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Using Studs to Increase Heat Transfer in Heat Muff

L

lr172

Well Known Member
I am thinking about strategies to increase my heat output for next winter. I have two heat muffs in series and the second helped, but it is still a bit lacking when the ambient's dip below 10-20* with cloud cover or at night.

I was thinking about welding a bunch of 1/2" or 3/4" long studs to the exhaust pipe under the muffs. In theory, all of the extra surface area should increase heat transfer. I am wondering if anyone has done this with success.

Larry
 
That has been done, as have things like coiled springs, brillow pads, and many others...with mixed to neutral results. As a fellow Frozen Tundra Flyer in Minnesnowta, I feel your pain. The reality is that below and in the single digits there just isn't that much that can be done beyond what you've already done with an RV6 that will provide a noticeable benefit (or at least anything other than an incremental improvement)...as soon as you pull the throttle at all you'll freeze anyway.

I'd almost suggest looking into heated seats (I know, a bit of heresy to even mention), but they are pretty easy to install in most seats and really work pretty darned well. Given that the firewall usually stays a decent temp, the seats help out the rest of it.

The other easy solution is to get an RV-10...you can almost fly in a T-shirt in those temps! :)

Just my 2 cents as usual.

Cheers,
Stein
 
I stuck a 1-1/2" SCAT duct up against the outlet side of my oil cooler and ran that air into the cabin. It works just fine in -10F OAT (the coldest I've flown in).
 
SteinAir said:
That has been done, as have things like coiled springs, brillow pads, and many others...with mixed to neutral results. As a fellow Frozen Tundra Flyer in Minnesnowta, I feel your pain. The reality is that below and in the single digits there just isn't that much that can be done beyond what you've already done with an RV6 that will provide a noticeable benefit (or at least anything other than an incremental improvement)...as soon as you pull the throttle at all you'll freeze anyway.

I'd almost suggest looking into heated seats (I know, a bit of heresy to even mention), but they are pretty easy to install in most seats and really work pretty darned well. Given that the firewall usually stays a decent temp, the seats help out the rest of it.

The other easy solution is to get an RV-10...you can almost fly in a T-shirt in those temps! :)

Just my 2 cents as usual.

Cheers,
Stein
Click to expand...

Thanks for the reply. Yeah, decents are painfull and realize nothing can be done there. I have heated seats, but haven't used them much. I will leverage them more next winter. I can't really run them with low RPMs, as the alternator can't keep up. Using both puts another 10 amp load on the buss and my daytime steady state is 20 amps with the boost pump on; More at night.

I don't think the steel wool would help much. I only thought that the studs, being attached to the tubing would heat up. I am very much regretting not buying the muffler option from Vetterman.
Larry
 
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Supposedly a Vetterman muffler system will generate more cabin heat capacity than standard heat muffs. Might be worth asking Clint?
 
Studs on exhaust pipe

Years ago I sent a tail pipe to another exhaust fabricating friend and he welded studs on the pipe for me. I tested the outlet air temps both before and after and there was no difference. At the time neither he or I had any data to know what it would do. He and I were both quite surprised at the results, so I'm saying from that test it's probably not worth the cost and labor.
 
Put Stainless Steel pot scrubber pads inside the heatmuff. Huge increase in heat output.

Don't pack it in too tight.

Carl
 
make your own stainless coil

Wrap some safety wire around a pencil diameter size rod. Use the resulting coiled spring to wrap around the exhaust pipe under the heat muff. You don't want to use a regular spring because it will rust and break up, or a Bril?lo pad size wire as it will chafe and break up. Good old .040 safety wire.
 
Cockpit well sealed?

