Copper Boiler Design

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James Powell
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Re: Copper Boiler Design

Post by James Powell » Sun Dec 09, 2012 1:45 pm

The answer is "it depends".

The Hoffman Hudson we had used 1/2 OD copper tubes- I seem to think 17 of them, but dad would know. That was with 22" of grate, and burning just about anything. That gave a free surface of around 15% of grate (22 sq inches of grate). (rather less, as I am using the full 1/2" for the free gas area). I'm not sure what #'s dad has used on more recent engines, but I would tend towards something around the 15% free gas area. BR standard railway engines used a figure of 16% as the design number...so, laterday steam seemed to think that was about right.

L/D ratio is also going to play into this. Emperically, don't use 3/8" OD tubes in a horizontal role, as they won't vent well enough to work. Renolds is NOT your friend in this case.

The last few boilers I have designed for dad were based around 16% free gas area. The worst effect of having oversized free gas area is going to be a hot smokebox, as per Little Johnny (Phil Soden's 4" Fowler, that I have). There are worse sins than having a hot smokebox- a pig of an engine to steam is far worse in my view than running at less than best N as a model. Full size has other considerations, and experimentation is possible.

Going back to the Hudson, tubes as small as 1/2" OD can have a superheater put through them, we had the 1/4" OD tube running the full length of the boiler as a single pass superheater. My thoughts would be to do something similar, probably with 5/8 OD flues, and 2, 2 pass superheaters, reversing in a block at the rear of the firebox. (close to what we had on the 1 1/2" traction engine after 1987 (?) .

There are no ultimate answers, as a lot depends on the coal you are going to burn, and how hard you are going to work the engine.

James

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Fender
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Re: Copper Boiler Design

Post by Fender » Sun Dec 09, 2012 1:56 pm

Seems to me there is another variable involved besides tube flow area and grate area. This is the area between the grate bars. If the coal doesn't burn well because the grate area is too large for the tube nest, its probably because the air velocity through the grates is too low. If the grate bars are then placed closer together (thus reducing the flow area through the grates), the air velocity will be increased.
Dan Watson

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Carrdo
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Re: Copper Boiler Design

Post by Carrdo » Sun Dec 09, 2012 2:07 pm

Jamie,

My Hoffman Hudson has 40 square inches ( 5" by 8") of grate area. It is big.

Now that you mention it, I seem to remember Dave may have blocked off some of the grate area on his Hoffman and converted it to more of a narrow type firebox boiler. And, at the time, it didn't have an ashpan so there was lots of circulating free air under the grates to support a roaring fire.

Anyway, once it got going, it was fast and powerful - too much so for me as I derailed but I was so impressed with it, it started my building affairs with Hudsons.

Dave denies it vehemently, but he bent all of the rules in the OMLET competitions we attended (1983 and 1985).

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David Powell
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Re: Copper Boiler Design

Post by David Powell » Sun Dec 09, 2012 3:54 pm

I did not bend any rules. I followed them to the letter. They said you could raise steam on charcoal up to 50 lbs, without ANY of it being counted. At 45 lbs the box was FULL of fresh unburnt charcoal, soaked in kerosene. Enough for a while without any coal. At RHLS the high line station is always home to a tin of charcoal soaked in kerosene. When we are busy we do not have time for loaded trains to stand in the station getting a few extra pounds of steam up, so the remedy is almost invariably add about 4 - 6 shovelfuls of soaked charcoal and send the train on the way. Provided that the ashpan is not full of ash and the fire not thoroughly clinkered most engines come round back to the station happily blowing off. Our experience seems to show that the most important factor in satisfactory steaming and consistent running , provided the engine is somewhere near the mark, is the skill and determination of the driver. We run engines with from about 10 sq" of grate area to about 50 sq" of grate area on the same duty( Pull one car, driver plus up to 3 adults only). In theory the larger engines are lightly loaded and the smaller are working quite hard, sometimes very hard. In practice the effort involved keeping each of them going for a 4 hr or longer run seems similar. Hope this is of interest David Powell.

