I have a pile of fine ss laying under my lathe. I will try that tonight and see if it works as good as the regular steel wool. It might be that any fine metal wool will burn up. I hope not because the fine material works better. The fine steel wool burns up as soon as it starts to glow red. I will post results later this evening.

[derwood]

I just completed a test using fine stainless. It ran better than the coarse stainless but not even close to the regular steel wool. Based on this test I think copper wool will do even better. I hope that it can withstand the heat and not burn up. Has anybody ever tried copper wool?

[fullofhotair]

Melting point of steel 2500f ,melting point of copper 1984f. An alloy is usually not as good a heat conductor as the pure metal . Stainless steel is an alloy. Tungsten 6000f melting point.and is a good heat conductor. Probably prohibitive due to cost. Old burn out light bulbs could be a cheap source.

[Hawke]

Just happened to be looking at melting points tody. New chart in my image gallery now.

[Ian S C]

If you were very lucky, you might get 1/4" of oxidised , and crumbling Tungsten from an old light bulb. Try a paint shop for stainless steel wool, they usually have some very fine stuff, it's not too cheap (well not here in NZ). You don't want too have anything with too much heat conduction, like copper, you want to hold the heat at the hot end of the regenerator core. Ian S C

[derwood]

I was able to do another test last night. I had some 14 ga. stranded wire. the strands were a bit bigger than the coarse ss wool. I formed them into a ball and compressed it into the heat exchanger. It out performed both grades of ss wool. The wire diameter was much bigger than the ss wool but I was not able to pack enough in to the exchanger to equal the the surface area of the ss wool. A rough guess, the surface area was not even ten percent of the ss wool but it started up quicker and maintained a constant speed. Remember, I am using a thinned down section of ss tube for the heat exchanger case, this keeps the heat contained to the hot side. The copper and ss wool both achieve the same temperature but the copper's ability to dump its heat into the cool air quickly makes all of the difference. It also recharges quickly for the next stroke. Both types of ss wool became very crispy around the outer edges and the copper did not. Don't be confused by the lower melting point of copper. It is very resistive to oxidation. The high heat causes the fine ss and regular wool to oxidize and burn up, It does not get hot enough to melt the ss or the copper. I will be trying copper wool this evening and will post results.

[Hawke]

[tab=20]From hands on experience Ive found copper and stainless steel do conduct heat very fast and a long way down the metal piece. Even brass. Regular iron is not as conductive to the touch. Copper has always been my choice to dissipate heat. I was wondering which property is better for displacer material myself. Slower or faster to dissipate heat. Then we have this deterioration of steel wool to consider.

[tab=20]Steel wool starts fires with a battery for campers. This video at 1:30 seconds in shows its volatility!
http://www.youtube.com/watch?v=JtRkdttaYqs

So for my low investment stirlings steel wool must be kept from too much heat, Your experiment there Mr Derwood is very helpful. Im thinking Copper Choreboys now uh?

FYI: Made this chart image for reference.(in My Gallery)

[derwood]

I picked up two boxes of chore boys today and am going to test later tonight. You read my mind...........

[derwood]

[tab=20] I was wondering which property is better for displacer material myself. Slower or faster to dissipate heat. ]

Based on my experience the best displacer material would be a material that conducts heat at an extremely low rate and does not store heat. stainless or mild steel doesn't really matter. Although the stainless conducts heat slower, it eventually achieves the same temp. Stainless will prolong runtime but once it heats up I can't see a big performance difference. High temp Silicone tubing would probably make a very good displacer body.

[derwood]

I tried the chore boys. The copper scrubber performed a little better than the wire. It out performed ss scrubber and the fine ss enough to say that copper is better in this case for the heat exchanger. It is puzzling why the regular steel wool did so much better than the others. I can think of many reasons but all just theory. Could it be that the burning steel wool had something to do with it. Or could it be that the crispy carbon framework that's left is superior material until it breaks apart. The fine stainless only achieved this state around the edges and the stainless scratch pad did not at all. I heated up all three materials with a torch until they burnt. the regular wool burnt up quickly but the material that was still remaining glowed white hot. The stainless did the same thing but was more resistant to burning. The copper scrubber hardly burnt at all but mostly melted into little beads that only glowed red. One fact remains and that is regular steel wool was far better. It kind of makes me think about the hard tungsten that was mentioned by Fullofhotair. It makes me think of a light bulb filament. Suggestions?

[derwood]

I would like to add that even though the copper showed better power it eventually got brittle and started to break up into pieces.

[Hawke]

Thx Derwood for passing the info along. I looked for copper choreboys in fact today at the grocery store and Lowes with no luck. After seeing your posts, I decided to go ahead and button up my displacer cylinder with steel wool except not at tightly packed as last time.

[bladeattila]

Hi Derwood,
the problem has complexity.
The thermal lag engine has a tube inside what make a laminar flow originally.
Should be SS material, and this connect to a red copper or aluminium heat exchanger/cylinder for the piston with thereaded connection. This tube is a preheater also preregenerator in this engine.
Also should be SS for the heated tube, because then lower the heat stress of cold side's seal rings and important for the compression stroke too.

And you need to make a real hot side heat exchanger but shouldn't be connecting to that inside SS tube, so if you can use some Ca-silicate there between the small diameter SS tube and the heat exchanger copper or steel bla...bla what you need to use. Because the cold side is your small tube's cold end also the cold heat exchanger/cylinder for the power piston.
(Ca-silicate is a fire security material, so light, and easy to use, also possible to use it for displacer, but the Al-oxide+Ca-silicate material is the top material for the home made hot air engines)( Not cheap:( )

Thicker thread has higher heat capacity, but I think the bigger open area was bigger help for the engine's speed.
The regenerator/heta exchanger open and closed area's ratio is important because the flow should be flush it perfectly...and the surface, but these parameters could be for speed increasing. Because bigger open area=faster gas flow, higher surface=better heat exchange
BUT! If your inward heat bigger then your outward then the engine will stop or will slower in the time, so my opinion is, the cold side much more important here, and the heat exchanger there. Should be use a plate with lot of hole between the piston and the smaller tube with a dsipenser cone for a great cold heat exchange.
As like this: http://www.youtube.com/watch?v=OLWmAE0FZJg

Another thing is for the heated tube cylinder and your heat exchanger wool heat contact. Should be higher surface for contact. But the wool thread has a lot of, but small contact surface. If the treads are dense (too much threads there) then that reduce the air flow, because close the tube. (A tip: If you can use some silver welding rod for better heat contact, then make a thin layer inside the tube and put in the SS steel wool, and heat it up again.)
This is a patience game with lot of compromise!
I hope this essay is not bored you!
Also I hope you never give up, because easy to build this engine also use for a home made stove.

Best regards!
Blade