What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Bronze Wool for heat muff

rmarshall234

Well Known Member
Has anyone tried using this instead of SS Wool for their heat muff? Anybody care to comment on bronze as a substitute for SS in this application?
 

Attachments

  • Bronze Wool.jpg
    Bronze Wool.jpg
    257.1 KB · Views: 159
Want more heat?..... Reduce the inlet size to reduce the speed of the air passing through the muff.
 
Want more heat?..... Reduce the inlet size to reduce the speed of the air passing through the muff.

Yes/no. It’s not quite so straight forward. Yes, reducing the mass flow will raise the HEx air outlet temp; however , the overall heat transfer goes down. Most of the related parameters are fixed. The primary heat transfer influence is the mean temperature difference (MTD); Heat exchanger temp to air temp.

Increasing surface area of the heat exchanger relevant surface area (A) can increase heat transfer though it would have to have physical contact with the hot surface or it could act like a sink.

Q=U*A*MTD. U being a function of film coefficients and fluid specific heats for those who suffered through Heat Transfer in college.

It’s a balance.
 
Yes/no. It’s not quite so straight forward. Yes, reducing the mass flow will raise the HEx air outlet temp; however , the overall heat transfer goes down. Most of the related parameters are fixed. The primary heat transfer influence is the mean temperature difference (MTD); Heat exchanger temp to air temp.

Increasing surface area of the heat exchanger relevant surface area (A) can increase heat transfer though it would have to have physical contact with the hot surface or it could act like a sink.

Q=U*A*MTD. U being a function of film coefficients and fluid specific heats for those who suffered through Heat Transfer in college.

It’s a balance.

Well, It's a good thing that I didn't read this fifteen years ago.

I installed a 1.5 inch scat to the inlet of my heat muff with no noticeable change to outlet temps or comfort in the cabin. What I do notice is way more service room to the back of the motor. I do live in mild climate.

I sure can't argue with the educated. Please disregard my response to the OP.
 
Well, It's a good thing that I didn't read this fifteen years ago.

I installed a 1.5 inch scat to the inlet of my heat muff with no noticeable change to outlet temps or comfort in the cabin. What I do notice is way more service room to the back of the motor. I do live in mild climate.

I sure can't argue with the educated. Please disregard my response to the OP.

Wasn't supposed to come off that way but it's my superpower and I had a few minutes before a meeting to get my geek on.

Think about the iterations/improvements of GA heat muffs over time. Smooth surfaces gave way to corrugated-ish ones to increase area and turbulation (air mixing.) Air is a poor heat conductor, an insulator actually so it's important to have a good bit of turbulence/recirculation/etc. to bring as much of the quickly passing air into contact with the hot surface. Studs were added to increase both of the aforementioned; A and a component of U in the equation.

Putting steel wool into the heat muff could help; the steel that has contact with the hot surface specifically. The heated steel wool would increase the hot surface area plus the extra turbulation would bring more of the air into contact with the hot surface. Too much would probably be detrimental relative to the optimal amount. Bronze would probably be great; at first. It's a better conductor but once the surface started to oxidize, that would quickly diminish.

A decent design shouldn't need it. As Vlad mentioned, another HEx in series will help but it is diminished returns because of the MTD in the second muff.

As mentioned, the overall Heat transfer is more important than the outlet temp alone. The air passes into a cabin that is basically a big heat sink.
 
Bronze would probably be great; at first. It's a better conductor but once the surface started to oxidize, that would quickly diminish.

Thank you.

So maybe it becomes a consumable and I replace it each year at annual. At $10 / year that sounds like a good investment to me. Except for the servicing issue, I'd pay that much _per flight_ to stay warm. :)
 
If it oxidizes, it eventually comes apart. The pieces have to go somewhere.

I haven’t tested such nor have any directly related experience. I have no idea how long it would be OK but I would be concerned.

Second or larger heat muff?

And/or heated seats?

Stay safe.
 
Darin, your modification looks great! could you in some way "quantify" the improvement? some or a lot more heat?
 