James Powell
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Re: Copper Boiler Design

Post by James Powell » Sun Dec 09, 2012 4:18 pm

We didn't break all the rules, as the Hudson had no hand pump ! Admittedly, we may have been...close...to some of the rules, but we just -read- them and followed them to the letter.

In any case, I know we talked about blanking off some of the grate area, but never did. Without an ash pan, she was a very free steaming engine, but certainly suffered in the way we would be cyclicly loading the fire.

The last time that Dad competed at OMLET with her, the fire went out, as he didn't have enough fire in to cover the grate, and it pulled cold air across the tubes. I think 1/2 of the heat that day came from the sun, I seem to think it was over 104F in the shade that day !

I would expect that for your design, that you will get away with a lower (10% ?) ratio of tube/grate, because the fire is going to be big and lazy, rather than small and forced.

Just don't try and deal with it like it is a Juliette, or similar...

James

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alanstepney
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Re: Copper Boiler Design

Post by alanstepney » Sun Dec 09, 2012 4:56 pm

Some of the most accurate and detailed work on boiler (and loco) design was done by Jim Ewins.
His articles were on the Internet, but you will need to search for them.

His conclusions on boilers were tested and checked by the late Prof Bill Hall, who found that his (Jims) formulae were valid within the ranges he had specified.

Might be worth locating the articles, and seeing how it relates to the numbers you have already.
http://www.alanstepney.info
Model Engineering, Steam and workshop pages.

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Bill Shields
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Re: Copper Boiler Design

Post by Bill Shields » Sun Dec 09, 2012 5:05 pm

If you want odd size tubing, Reeves in the UK does have it.

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gwrdriver
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Re: Copper Boiler Design

Post by gwrdriver » Sun Dec 09, 2012 5:28 pm

Carrdo,
Thanks for the compliment of inviting me by name to offer comments, and I will if I can come up with something useful to add, but so far it appears the subject is being covered well.
GWRdriver
Nashville TN

kvom
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Re: Copper Boiler Design

Post by kvom » Sun Dec 09, 2012 6:09 pm

This thread incited me to measure my boiler and grate. The flue/grate ratio is about 30%, so a lot more than the 12% cited previously. It's a small boiler though.

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Carrdo
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Re: Copper Boiler Design

Post by Carrdo » Sun Dec 09, 2012 7:03 pm

Alan,

Your reply prompted me to look through the papers here and I did find a couple of articles by Jim Ewins namely, Model Locomotive Research A brief survey and An Experimental Model Based on the BR 2-10-0 Class of Locomotive a series which was originally published in Engineering in Miniature Vol.3, No.9 to Vol.4, No.2 Feb. 1982 to July 1982.

I will have to review the Ewins work again.

Thumbing through the series I noticed that the only wide firebox American locomotive Martin Evans ever did a construction series on, Columbia a 4-8-4 Northern in 3/4" scale has (based on Kieller) a "Tube Factor" of 98.

Coincidence?

There is much much more of course to Ewins and I remember him coming to the "conclusion" through all of his research that everything done to date in every aspect of model locomotive design and practice left an awful lot of room for improvement even with the work of such notables as LBSC (one has to read this between the lines, Ewins is too scholarly and subtle to say this directly but the results of his research certainly do).
Last edited by Carrdo on Mon Dec 10, 2012 7:43 pm, edited 2 times in total.

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David Powell
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Re: Copper Boiler Design

Post by David Powell » Sun Dec 09, 2012 8:06 pm

While being the first to admit that some parts of some of the locos designed by LBSC make me shudder a bit I have become a great admirer of his work in general. We run a 3 1/2" Britannia to his design, and apart from a water gauge which is wrongly placed, showing water when the crown is uncovered! there have been no problems, the design has given us a powerful smooth running and economical engine. I ran a Maisie for two years on hard heavy work and found her a delight to manage and handle.Her new owner is equally happy with her performance in a less demanding role.I know some of the designs have been criticised and improvements published, but I have seen a Juliet, woken from a 30 yr sleep, pull 3 large adults round the Richmond Hill track faster and more smoothly than the large American style engine on trial at the same time. As LBSC himself would have said " Nuff Said " regards David Powell.

