Need more heat...2nd muff series or parallel?

Getting colder up here in Canada and am looking to add a second heat muff without too much modification. For those who have done this I would like to know the pros and cons of muffs in series vs parallel; and of course learn from your experience.

Darin Watson said:

Getting colder up here in Canada and am looking to add a second heat muff without too much modification. For those who have done this I would like to know the pros and cons of muffs in series vs parallel; and of course learn from your experience.

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Darin

I tried both on my old RV6. I did get higher temperature in series through one outlet, however the volume of air was reduced.

I went back to two individual muffs each with there own vent, pilot, copilot.

This was in Toronto

Peter

In an attempt to gain more heat in my RV8, I ran 2 muffs in series for the last 5 years. When I switched to inverted oil and fuel injection, I got rid of the second muff to help declutter the firewall fwd area to more easily route all the new hoses. What I found was absolutely no change in volume or temperature of the incoming air from the now single heat muff. I am in the process of my condition inspection and have reinstalled the second muff and have added a second cabin heat box on the firewall. I will now have hot air to my right foot in the original RV8 heat box area, fed from the aft #3 baffle. The original heat box is now fed from the #2 horizontal baffle. The heat box that is in the now standard RV8 location by plans is the triangular box. I substituted a square box with a 2 inch flange that sticks thru the firewall. This box is above my left foot. I have a scat hose attached to this flange and it runs aft to Vans black plastic eyeball located on the aft gear tower cross brace. Scat hose is about 2 feet long.
The air at this eyeball, with a 20 degree OAT was so hot that I was concerned that the plastic vent might melt!!!!
There is no reason whatsoever to run 2 muffs in series. I was going to look into the new Vetterman muffler/heat muffs, in hopes of getting hotter air into the cabin. I no longer believe that will help.
Also, for what it's worth both my baffle inlets are 1 inch diameter holes servicing my 2 inch scat flanges.

I have 4 into 4 exhoust and posted this a while back to show how I routed more heat into the cabin away from my feet.

http://www.vansairforce.com/community/showthread.php?t=105845&highlight=rv8+heat

I concur with Jon Thocker on this. I used to have two muffs in series and noted no change in heat or amount of air when I went to one muff. This was on my old 4 into 2 system.

It's more about volume than it is heat. I installed dual heat muffs in parallel and set it up for cross-flow to maximize heat transfer. That, in addition to sealing the cockpit, lets me stay nice and warm even when it's -12?F OAT during a cold Minnesota winter.

I would like to handle outside temps of 0 deg F, So let's see if I got this right.

One muff is adequate if the air volume through it is lowered (1" inlet, per John T, thanks) so the temperature rise of the air through it is greater (than maximum flow), thus providing lower velocities, but higher temps into the cabin. Right?

I assume the muff is like baffling - it must be sealed well. Also, is it better to wrap the outside of the muff to help keep the external losses down?

I have the AWI 4-into-1 and one muff, but they weld the threaded pin fins on the exhaust pipe for increased heat transfer to the air flowing through the muff.

N8RV said:

I think I have a solution, however. MOVE TO ARIZONA!

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...or Florida!

Dang it, just received a second muff ($130) from Spruce and $30 worth of Scat. Looks like I'll still be freezing my buns off! FYI, the last several seasons I had Vetterman mufflers installed. The heat was MUCH better than with a single stock muff.

Someone suggested filling the muff with aluminum "wool". Any PIREPS on that?

Just my opinion, but if you're looking for a little extra heat for the winter months consider the light weight electric vest and insoles for your shoes. Adds no more weight to the ship than an electrical outlet and some wire.

I found them to be so effective that I completely removed the heat muffs from my airplanes. And while I don't live in the frozen tundra, it does get into the teens here, and flying at altitude means well below freezing for hours on end. The electric vest and feet (set to "low") is all I need.

