Author Topic: 3 inch boiler build  (Read 53202 times)

Offline gary.a.ayres

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Re: 3 inch boiler build
« Reply #210 on: December 28, 2018, 09:43:13 PM »
So - back on topic.

I got a modified version of the firebox set up on a provisional basis. The apertures for the door and the ashpan are rough, and instead of a door there is a makeshift cover fashioned from a baked beans tin. In lieu of a proper grate there is a grungy bit of thin sheet steel drilled with lots of holes and supported by three coach screws. I saw no point in spending unnecessary time on details before knowing that this boiler (being only 3 inches in diameter with only five tubes) would run on solid fuel. However, the setup was designed to be as functional as possible on that basis:





Peter (aka gas_mantle) very kindly sent me some Welsh steam coal. Here it is, accompanied by the obligatory sawn-off spoon:



Call me idiosyncratic (or just plain stubborn if you will), but I do not wish to use an electric blower, so I came up with the idea below, which uses a hand-cranked barbecue fan and plumbing fittings. The business end slips into the hole for the ashpan, below the grate:



I was pretty pleased with myself until I discovered that it was virtually useless. I later found that a draught up the chimney is way more effective at pulling the fire than a contraption like this is at pushing it. My apologies to Signor Venturi for thinking I knew better...

Due to the combined narrow diameter of the blower, the small number of firetubes and the dysfunctional blower arrangement I struggled for a good two hours to raise a decent fire. The white spirit blazed, but the charcoal kindling and the coal just sat there. Finally, I resorted to some small pieces of softwood which - to my surprise - burned up nicely with a bit of huffing and puffing and carbon monoxide inhalation on my part. When I got to the point at which I could hear some activity within the boiler, I turned on the steam blower and was astonished by its effectiveness. It was dark by this time. The system became a tiny furnace, roaring away on the barbecue table, sending a jet of steamy smoke and sparks blasting from the chimney:



This was the high point of my day. It was also an opportunity to add more coal and move on to the next level...
... an opportunity which in my inexperience I missed. The fire dropped quickly, and I spent another hour or so trying in vain to recapture that moment.

So, before I give up on coal and hit the meths I will drill a second hole in the chimney and run a small pipe into it from the hand-cranked fan via a reducer. At this point I see no reason why that shouldn't provide a nice Venturi to get the fire up to steam blower level. Or am I always going to struggle to raise steam using solid fuel with this boiler...?

We shall see...

All ideas welcome, of course   ;)

Offline Gas_mantle

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Re: 3 inch boiler build
« Reply #211 on: December 28, 2018, 10:03:10 PM »
Gary, I'd be tempted to try your hand operated blower temporarily connected to the steam blower pipe (ie bypass the steam blower) that ought to give you at least some idea whether it will create sufficient updraught, if so it ought to be a simple job to route it up the chimney properly.

I know you don't want to try an electric blower but I do think it worth rigging up some sort of electric fan on top of the chimney - even modifying one of those cheap hand held battery operated fans they sell at the seaside may be better than nothing and they only cost about a quid.

As for the firebox can you temporarily make it less tall? my guess is if the fire was closer to the boiler tubes it may help  :headscratch:
« Last Edit: December 28, 2018, 10:49:02 PM by Gas_mantle »

Offline gary.a.ayres

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Re: 3 inch boiler build
« Reply #212 on: December 28, 2018, 10:41:06 PM »
All good ideas Peter. Probably better to try routing the hand cranked blower up the steam blower pipe as a test than to start making more holes in the structure.

I have already reduced the height of the firebox significantly, but your point about a further reduction makes sense to me - will give it some thought.

Fan? Well, maybe - if I have to, having come this far...  ;)

It would be easy for me to just switch to meths at this point, but there was something so compelling about the way it burned and steamed when the steam blower was at full whack. Dynamic and alive. I had never seen that before - but I want to see it again!   :Mad:

Cheers!

Offline Gas_mantle

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Re: 3 inch boiler build
« Reply #213 on: December 28, 2018, 10:46:54 PM »
It would be easy for me to just switch to meths at this point, but there was something so compelling about the way it burned and steamed when the steam blower was at full whack. Dynamic and alive. I had never seen that before - but I want to see it again!   :Mad:

Cheers!

Yea coal firing is messy and far less convenient but I think it is a lot more fun  :)

Will it run on wood without the blower once the fire is established? Presumably running the blower extensively uses a lot of water?

Offline Zephyrin

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Re: 3 inch boiler build
« Reply #214 on: December 28, 2018, 11:09:31 PM »
Gary, testing you boiler with coal firing first looks brave...
coal firing is temperamental on such small sized grate, too many parameters to control.
I also agree that a electric fan on the top of the chimney to induce draught is a must as the long as the steam blower or the exhaust are not blowing.

And you most probably would need it too with a meths burner in a internal firebox.

You will notice that pressure is still raising with the blower valve just open, with a very low steam output. and steam from the exhaust will also induce a powerful draught, that allows to close the blower
 
the steel sheet with holes which stands for grate will not last more than 5 coal fires at a glance...and a little more with charcoal !
 

