Posted: 7/10/2011 20:24:17
I have limited space for the cabin heat muff I'm building. I plan to locate it a half inch in back of an EGT probe, using one clamp for both items. I'm leery of enclosing the probe inside the muff. For the same reason, I'm reluctant to place a muff downstream of the 3:1 merge collector slip fitting, which may leak. But I'll have higher pressure air in the muff than the cowl, since I will be steeling the air from the ramp in front of the #1 jug.
Are my fears well grounded, or am I worrying too much? In it's current planned location, I have about 5.5" of length with a 2 5/8" muff tube around the 1.25" exhaust pipe. If I wasn't concerned about CO, I'd be able to get a couple more inches length for the muff.
Posted: 7/11/2011 11:45:56
You are right to be concerned about CO getting into your cabin heat system. Hypoxia can kill you. I would not recommend enclosing the EGT prob inside the cabin heat muff. I think the chance of an exhaust leak at the probe hole is an unacceptable risk.
I don't think that locating the heat muff downstream from the 3:1 fitting is necessarily a problem. However, enclosing the fitting inside the heat muff would cause me concern. Whether or not it woudl be safe to locate your heat muff drownstram of the 3:1 fitting really depends on the integrity of the fitting. I have seen some that do not leak and others that leak like a seive. For instance, my Jabiru exhaust leaks so bad that I removed the cabin heat system from my plane entirely. Even a new muffer didn't solve the problem. On the other hand, the Power Flow system on my Lyc. O-360 has no leaks at all, so cabin heat CO is not a problem.
One thing you can do to make the most of a short cabin heat muff is to increase the surface area of the exhaust pipe inside the muff. You can do this by welding fins onto the exhaust pipe or welding on mesh -- anything to put more hot metal in contact with the air flowing through the muff.
Be sure to use a cabin CO detector after you install the heater muff to make sure yor system isn't allowing CO into the cockpit.
Hope this helps.
Posted: 7/11/2011 12:32:11
Thanks, Dave :-)
I have a Jabiru 3300 going into a Sonex. No muffler, just a straight (actually bent) pipe after the 3:1 for the exhaust. I've read of some issues to seal this, but don't know how bad of a problem it will be. The merge collector doesn't look very tight. I plan to get a CO monitor and also to clamp fins around the pipe in the muff and fill the voids with as much aluminum wool as needed.
Posted: 7/12/2011 18:16:37
Let me know if you come up with a good solution. This is a major area of disappointment for me re: the Jabiru engine. I have talked to Jabiru USA about it and they were no help at all. It is incongruous to me how the engine itself can be the model of precision machining, but the exhaust system looks like it was made by a drunk blacksmith who was in a hurry for his shift to end.
Posted: 7/15/2011 14:15:31
As for increasing the surface area of the pipe inside the muff, there is a precaution out there somewhere about letting anything made of zinc or plated with it come in contact with the pipe. I hadn't heard of it when I wrapped a screen door spring around the pipe inside the muff on a Continental A-75 installation, a standard zinc-plated one, and not too many hours later while flying I pulled out the control and got an exhaust smell along with the sound of half the engine firing. Pushed the control back in, landed, removed the muff and sure enough, the pipe was cracked and looked corroded at the site. Could have been coincidence, I don't know, but I learned of the possible zinc problem while waiting for the replacement pipe and stayed away from it the next time around.
Posted: 7/15/2011 15:20:16
Modified: 7/15/2011 15:22:01
I'll be using SS clamps. Muff and fins will be of 025 6061. I figured I could put 3 or 4 fins like the one shown on each 1/2" wide clamp. Plenty of room for clamps, since they're 1/2" wide. My muff is only 5" long, but that would mean 8 or 10 clamps.
I know people have had problems getting heat transferred and have used springs and SS wool to slow the air down. I'm thinking of trying the fins, then if I need any slower air, I'd stuff it with aluminum wool. Not sure why SS has been used. Al transfers heat much better. Temps inside the pipe are 1200F, but that'd drop of very quickly on the outside, so I doubt melting the Al wouldn't be a problem.
Sorry to hear about your exhaust problem. Live and learn.