Heat muffs

I have an RV-6 with Vetterman 4 into 2 exhaust and a single heat muff on each side (two total),feeding into one heat box. I have flown down to -15C and been toasty warm, the heat control was not even full open. The cabin walls and firewall are insulated, with a good canopy seal, boots on the ailerons, boots on the flaps and the rear fuselage sealed as well, so the cockpit is not drafty at all. I have heard of guys taking coiled stainless steel wire and filling their heat muff as a trick to get more heat into the air around the muffs. I have also been told the large oval muffs that encompass two pipes side by side work well, but have no experience with these methods. My RV-3 is quite drafty and has a single muff, I havn't flown it down to these same cold temps we are seeing now, but it will probably be cold, simply due to how drafty it is inside the cockpit.

apkp777 said:

. . . . . Someone suggested filling the muff with aluminum "wool". Any PIREPS on that?

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Yes, they have, but what ever is in there needs to be able to handle EGTemps and aluminum does not sound good for that. Others have used stainless scrubbers but most reports (in my memory) did not report it as a solution.

Use this search in google - - - "site:vansairforce.com term1 +term2" etc and it will return the threads with these terms, even in the archives.

Well, my experience has been a little different. EVERY TIME I add a second heat muff in series I get MORE HEAT.......period . No exceptions.

N8RV said:

Jon, that's good info about two muffs in series not increasing heat appreciably -- but disheartening, because I've been meaning to do just that for the last few years. When it gets below about 25 degrees up there, my single heat muff and even electrically-heated seats just don't work that well. I end up feeling like I'm sitting in a refrigerator!

I think I have a solution, however. MOVE TO ARIZONA!

Besides, with the number of friends with -8s who have suffered from cracked canopies from the cold, I avoid it like the plague.

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rv6rick said:

Well, my experience has been a little different. EVERY TIME I add a second heat muff in series I get MORE HEAT.......period . No exceptions.

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Yes, of course. I have two heat muffs, and can "add" or "subtract" one at will by closing it off with cockpit controls. Huge difference in cabin warmth between one or two.

Also, be sure to plumb them in counter-flow condition (meaning exhaust goes aft, heated air goes forward in the muff). For some reason everyone seems to have supply air go in the front of the muff, with heated air out the back. I can't say how much more effective counter-flow is, I can only say that it will indeed be more effective. Heat exchanger 101 stuff.

Wish I could help but I have a dual muff installed on two pipes of my 4 pipe vetterman exhaust. But it only has one inlet and one outlet... it will drive you out of the cockpit. The coldest I have used it is in 20 F weather. But at that temp we only crack the heaters door about a 1/4 open.
Larry Vetterman supplied it with my exhaust.

perhaps he has one for a dual exhaust.?

Smilin' Jack

Counterflow...??

To Alex's point, counterflow is theoretically more efficient. However, in this application, it is insignificant, methinks. The velocity of the primary fluid is very high, the amount of heat being transferred is a small percentace, and the exit delta Ts are still huge. SCAT is a draggy conduit, minimizing its length will increase flow on the secondary side and may deliver more heat to the cockpit.
I put some guesses together. A good cockpit heater might produce as much heat as a big hair dryer. Call that 1500 watts or 1.5 kW. A 200 hp Lycoming produces 50 hp per cylinder. Internal combustion is very inefficient from a thermal perspective, so let's say twice as much heat goes out the stack as is converted to useful shaft power. 100 hp is about 75 kW. So it looks like 3 % of the heat in an exhaust pipe is recovered by the muff. FWIW.
Reducing cockpit draftiness made a huge difference.
Stay warm!
Roger

apkp777 said:

...or Florida!

Dang it, just received a second muff ($130) from Spruce and $30 worth of Scat. Looks like I'll still be freezing my buns off! FYI, the last several seasons I had Vetterman mufflers installed. The heat was MUCH better than with a single stock muff.

Someone suggested filling the muff with aluminum "wool". Any PIREPS on that?

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I used Stainless Steel Wool and that noticeably helped on our single muff heat system.

In all our years of flying back east during our winter trips, we've found that the single muff does not work well when the OAT gets near freezing and below. Our heated seats (found in this thread, page 2) that I added last year work great for OATs 10deg and above (that's the coldest OAT we've seen).

