Gotta get me one of these!

[Barneydog]

Hi Guys
A few months back my mate Rich got the plans for a 'simple' traction engine called PYRTE. Gas fired simple traction engine with a simple straightforward 4" boiler. We started looking at it and planning improvements to the design that were immediately apparent. One issue was the boiler which pushed us to do what we have been promising for years. This was to join our local model engineering society. We joined and went along to one of the member only steaming days. Though there were only four engines there that day we were bitten. The words in the subject for this topic were our reaction within a few minutes...."Gotta get me one of these!!!!"
We have scoured the internet for free plans for various engines and took several files away on a recent holiday to the Carribean. While the women were sleeping in the sun we were avidly reading and rereading these articles. We have both settled on Conway by Martin Evans. Unless we can find some very cheap casting we intend building from scratch. Personally I have decided to base my loco on Conway but only loosely. I intend building it as a 060 or a 260 or a 062 rather than 040. I also intend moving it to 5" rather than 3 1/2" guage. This will allow me to keep the rough size of the engine nearly the same but to mount the wheels outside the frames with the cranks on the wheels. I realise some of you will say that this will effect the scale and not conform to the recognised scale per real foot. Sorry but this is not a concern to me. I just want to build a loco that looks ok, that runs well and that will run around the track well. It has to be functional and easy to operate with the necessary controls where they work the best and of a size that are easy for my big hands to work. I also want the build to be at a reasonable cost with as much use of modern materials and processes. I also want it as maintenance free and clean as possible. I am going to use as many bearings as possible rather than bushes. I am going to use  my cnc mill and 3D printer for as many parts as possible. I have a piece of 4" copper 14" long that will become the simple boiler as used on PYRTE and will be propane fired.
I am in the design and trial stage at present. I have cnc'd some wheels to try with a 3D printed strap for the bearing I am using for the excentrics. These trials had to be done before a full scale design drawing can be produced. I will keep you posted on progress and will put some pics on when I can remember how!

All thoughts welcomed but not if it includes comments like "If this was full size the driver would have to be less than 4ft to get in the cab!". Look at this build as a modern take on older processes and designs with plastic, metal, bearings ect. I am intent on enjoying this build so join in please.

Julian

[joe d]

While I am entirely unfamiliar with the engines you have mentioned (more internet research for me)
I see nothing but enjoyment ahead in building to YOUR desires.

I look forward to following along!

Cheers, Joe

[Barneydog]

Thanks Joe.

Hopefully attached is a pic of pyrte made by the designer before it was painted. Conway is a 040 two cylinder narrow guage loco with inside wheels and outside cranks. 3 1/2 guage with 3 1/2 diameter wheels. Stands about 26" long and 10" wide

[Barneydog]

finished a bit of test work today so I can move on with drawings.

Cnc milled the insides of the wheels to get crank and spokes. Turned on lathe to finish tread and flange. At the moment tread diameter is about 92mm. Once all are ready they will be finished to 90mm. Spokes need a bit of fettling with a file or dremel. Axle size is 10mm and crank is 8mm with an offset of 25mm.

3D printed bearing holder for excentric. Centre hub turned on lathe for an offset of 6mm giving a total movement of 12mm.

Julian

[Jasonb]

The PYRTE boiler is not that efficient and does not produce a lot of steam, before you get carried away it would be worth doing some calculations on what amount of steam your loco is likely to need if you want any sort of reasonable performance out of it

[Barneydog]

Boiler is on my to do list for calculations. Planning to use pyrte boiler with porky on the heating area to get more heat into water. Also going to be well insulated to hold heat. Gas fired so hopefully more available heat.

Julian

[Jasonb]

Even with all the studs like his coal fired one it is not that good a design a multi tube or cross tube design would be better. Also if you are designing your own make sure you run it buy the club boiler inspector before buying materials or cutting metal as they will have the final say when it comes to getting it certified.

[Barneydog]

At the moment inspector is happy. He will see everything and I will not start building boiler until he gives ok.

Can someone please share with me the formulas etc for working out steam requirements of an engine  and the steam output of a boiler. Then I can get calculating!

Cheers
Julian

[Jasonb]

K.N.Harris's and Martin Evan's book have a lot of info on boiler design. Also Model Engineers Handbook by Tubal Cain has quite a large section on steam and boilers

[SandCam]

At the moment inspector is happy. He will see everything and I will not start building boiler until he gives ok.

Can someone please share with me the formulas etc for working out steam requirements of an engine  and the steam output of a boiler. Then I can get calculating!

Cheers
Julian

Hi Julian,

I have just sent a lengthy email to Rich which show the calculations (or some of them) so I am sure he will be in touch.

Best regards.

Sandy.

[SandCam]

Hi Guy's,

For the benefit of any of you that may be following this thread here is the main text of the information I sent to Rich (firebird).

He asked the question... did I think a PYRTE type boiler would be suitable for a 3 1/2" gauge locomotive based on Martin Evans 'Conway' narrow gauge locomotive model...


Short answer... NO... not much chance at all since it just does not have sufficient ‘Effective Heated Surface’ this being only approx. 38 - 40 sq ins**. (at best 90sq in with studs fitted)

 ** approx. Effective Heated Surface area = half circumference of barrel x length of firebox.

 PYRTE boiler is 4" in dia. and the firebox is around 6" long and is fired from underneath similar to a 'Mamod' style boiler.

