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 Post subject: Re: NH&I 40 exhaust
PostPosted: Tue Sep 03, 2013 1:28 pm 

Joined: Sun Aug 22, 2004 8:31 am
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Location: South Carolina
Kevin Gilliam wrote:
Can't speak for all of the branches, but the Abingdon branch job started in Bristol. That's 15 miles or so on the mainline, and there are at least a couple of times where the Ms equaled the speed of the Js on that section.
Kevin


I, for one, would like to see an M run 80 MPH. :-)

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 Post subject: Re: NH&I 40 exhaust
PostPosted: Tue Sep 03, 2013 10:03 pm 

Joined: Thu Nov 04, 2004 10:28 pm
Posts: 205
whodom wrote:
Kevin Gilliam wrote:
Can't speak for all of the branches, but the Abingdon branch job started in Bristol. That's 15 miles or so on the mainline, and there are at least a couple of times where the Ms equaled the speed of the Js on that section.
Kevin


I, for one, would like to see an M run 80 MPH. :-)



so would I....but seriously, those Ms got to moving out a couple of times on the Bristol line. Given the speeds on the Bristol line, I'd say 50mph or so would be a good guess for passenger speeds at that point--and it's downhill from Abingdon to Bristol once you get over the little hump leaving town. Probably one of those cases of a long day, and the local either needed to make quick work from Abingdon to Bristol or had to wait for the westbound passenger and however many freights.
Kevin


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 Post subject: Re: NH&I 40 exhaust
PostPosted: Wed Sep 04, 2013 8:57 am 

Joined: Sat Oct 22, 2011 5:07 am
Posts: 60
Dave wrote:
I find myself sharing Dr. Koopmans' perspective on the difference between the bottom of the stack that is inside the smokebox as the bottom of the stack, and a separate fabrication that doesn't directly attach to the stack but allows smokebox gasses to pass between it and the stack as a "petticoat".

For purposes of clarity in this discussion can we agree on that?

dave

I have been looking around for proper nomenclature, it is quite disturbing what can be found. However, I have this copy of the Association of American Railroads on the revised Master Mechanics Front End of 1936. On p. 22 of their 1937 publication they state: "A two-piece smokestack, consisting of the stack proper and stack extension, is recommended".
It is very clear from the accompanying figure that the stack extension is the part inside the smokebox.
Just for the record!
Kind regards
Jos Koopmans


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 Post subject: Re: NH&I 40 exhaust
PostPosted: Sat Sep 21, 2013 10:43 pm 

Joined: Wed Feb 20, 2013 9:26 pm
Posts: 18
A few thoughts, from what I was taught on the subject:
1. As was said once already, a piece directly connected to the stack and the same diameter would be termed a stack extension. A separate fixture, with an air gap from it to the stack, and generally at least slightly smaller diameter, usually with a flare at the bottom, is termed a petticoat pipe.

2. Petticoat or not, the whole premise that these front ends were designed on was the fact that the jet of steam leaving the nozzle expands at a 6-7 degree angle as it goes up. It needs to strike the inside of the straight portion of the petticoat appr. 12 inches below the top, so as to seal it completely. It them exits the petticoat, expands at the same angle again, and needs to strike the stack 12 inches below the top, in order to seal completely and produce the vacuum inside the smoke box. Some locomotives had the right proportions to where the petticoat step could be eliminated entirely as long as the steam jet struck the stack correctly.

I've studied this quite a lot, even going so far as to take measurements from about ten locomotives' front ends that I've had access to and crunch the numbers to see if my understanding is correct, and it seems to be. Another interesting piece to this is the relationship between the cylinder diameter and the diameter that the nozzle "should" be. Two locomotives from the Cowlitz, Chehalis and Cascade are now at a tourist line in Chehalis, Wa, where I have spent some time over the years on the crew of 2-8-2 #15. Harold Borovec, the longtime overseer of the operation, always said that 2-8-0 #25 didn't steam very well back in the days when he worked at the CC&C, but my time in recent years on #15 proved it to be quite the good steamer. So I studied both engines' front ends and found that the 25's nozzle was larger than it should be, and that the 15's is smaller than it should be. In fact, it looks like on the 15 the steam actually goes straight through the petticoat without even touching and then strikes the stack in a suitable place, so that in theory you could remove the petticoat and the engine would draft the same. I have a hunch the tone of the exhaust would be different. If the nozzle were the correct size then the petticoat would be struck and then step to the stack as per normal.

Also, a saturated engine WILL be quieter than a superheated one, all else being the same, but there are several other factors (petticoat, netting, design and condition of valves and rings, etc.)

Scott Gordon


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 Post subject: Re: NH&I 40 exhaust
PostPosted: Tue Sep 24, 2013 7:30 am 

Joined: Sat Oct 22, 2011 5:07 am
Posts: 60
I would like to draw your attention to the Proceedings of the Master Mechanics Association of 1906 where the results of the tests of Goss with stacks and "draft pipes"=petticoats are discussed. They used NYC Atlantic 3929. The orifice was 5 11/16 in. and standard 3.5 psi back pressure was provided for.
Best result was 5.2 in.(water) vacuum with an outside stack on top of the smokebox, this was however outside the allowed loading gauge. Next best was a 29" stack with 24" extension inc. bellmouth, 5.05 in. vacuum.
87 different configurations were tested, 15 with stacks only, the remaining 72 with draft pipes. Best result with draft pipe was a 10% lower vacuum and worst was 2.6 in. of water. A draft pipe is plainly a nuisance!!!
Goss ea. went astray with their dimensional suggestions which were partially corrected by the MMA some 20 years later. Present know how, based on later tests, would consider the chimney to orifice diameter ratio of 17/(5 11/16) = 3 as good since it reflects the ratio between steam and mixture to be expelled. The orifice to throat distance ratio would be (25+16.5)/(5 11/16) = 7.3, slightly too large. The chimney length to diameter ratio would be (29+24-16.5)/17 = 2.15, sufficient.
So we know for over a hundred years what a good front-end should look like.
Please note that the exhaust orifice diameter would be determined from the heating surface, not from the cylinder diameter which is the prerogative of model engineers.
Kind regards
Jos Koopmans


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