Scotch Type Marine Boiler

[doubletop]

It's time for another boiler now that my last engine is finished. http://www.homemodelenginemachinist.com/index.php?topic=9412.0

Its a marine engine so needs a marine boiler. I've done a vertical boiler so this next one has to be horizontal. I've recently been reading K N Harris "Model Boilers and Boilermaking" and it has examples of a Scotch Marine boiler and the Inglis variant. The trouble is the plans require something like 6" tube. I've got 3" tube so it's going to be that.

The plan is to fire it with LPG gas (propane/butane) and a through flue burner much like SandyC's plans for 3.5" burner http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item175 however the burner flue is going to be 1" tube as I'm only using a 3" main tube. So the first thing was to try out a burner design and see if it would work enclosed in a length of pipe. First attempt was a non starter. I based it on a small Sievert burner which worked well when poked up the pipe. But my copy was a disaster and flashed back as soon as you looked at it.

Next was something based on this old torch that was lying in the junk box. It has a #10 jet which will do fine

So I set about copying the parts. Which is basically a hole for the gas to come though and mix with the air.

All the parts just push fit together for now

Then light it up. It needed a bit of adjusting of the size of the second set of air holes but it fired OK just by lighting the end of the tube. To allow for a bit of tuning I have already made an adjusting sleeve for the air flow ready for some bigger air holes and hopefully to deal with the "pulse jet" noises it makes when the gas is turned up.

[ame=http://www.youtube.com/watch?v=LIvCLxzY0I4]http://www.youtube.com/watch?v=LIvCLxzY0I4[/ame]

There may be an issue with flashback when it all gets a bit hot but successful enough for now to proceed with the boiler design............

 

[doubletop]

So that was yesterday. Today I made a start on the major components and sketched out a design. I make no appologies for a bit of plagiarism as some of the bits come from the SandyC designs. The first draft of my design can be found here http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item263

It remains to be seen it it works but I won't find out unless I build it, so here goes....

Cut out the plates

First phase of anneal/bash/anneal cycle

Four plates at one point in the cycle

Finishing the edges in the lathe

Then I realized I now had a new mill so try that instead. Much better

Finishing the ends of the main tube in the lathe

After an hour in the pickle

I just love that clean pink finish after the pickle. You'll see there are now 5 plates when I need 4. I had a spare one after may last boiler cock up. No doubt there be another cock up at some point so the extra plate may as well come on the journey.

That's going to be it until next weekend. People have decided they are willing to pay me for work so I'd better not disappoint them

Pete

 

[doubletop]

Had a chat this week with our boiler inspector and he was OK in principal with my plans so this weekend I set about drilling the end plates and boiler barrel.

I don’t have a step drill so I did the holes in the end plates for the burner tube in the 4 jaw chuck with the boring bar.

Setting the centralizing the hole using a center between the punched centre point and the tailstock.

Boring the holes

Trouble is my lathe is too small so I couldn’t get the holes for the two return flues centered on the 4 jaw so over to the mill and the boring head.

The finished plates; The square hole is for access to the super heater and will have a door on it eventually.

On to the boiler barrel. That was relatively easy. Like my last boiler I am going to use rivets to keep the plates aligned while I solder them up.

When it came to the hole for the steam dome the boring head wasn't going to work as the head hit the curvature of the barrel and tried to move it on each rotation. Many ways of skinning the cat and I found I had a ¾” slot drill that would to the job.

All the parts ready after a while in the pickle.

Loose assembled boiler

This boiler has a central flue and rather than the smoke stack being at the opposite end the Scotch boiler has the flue at the burner end so the got gasses from the burner go down the main burner tube to a cavity at the opposite end (where the superheater will be) and then fold back down secondary flues (I have 2 on this boiler) and into a 'smoke box' at the burner end. You'll see two matching sets holes in the both end plates. These are for the water gauge. The intention is to have long bushes that extend from the main end plate through the smoke box and out to the secondary end plate. Check the outline drawings http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item263

That’s it for this week I may get the main burner flue cross pipes soldered up but I’m waiting for the bronze for my bushes to arrive and I’ll do those next weekend.

Pete

 

[Diversion900]

Looking good so far.

I am looking into building an Inglis variation, so I will be watching with interest.

What size is the tube you are using ?

I have a piece of 4" Diam. copper I am thinking of using for mine, but most of the pictures I have found use 3".