Offline gary.a.ayres

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Re: 3 inch boiler build
« Reply #215 on: December 29, 2018, 12:08:13 AM »
Will it run on wood without the blower once the fire is established? Presumably running the blower extensively uses a lot of water?

Peter - without some kind of external encouragement (including the blower) I couldn't keep the fire alive. This was so even with wood, but even more so with charcoal and coal. I'm hoping that if this works it will be straight from hand-cranked blower to steam blower. Today I saw the power of the steam blower for the first time - once it takes hold, no problem I reckon.



I also agree that a electric fan on the top of the chimney to induce draught is a must as the long as the steam blower or the exhaust are not blowing.

And you most probably would need it too with a meths burner in a internal firebox.

Thanks for your input, Zephyrin. I still think it's worth trying my hand-cranked bbq blower routed up the chimney. If it doesn't work I can reconsider.

Interesting to know that about meths burners - I hadn't realised that they could benefit from the assistance of a blower - I thought that was only applicable to solid fuel. However, I'll probably use a different arrangement for meths (or French alcool a bruler!) - am thinking of using a Trangia in a fairly open setup with maybe just a partial windshield but definitely not shut inside a firebox. Having tried the boiler on meths before I'm reasonably confident:

<a href="https://www.youtube.com/watch?v=9QwzpgftKic" target="_blank">http://www.youtube.com/watch?v=9QwzpgftKic</a>

Offline Zephyrin

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Re: 3 inch boiler build
« Reply #216 on: December 29, 2018, 07:35:16 AM »
No problem with alcohol to fire an open fire or a plain pot boiler from underside, I agree, but with an internally fired boiler a vacuum fan in the chimney is mandatory to start steaming !
I also have a lot of excitement when models start to breath...

Offline gary.a.ayres

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Re: 3 inch boiler build
« Reply #217 on: December 29, 2018, 08:41:48 AM »
Zephyrin -

Yes, I got a taste of that excitement yesterday when the steam blower started to draw the fire!

However, regarding alcohol: in the video in my post above it's the same boiler, with five firetubes. It raises steam on alcohol without a fan or any other assistance, as can be seen when the safety valve blows off near the end of the video (9 min 17 sec).

My aim is not to contradict you, but I would welcome your further thoughts as you have considerable knowledge on the subject of alcohol firing.

Salut!

gary

Offline gary.a.ayres

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Re: 3 inch boiler build
« Reply #218 on: December 29, 2018, 09:45:39 AM »
Zephyrin -

sorry - I may have misunderstood you:

No problem with alcohol to fire an open fire or a plain pot boiler from underside, I agree, but with an internally fired boiler a vacuum fan in the chimney is mandatory to start steaming !

You say an open fire *or* a plain pot boiler. So I guess that would mean an open fire would be ok with my 5-tube boiler (as in my video)?

I also assumed by 'internally fired' you meant one with tubes, but perhaps you meant a closed-in firebox...?

In any case, my thinking at this point on alcohol is to persevere with a fairly open burner and assess it as I go along. Some kind of fan assist could always be added afterwards if necessary.

Cheers.

gary

Offline MJM460

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Re: 3 inch boiler build
« Reply #219 on: December 29, 2018, 11:24:02 AM »
Hi Gary,

The issue basically hinges on the necessity for all the combustion gases from your fire to flow through the five small diameter tubes.  If you calculate and add up the cross sectional area based on inside diameter, or flow area, I suspect it will be less than the cross sectional area of the chimney, so more of a restriction to flue gas flow.  On the other hand, if you calculate the surface area of those tubes for heat transfer, which is also the area that provides friction resistance to flow, it will probably be more than the surface area of the inside of the chimney.  Even more flow restriction.  Without gas flow out the top, there is no air flow in at the bottom, so no combustion.

If the combustion chamber is not sealed, combustion gases can escape, so you might get slightly more fire, but unless the gas flows through the tubes, it will not help.

The pressure outside the firebox is the same as the pressure at the top of the chimney, so you have a column of air from the grate to the top of the chimney at combustion gas temperature and cooling as it rises, so the density of air at that average temperature, compared with an equal height column of air outside the boiler at the density for ambient temperature, as all the pressure that is available to overcome the friction of the tube walls and cause the flue gases to flow through the tubes.  It is not hard to imagine that some assistance might be needed.

If you don’t like an electric fan assist, and have a handy length of pipe say 2 m long, you could try turning a plug to make a reasonable seal at the top of the stack to increase the effective stack height and hence increase the draft.  I have seen pictures of this arrangement used on locomotive steaming bays.  No idea how much extra draft you need but it is easy to try if you have a length of pipe lying around.  Just remember to insulate it in the section you will hold to support it, or lift it off, as it will get quite hot.

Once your engine is piped up with the exhaust aimed up the chimney to induce some draft you will probably be fine, though some locomotive builders spend a lot of time refining the detail of the exhaust exit nozzle to optimise the effect.    Before the engine starts, a steam or air blower is normally needed for a coal fire and probably for meths.

