steam boiler fo boat

Hello, new on this sight. I have been into steamboats for some 15 yrs. and have been building one for just as long. (a large steamboat) Am now ready to build a boiler and have recently been looking into rocket stove technology as an option. Am intised by this technology because the firebox is outside the generating tube area and the gassas could easily be routed around them during stand-by or emergency situations.
On the down-side, this means that the tubes will only be heated by convection and not radiation. This in turn would call for more generating tube surface, convective gassas around 1200 deg. and ample velocity of gasses.
From reading about rocket stoves, I gather that the gasses from the fire box want to slam into the bottom of the riser in order to create turbulance and then as it exits the riser it again slams into a top-plate surface. This slamming and the additional torres created outside the riser help create a clean burn and is possibly a main facter in the high temperatures of the gasses. O.K. so far assuming this is correct thinking.
What I am wondering is weathwer this slamming reduces velocity. What would theoreticly happen if there was a gentle curve transitioning the fire box to the riser. I am imagining a design that would put another gentley curving elbow at the top of the riser that would disperser the gasses onto a rectangular generating tube configurationin of a down-draft type? Am also wondering about riser heighth and the roll it plays in increasing velocity and or heat? If it is kept to a minimum I am wondering weather the top surface of the generating tubes might actually see some radiant flame but perhaps at the loss of velocity?
The floor is now open for opinion!

Richard, this post was a long while back. Clearly I missed it. I have also considered a rocket stove for a steam boiler. In my case, I believe it should be done with the drum in place and with the drum highly insulated (of course). Placing a fairly large monotube steam generator in the annular space between the insulated fire tube and the insulated drum would cool the gases to induce the flow for the system. There would be a counter flow heat exchange with the water feed pump supplying the bottom of the tubing coil. I considered that a lot of radiant heating can be provided by concentrating several tubing coils at the top of the drum above the rim of the fire tube, and this should provide some superheat. The design I imagine would use fairly large tubing to provide some reserve capacity, so it would be functionally more like a water tube boiler - but with a monotube configuration. I think it can be made to work exceptionally well for fairly low pressure steam under 500F or so. Normally, you want a lot of surface area exposed to the flame for radiant heating, but this is really only emphasized where the steam generation rates desired are very high for the size of the steam generator (i.e. automotive applications). However, this is not required by any means. As long as the temperatures of the combustion gases are high enough, the boiler insulated well enough, and the heat transfer surface area exposed to the furnace gases are high enough (and assuming good turbulence for good heat transfer), then I believe it can be made to work well enough within limits. I would keep the drum and insulate it inside and out while highly insulating the fire tube (such as using fire brick or insulating furnace cement) to get extremely high temperatures and a powerful draft for a lot of turbulence.

All that you have stated I agree with. If; however, one is using the boiler in a stationary capacity ( speaking in terms of theoreticly ideal ) and primarily for the purpose of generating steam, then it seems optimal to place all generating tubes above the heat riser and continue the flow of gasses upward. That configuration would allow for the greatest velocity.It is when using a boiler in a marine application that over all shape has a bearing on design due to the need of acheiving a low center of gravity. In other words, I see no advantage in using the drum unless on is using the furnace for home heating also.

richard orr wrote:All that you have stated I agree with. If; however, one is using the boiler in a stationary capacity ( speaking in terms of theoreticly ideal ) and primarily for the purpose of generating steam, then it seems optimal to place all generating tubes above the heat riser and continue the flow of gasses upward. That configuration would allow for the greatest velocity.It is when using a boiler in a marine application that over all shape has a bearing on design due to the need of acheiving a low center of gravity. In other words, I see no advantage in using the drum unless on is using the furnace for home heating also.

The reason I considered that configuration is to use fairly large tubing coil for a high reserve capacity, the drum is an easy way to achieve a counterflow exchange, and a simple coil can be placed in the annular space between the fire tube and drum with water supply and steam outlet penetrating the top of the drum. My concern is primarily a system that is easy to build. This is more like a water tube boiler rather than a monotube steam generator. I believe this can be built very easily.

First of all please understand, unless you are recirculating the water, the configuration you describe is a once through monotube. A water tube is not defined by diameter of pipe or necessarily volume of water. Also keep in mind that you are talking about generating steam, therefore you have to adress latent heat, I.E. much more heat needed.I agree about the configuration you described as being simplier in construction, but it also means that you will need much more square footage of tubing if you go this rout. Every inch the tubes are from the fire, the more heat will be lost. Also every bit of loss of velocity will reduce the amount of heat transfer to the tubes. One of the critical questions to be asked is how many pounds of steam per hour do you need to generate and at what pressure?

richard orr wrote:First of all please understand, unless you are recirculating the water, the configuration you describe is a once through monotube. A water tube is not defined by diameter of pipe or necessarily volume of water. Also keep in mind that you are talking about generating steam, therefore you have to adress latent heat, I.E. much more heat needed.I agree about the configuration you described as being simplier in construction, but it also means that you will need much more square footage of tubing if you go this rout. Every inch the tubes are from the fire, the more heat will be lost. Also every bit of loss of velocity will reduce the amount of heat transfer to the tubes. One of the critical questions to be asked is how many pounds of steam per hour do you need to generate and at what pressure?

Yes, that classification is strictly true. I do not disagree. I wished only to emphasize that a small monotube configuration with a large diameter tube changes the dynamics in many respects and loses some benefits that an ideal monotube can provide in other settings. As you emphasized, heat transfer rates would be lower. In my particular case I'm considering a stationary unit operated at a more or less constant output with steam generation rates less than 30 lbs/hour at 250-350 psig. Tubing is 304 stainless with 4.9 square feet surface area (50 feet of 3/8" tubing). I hope to concentrate several coils near the top of the fire tube for exposure to some radiant heat and the highest temperatures. Again, this approach is all about simplicity in fabrication. It just has to work. If what I've described is not suitable for the purpose, then please offer feedback.

