Supporting > Boilers

Scotch Marine Boiler

(1/30) > >>

Maryak:
Hi Guys,

The boiler I am attempting may be familiar to some of you and below is where I am at.

Machining the shell.









In addition I have cut the 2 shell end plates and will be marking them off and machining them in the near future.

SandCam:
Hi Bob,

Just been looking at your power station small boiler design and must say you have got it pretty well sussed out  8) 8) 8).

One or two small observations, if I may... On your calculations file (obtained from the other site... yes!! I have been keeping an eye) you show the furnace tube as being 3" OD with a wall thickness of 1/8" ( as against the 0.010" calculated thickness required).

This would make the ID 2 3/4".

On your drawings, however, you show it as being 3" OD with 2 1/2" ID... making the wall thickness 1/4" which one is correct?.

If the 1/8" figure is correct, then the dimensions shown for your GRATE and ARCH are incorrect.
If the 1/4" is correct, then I would suggest that you try and find some 1/8" wall tube as 1/4" is a bit on the thick side for heat transfer.

Moving on... Fire tubes and Grate...

As drawn... the area of your grate is 10.839 sq ins.

In your calcs. you show the wall thickness of your fire tubes as being 0.060" (just shy of 16swg @ 0.064") and the OD as 1/2"... This yields an internal tube cross section of 0.11045 sq ins.
you have 18 tubes making a total area for combustion gas flow of 1.9881 sq ins.

If you are planning on using natural draught for combustion then the tubes cross section total should really be approx. 1/4 of the grate area... as things stand you are somewhere between 1/5th and 1/6th... which may mean you will need to use forced draught (fan or some such).

If you could reduce the wall thickness of your fire tubes to say 0.036" (20swg) this would increase your cross section total to 2.5897 sq ins... which is much closer to the 1/4 grate mark.
The only other way would be to increase the number of tubes... and it should be possible to fit 8 more between the existing rows if you move them apart a little more... or above the top row.

I don't know how you are planning on fitting your fire tubes (but imagine you expanding them) in which case the thinner wall tube would again be a better option.

The above ratio's are based on your DRAWING dimensions... if your furnace has a larger ID ( as per the first observation) and you keep your Grate height as per your drawing... then the grate will have a larger area... making your tube/grate ratio even worse.

I hope this all makes sense and doesn't come to late to make any corrections to parts made... just thought I would point them out.

Other than the above, it looks like you are well on your way... well done.

Best regards to you and your good lady.

Sandy.

Maryak:
Sandy,

Thank you.

My drawings were done without reference to the materials I could scrounge and I must admit with no thought as to the grate/ flue ratios.

I have now scrounged a piece of boiler tube which is  high pressure. I have started to machine it and it should end up with a wall thickness around 1/8" and an OD around 2-5/8"

My reasoning is that the smaller I can get the OD the better will be the clearance between the bottom of the furnace and the shell. It should also allow more room for the welds shell to tubeplate and furnace to tube plate especially at the front where the furnace passes through the airspace.

Thanks once again for your very valuable input.

steamer:
Whats the plan for the weld prep and ends on the boiler?
Looks like flat ends....what kind of stays?......just curious....

 :)
Dave

Maryak:
Hi Dave,

Not to put too fine a point on it but my original PDF shows weld chamfers for the tube plate. Solid stays 5/16" dia.

Attached is a new PDF of the tube plates with 27 tubes. In addition I have done some checking as recommended by Sandy and here's the rub. My tube wall thickness is 0.028" so this gives me a X section of 0.1548 ins2. My grate area will be 10.98 ins2 so I should end up with a tube to grate ratio of around 3/8 which should satisfy the draught OK.

Navigation

[0] Message Index

[#] Next page