The horizontal pot boiler does not pose too much restriction to the gas flow which goes around the outside the boiler with much more flow area available than your tubes.  The furnace casing confines the gas to somewhere near the shell for heat transfer, but does not cause much flow restriction.

Two of my boilers are shown in the attached pictures. (Oops, files too big.  Will try agin otherwise they are in the showcase).  Both minor variations on the pot boiler, and one with a stack about 150mm high, the other with a less sophisticated firebox.  The burners are sitting beside the boilers.  My third boiler is a horizontal centre flue type with cross tubes and works quite ok without a blower, but it is gas fired.  The gas fuel is supplied under pressure as a jet along the centre of an air mixing tube.  This gas jet supplies the momentum to overcome the friction through the centre flue tube.  That is a critical difference.  I doubt that the draft would be enough to draw a meths or coal fire through the tube, though Paul and Zephyrin  both run little locomotives which run on meths.  I will leave Zephyrin to tell us if he uses any draft assist.

I hope that helps you understand what is going on

MJM460
The more I learn, the more I find that I still have to learn!

Offline MJM460

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Re: 3 inch boiler build
« Reply #220 on: December 29, 2018, 11:28:17 AM »
Got one to post, now I will see if I have one of appropriate size for the other.

MJM460
The more I learn, the more I find that I still have to learn!

Offline gary.a.ayres

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Re: 3 inch boiler build
« Reply #221 on: December 29, 2018, 12:50:50 PM »
Hi MJM.

Thank you for your detailed explanation of boiler and flue dynamics.

I think the combined cross-sectional area of the tubes may actually be wider than that of the flue because the middle tube is wider than the other four and is not much narrower than the chimney, which it sits directly underneath. I suspect you are right about the heat transfer area though. Also, there could be quite a bit of friction drag between the tops of the smaller peripheral tubes and the cap before the gases find their way to the stack, as there isn't a great deal of room in there.

However, my boiler is what it is, and short of a major overhaul (which I really want to avoid) it's probably a matter of finding my way round things. I like your flue extension idea, and will use it if my next experiment fails. This afternoon I'm going to jury-rig some tubing which will enable me to direct the air from my hand-cranked fan up the chimney via reducers ending in narrow-bore pipe. I have decided to go ahead and drill a hole in the chimney for this, as the hole can be used for the engine exhaust in due course if my idea doesn't work. I was encouraged by the performance of the steam blower yesterday, and am hoping that my fan arrangement will get the system up to the temperature/pressure at which the steam blower will take over. I'll start with small pieces of softwood (which worked well yesterday), gradually introducing small pieces of coal when the fire really starts to glow. Irrespective of my results with this, I'll probably try your chimney extension suggestion anyway, as overall it could be a simpler solution.

As for meths, well again I refer to my video above (which you have seen before). The thing steamed plenty on a crude jury-rigged burner with no blower of any description to help it. To me that seems promising, I have to say. I have since followed your earlier advice and bought a Trangia but haven't tried it yet.

However, I aim to take the coal trials as far as I can. Once I feel I have done so (whether it succeeds or fails) I'll come back to meths, as I'd like to use both interchangeably if possible. One thing that I have learned from both yourself and Zephyrin is that blowers and exhaust routed up the chimney are helpful with meths. Up to this point, I thought they were only useful with solid fuel.

It's a learning curve, and I have several excellent teachers on this forum!

Love the pictures, by the way. Your engines have an elemental, 'real' quality to them - an aesthetic which very much appeals to me.

Cheers,

gary

Offline gary.a.ayres

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Re: 3 inch boiler build
« Reply #222 on: December 29, 2018, 06:01:21 PM »
I aim to take the coal trials as far as I can.

Well, I feel I did that this afternoon. Again, several hours of frustration, trying to coax the fire to grow and raise steam. I couldn't even repeat my limited success of yesterday. I tried my hand-cranked fan setup - it didn't work. I extended the chimney with a long tube - little improvement. I think this boiler is just too small and lacking enough tubes, as Peter (Gas_mantle) thought might be the case from the outset. Peter - I must add - has been hugely supportive of my efforts with this and has encouraged me to give my plan for solid fuel firing the boiler my best shot, while at the same time being realistic about the possibility that it might not work.

Might it work with an electric blower? Well, I have resisted that option, but given the lack of alternatives I'll think about it over time.

Another option would be to keep the firebox and see if it works with a four inch boiler in the future - all I'd need to do is widen the hole at the top.

But now it's time to draw a line and go on to meths. Back out to the workshop this evening to clean the boiler up and make a start on some trial setups with the Trangia I bought.

simplyloco

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Re: 3 inch boiler build
« Reply #223 on: December 29, 2018, 06:13:43 PM »
SNIP
But now it's time to draw a line and go on to meths. .

I know the feeling well, but these days I prefer my own gin... :cheers:
John

Offline Jo

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Re: 3 inch boiler build
« Reply #224 on: December 29, 2018, 06:19:51 PM »
I know the feeling well, but these days I prefer my own gin... :cheers:
John

Its always nice to know there are uses for those off cuts when we make a boiler   ;)

Jo
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