Almost 100% of the RVs I look at have what is called, in the heat exchanger world, concurrent flow set up for the heat muffs. This is where the exhaust gas and the heated air go through the muff go the same direction. This is sub-optimal. Counter-current is going to result in hotter air. Put the supply air into the aft end of your muff, with heated air coming out the forward end of it. I've not measured how much difference this would be, but it is likely quite a bit.

Avoid putting the heater on a crossover pipe - there is noticeably less heat from a heater on a pipe fed by one cylinder as compared to two, even though it is closer to the cylinder on the crossover.

Use caution with the bronze wool - it might only be a plated mild steel. I've found many "SS" wools to also be just a plated mild steel, which will rust fairly quickly in this environment. (Edit: it appears that the OP's bronze wool is indeed just plated - the label says "rust resistant").
 
I have the vetterman mufflers on my 6A (O-360). I only have 1 plumbed with a heat shroud. I get so much heat, I rarely crack the heat more than 2-3 clicks on the detent push/pull cable. That is even when cruising at 13K in winter under an overcast layer. That said, I would like to add a liner or carpet under the seat, the seat pan still gets cold even with the pushrod boots.
 
Put the supply air into the aft end of your muff, with heated air coming out the forward end of it. I've not measured how much difference this would be, but it is likely quite a bit.


Use caution with the bronze wool - it might only be a plated mild steel. I've found many "SS" wools to also be just a plated mild steel, which will rust fairly quickly in this environment. (Edit: it appears that the OP's bronze wool is indeed just plated - the label says "rust resistant").

Excellent. Thanks Alex. I will absolutely go with the Counter-current ducting design - that makes perfect sense to me. And, where's my magnet to check this "Bronze Wool". Geezz...whatever happened to truth in advertising. Ha!

As for an optimal design...it looks like Darin has the best approach with his aluminum fingers that stick up into the airflow. Brilliant!
 
I tried to pick up a single strand of my Bronze Wool with a big ol' magnet and it just sat there on the table so it does appear that I have the real deal. I completed the installation this am and will report back if there is anything notable.
Thanks guys.
 
I have 2 heat muffs in series, both stuffed with steel wool - it is OK, but never gets truly warm. The canopy is sealed, the rear baffle is sealed, no boots on the control rods.
 
Tension

Very nice.

Key to maintaining its effectiveness would be maintaining solid contact with the pipe. Did you do anything to tension it? Stainless strip?

With the bent up ends for the #8 threaded screw, and the fact both the pipe and material for “finned clamp” are both SS, there does not seem to be any loosening due to thermal cycles.
 
Quantify

Darin, your modification looks great! could you in some way "quantify" the improvement? some or a lot more heat?

Somewhat challenged to quantify, but I would say it was chilly at -10C with “normal chothing” prior to shirtsleeves now and at -20C you want to start to zip up your jacket. My wife and I were up at 17,500 on a cold day over Montana once and the OAT was reading -53C on the way to Mexico, and it did get uncomfortably cold…I think it was -30ish on the ground that day.
 
Somewhat challenged to quantify, but I would say it was chilly at -10C with “normal chothing” prior to shirtsleeves now and at -20C you want to start to zip up your jacket. My wife and I were up at 17,500 on a cold day over Montana once and the OAT was reading -53C on the way to Mexico, and it did get uncomfortably cold…I think it was -30ish on the ground that day.

Is that a typo, -53C? That is -63F. Count me as impressed if you survived a flight in that OAT in an RV!
 
I once made a skydive from 25,000 msl when the OAT was -40F with a windchill factor of over -100F. That was cold! See why I want a heat muff in my airplane.:D
 
Horton to the rescue

I am waiting for Dan Horton to come out of retirement and help with this thread.

IIRC - He torched the bronze already or doing it now.:rolleyes:

Boomer
 
Are there any metallurgists out there that could give an opinion on making Darin’s fin assemblies out of Aluminum vs. his SS? Obviously, AL is much easier to source and work with. Thanks in advance.
 
Early Mooney's used a very long spring tightly wrapped around the 2 cross-over pipes and then the heat muff fit over the spring. The heat seemed very good with that set up.
 