RET
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Re: Copper Boiler Design

Post by RET » Sun Dec 09, 2012 9:58 pm

Hi,

I'm the fourth person Don is referring to back on page 1 of this thread. Don told me about his progress, or lack of it because of tube plate problems so I invited myself to be part of the group. We met last Sunday and came up with a revised CAD tubeplate design which Don & I proceeded to make last week so Don can go back to help with the boilermaking process this coming week. He thought I should add my 5 cents to this discussion.

Ted and Wolfgang were a little doubtful about the superheat idea so I told them of my experience and experiments with the Boston & Albany. When I first started to run it, I was told that you had to really work any wide firebox locomotive to get it to run at all well and I found that this was true. I didn't think it had to be that way so I tried a number of things, some of which worked and some others that didn't. The Boston &Albany grate is 5" wide and 6" long, so I tried blocking off part of the outside of the grate on each side. That didn't help at all, it just allowed cold air to come in along the sides and made the steaming even worse.

Next, I started playing around with the draft in the smokebox. To make a long story short, I figured that in the smokebox, if you have a lot of free volume and significant space between the petticoat and the blast pipe, its easy to establish a torus or smoke ring and the products of combustion just go round and round inside and never go out the stack. I decided that you needed to have as complete mixing as possible between the blast and the smoke gasses for best momentum transfer so I put in a secondary petticoat and made the blast nozzle into 4 separate nozzles by filing a cross in the center hole and plugging the center with a pin. I also put a step in the nozzle so I could file 4 grooves in the outside of the larger part to serve as the blower nozzle (the step is to prevent the oil in the blast from plugging up the blower). The first mixing happens in the small secondary petticoat and further mixing occurs when the blast from the secondary goes into the primary petticoat.

The combined effect of these changes is to make the locomotive into a "pussycat." Anyone can run it. I have also found that with the proper amount of draft, the tubes don't plug up, if they do, you have too much draft.

As I said before, Boston & Albany has superheaters, 3 of the "hairpin" type made from stainless thin wall tube that is just over 1/4" od. They are TIG welded at the tip in the firebox and extend 1 1/2" into the firebox, but I've found this isn't enough superheat. Actually, all its doing is turning the water droplets into steam, no superheating is taking place at all.

I say there isn't enough because if you are running at a medium speed (about 5 mph. or more) and you rake the fire so the coals are bright, you can feel the engine pick up speed immediately which tells me that more water is being turned to steam in the superheaters. I plan to extend them a further 1 1/2" to 2" into the firebox to see what that will do. Like a lot of other things, its on the list, I just haven't done it yet.

Ted and Wolfgang were also concerned about having too much superheat; they said that had happened to someone they knew with a slide valve engine and the slide valve had worn into the cast iron of the cylinder face because there was no water left for lubrication. I suggested that you can vary the superheat by how far the superheaters extend into the firebox, but I also suggested that it might be better to use a heavy gear oil for lubrication since it isn't water soluble like steam oil is (at least partly) so it wouldn't get washed off and the extreme pressure additives in gear oil would also help with lubrication under those conditions. I don't pretend to be an expert, but that does make a kind of sense to me. I haven't run Boston & Albany a lot, but that's what I've been using since the start, not because I'm a clairvoyant expert, but because it was handy.

When I started running the engine, I had problems with the mechanical lubricators on the engine; with the least bit of dirt, the little ball check valves would leak back and force all the oil out of the lubricator reservoirs. After thinking about a better check valve for a while I came up with a design that works very well, the only moving parts are two "O" rings; you install the check valve & forget about it, it just keep on working.

Its time to quit. I hope some of this may help a bit.

Richard Trounce.

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