Hotwell position

[quinten1231]

Hi

I was just thinking on how to build my boiler in the future, and since i want to build a boiler without a water leg first i was thinking of maybe placing the hot well around the bottom of the boiler where the firebox would be.

Do u guys think it will get so hot that it will just boil and evaporate or would it be able to keep the temperature below boiling point.
It would be rather large so that i have enough space to pump in some feed water to warm it up before pumping into boiler... Lets say roughly 5 gallons or so (guessing wildly here)

The main point of this is that i don't want to build a water leg boiler for my first boat and thought that i might aswell use the lost heat to warm up feedwater so that it would enter the boiler at a fairly hot temperate therefore reducing the heat necessary to keep the boiler pressure up (I guess)

Of course the ring surrounding the boiler then would be covered with a safety lid or something of that sort to ensure nobody gets hurt, and reducing the evaporation.
I am sure most u will suggest to just build a water leg boiler in the first place, but I want to build a simple boiler for a small beginner boat.

Regards Quinten Versluis

[fredrosse]

Taking heat away from the furnace to heat unpressurized feedwater is not the way to go. Two better methods are to heat feedwater with exhaust steam, feedwater temperature approaching 200F can be had this way on an atmospheric pressure exhaust engine, without robbing any heat from the fire. Beyond that, an economizer coil on the outlet side of the boiler can heat the pressurized feedwater well above 250-300F, and these two methods provide good economy.

The furnace section (usually a wet leg on a VFT boiler) provides intense heating surface, if it is refractory lined (or firebrick lined), then the heat of the fire is available to the tubes in the VFT boiler. But if you surround the firebox with hotwell water, you will have way too much heating of the feedwater, and you will take away some fire heat that should be used to generate steam. The feed pump will probably not work with such hot water entering the pump.

Putting a pressurized economizer coil surrounding the firebox would be OK from an efficiency standpoint (downstream of the main feedwater pump), but those coils would tend to overheat unless a steady flow of feedwater was continuously assured. Better to just use a steam feedwater heater, and/or an economizer coil on the outlet side of the flue gas that has already passed thru the firetubes.

[quinten1231]

Ok, thank you.

Yeah im new to steam engines, was just trying to think of some designs, but now i know i can scrap that and just go the classic way.

Regards Quinten

[Lopez Mike]

I'd like to know why you are avoiding a wet leg. It involves only two additions to a simple VFT and can double the steam output. Surfaces "seeing" the flame are so much more effective in transferring energy that including it is well worth doing. Flues do transfer heat but not so well. Radiant heat is best. There are seldom any flames in a flue.

The building of the wet leg itself is trivial. Just a couple of rings and the firebox wall. No stays needed on a round boiler. The other change is the firebox door. Many of the boilers I've out together have a round or oval opening so that I can make the penetration out of ready made bits of pipe and plate and minimize any flat surfaces.

The only reason I can see for leaving off a wet leg is when you are trying to burn some fuel that is grate area limited like wet wood or some hard to burn coals. I have a wet leg and burn some rather indifferent woods (like anything that is loose at one end!). There are occasional moments when I pitch in some wet and rotten bug infested lump and things pause for a moment. But most of the time I need to throttle the air a bit.

The annular water space doesn't need to be very thick. An inch or so is plenty and provides good circulation. Also, it localizes any deposits so a quick blow down suffices.

Mike

[quinten1231]

Mike

That is very interesting, since i thought that the annular space for the water leg would have to be much bigger.
I also didnt know that effeciency would increase that dramatically, i knew it would increase but almost double the output, wow. Always thought a waterleg would be more of use in large steam plants with larger fireboxes.

Well then i guess i might aswell build a waterleg, i suppose the resell value of the boiler itself would rise a little aswell since most people seem to care about effeciency. (Guess I would aswell, but at the moment i have no clue how much steam would be used on a day of steaming)

Still have ALOT to learn.

Regards Quinten Versluis

[Lopez Mike]

I would be interested in what Fred has to say on this subject. One of the great things about this forum is the extent and variety of expertise.
Mike

[dhic001]

A wet leg is something I will always avoid if at all possible. Just take a look at a wet leg vft from the top, note the area of boiler that isn't close to a tube for most of the height of the boiler. Then take a note of the area of grate, and the available combustion room, can they be increased if you want to? No. Also consider that the area of the fire directly beside the wet leg will be cooler than the rest of the fire, because the heat is being pulled out of it before combustion has properly taken place. Not a problem when its a huge fire in a large boiler, but a problem in a small boiler with minimal grate area. Finally, consider the actual grate you fire on. What happens if you overheat your grate, simply it expands outwards quite considerably. If it can expand outwards, and a wetleg is in the way, either the wetleg is going to be damaged (unlikely) or your buckled grate will become a permenant fixture of your boiler.

Much better to have a separate dry furnace that can easily be changed if the fuel changes, or if you deside to alter the way you operate. Its also easily repairable if something untoward happens. Sure, it has to have insulation inside it, but this doesn't need to impinge dramatically on combustion space, as the firebox can be considerably larger in diameter than the boiler.