Cheers, Neil

 

[doubletop]

Neil

I wanted a basic marine boiler and found the K N Harris book "Model Boilers and Boiler Making". From what I could establish his sketches were in the area of 6" dia. I had 3" tube and that's what I had and wanted to use. (also in some jurisdictions there are boiler regs considerations going above 3")

I looked at the commercial offerings and had done the SandyC 3" vertical so thought I'd base this one on his design, pinching the basic components. You'll see the sketch I've posted and progress so far, I'm using 16gauge for the body and 18 or 20 gauge for the tubes (there's a bit of margin there). I had a chat with the club boiler certifier last week and he thought the safety margins will be OK as he proposed to use 16 rather than 8 as the safety factor for the flue tube calcs as they are under compression and not tension, but still OK. Also this boiler is for 50psi max as its for my double acting oscillator so too much pressure is not required.

So this is very much what you see is what you get, a design/build as you go project. Maybe it will work OK, maybe not. However, the defining moment was the burner as I have realized that is critical in the success of any boiler. Once I had got the burner working doing the boiler was to some degree secondary.

If you are doing an Inglis are you planning it to be detailed down to three sets of 'folded' flues or a basic representation? How do you plan to fire it. No doubt you know this but if you have a piece of 4" copper the thickness and planned operating pressure are key factors on deciding if its suitable for your planned boiler.

Welcome on the journey

Pete

 

[Diversion900]

G'day pete,
I am still in the planning stages at present but I have planned on a ceramic gas fired tube, with a slightly smaller return tube, maybe with cross tubes ?, then back the other way through some smaller tubes either side of the second one.

Most of it is still in my head at present, but I have most of the components at hand, I just have to get hold of some Phospher Bronze.

I am not in any club yet so don't know of any restictions on size, and I can always make a smaller one to satisfy club rules at a later date.

Its just that I would dearly love to have a large-ish boiler at my disposal to run anything I throw at it, and my piece of 4" is just begging to be used ;D.

Cheers, Neil

 

[doubletop]

Neil

Sounds like you are in the same place I was. I couldn't find plans for what I wanted so rather than pitch in and find I couldn't fire it, I did the burner first. Once I'd got burner sorted I then turned to the sketches of the boiler.

If you want a burner design for a ceramic,horizontal in tube the plans for one are here http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item175

However, number one consideration for now is what pressure are you planning to work at and how thick is your 4" tube. I'm no expert but have established that for a drawn copper tube under expansive pressure (not compression) and having a safety factor of 8 (from what I can establish the accepted safety factor) is given by:

Thickness = max working pressure (psi) x inside diameter of tube (inches) / 2 x 3125 . If your tube is thicker than the result you are in business.

On this site somewhere are the calcs for the SandyC 3" vertical boiler but I can't find them. One point of debate is the calcs for the internal tubes. The internal tubes that are under compression and they can collapse a lot more easily under pressure. Sandy used the same safety factor of 8. My club boiler tester has proposed a factor of 16 which really means make these tubes twice as thick as the calculation, or half the max working pressure. As all these posts will tell you boilers are dangerous don't underestimate what you are doing.

Like you I also need bronze for the bushes and am on hold until MBM deliver it.

Pete

 

[Maryak]

Hi guys,

An extract from an old post of Sandy C's on copper tube wall thickness.

T = (PxD / 2xS + P) + 0.005 x D

And for pressure this would be: -
P = S [2xT –0.01 x D / D - (T - 0.005 x D)]

Where S = Maximum allowable stress value of the design material at design
temperature.

For copper @ temperatures below 450deg F it is reasonable/acceptable to use: -
S = Tensile Strength / 3.5. (Above this temperature, then reference to stress tables
would be required).

For copper with an 8 times safety factor this would amount to: -
S = 3125 / 3.5 = 892.857psi.

Also in this case D = Outside dia.

This is in fact a much-simplified version of the formulae, however, for the purposes of
designing of model boilers, operating at relatively low pressure and temperatures and
which are using silver soldered jointing methods, it is perfectly satisfactory/adequate.

Hope this helps

Best Regards
Bob

 

[Diversion900]

Thanks guys

I will do some calculations on the tube I have to see if it is worthwhile using.

I was thinking maybe 50 or 75lb but I wll see what the fugures say.

Cheers, Neil

 

[cfellows]

Pete, what is the purpose of the two return flues?

Thx... Chuck

 

[doubletop]

Chuck

Its a bit of artistic licence on the original design of the full size boilers. I am basing my design on the outline of the boilers in the the KN Harris book. In that book there are photographs of a model boiler similar to the one I'm doing. I'm making use of the info available and building it and seeing what happens without paying much attention to the detail of the theory.

Without going back into book and paraphrasing it my take is:

These boilers are compact and all the functional parts are at one end so don't need access to "the other end". In the full size situation they would install two back to back. The access door at the back end of my design has no functional use and probably would be detrimental to the thermo dynamics but what the..?