Is your cockpit well sealed? Canopy, Stick boot, etc...? I live in western Wyoming and regularly fly in ground temps around zero. At altitude often well below zero. When I first bought my 6 it was unbearable at even 15 above. Sealed the canopy and it is perfectly comfy. Even my wife, who does NOT tolerate cold, finds it perfectly comfy in seriously cold temps.:)
 
I bought a heated battery powered jacket for my friend who has no heater, he says works great, battery last for hours and it probably can be tied in to A/C power. Ordered jacket from Home Depot
 
Where are you feeding the air from? Low on the rear baffle bulkhead will let the cylinder fins pre heat the air before it gets to the heat muff.

Electric seats and electric snowmobile underwear are very effective and let you keep your head in cooler air.
 
vetterman said:
Years ago I sent a tail pipe to another exhaust fabricating friend and he welded studs on the pipe for me. I tested the outlet air temps both before and after and there was no difference. At the time neither he or I had any data to know what it would do. He and I were both quite surprised at the results, so I'm saying from that test it's probably not worth the cost and labor.
Click to expand...

Thanks for posting. Very helpfull to hear of actual test data. Maybe a bigger alternator is in my future.

Larry
 
sibriggs said:
Wrap some safety wire around a pencil diameter size rod. Use the resulting coiled spring to wrap around the exhaust pipe under the heat muff. You don't want to use a regular spring because it will rust and break up, or a Bril?lo pad size wire as it will chafe and break up. Good old .040 safety wire.
Click to expand...

Thanks Steve and Carl. Maybe I'll try stuffing something in there and see if it improves the heat output.

Larry
 
WyoDave said:
Is your cockpit well sealed? Canopy, Stick boot, etc...? I live in western Wyoming and regularly fly in ground temps around zero. At altitude often well below zero. When I first bought my 6 it was unbearable at even 15 above. Sealed the canopy and it is perfectly comfy. Even my wife, who does NOT tolerate cold, finds it perfectly comfy in seriously cold temps.:)
Click to expand...

Yes, cockpit is well sealed. Zero air from the canopy. Almost zero air from the spar area or footwell area. I have aileron boots and a fair amount of RTV sealing all sorts of little openings, including around the spars. I could have air from the baggage area, but I don't feel it.

The heat is fair, but had a few days around zero and could have used more heat. I am using a 1.25" restrictor on the air inlet and could consider opening that up a bit now that I have two muffs.

I also have no carpet or interior panels and I suspect the lack of insulation allows a very large amount of heat loss through the aluminum and plexi. The air near the cabin sides is MUCH colder than the rest of the cabin air.
Larry
 
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F1R said:
Where are you feeding the air from? Low on the rear baffle bulkhead will let the cylinder fins pre heat the air before it gets to the heat muff.

Electric seats and electric snowmobile underwear are very effective and let you keep your head in cooler air.
Click to expand...

I am taking air from the rear baffle just above the fins on #3. I like the idea of taking air from the oil cooler, but can't think of a way to maintain enough pressure/velocity without some mechanism to switch from cold to warm weather (need more air flow through cooler).

I have seat heaters, but never used them. Pretty foolish on my behalf, but my alternator can't handle the extra load at lower (idle/taxi) RPMs. Therefore they are off at T/O and I always forget to turn them on once moving.

Larry
 
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Be care with using steel wool, pot scrubbers, etc. inside your heat muff. Put a torch on the stuff first and see what happens. I've seen steel wool catch fire and burn like a sheet of newspaper with just a pass with a torch. You don't need that kind of heat!
 
Heat

Ok guys, I see a lot of ideas being kicked around here, some good some well here's what I did years ago on my -4 to try to increase the cabin heat. I took one of those stainless pot scrubbers and neatly pushed, pulled, and prodded it into the heat muff. Yep, it did increase the temperature of the cabin heat. Seems it was about maybe 50 hours later, flying along happy as Jay bird when I noticed I was being hit with something coming out of the heater. I captured one of them and discovered it was little C shaped steel things. That's right, the heat deteriorated the pot scrubber and it was literally coming apart and attacking me. If I remember it wasn't a cheap one, I'm thinking it was a chore boy brand. It was removed from the heat muff, or what was left of it. So the next step was to close up all of air leaks, which really helped. I've always said that little leaky breeze that you can feel in the summer time, become an unbearable arctic blast in the winter time.
 