Are there any metallurgists out there that could give an opinion on making Darin’s fin assemblies out of Aluminum vs. his SS? Obviously, AL is much easier to source and work with. Thanks in advance.

i also wonder to make these things from aluminum sheet. melting temp of 2024 is around 900 - 1200 F. we know that exhaust gas temps can get higher but i have no idea how hot the exhaust pipe itself gets. stainless steel has around 2500 F melting point. so there's a significant difference. but my gut feeling tells me that we should get away with aluminum.

anyone could shed some light?
 
Last edited:
i also wonder to make these things from aluminum sheet. melting temp of 2024 is around 900 - 1200 F. we know that exhaust gas temps can get higher but i have no idea how hot the exhaust pipe itself gets. stainless steel has around 2500 F melting point. so there's a significant difference. but my gut feeling tells me that we should get away with aluminum.

anyone could shed some light?

You nailed on the first strike. Significantly different alphas, expansion rates as related to temperature, would be strike two (but overcome-able). Solid contact between the tube and fin material is important. Tensioning the fin wrap is the typical answer.

Strike three would be stainless and aluminum don't play well from a galvanic standpoint. The heat they would be exposed to would greatly accelerate this electrolysis/corrosion. Once this corrosive layer separated/insulated the "feet" of the fin from the hot exhaust tube surface, the local heat conduction (tube to fin) would rapidly shift towards zero making the fins next to worthless.

Designed a lot of HEx's a while back; one of the many things I've gotten to do as a professional. Most finned tubes utilized "wrapped" fins along the tube. For some critical applications, the fins and tubes were a single element. We had a vendor in Tulsa OK that would mechanically extrude fins from very thick walled pipe. I was an impressive feat of machining, if you're a geek.
 
Last edited:
You nailed on the first strike. Significantly different alphas, expansion rates as related to temperature, would be strike two (but overcome-able). Solid contact between the tube and fin material is important. Tensioning the fin wrap is the typical answer.

Strike three would be stainless and aluminum don't play well from a galvanic standpoint. The heat they would be exposed to would greatly accelerate this electrolysis/corrosion. Once this corrosive layer separated/insulated the "feet" of the fin from the hot exhaust tube surface, the local heat conduction (tube to fin) would rapidly shift towards zero making the fins next to worthless.

Designed a lot of HEx's a while back; one of the many things I've gotten to do as a professional. Most finned tubes utilized "wrapped" fins along the tube. For some critical applications, the fins and tubes were a single element. We had a vendor in Tulsa OK that would mechanically extrude fins from very thick walled pipe. I was an impressive feat of machining, if you're a geek.

Copper might be an ideal material for this. The melting temp is much higher than egt and the thermal conductivity is excellent, about 20x that of SS and almost double that of aluminum.

Re galvanic corrosion - it requires that liquid be present, so it won't occur in this application.
 
Re galvanic corrosion - it requires that liquid be present, so it won't occur in this application.

I won't get into a debate but I'm going to respectfully disagree, Sir.

Catalyst like water or even more so, the contaminants that water tends to deposit, affects the reaction rate (definition of a catalyst). It does not have to be present for electrolysis to occur. It would usually be limited to the first few mills of the surface before the new layer provided some isolation for lack of a better word. The preservative oils that both wick well and have good dielectric properties are a good approach (and the fact that they repel water). At colder temps, the rate could be measured in decades. In exhaust conditions, weeks, days or even hours to start seeing the consequences. Relatedly, there are plenty of nasties in hydrocarbon exhaust that are both corrosive on their own and catalytic to the related chemistry. Water is also a byproduct of combustion. It's a rough environment for most any alloy.

I mentioned before in response to the original post, the improvement of bronze wool over stainless (everything else being equal) would probably be quite measurable; at first. I wouldn't expect that to last.

Try your approach. I've been wrong before and that won't change in the future.
 
Last edited:
sounds convincing enough. will check ifsome SS is laying around in my scrap metal box.

Thank you for taking the time.
 
As a point of reference, I have an 0320 E2C with a cross over exhaust system. I have two heat muffs in series on the tailpipes. The one on the left has the inlet in front and outlet in the rear. It feeds directly across (around the back of the airbox) and into the rear of the one on the right. From the front of the right one, it goes into the cabin.

