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Post by baggo on Jul 3, 2010 16:31:21 GMT
Have just sat and read an interesting letter by Andrew Gully in the latest copy of ME in which he highlights the work of Dr. Richard Wagner of the German State Railways. Dr. Wagner advocated that the total free gas area of the tubes and flues should ideally be around 15% of the grate area when using a conventional single chimney draughting arrangement. Lower than 15% would require a stronger blast and inhibit steaming whereas more than 15% would cause cooling of the firebox because of excessive air drawn through the firebed.
Peter (Seager) mentions above that all the BR standards were built using this ratio or very close to it.
So, perhaps we should be looking at this ratio of 15% as well. As I mentioned above, a lot of boiler designs average only 10% or less which may require much stronger blasts to draw enough air through the fire.
In the letter, Andrew also mentions the actual effectiveness of tubes in 'our' sizes for transferring heat into the boiler. He suggests that they only supply about 25% of the total heat input (the rest from the firebox). It has been often stated in the past that the tubes in our boilers contribute very little heating effect. So, is the length of the tubes not actually very critical at all? Can we safely ignore the Kt factor and just worry about the total gas area of the tubes?
All interesting stuff.
John
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Post by havoc on Jul 3, 2010 18:15:24 GMT
Problem is that a lot of these "formulas" don't scale, we need our own ones. But ME's aren't interested in the 'E' part of it.
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Post by alanstepney on Jul 3, 2010 20:45:57 GMT
That is precisely why Jim Ewins did all the experiments that he did, and wrote about them for others to follow.
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Post by baggo on Jul 3, 2010 23:49:55 GMT
And why some of us are trying to continue with what Jim started John
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Post by havoc on Jul 4, 2010 9:19:23 GMT
This is what I was replying to, don't take everything out of context.
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steam4ian
Elder Statesman
One good turn deserves another
Posts: 2,069
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Post by steam4ian on Jul 4, 2010 13:27:54 GMT
G'day.
Me again.
As I have read through this thread I came to the realisation that what matters is the pressure drop across the tubes. This is a function of two things, the tube area and the tube length. Longer boilers need more tube area so Simplex with 11" tube length needs more area than mine with 8.5" tube length. The Kt factor tends to even this out. A large pacific of of the type to which Wagner refers would have at least 16' between tube plates which is 16" at 5" gauge. For this reason at least 15% of grate area is required whereas 10% will work for shorter boilers. Another reason to have combustion chambers, to shorten the tubes to get a practical tube area.
Too many tubes causes other problems like raising the crown sheet restricting the water space. If there is trouble lifting the fire then there is a chance to soften the blast.
Regards Ian
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