I used a couple of additional stainless steel hose clamps and some bits of stainless.

The hose clamps hold short L-shaped strips of the SS around the pipe, with the short end of the L sticking up vertically and with 6-8 of these things distributed around the circumference of the pipe. This whole thing is kind of a like a diffuser.

Two such diffusers are in place, just inboard the inlet/outlet openings of the muff. The screw for the clamp is inline with the inlet/outlet, and both serve to not only provide additional surface area but also to force a longer route for the air to take across the hot exhaust pipe.

Coupled with the cross-flow arrangement and a not-drafty cockpit, I'm pretty comfortable down to about -5ºF during a sunny day and perhaps 10ºF at night.

-Brad
 
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Brad's (ChiefPilot) idea is similar to one I've thought of before. Think of the diffusers as fins, as those on our cylinder heads. First you need a good contact area for each piece of SS to contact the pipe for good conduction heat transfer. Then you need enough fin area for good convection heat transfer into the air stream.

There are equations from too far back in my education to calculate the optimum size of the fins. Also, I'm not sure SS offers the best Thermal Conductivity compared with other alloys like Aluminum or copper. But if Brad says it works, then I believe him. (P.S. The Thermal Conductivities of Aluminum and Copper are about 5X and 8X, respectively, that of Stainless Steel, per The Engineer's Toolbox.)

Fin design is a science, although there is room for empirical development. The thickness of the fin is as important as the height and length of the fin.

The problem I see with the steel wool and coiled safety wire is inadequate contact area (square inches) with the pipe to assure good conduction heat transfer from the pipe.

Back to the original topic regarding welded studs. IMO, the studs won't have enough surface area for good convection heat transfer into the air stream unless you use a fan to increase air velocity. And the fan would draw more electrical load. And the hose clamp idea offers less chance of damaging the exhaust pipe than welding. You don't want any cracking of the exhaust pipe especially in this area. (Engine exhaust to the cockpit.)

Disclaimer: Comments appearing here are the opinions of the author. Any use of these comments for commercial or private endeavors are done at the risk of the reader and the author will not be held responsible for any resulting losses or harm.
 
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This thread is deja vu all over again.

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Carl Froehlich said:
Put Stainless Steel pot scrubber pads inside the heatmuff. Huge increase in heat output.

Don't pack it in too tight.

Carl
Click to expand...

...Actually the best for this is the copper ones (Chore Boys). You still will be lacking when the throttle is retarded. Another option that works well is a 12 volt car heaters of the 300 watt variety. Many available on E-Bay under $20. I have also used 12 volt electric hair dryers for heating, and they lend themselves well to attaching a scat tube for heat placement. Cheap, portable and easy fix. Thanks, Allan...:D
 
Heat transfer in muffs

Fins as Don calls them do work. They not only provide more surface area for heat transfer but they redirect or in some cases restrict the airflow. We know that heat muffs with the inlet and outlet in line simply allows the air to flow directly from one to the other and the air is not in the muff long enough to heat up. I placed partial heat muff end plates close to the inlet and exit by riveting them to the heat muff and using a radiator clamp right above them to draw them down tightly on the exhaust pipe. The temperature of the air increased more than other things I tried.
Then mufflers came along. I use that term loosely as most units with a flame tube or flame cone started out as spark arrestors. Then one day they were called mufflers. When we first tested mufflers we were surprised that there was no power loss, it did quiet the exhaust by a few decibels and the cabin heat was greatly improved. It took quite awhile for mufflers to be accepted but when the RV-9 came out, a lot of builders decided to get them. Also most builders in Canada wanted them to be able to fly in really cold weather. So why do they put out so much more heat? Mufflers use a 3 inch piece of tubing vs 1 3/4 inch common on most systems. The heat transfer area is simply much larger. Hope this explanation helps a little
 
On a similar note...