Everyone has there own ides, but the most successful steamer i've ever operated, (and i've operated a good number now) has a big diameter boiler, lots of small short tubes, a large dry furnace. The boiler steams like an absolute witch, comes into steam extremely fast, 25 minutes from cold to full working pressure on natural drafting, and will easily maintain full outfut while burning fairly average quality coal. The boiler is pictured here http://www.warkworthsteamboat.net/Album.html#26 on Kapanui. Please note that the boiler barrel ends just above the outlets shown, the lagging continues up around the smokebox. The furnace is slightly larger in diamater than the boiler, but also has 2 inches of lagging outside it, plus the sheet metal cladding. While it is considerably larger than most people would need, I would have a scaled down version in an instant if I had the finance for a new boiler for Zeltic.

Daniel

[Mike Rometer]

[quote] "What happens if you overheat your grate, simply it expands outwards quite considerably. If it can expand outwards, and a wetleg is in the way, either the wetleg is going to be damaged (unlikely) or your buckled grate will become a permenant fixture of your boiler." [quote]

I have to say I don't agree with this part. If your grate is so close a fit as to be able to expand that much, it is far to big to start with, and stands a very good chance of being jammed by ash or clinker, even without getting overheated first. Allow plenty of clearance to start with.

[fredrosse]

Mike and Daniel both make good points for consideration with VFT (Vertical Fire Tube) boilers. Both make arguments which are basically true, although some clarification is probably required here. So here goes another "long winded" writing from Fred:

A wet leg boiler does enjoy extra furnace heat transfer surface area, and with fuels having a high proportion of radiant energy transfer (coal and wood burners), this surface is very effective at absorbing heat. The enclosed furnace area does limit the size of the firing grate, and fire access through a necessarily small firing door can be limiting for some conditions. For any wet leg VFT boiler, the furnace must be at least about 3 inches smaller diameter than the outer shell, and this results in less area for installation of firetubes. In effect, the extra heat transfer surface of the wet leg is somewhat canceled out by the fewer number of firetubes that can be fitted.

Stay bolts between the water leg furnace pipe and the outer shell may indeed be required, although for most of our small steamboat boilers they would not be required. A circular pipe can withstand far more internal pressure than the same pipe with external pressure applied, as is the case with wet leg furnaces.

As to the concept of the fire grate expanding so much as to push against the furnace pipe, this is a possibility, however allowance of proper clearance will preclude this problem. A 24 inch diameter grate, heated to 1200F above the steam temperature, could expand and reduce clearances by as much as 3/8 inches, so just make the grate 1/2 inches smaller diameter than the furnace and there is no problem.

With respect to solids accumulation, the wet leg boiler is better, and will tend to accumulate “mud” in the lower area, and this should be below the furnace area of intense heat (below the grate). The VFT without a water leg tends to accumulate this material on the lower tubesheet, just where the fire heat transfer is at its maximum. In either case proper blowdown procedures are required to prevent accumulation of solids within the boiler, and either arrangement can give long service life.

As far as construction considerations are concerned, the simple VFT (similar to the classic “Stanley Steamer” boiler layout) without a wet leg is much easier to construct. Basically a single shell pipe, two tubesheets, and a bunch of tubes.

While the wet leg boiler only adds the furnace pipe, lower “Mud ring”, and fire door penetration, these are not so easily accessible for fabrication and welding. In particular, the firetube to furnace tubesheet joints can be troublesome, rolling the tubes in place and welding them involves very close quarters within our small boilers, vs. a very easily accessible task for the Stanley type VFT layout. This means that there is a generally higher probability of tube rolling/welding errors for tubes inside the furnace, near the furnace pipe. These errors are also much more difficult to correct.

The simple “Stanley Type” VFT has both advantages and disadvantages with respect to the furnace. While it can be made larger to suit any fuel, it also involves supports and insulation far more complicated than is required of the wet leg design. The insulation in this area must be “firebrick” or other high temperature insulation. A wet leg VFT can get away with simple wood strips over the entire boiler shell, except for the ash pan under the boiler, which needs some insulation and ventilation to avoid burning a hole thru the boat hull. But either type boiler needs this protection for the hull.

In terms of construction costs, the VFT with water leg will cost at least twice as much in terms of fabrication labor. Materials costs are just about equal as far as the boiler pressure retaining part is concerned. Some of this extra cost for the wet leg boiler would be offset with the labor and materials needed for a furnace in a non-wet leg VFT.

In conclusion, for me it was nearly a flip of the coin, I would have wanted a wet leg boiler, but the ASME Code shop wanted thousands more to make that, compared to the simple boilers (Stanley type) that they routinely manufacture. I am happy with my simple boiler on the Margaret S. but then I burn propane, so tending the fire is not any issue for me.

[S. Weaver]

I'm building two wet-leggers right now, in addition to my scotch. I echo what Fred says about complexity ...