From: wkaiser@MtHolyoke.edu (William F. Kaiser)
Newsgroups: rec.models.railroad,rec.models.scale,rec.crafts.metalworking
Subject: Porcupine Boilers?
Date: 28 Jan 1993 14:30:56 -0500
Organization: Mount Holyoke College
NNTP-Posting-Host: slab.mtholyoke.edu
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A few years ago I built a simple gauge 1 live steam locomotive, with pot
boiler.  Since then, I have read a few things about porcupine boilers for
Gauge 1 live steam locomotives.  Basically, these are pot boilers with
copper rods, quills, sticking through the boiler shell.  The idea of this
boiler type is to increase the heating surface without getting into the
complications of fire or water tubes.  I am starting to think about
building another loco, and am wondering about a porcupine boiler.

I have plenty of information about designing conventional fire tube loco
boilers, and water tube boilers for the smaller scales, but nothing on
designing porcupine boilers.  Does anyone have any practical experience
with this type of boiler?  If not, can anyone apply some good model
engineering theory?  The following is a list of questions that come
to mind as important in designing a boiler. 

1  Is there an optimum diameter of quill?  Very large diameters would
   encroach on the boiler's water space.  Very small diameters could burn
   off in the fire.  Can this optimal diameter be expressed as a proportion
   of boiler barrel diameter? 
2  How much quill should be in the water and how much in the fire?  Half
   and half would be a logical guess, but is there a better ratio?
3  Is the part of a quill in the steam space of any use, or will only the
   submerged part be of use in raising steam?  If this is the case, a
   boiler with long quills should steam well when full and have a
   decreasing steaming capacity as the water level falls.  Would a boiler
   with short quills maintain a constant steaming rate?
4  Numbers 2 and 3 are related to the question of how long should the
   quills be.  Can this length be expressed as a fraction of the boiler
   diameter or as a multiple of the quill diameter?
5  How many quills should a boiler have?  Will just a few quills make a
   boiler that is a noticeably better steamer than a simple pot boiler, or
   does it take a great many quills to make a difference?  If there are too
   many quills will water circulation be impaired or strength of the boiler
   be decreased?  Can a minimum quill spacing be expressed in terms of
   quill diameter?  Is there an optimal ratio of quill heating surface to
   boiler barrel heating surface?
6  What is the best quill orientation?  Should they go through the barrel
   in a radial direction, or would circulation and steaming be improved if
   the quills were vertical?
7  How should quills be arranged in the boiler?  Should they be in rows,
   staggered rows, etc.?
8  What is the best firebox arrangement?  Should the hot gasses pass around
   the sides of the boiler and be vented at the top?  If so, how big should
   the fire space around the boiler be?  If the space is too large the
   heat transfer to the quills may not be optimized.  If the space is too
   restricted, there may not be enough draft for efficient burning.  Can
   the boiler be enclosed in an outer barrel with the hot gasses passing
   under the boiler to a smoke box and out the stack, such as is done with
   water tube boilers?  Then what is the size relationship between inner
   and outer barrels?
9  Given the volume of the boiler, its heating surface, and desired working
   pressure, how much fire will be needed to make steam?  That is, what
   volume of fuel (presumably alcohol) must be burned per minute to
   maintain pressure?  How many wicks and of what size would burn this much
   fuel?

Thanks for any help I can get on this topic.

William Kaiser
wkaiser@mhc.mtholyoke.edu
Newsgroups: rec.models.railroad