Minimum Tube Diameter

[Brenton Baker]

What seems to be the practical minimum tube diameter for boilers?
After buying and reading Audel's Engineers and Mechanics Guide (1938) and Steam Boats and Modern Steam Launches, and attending the Great Delta Steamboat Meet (videos and pictures when I get editing software), I'm redesigning the engine and boiler along more practical lines.
I was originally considering propane atomizing diesel, but it seems that oil burners are loud. I am considering straight propane, though for purposes of the thread I have to wonder whether anyone has experience with soot from a wood, coal, or oil fire affecting efficiency noticeably; I do not recall anything about this in the aforementioned literature, nor have I been able to find an in-depth discussion elsewhere in the forum.

To restate: what is the practical minimum ID for boiler tubes, and (how) is it affected by the type of fuel on which the boiler is designed to run?

[DetroiTug]

What seems to be the practical minimum tube diameter for boilers?

It depends on the fuel and how it is fired.

Typically oil and gas fired boilers can use small firetubes - down to 7/16" as many of the early steam car boilers had (Google Bourdon boiler). Sooting was not a problem as long as the gasoline/oil fired burners were tuned properly and clean.

For wood and coal- solid fuel, a larger tube is desirable. In the range of 1" -1-1/4" ID. These fuels can soot easily and smaller tubes can become restricted quite fast with slow drafts and/or green woods.

I burn wood, but I'm also exhausting up the stack which in turn creates a very high velocity forced draft, which provides almost complete combustion. Plus with typically burning well seasoned wood, sooting up has not been experienced to any extent. I cleaned the flues last fall and got about a half cup of carbon out in total. I don't recall any big change in performance after. That's one of the downsides of using wood, No two sticks are identical in their BTU output, so making changes to the system, one is not sure if it's the change or the wood being used presently providing a result. And it's the lowest BTU output of all the aforementioned fuel types per pound. My opinion, but I think it is the best for the boiler as it is slow to rise in temperature and slow to cool down.

[barts]

Oil burners don't have to be loud; it depends on the volume and pressure of the steam atomizing jet. You need to bring sufficient air into the boiler with the steam/oil jet; placing the burner at the end of a tube will help your burner draw in more air.

The trick is that the volume of steam controls speed of the jet and the amount of air carried along; many burners do a poor job of drawing in air so they use large amounts of steam (and resulting noise) to draw in the required combustion air.

Above a certain level, using a stack blower is a quieter source of additional draft.

I start my steam atomizing burner using propane to spray the oil; this works well. It would definitely take a different 'tune' to use this all the time; getting the right propane/oil/air mix under hard firing is difficult w/ my current burner.

Remember, when building a oil burner leave sufficient flame space; flames impinging on cool boiler surfaces will always produce smoke & soot.

- Bart

[fredrosse]

See a previous thread: "Fire Tube Diameter Question"

If you are burning Propane, smaller diameters would be better, down to about 3/8 inch. The smaller diameters give more heat transfer surface area per unit volume of the boiler proper, and smaller tubes generally enjoy somewhat higher heat transfer coefficients. The Stanley steam automobiles of 100 years ago used 31/64 inch OD tubes, gasoline or kerosene fired. In a simple VFT with 1/2 inch outside diameter tubes, 0.049 wall, with a shell 16 inches in diameter and 12 inches high, 35 square feet heat transfer surface area can be had.

Burning wood or coal, as previously stated, the soot problem becomes significant, so probably 1-1/4 inch OD x 0.095 wall recommended minimum. This is for firetube boilers, but also similar recommendations for watertube boilers. For comparison, the Margaret S has a simple VFT boiler with 1-1/4 inch outside diameter tubes, 0.095 wall, with a shell 16 inches in diameter and 18 inches high, but only 20 square feet heat transfer surface area.