Long answer, to clarify the above... and to answer SOME of Julian’s request for calculation methods... Boiler design is complex I am afraid with a huge number of variables.

 'Conway' cylinders are, if I recall, 1 1/4” dia with a stroke of 2”.

Piston area x stroke x 2 for double acting x 2 for 2 cylinders = Total swept volume per revolution.

 : - The total swept volume per revolution is 9.8176 cu in per rev.

 Assuming a max wheel speed of around 100rpm, since the cylinders are directly coupled, this would amount to 981.76cu in steam required per minute (total swept volume x rpm).

How much steam a boiler can produce is a product of the working pressure, it’s conversion factor (type specific) and the available effective heated surface area (HS).

The volume of steam produced by a given volume of water is determined by the working pressure...

 Assuming a working pressure of 80psi (as per ‘Conway’) then 1 cu in of water will produce 289.65 cu in of steam.

(Note... this is calculated from steam tables which for a given working pressure give specific volume of steam in cu ft/lb x 1728 (cu in per cu ft) to give cu ins/lb divided by 27.741 (since 1 lb of water occupies 27.741 cu in) which gives cu in steam per cu in water.)

 : - 981.76 / 289.65 = 3.3895 cu ins water need evaporating per minute.

 
The PYRTE boiler type even with studs fitted would have a maximum conversion factor of x2 (even if fired very hard... and that is being generous) typically it would be more like x1.5.

The conversion factor is the number of cu in water evaporated per 100 sq in HS a particular type of boiler can evaporate per minute.

 The required heated surface required would then become: -
 

HS required = the number of cubic inches of water to be evaporated per minute x 100 / conversion factor...

therefore heated surface required = 3.3895 x 100 / 2    =  169.475 sq in HS required for a x2 conversion factor.

At the more typical x1.5 conversion factor this becomes: -

              3.3895 x 100 / 1.5 = 225.966 sq in HS required.

You only have around 40 sq in HS.... and even with studs added it would not exceed 90sq ins at best... so either way you are well short I am afraid.

You could choose to reduce the working pressure which will help a little, however, you will loose tractive power if you go much below 60psi.

 At 60 psi 1 cu in water will give you 363.753 cu in steam.

So evaporated water required becomes 981.76 / 363.753 = 2.6989 cu in water per minute.

 : – Heated surface becomes 2.6989 x 100 / 2 = 134.945 sq in HS. for a x2 conversion factor.

or  2.6989 x 100 / 1.5 = 179.926 sq in HS. for the more typical x1.5 conversion factor.

So... still well short of HS.


If you consider the actual heated surface available then the actual number of cu in water evaporated per minute becomes... (Available HS in sq in / 100) x conversion factor cu in per minute.

e.g.... for 40sq in heated surface you would only be able to evaporate (40/100) x conversion factor cu in per minute.

hence for a x1.5 conversion rate this becomes (40 / 100) x 1.5 = 0.6 cu in water evaporated per minute rather than the 1.5 cu in per minute per 100sq in HS required... i.e. well under half the required amount.

When considered in terms of the actual conversion rate required at 80 psi being 3.3895 cu ins water per minute then this 0.6 cu in per minute clearly indicates the effect of having too small a heated surface area.

Similarly for the 60psi case being 2.6989 cu in water per minute required then again the 0.6cu in per minute is clearly not going to work.

I hope this is of some help to anyone following.

and to everyone... Have a very Happy Christmas and a peaceful New Year.

Sandy.

[Jasonb]

Thanks for that Sandy, even less efficient that I had thought

I did not want to be seen as putting a downer on Julian's plans but at the same time would have hated for him to be disappointed after spending a lot of time and effort making a Loco that would be a Lemon on the track.

J

[Barneydog]

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

[Gas_mantle]

I don't want to put you off building a larger Pyrte engine but I'd think very carefully about scaling up something that is already a fairly large model. I'm making a small stationary engine from another members plans and have scaled up by 50% - it's a small model to start with so the larger version isn't a problem but it is considerably larger that the original. 50% doesn't sound a lot but it equates to nearly 3.5x the volume and weight.

If I remember rightly the Pyrte has wheels fabricated from gas bottle bottoms, in a scaled up version they could to be fairly big to handle depending on the tooling / machinery you have.

 

[Jasonb]

And scaling it up 50% will make for an even less effient boiler on the traction engine as the volume of steem increases more than the low heating surface available.

Have you calculated if your bit of 6" tube is thick enough? Also distance between stays will increase so probably more needed rather than keeping the same number but making 50% larger dia.

There is a lot more to be considered when scaling up a boiler than the rest of the model.

[SandCam]

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.

Keep Happy.

Have a great Christmas and New Year.

Sandy.

[Barneydog]

Got a choice of 6" x 10swg for 50% increase in size or 5" x 10swg for 25% increase. Any ideas?

Julian

[Barneydog]

One choice would to go 50% bigger but use coal fired marine style boiler instead of enlarged pyrte boiler

Julian

[Gas_mantle]

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 ?

[SandCam]

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.

[Barneydog]

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

[Barneydog]

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

[Barneydog]

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

[Gas_mantle]

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 ?

[Jasonb]

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.

[Barneydog]

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

[Jasonb]

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.

[Barneydog]

Jason,
Thats correct its the firebox tube. The boiler tube is available


Julian

[Barneydog]

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