The folded design allows for all the heat to be exttacted from the heat source as it travels first in one direction and then the other through the boiler. Many smaller tubes have a greater surface area than one tubes allow more heat transfer into the boiler. OK two is only just more than one.

The Inglis version extends this approach and there are three sets of tubes getting progressively smaller on each 'fold'. That's what Neil is planning to make. Why do they need to be smaller on each fold? From my point of view it feels right but the theoreticians can probably give us chapter an verse.

So the idea is a compact marine boiler that can be installed in a small space and extracts as much heat as possible from the heat source. As we know the vertical center flue boiler is very inefficient, this type should be somewhat better. Can I prove it? Probably not but its going to be fun doing it anyway.

Hope that helps?

Now MBM where is that bronze?? The weekend is coming up fast.

Pete

 

[doubletop]

Update to the drawings

http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item269

Pete

 

[cfellows]

OK, I get it. The burner gases go through the large tube into a closed chamber on the other end of the boiler then come back through the return tubes to be exhaused on the same end as the burner. Very clever!

Chuck

 

[doubletop]

Chuck

That's it; and the Inglis does three passes and then exhausts at the 'conventional' end. We'll let Neil show us that when he gets his build going.

Bronze hasn't arrived for whatever reason, no bushes this weekend so no soldering, or not much anyway. I'll do what I can on some other bits.

Pete

 

[Diversion900]

Sorry to hijack your post again pete, but I'm a little confused with the figures I came up with..... ?

The Copper tube I have is in fact a metric 4" diam. 16SWG
which is actually 101.5mm OD and wall thickness is 1.5mm
converted to inches makes it 3.996" OD wall thicness 0.059055"

If I use your formula I get a max pressure around 95psi which works out fine for me,

but using the other formula from Bob I get about 17psi ?

I am in need of some expert advice here I think.
Maybe we should move this question to the Q & A section so we don't get too far off topic with your build.


Cheers, Neil

 

[Maryak]

Neil,

I may have confused the issue here :-[

The formula for the boiler shell is P=(2T x t)/D so for your example

P = (2 x 0.059" x 3125)/4

P = 368.75/4

P = 92 psi.

The formula I quoted in my previous post is for the boiler tubes i.e. furnace tubes/return tubes.

e.g. if your flue tube is 1.5" Dia and using 1.644 as the OD

T = (P x D / 2S + P) + 0.005D

T = (92 x 1.644/ 2 x 893 + 92) + 0.005*1.644

T = (151.248/1878) + 0.0082

T = 0.089"

As 1.5" copper pipe has a standard wall thickness of 0.072" this is below the thickness required for a boiler at 92 psi. so use high pressure copper pipe, or reduce the boiler pressure, or use a smaller flue tube etc.

similar for the return tubes using 0.5" copper pipe and using 0.598 as the OD

T = (92 x 0.598/2 x 893 + 92) + 0.005*0.598

T = (55.016/1878) + 0.003

T = 0.032"

As 0.5" copper pipe has a standard wall thickness of 0.049 then this is OK for a boiler at 92 psi.

I hope this helps and sorry if I confused you previously.

Best Regards
Bob

 

[doubletop]

Bob

Thanks for this it has helped me confirm the working pressure for flue tubes for this boiler. I had to add some extra brackets in your formula to get the same results as you.

T = (92 x 1.644/ (2 x 893) + 92) + (0.005*1.644)

I am using 1" tube for the main flue OD = 1.078 and wall thickness of 0.036" ignoring the cross tubes the calculation for 92psi comes out as requiring a wall thickness of 0.058" however this boiler is for my small engine so don't plan to go above 50psi and for 50psi 0.034" is required.

The return flues are OD .0579 so all is well there.

Pete

 

[Diversion900]

Thanks guys, I feel a bit happier now, I didn't think my maths was that bad. ;D

I had planned on a max pressure of 50 to 75psi as previously stated, so with these figures I could still work on a 75 psi working pressure, providing I use a pump and a good burner to keep up steam.

Looks like my 4" tube will do the trick after all.

Cheers, Neil

 

[Diversion900]

To satisfy my curiosity, and maybe some others out there as well.....

Does this mean that when testing, we shouldn't go past 92psi, thus having a working pressure of half this amount, being 46psi. ?

Or am I stretching the safety thing too far ?

Cheers, Neil

 

[Maryak]

Bob

Thanks for this it has helped me confirm the working pressure for flue tubes for this boiler. I had to add some extra brackets in your formula to get the same results as you.

T = (92 x 1.644/ (2 x 893) + 92) + (0.005*1.644)

Well of course you did. : If I was an artillery man I would have bracketed it correctly but being an old steam plumber scratch.gif my apologies for the unbracketed assumption.

Best Regards
Bob