The Steamboating Forum

Not getting into the debate, but just quoting from the ASME rules, from a previous post on this forum:

"Perhaps the best perspective as to steam generating capacity of various heat absorbing surfaces in power boilers is the current ASME Boiler and Pressure Vessel Code. When determining the required safety valve capacity, the code requires consideration of the following rules:

Steam Generating Capacity in Pounds Per Hour (PPH) based on 1 square foot (non furnace) heating surface.

Hand Fired Solid Fuel Boilers – Firetube 5 PPH, Watertube, 6 PPH
Stoker Fired Boilers Firetube 7 PPH, Watertube, 8 PPH
Oil, Gas, PC Fired Boilers Firetube 8 PPH, Watertube, 10 PPH

Waterwall / Furnace Surface, receiving radiant heat from fire.

Hand Fired Solid Fuel Boilers – Firetube 8 PPH, Watertube, 8 PPH
Stoker Fired Boilers Firetube 10 PPH, Watertube, 12 PPH
Oil, Gas, PC Fired Boilers Firetube 14 PPH, Watertube, 16 PPH

From these numbers you can see that radiant heat absorbing surface carries much more evaporation potential than ordinary tube surface that does not “see” radiant heat from the fire. Note that one could easily build a firetube boiler with a big water cooled/boiling water furnace that would out-perform a typical watertube boiler that has allot of ordinary (non-radiant) tube surface."

Another issue that is not addressed here is the radiation heat transfer potential based on the fuel used. Propane and anthracite coal flames are very poor at emitting radiant energy, while wood and bitiminous coal flames are much better. An oil flame is generally best, giving off much radiant energy. My propane fired dry leg VFT cut fuel consumption (for the same steaming capacity) in half after I added turbulators. The hot propane gas heated the turbulators red hot, so the firetubes then got some radiant energy transfer in addition to convective heat transfer.

I suspect, what with my less than focused job of hand firing, I seldom reach 5PPH.

Fred,

Thanks for that info. As I stated early on in this thread, I merely question how much a small waterleg is doing and we shouldn't be so quick to discount a dryleg VFT simply because it is one. I never said I was ready to hotwheel mine off .

The ASME numbers as presented don't really say how they were arrived at. Was it natural draft or with draft induced which makes a significant difference. Were turbulators in the tubes considered? Same result here, when I installed them, there was a marked improvement. One would assume their test was performed just as boilers come from whomever built them, natural draft and nothing in the tubes as those are really performance modifications.

The numbers indicate that the furnace wall seeing radiant heat is 37.5% better at generating steam. By Fred's account there was a 50% increase in efficiency by adding turbulators in the firetubes. For sake of discussion, lets remove the water wall, add another ring of tubes which in some designs would provide more heat transfer area than the waterleg would, then add turbulators and induce draft . As I said earlier in the thread it's "case by case", but I think in some configurations, the dryleg VFT will do better with more heating surface and larger fire.

Thousands of steam cars were built with a dryleg VFT, there is not one production steam car that I know of that used a waterleg of any sort. The most successful steam vehicles used a watertube of various design with complex control schemes. These folks were very familiar with boiler design as there are hundreds of different types of boiler designs that came out of that era. Those VFT's had many hundreds of tubes and some over a thousand.

-Ron

There are really two issues being discussed here, a) the benefits or not of a wetleg and b) tube size and the need for tabulators or retarders as I know them.

Here's my two cents worth, based largely on operating a variety of dry furnace vertical firetube boilers, and some smaller wetleg variants.

A) Wetlegs or not. in my view, a small vertical wetleg boiler has a number of disadvantages over a dry furnace type. The wetleg itself restricts grate area and combustion space to a diameter smaller than the boiler. grate area and combustion space can be critical with some fuels, and having a wetleg prevents this area being increased. My furnace on Zeltic is larger in diameter than the boiler, and has as a result a larger combustion area, which helps with the poor coal we deal with. The wetleg is also colder than the fire beside it, so it naturally tries to suck heat out of the fire, making the ring of fire beside the wetleg less inclined to burn well. The fire in a dry firebox is very even in burning. The wetleg also reduces the number of tubes available within the diameter of the boiler, and creates a large volume of water above the wetleg that isn't seeing much heat from the tubes.

B) Tube sizes and retarders. I've run VFTs with boiler tubes of 3/4 inch ID up to 1 3/4 inch ID. In my experience it is only the larger tube sizes, over 1 1/2 inch ID, that need retarders. Zeltic has 1 1/4 ID tubes and the retarders actually have a negative effect, as they reduce the draft and contribute to soot build up in the tubes. As a result the draft enhancing blower is used more often. The same performance can be achieved without any blower with no retarders in the tubes. By contrast another boiler with 1 3/4 ID tubes was hopeless without retarders, the gasses just raced up the middle of the tube and out the top. For no extra coal, the boat speed increased by at least a 1/4 by using retarders, although soot blowing needed to be more frequent. So the conclusion is, have smaller tubes, no need for retarders.

The smaller tubes combined with no wetleg results in a lot of effective heating surface within the diameter of the boiler. Now if we make the diameter as big as possible and keep the tube size down that results in a lot of tubes, and a very effective boiler. The best steaming boiler I've ever operated is the big dry furnace VFT boiler in S.S. Kapanui. The boiler has I think about 125 1 inch ID tubes, all only 25 inches long. It provides more than enough steam for the 30hp Simpson Strickland compound driving a 37 foot boat. It is also a fast steamer, coming into full steam in 25 minutes if we set a good fire. If i were able to reboiler Zeltic, i could just build a smaller version of it as it is by far the most effective boiler of its type.

Daniel