I forgot to cut holes in my carpeting for the heater discharge ports in my -10. I only noticed it When I opened the heater valve for the first time. I had warm air rising all around me instead of just on my right calf. The warm air is flowing under the carpet and escaping around the edges into the cockpit. It's like a radiant heat floor! The mistake has turned into a really nice feature!
 
FWIW custom aircraft parts in El Cajon, Ca will install the studs if someone wants them.
No endorsement but I did ask them once if they could instal lthem.
Tim Andres
 
Increase Heat Transfer in Heat Muff

Actually I think that using a Nichrome heating element wire is the best(cheapest) heat transfer method because the wire in a normal application could be somewhat "red hot" in normal usage. This type of "heat element restring kit" can be purchased at wholesale heating and air-conditioning distributors or appliance distributors( clothes dryer element restring kit) pretty reasonable (<$20.00) voltage and wattage doesn't matter in this case, but may be available in 3/8,7/8 or other diameters. You just need to pin each end down and put as much as you can inside your heat muff. The contact area of the nichrome wire with the exhaust is what will get the heat in to the air. I have tried a few other methods since if been in HVACR all my life and this is the least amount of work, cost for what you get.


Keith Rhea
RV7
 
Heat transfer

Tim 2542. You are talking about Clinton at custom aircraft, he's been a friend for many years and does excellent work.
 
rv7boy said:
There are equations from too far back in my education to calculate the optimum size of the fins. snip

Fin design is a science, although there is room for empirical development. The thickness of the fin is as important as the height and length of the fin.
Click to expand...

Don,
I was in school about the time Mandelbrots stuff was coming out but fractals were not recognized as being as applicable to the physical world as they are today.

I remember a presentation on cooling fins that discussed the math to optimize heat rejection. The resulting fins had exponentially curved walls (wide base and pointy tip) instead of the parallel ones we have on our motors. The funny bit is that they recognized data that did not fit the exponential curves. When the calcs were run at a finer sample rate, the fins had what looked like a sinusoid overlaid on the exponential sidewall. They looked like christmas trees. So... there were still some points that did not fit and the engineers were still scratching heads. Unfortunately, that was the end of it and my career has not included fin design. I strongly suspect (but have no proof at hand) that fins are a fractal.


Somebody talked about using oil cooler air. Here is a link to the first page of my oil cooler and double heat muff revision.
Heat revision
 
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rv7boy said:
Obviously my description of fins was not understood. Vetterman indicated the studs don't work.
Click to expand...

Your description was understood Don and match my recollection of heat transfer classes, and I get that SS is not a good conductor, and Mr Vetterman has a data point, and that we need come baffles within the muff to create some multi-pass for the air internally. The picture was just a visual for what AWI does to their exhaust. I have no illusions about this being some panacea. Alternating baffles will be added, insulation of the outside of the muff will be done, and in the winter data will be taken. Mr V has always done his homework, so that comment certainly lowered my expectations of this design.

Heat transfer can be limited on the exhaust side too. In a past life, a project needed to recover heat from the exhaust of a wankel engine. The exhaust temps were around 1800F. As an early experiment, the lab guys put a water jacked on a 1 ft section of exhaust and ran 60F water through it. The exhaust temp only dropped 100F. Not much, I surely believe that the exhaust wall temperature was much closer to the water temp than the exhaust, as the heat transfer coefficient is much much greater. We applied an alfa-laval plate type HX and dropped the exhaust to 300F and recovered a huge fraction of the lost energy. Using this reference in our situation, with air on both sides it seems we just need more area on both side - the low and high temp side.

Let the experiments continue and successes be reported.
 
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Custom Aircraft Kudos

vetterman said:
You are talking about Clinton at custom aircraft, he's been a friend for many years and does excellent work.
Click to expand...
I concur. He built a custom crossover exhaust for my Lancair 235 and I'm very pleased with it and the service he provided.
 