Good luck Rosie

screen door spring

IIRC, Tony Bingellis (Firewall Forward) recommended wrapping screen door spring around the pipe inside the muff to increase heat transfer. Have not tried it, but it looks like it would be effective...
Roger

I have the stock Vans heater valve on the firewall of my 7A and the stock single muff on one pipe of my IO-360-M1B.

I am 100% convinced that at anything less than full open, I am actually getting gobs of cowl air back flowing through the valve into the cabin. Plenty of air, but only medium warm. In fact, the pressures of the cowl flow on the flapper door were so great that I had to replace the stock actuator cable with a button locking unit from Spruce in order to keep the door from popping shut on its own. Plus, once I get the door all the way open, the flow is actually reduced from its mid position flow rates. I can only assume it is because I have the cowl air path fully blocked at that point.

On consideration, I wonder how it is that ALL RVs are not like mine since the valve design does nothing to prevent cowl air intrusion. The only thing to prevent it would be prodigious flow through the heater muff and scat tube.
I have put my fresh air inlet on the #1 cylinder ramp per vans instructions for injected engines. Due to the crease one puts in that ramp, it forces the inlet position mounting towards the spinner which is an area of possible stagnancy or backflow out of the cowl. I made a crude test scoop for the inlet but could not tell any difference by observation of operation during flight (no objective measurements).

With that one might say "Hmm, Bills baffles are FU'ed" to give high post cylinder cowl pressures. Well... they are not. According to my CHTs, they perform on par with the best that post on this website.

Even with this anomaly, there has been plenty of heat for my mid atlantic adventures. Rarely use it because the greenhouse canopy is so good. Until about a month ago. Diane and I went to 7G0, near Rochester, NY and the second half of that flight was darned cold in the cabin.

Since I have not come to a solution, I think two heated vests are in our future (thanks to rocketbob for his post )

BillL said:

Yes, they have, but what ever is in there needs to be able to handle EGTemps and aluminum does not sound good for that. Others have used stainless scrubbers but most reports (in my memory) did not report it as a solution.

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I did the 'chore boy pad thing' in my single heat muff - that plus slightly covering the air inlet gave me PLENTY of heat in my RV-7. I've flown it to Texas at high alts (I forget the the temp but _cold_) and I had to take off my sweater to fly in my T-shirt.

The other thing I did was to use some foam to seal the V openings on the baggage bulkhead and aileron pushrod boots.

Buy a 10!

RKellogg said:

To Alex's point, counterflow is theoretically more efficient. However, in this application, it is insignificant, methinks. The velocity of the primary fluid is very high, the amount of heat being transferred is a small percentace, and the exit delta Ts are still huge. SCAT is a draggy conduit, minimizing its length will increase flow on the secondary side and may deliver more heat to the cockpit.
I put some guesses together. A good cockpit heater might produce as much heat as a big hair dryer. Call that 1500 watts or 1.5 kW. A 200 hp Lycoming produces 50 hp per cylinder. Internal combustion is very inefficient from a thermal perspective, so let's say twice as much heat goes out the stack as is converted to useful shaft power. 100 hp is about 75 kW. So it looks like 3 % of the heat in an exhaust pipe is recovered by the muff. FWIW.
Reducing cockpit draftiness made a huge difference.
Stay warm!
Roger

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Roger, I think a 1500 watt hair dryer would hardly dent a cockpit's temperature. I recall decades ago putting a 1500 watt electric heater in the back of a station wagon, and leaving it on as a way to pre-warm the interior on a 0F day. It did almost nothing. Car (and airplane) heaters are in the tens of thousands of btu's per hour range. 30 to 50k btu is probably a reasonable guess.

I did an estimate a few years ago, and the average mass flow of gasses is in the same range inside the pipe vs through the heater. What motivated me was this: I moved one of my heaters from the tailpipe (fed by two cylinders) to a crossover pipe (fed by one of course), and the heat was very noticeably less. I had to change it back to tolerate flying it in the winter. I thought that being farther upstream would would make it be more effective, since the exhaust temps would be a little higher. It wasn't the case.