I originally thought about the wound spring idea but didn't want a bunch of stuff inside the heat muff. I fly all winter in Wisconsin and really don't have any issues keeping warm even 0 degrees F. What is interesting and mentioned in this thread is that if I "throttle" the cabin heat valve closed, the temp of the incoming air goes up because the air is moving slower thru the system and picks up more heat. The downside is that less air is coming in so the total amount of heat in the cabin is less even though it is hotter. I am glad we are having this conversation as many people completely shut down when it gets cold outside and miss out on some great flying.

Keith
 
Pattern

Attached are the dimensions I used. As it was 2014 I don’t recall if the shop I had do them used 1/16”, 1mm or 1.5mm, as I told them any of those would be fine with whatever they had available. I did find that they charged $4.50/each (CAD) to laser cut them.

Good luck and use at your own risk, no warranty is implied nor given, your specific results may vary, side effect may include dizziness, disorientation, vomiting, diarrhea, headaches, nausea and erectile disfunction, see your doctor if conditions persist. :D
 

Attachments

  • 852B3D32-1F21-4B32-977B-B21DEB958DD9.jpg
    852B3D32-1F21-4B32-977B-B21DEB958DD9.jpg
    299 KB · Views: 101
Attached are the dimensions I used. As it was 2014 I don’t recall if the shop I had do them used 1/16”, 1mm or 1.5mm, as I told them any of those would be fine with whatever they had available. I did find that they charged $4.50/each (CAD) to laser cut them.

Good luck and use at your own risk, no warranty is implied nor given, your specific results may vary, side effect may include dizziness, disorientation, vomiting, diarrhea, headaches, nausea and erectile disfunction, see your doctor if conditions persist. :D

i am ok with the side effects, except for the last one.

1/16" sounds a bit beefy, i think 1/32 or so should do. the airflow in the heat exchanger shouldn't be that violent.
 
Last edited:
Hi temp corrosion

Are there any metallurgists out there that could give an opinion on making Darin’s fin assemblies out of Aluminum vs. his SS? Obviously, AL is much easier to source and work with. Thanks in advance.

I am NOT a metallurgist, however, I have invested a career in the power industry dealing with Gas Turbine, coal and pet coke boilers, etc. and suffice it to say I have seen some pretty weird and surprising things with mixed metals, even at trace levels, when they are at high temperatures. Granted we are not talking 2700-3000F, but rather 12-1400F, and I also recognize there are better heat conductors such as Copper and Aluminum. All that said thin laser cut SS, does the trick, clamps are easy to keep tight as there is little to no differential expansion of dissimilar metals, and very most importantly, minimal corrosion or risk of creating a leak through a lose clamp vibrating and abrasion.

IMHO low risk, simple, reliable AND safe trump a theoretically more optimal design from a heat transfer POV, as all designs there are trade offs and i specifically choose the safer one….and it more than meets the heating needs. Please consider this in your own evaluations.
 
Somewhat challenged to quantify, but I would say it was chilly at -10C with “normal chothing” prior to shirtsleeves now and at -20C you want to start to zip up your jacket. My wife and I were up at 17,500 on a cold day over Montana once and the OAT was reading -53C on the way to Mexico, and it did get uncomfortably cold…I think it was -30ish on the ground that day.

I think this re-qualifies your wife as a keeper.:D A lot of wives would consider a flight in an RV at -53C OAT and monitoring that the O2 was still flowing a deal breaker.
I do like your brilliant SS heater strips and the helical airflow and heat transfer they provide.
 
Last edited:
Galvanic potential

Took me a few days to get around to looking for the galvanic potential between steel and bronze. Looks like there is 0.45 volts potential between them. So galvanic corrosion between dissimilar metals is possible.
 
She is a keeper!

I think this re-qualifies your wife as a keeper.:D A lot of wives would consider a flight in an RV at -53C OAT and monitoring that the O2 was still flowing a deal breaker.
I do like your brilliant SS heater strips and the helical airflow and heat transfer they provide.

Oh she is a keeper…30 happy years, though it did help to keep a mistress for about 18 of those years….aluminum mistress that is :rolleyes:
 
Back
Top