Phil;

 

I know you’re asking Ed, but hope you don’t mind another opinion.

Both 304 and 321 SS are subject to inter-granular stress corrosion at the temps we’re talking about.  321 is a better choice and will last longer, and could be lighter by some reduction of thicknesses.

 

Yes, making it thicker will make the 304 last longer by lowering stress, but it is still subject to limited life, and at the thicknesses you suggest, my estimate is your system will be rather massive at close to 30 lbs.

 

I don’t know the details of his design, But Dave Atkins had a SS tangential man/muff that basically crumbled to pieces within a couple hundred hours.

 

I’d also suggest a larger diameter for the tangential muffler; like 5+”.  This allows for more expansion which lowers both temp and pulse strengths. Flat ends would definitely be a failure point.  Reinforcing with a central tube would be a big help, but I would still make them conical, or concave inward.

 

I know that inconel kicks up the cost a fair amount, but it gets you away from the stress corrosion issue and allows a system of about 1/3 the weight of what your considering.  When you consider the overall time, effort, cost, risk, and weight, I still think inconel comes out on top.  My inconel tangential muffler weighs in at about 12#; including the rather massive RB exhaust flange.  The 321 secondary muffler and exit pipe add another 3 ½#.  It has now been running for 170 hours; and as far as I can tell, is about as sound as when it started.

 

Al

 

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Phil White
Sent: Sunday, December 06, 2009 3:53 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Exhaust Manifold material

 

Ed Anderson:    I recall (if memory can be trusted?) that you are using

304 stainless for your exhaust manifold.  I am wanting to make a

tangential muffler for my 20B, and wonder if the following might last a

reasonable time: 

1.   From the ports;  2" x .150 wall

2.   Tangential tube;  4" x .120 wall

3.   Exhaust pipe;  2.5" x .120 wall

4.   End caps on tangential pipe;  .125 plate

5.   Center tube (to help swirl, and to reinforce end caps);  3/4" x

.080 wall  (wondered if it would be a good or bad idea to run cool air

through this to cool it and the exhaust a bit, or better to leave the

ends closed??)

 

    Are these walls heavy enough to last?  Or is using $175 worth of 304

a waste of time and money compared to getting the more expensive 321

stainless?  Your opinion is most welcome.

 

Phil in IL

 

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