An interesting experiment would be to mount a thermocouple in the exhaust pipe, and fly with and without flow in the heat muff. I can't do that in the same flight, but will contemplate.

In RV's I've inspected or just looked at, almost all have concurrent flow on their heaters. It is utterly free to use counter-flow, but there is apparently a huge resistance to doing so. I'm not sure why.

Getting to some facts - - has anyone measured the outlet temperature from the cuff? I did a few rough calculations and it seems like there is only about 900 watts (Wrong!!) of energy to be captured from the exhaust pipe area of 50 sqin for a 9" length and 1.75" diameter. This according to maximum convective heat transfer coefficients.

Update: Alex was pretty close, my heat transfer coefficient was way off. After hours of searching and using a spreadsheet with Reynolds, Prandel and Nusselt numbers . . . The heat is more like 45000 BTU/hr. But - -with a 0 F ambient, one would have to reduce the flow to 25 cfm through the muff to get a temp rise of about 100 F. This goes back to an early post restricting the inlet air to about one inch diameter.

Any actual temps?

Bill, thanks for the clarification. I have a Fluke thermocouple, I'll drag it along next time I fly (this is a big assumption with the cold weather we've been having...).

I do know for a fact that the outlet (more correctly, what dumps into the cabin) is not at 100F when OAT is 0F. Probably something like 70F. There is an optimum airflow rate for cockpit comfort - too little air or too much air is less comfortable (cold cockpit). I have most of the inlet of the heaters blocked at their source in the baffles, but it is still a lot of air flow. The point is that max temperature is not the goal, max average cabin temp is.

BTW: I have one heater at 2" scat, the other at 1.5" scat. I "insulated" the 1.5" one with another wrap of siliconized fiberglass. If I build another one, it will have 1" scat.

getting warmer...

Okay, so I haven't measured yet either. But from putting a hand at the heat valve, the temperature and mass flow feel more like a hair dryer than a 75, 000 btu torpedo heater. I am in russia for two weeks and surely someone will instrument this before I get a chance to. Look forward to more data.
Roger

air flow rate

Bill, is 25 cfm a known value? That would require 19 ft per sec airflow through a 2 inch scat, which sounds a bit high to me. Years ago I melted my crew id tags by leaving them too close to the heat inlet in a 172...
Roger

RKellogg said:

Bill, is 25 cfm a known value? That would require 19 ft per sec airflow through a 2 inch scat, which sounds a bit high to me. Years ago I melted my crew id tags by leaving them too close to the heat inlet in a 172...
Roger

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I really just did calculations on the max heat flux possible. 25CFM was just a number for comparison. I suspect that the cool side is most limited. Vetterman exhausts are smooth pipe under the muffs. Certified aircraft have pin fin studs welded to the pipe to increase cool side surface area and heat transfer. I have no idea what area they have.

Here is the AWI exhaust, it has provision for a single muff and pin fins.



Onset of winter always brings these threads to life and it seems there is either an adjustment or modification needed to match appropriate ambient temps. Maybe it is like spring and baffles, always the same overheating and same answers. (different posters thought ) As an observer it is hard to see a successful, definitive modification with data.

heat transfer problems...

are always complex, multi-physics, multi-variable, and difficult to generalize.
Maybe the suggestion about moving to the south was good advice...

Bill, how close is your 7 to first flight? I painted mine at Thanksgiving and now finding it hard to get home to work on it. Lots of loose ends that need to be tied up. Eager to fly it soon.
- Roger

I ordered a 10" heat muff from Robbins Wings to replace the stock 8" to be able to get 20% more heating area. It increased the temperature of heated air but not enough for my cold blooded taste. Used both muffs in series and removed restrictors in muff outlets and now am finally able to get enough heat to be toasty warm at 10*F OAT. Because of routing simplicity and lack of scat on hand I did not plumb it for counter-flow but would have tried to do so if this setup didn't net enough heat.