Author Topic: Gotta get me one of these!  (Read 5086 times)

Offline SandCam

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Re: Gotta get me one of these!
« Reply #15 on: December 14, 2016, 04:51:07 PM »
Thanks guys.
Dont worry jason i regard failures as learning but will not start until ready.
Thanks Sandy for excellent calcs and explanations. Now looking at loco or marine style boilers. May move to bulld pyrte first as just got hold of decent piece of 6" tube. I intend building 50% bigger so it becomes 3" instead of 2" scale model. Still intend loco but more needs planning and sourcing before starting. Will leave loco on this topic and start new for traction engine when ready.


Julian

Hi Julian,
I am afraid 'Jason' and 'Gas Mantle' are correct, you will need to consider a much better boiler design than the one offered by the PYRTE, especially if you go 50% larger.

Firstly you would need to increase the thickness of the barrel due to the additional dia + you will also need to allow for the fact that the boiler is also the chassis on a traction engine and since you propose adding approx 6" to it's length this would also need extra thickness to be added.
At 1/4 scale (3"/ft) you are close to the maximum for a copper boiler.

A few quick calculations on a 6" boiler x 18" long at 75psi ( the higher pressure specified for PYRTE) would require the thickness to be increased to 0.108" minimum... the nearest easily available standard size would be 10swg @ 0.128".

Secondly... a 50% increase would imply a firebox length of 9" which, with a 6" dia. barrel, would give you roughly 85sq in heated surface without any studs... perhaps 160 sq in if you managed to fit enough studs.

Cylinder size would increase to 1 1/2" bore x 3" stroke giving a total swept volume per revolution of 10.6 cu in.

Rear wheels would become 15" dia. and assuming a scale speed of 1 1/2 miles per hour (6MPH full size) this would equate to approx 11.2 revolutions of the rear wheels per minute.
Assuming also that the gearing remained the same i.e. 16:1 as per PYRTE then the engine speed would become  179.2 RPM.
179.2 x 10.6 = approx 1900 cu ins steam per minute required.

At 75psi 1 cu in water gives 306 cu in steam.

You would need to evaporate 1900/306 = 6.21 cu in water per minute.

Assuming the basic x 1.5 times conversion factor this would require a heated surface area of: -

       6.21 x 100 / 1.5 = 414.0 sq in H.S.

You only have 85 (or 160 at best with studs)

You would therefore be wasting both your time and money going down this route with such a boiler.

Unfortunately there is no easy solution and a proper locomotive style boiler with a fully wet firebox and multiple fire tubes will be required if such a model is to be successful and almost certainly a coal fired version will be the best option.

Sorry and all that... but there just are no free rides in this game. :'( :shrug:

Keep Happy.

Have a great Christmas and New Year.

Sandy. ;D

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #16 on: December 14, 2016, 10:00:41 PM »
Got a choice of 6" x 10swg for 50% increase in size or 5" x 10swg for 25% increase. Any ideas?

Julian

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #17 on: December 14, 2016, 10:20:52 PM »
One choice would to go 50% bigger but use coal fired marine style boiler instead of enlarged pyrte boiler

Julian

Offline Gas_mantle

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Re: Gotta get me one of these!
« Reply #18 on: December 14, 2016, 10:45:56 PM »
I'm no expert here and I'm sure someone with more experience will put it better but the issue is, the boiler is inefficient to start with and scaling up will only amplify the short comings.

The simple way I see it is by scaling up by 50% you increase the volume of water by 1.5 cubed - ie 3.375x but the surface heating area is only increased by 1.5 squared -  ie 2.25x

To get a corresponding increase in heating area you will need tubes. As someone said earlier there is no such thing as a free lunch.

Why not make a normal sized Pyrte then use the time and money saved to get someone to make a more efficient boiler ?
« Last Edit: December 14, 2016, 10:49:03 PM by Gas_mantle »

Offline SandCam

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Re: Gotta get me one of these!
« Reply #19 on: December 14, 2016, 10:56:30 PM »
One choice would to go 50% bigger but use coal fired marine style boiler instead of enlarged pyrte boiler

Julian

Hi Julian,

Yes that would be a much better option... especially if you don't want to build a full loco type boiler... tie up with Rich as he is now considering a 5" version of this type for a 3 1/2" gauge loco.
This type would at least get you in the right ball park regarding H.S. if you get the number of firetubes right.

For coal firing I would not go smaller than 3/8" OD for these.

Keep Happy.

