W Melville scotch type return flue boiler.

[London_lighterman]

Good afternoon one and all.

I purchased a sheet of a scotch return flue boiler drawn by a W Melville.
On the drawings there is no where a safe working pressure so I did some calculations and I worked it out as 117.3 Lb per sq Inch. so I think 90 will do ok.

Copper tube now this may cause some eye rolling and some tut tuting but we get to the bit about its provenance. From what I have read its materials for the welding process be it steel or copper, and as long as the copper is drawn and the right thickness.
could the panel give me some pointers.

[Jasonb]

He designed it for 60psi working pressure.

Ideally you want C106 grade copper. Unless you are going to get it inspected and certified then it is upto you what you get upto in your shed but for insurance and peace of mind it would be worth having it tested by a third party should something go wrong and a neighbour or family member get hurt.

[Jasonb]

It is unlikely that you will find 5 1/2" x 14g tube these days so the barrel will have to be rolled with an external butt strip. This joint  will also need an allowance in the calculations so you are likely to be looking at rolling from 2.5mm sheet

[London_lighterman]

Jason.
I will have a look about and I think the neighborhood is quite used to the pops and bangs by now.
The boiler inspectors can have a good paw over the drawings with me and push me in the right direction I am sure.
Drawings call for a thinner gauge backhead and front tube plate. and I would say I will match the barrel thickness for a start. There are Two stays on the water and steam space above the tube nest which is 30 strong.

I would share some of the drawings here but I would be in breach of copyright and going to chokey at this stage in life holds no thrill.
reading Tubal Cains book he recommends rings around the main flue of a marine or Cornish boiler. Does the famous sweet pea design have rings round the fire box tube?

Strange how the nomenclature changes on boilers. "furness" for one.

[London_lighterman]

He designed it for 60psi working pressure.

Ideally you want C106 grade copper. Unless you are going to get it inspected and certified then it is upto you what you get upto in your shed but for insurance and peace of mind it would be worth having it tested by a third party should something go wrong and a neighbour or family member get hurt.

I did the calculations as per the southern fed's seminar notes and used Harris' calculations  and landed at 117.
Where did you find out about how many PSI he designed it for?
Was this design published in the ME?

[Jasonb]

Yes it is in ME

Depending on what formula you use it can vary .

If you strip Evans formula down to the bare bones you get thickness of barrel = (WP x boiler Dia)/5000

So (60 x 5.5) / 5000

330/5000

0.066" as minimum thickness

That is just over 16swg so probably went for the next available tube thickness up so 14swg on the design.

If you use the spreadsheet that some of the commercial model boiler makers use then 14swg would be good for 116psi for drawn tube but that drops to 96psi if rolled.