Maybe too simple .....

With my RV7a, I theorized that the problem might be too much air velocity through the muff. It's not really controllable with the flap because the bypass air is continuous and it takes a lot to slow down the high volume high pressure air. If you slow it down with pot scrubbers that concentrates hot spots in the muff and exhaust pipe.

So, instead of cutting one big hole in the rear baffle where the scat tubing connects, I just drilled about 6 3/8 inch holes which reduced the flow of air into the heat muff. I have flown in 0 degree weather and had plenty of heat with no other modifications. No indications of hot spots in 600 hours. If you already have the 2" hole, you might try a small restrictor plate to cut the entry flow down by maybe 50-60%.

Just my .02 YMMV

Bill S.
 
rzbill said:
Don,
I was in school about the time Mandelbrots stuff was coming out but fractals were not recognized as being as applicable to the physical world as they are today.

I remember a presentation on cooling fins that discussed the math to optimize heat rejection. The resulting fins had exponentially curved walls (wide base and pointy tip) instead of the parallel ones we have on our motors. The funny bit is that they recognized data that did not fit the exponential curves. When the calcs were run at a finer sample rate, the fins had what looked like a sinusoid overlaid on the exponential sidewall. They looked like christmas trees. So... there were still some points that did not fit and the engineers were still scratching heads. Unfortunately, that was the end of it and my career has not included fin design. I strongly suspect (but have no proof at hand) that fins are a fractal.


Somebody talked about using oil cooler air. Here is a link to the first page of my oil cooler and double heat muff revision.
Heat revision
Click to expand...

Thanks for posting the details Bill. Did you notice how the airflow compared with the oil cooler feed vs the standard entry from the rear baffle?

Larry
 
lr172 said:
Thanks for posting the details Bill. Did you notice how the airflow compared with the oil cooler feed vs the standard entry from the rear baffle?

Larry
Click to expand...

No noticable difference in air flow rate, Larry.
My original was front baffle entry.

It is not in the images but please note that my oil cooler is blocked off 50% with aluminum tape.
The heater hose gets the majority of airflow through the cooler.
 
i guess if i were trying to improve temps in an ice shanty etc. instead of trying to soup up the furnace i would try to improve the insulation.
 
vetterman said:
Fins as Don calls them do work. They not only provide more surface area for heat transfer but they redirect or in some cases restrict the airflow. We know that heat muffs with the inlet and outlet in line simply allows the air to flow directly from one to the other and the air is not in the muff long enough to heat up. I placed partial heat muff end plates close to the inlet and exit by riveting them to the heat muff and using a radiator clamp right above them to draw them down tightly on the exhaust pipe. The temperature of the air increased more than other things I tried.
Then mufflers came along. I use that term loosely as most units with a flame tube or flame cone started out as spark arrestors. Then one day they were called mufflers. When we first tested mufflers we were surprised that there was no power loss, it did quiet the exhaust by a few decibels and the cabin heat was greatly improved. It took quite awhile for mufflers to be accepted but when the RV-9 came out, a lot of builders decided to get them. Also most builders in Canada wanted them to be able to fly in really cold weather. So why do they put out so much more heat? Mufflers use a 3 inch piece of tubing vs 1 3/4 inch common on most systems. The heat transfer area is simply much larger. Hope this explanation helps a little
Click to expand...

Don/Larry,

Interesting idea with fins/diverters. It would make sense that forcing the air around the muff more than the short path that it currently takes would make a difference. I will play around with that a bit and see what kind of improvement I can get.

Larry
 
BillSchlatterer said:
With my RV7a, I theorized that the problem might be too much air velocity through the muff. It's not really controllable with the flap because the bypass air is continuous and it takes a lot to slow down the high volume high pressure air. If you slow it down with pot scrubbers that concentrates hot spots in the muff and exhaust pipe.