Sandy. ;D

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #20 on: December 14, 2016, 11:31:10 PM »
Thanks guys for the replies. It's most of the way towards what I expected. I think rich has now decided to build Conway with the specified 4" loco boiler.  We are trying to locate suitable materials with our new supplier at the moment. I don't want to spend too much money on these projects especially as I want to do it all myself including the boilers. I always look for the simplest effective option for everything. I will not pay someone else to build one but equally need to ensure I don't waste money by the boiler having an inaccessible leak!. The simpler the better for me. Next route is probBly the marine style boiler in 6" for pyrte and 5" for loco. Will look at email sandy has sent rich for calculations and see if this is viable.

Thanks
Julian

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #21 on: December 15, 2016, 12:08:48 PM »
Hi guys,
For the firebox on a 5" marine boiler 3" od 8swg copper tube is needed. Nothing this size is made or available any more. I can get everything else i need except this. I have plate knocking around this thickness and thicker. If made using square staggered joints is there anything wrong with rolling and silversoldering this tube? I have rollers that can do this.
My plan is a 5" marine boiler sitting inside a 6" steel tube to give structural strength to the traction engine and a layer of insulation in between.

Same boiler for loco but no steel outer tube.

While the tubes are available to me at a good price i will probably buy them.

Julian

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #22 on: December 15, 2016, 12:36:12 PM »
Sandy,

Please is there any chance of a quick calculation from you for 5" x 18" marine boiler as discussed with Rich for the steam production figures running on coal.
If it can produce enough I would go with this style for both large pyrte and loco. Problem as stated is sourcing suitable tube for firebox unless it is allowable to roll and fabricate it.

Thanks

Julian

Offline Gas_mantle

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Re: Gotta get me one of these!
« Reply #23 on: December 15, 2016, 01:05:55 PM »
I'm kind of getting confused here with the talk of 2 different projects in various different sizes. Are you intending to build a 150% scaled up version of the Pyrte using the same pot boiler principle ?

Online Jasonb

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Re: Gotta get me one of these!
« Reply #24 on: December 15, 2016, 03:23:46 PM »
You would be better off rolling teh sheet and then using an external butt strap with some rivits to hold it in place while you solder the joint.

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #25 on: December 15, 2016, 03:39:58 PM »
Jason. Thanks. I am happy to roll the sheet to make the 3" firebox but need to know its permissible for regs. I originally thought of straight butt joint with reinforcing strap but then thought sawtooth shape would be stronger. Yes straight joint would be much easier.

Gas mantle. Its not confusing it just seems that way!! I intend in the near future to make both a loco and a traction engine. The first to be produced is the one that I get all the materials for first. Problem is the boilers. I had hoped to use the pyrte style boiler for the loco as its an easy boiler to build but now find its not suitable for the loco and as you have stated cannot be resized from 42 to 62 due to efficiency problems. Next easiest to produce is the marine style with a round firebox and tubes which is already used on some models such as Station Road Steams "Stafford". The problem here is that the round tube at 3" diameter for the firebox needs to be 8swg or thicker to withstand the external pressures on it within the boiler. This is why the question over rolling my own tube as this size tube is rarer than Rocking horse p**h. If I go with the loco first I will detail it here on this topic. When I go with the traction engine it will be on a new topic. At the moment they are both on this as they are related by the boiler problems.

Regards

Julian

Online Jasonb

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Re: Gotta get me one of these!
« Reply #26 on: December 15, 2016, 06:33:41 PM »
Julian, I read it quickly at lunchtime and thought you were talking of rolling the main boiler barrel. Not sure if rolled & jointed will be OK for firebox, best ask your club inspector.

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #27 on: December 15, 2016, 04:36:20 PM »
Jason,
Thats correct its the firebox tube. The boiler tube is available


Julian

Offline Barneydog

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Re: Gotta get me one of these!
« Reply #28 on: December 26, 2016, 01:15:39 PM »
Merry Christmas every one.

Plans have changed over Christmas. I won on eBay a partbuilt 5" Simplex loco including a majority of parts. Because I won this I have decided to take the plunge and build a Simplex boiler from scratching the next couple of months. The frames are already built and assembled. The cylinders are ready for assembly and fitting. This is after everything has been checked and corrected if necessary. Boiler needs to be soon so all other bits can be made and fitted. I am collecting it all tomorrow so will see on Wednesday what state it is in!!. I M hoping that some of you will help me with advice and suggestions especially Stu who rebuilt one on here several years ago. Needless to say I have printed out his entire topic as an aid to the build.
This change of plan has now pushed the building of the Pyrte back a while. I still have a traction engine on my bench to finish which is electric powered. I suppose I ought to finish that first to make some space. May put some pics on here to show where it is at the moment soon.

Julian