So, instead of cutting one big hole in the rear baffle where the scat tubing connects, I just drilled about 6 3/8 inch holes which reduced the flow of air into the heat muff. I have flown in 0 degree weather and had plenty of heat with no other modifications. No indications of hot spots in 600 hours. If you already have the 2" hole, you might try a small restrictor plate to cut the entry flow down by maybe 50-60%.

Just my .02 YMMV

Bill S.
Click to expand...

I have a restrictor over my inlet. I can't remember if it was 1-1/4" or 1", but I think it is the former. I meant to try closing it off some for a test, but the cowl didn't come off much during the really cold periods last winter.
 
snopercod said:
Very true. The thermal conductivity of SS is 16, carbon steel is 42, and copper is 400. They use stainless steel in cryogenic valves and transfer lines because of the low heat transfer.
Click to expand...

The point wasn't so much heat transfer via the diffusers themselves, but to force the air to take a less direct route between inlet and outlet. More time in the muff = more contact with the hot exhaust pipe = warmer air.

Not really different than necking down the inlet/outlet to reduce the flow of air, but I tried this approach to have the same volume of air except at a overall higher temperature.
 
One data point

I've had both Vetterman and AWI exhaust systems on my RV-8. I changed to the AWI 4-1 in large part for the studs. I was living in Northern Illinois at the time and wanted to fly down to about 10F. I got almost no heat from the heat muff on the 4 into 2 without studs or steel wool in it but get a LOT of heat from the AWI stud system. The heat comes out right at my right foot and it can be hot enough to be uncomfortable.

It has been 30 years since I did heat transfer work, but if I remember correctly, surface area, temperature difference between source and sink, heat transfer coefficient of the materials and mass flow rate all affect the temperature rise of the fluid/gas (and temperature decrease of the source).

Studs increase surface area, baffles and metal wool do too plus may slightly slow the flowrate down which would increase outlet temperature but not necessarily increase total energy transferred. Do I want a small amount of really hot air or a large amount of warm air?

The AWI 4 into 1 is stainless steel but I don't know what kind. I don't know how the thickness of the material compares to what Larry uses, and the different geometry compared to Larry's may mean the heat muff sees a higher exhaust gas temperature which would also help it pump out more heat.

Lots of variables. Be careful drawing conclusions other than "this works". In my case I am fully satisfied with the heat I get from the AWI system.
 
ChiefPilot said:
The point wasn't so much heat transfer via the diffusers themselves, but to force the air to take a less direct route between inlet and outlet. More time in the muff = more contact with the hot exhaust pipe = warmer air.

Not really different than necking down the inlet/outlet to reduce the flow of air, but I tried this approach to have the same volume of air except at a overall higher temperature.
Click to expand...

Your approach of getting the air to take a longer route through the muff chamber seems to me like the best method to increase the heat transfer and will try that. I may dabble with a method to get some heat transfer from the same metal used to divert the air, as you did. I prefer fins or channels over stuffing with material.

I think that I will order a few of the two piece muff end caps from Rick Robbins. I will install two in each muff and drill a series of strategic holes in them to create a chamber approach like a modern muffler. The plan will be to slow the air down just a bit and force it into each chamber for better heat transfer and a greater potential dwell time.

I'll report next winter if it actually works for me.

Thanks for sharing.

Larry
 
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I know from welding that SS has very poor heat transfer. You only need about 60% of the current to weld it that you need for low carbon steel. The heat just stays right where you put it and does not dissipate into the resp of the part. It is easy to damage the material and reduce its corrosion resistance by precipitation of the nickel. Aluminum is the opposite - you need more current because the adjacent material sucks the heat away. Copper is the best for hear transfer. It melts at 2000F so it should hold up. I will try taking a long narrow strip of some thin sheet and bend a flange in it, then stretch the outter edge of one side to make a helix or auger to wrap around the pipe. Easy to do with soft material. It would transfer heat from the pipe very well and slow down the flow through the muff.

Of course there is the matter of 10,000 more rivets to buck before I can report on results:eek:
 
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