Also, I have considered this configuration primarily as a steam generator to provide heating applications (saturated steam at little more than atmospheric pressure), but believe it can be configured for powering a small piston steam engine using saturated steam at modest pressures. However, I don't generally consider small steam power to be practical (even though it's interesting).

BTW, there is another configuration that you might consider interesting (although, it's more complicated). I considered a steam generator fired by a gasifier (much like a FEMA-type fire tube mounted on a refractory base to route the fuel gases to the combustion chamber). The combustion chamber is completely surrounded by a tubing coil for maximum radiant heating, and the base of this coil is connected to a tubing coil in the flue positioned for counterflow heat exchange and turbulent convective heat transfer. Also, the flue and combustion chamber shroud is surrounding by an insulated outer shroud. The annular space between the flue and outer shroud is used to preheat the combustion air that is drawn down to the base of the combustion chamber to mix with the incoming wood gas. This configuration allows for providing a tall flue for a lot of draft that could be sufficient to draw air through the system at a fairly high rate, and the high radiant heat and especially the air preheating should see a good turndown ratio. Use damper(s) to control the output.

How many h.p do you design to have ? What are the particulars of the engine you intend to use?

With gasification there is a loss of effeciency in terms of heat delivered. It doesn't mean that it's a bad idea all-together but does raise questions as to why one intends to use it. There could, for example, be a hightened control in terms of being able to isolate the fire from the generating tubes or perhaps a gain in the ability to have a cleaner burn when using wetter fuel.

richard orr wrote:How many h.p do you design to have ? What are the particulars of the engine you intend to use?

In my case I've considered only a single cylinder uniflow bash valve configuration for its simplicity (limited to 1 hp at 500 rpm and 400 psig steam). It has a 1:4 nominal expansion and is a closed system - condensing. I can't get into the specifics of its design, but it is very simple.

In general, I think small scale steam is not practical. However, there is one configuration that I believe can be very useful in some off grid settings. I'm considering a system fueled by fire wood or stick wood (biomass with little processing) and set up to run at a more or less constant low output for battery charging and extensive waste heat recovery. The design I've considered emphasizes simplicity above everything else and ease in maintenance and operation. It just has to work, and it has to be easy to fix. I don't care much for high "efficiency", but I do hope for 7% net thermal efficiency.

richard orr wrote:With gasification there is a loss of effeciency in terms of heat delivered. It doesn't mean that it's a bad idea all-together but does raise questions as to why one intends to use it. There could, for example, be a hightened control in terms of being able to isolate the fire from the generating tubes or perhaps a gain in the ability to have a cleaner burn when using wetter fuel.

I disagree. The thermal losses in gasification generally come from cooling the fuel gases for use in powering I.C. engines. In this case the hot pyrolysis gases need not be cooled. Also note that sending hot pyrolysis gases to the combustion chamber to mix with preheated air should achieve higher temperatures in the furnace. With radiant heating (the fire box surrounded by tubing) this configuration should see higher heat transfer rates.

Note that I realize there are all sorts of sophisticated ways to optimize things, but I don't consider configurations that are outside my price range. I'm interested only in configurations that might be possible with very limited funding. If it's not simple, then I'm not interested.

When you talk about gasification do you mean partial or complete combustion?

richard orr wrote:When you talk about gasification do you mean partial or complete combustion?

Both. Complete combustion in the furnace where hot fuel gas mixes with excess preheated air, and incomplete combustion in the fire tube (where the wood fuel is pyrolysed and reduced to generate the fuel gas). The fire tube for the gasifier is insulated and next to the combustion chamber. The hot fuel gases transfer directly to the base of the furnace (which is also highly insulated, of course). Think of it as an updraft gasifier furnace with the fire tube inverted.

http://www.youtube.com/watch?v=-ukKHTgiNsE Simple video that illustrates principles of gasification well.

If I am following you correctly, you are ending up with char at the end of the process? If so that char represents unused btu's for a given amount of fuel.

richard orr wrote:If I am following you correctly, you are ending up with char at the end of the process? If so that char represents unused btu's for a given amount of fuel.

Gasifiers don't normally generate a lot of char. Now, it's possible to configure a gasifier to increase char formation. Conversely, it's possible to configure a gasifier to generate almost no char or to consume any char that might form and drop below the grate. For example, some air can be admitted below a secondary grate that would catch any char that forms. It's also possible to use an updraft configuration where a hopper contains the fuel that drops onto the grate with the pyrolysis gases shunted to the attached furnace to be mixed with preheated air, and I've seen a system like this. It worked well on small wood splits. Seriously, char formation is not a problem at all.

O.K. Sounds like a do-able design stratigy. It will even allow for a a certain safety facter as you will be able to more control and isolate the fire. Steady burning solves a lot of the inhearent problems with monotube configuration and larger tube dia. does increase the ratio of water to heat surface thus settling the fluctuations down some. What is the perentages of radiated heat surface to convected heat surface?

richard orr wrote:O.K. Sounds like a do-able design stratigy. It will even allow for a a certain safety facter as you will be able to more control and isolate the fire. Steady burning solves a lot of the inhearent problems with monotube configuration and larger tube dia. does increase the ratio of water to heat surface thus settling the fluctuations down some. What is the perentages of radiated heat surface to convected heat surface?

For myself, I haven't considered the details of anything more sophisticated than my 1 hp steam engine battery charging/cogeneration system for a while. I suppose all I wanted to emphasize is that using a gasifier with preheated combustion air can show high performance. For example, if I were devising a steam boat, then I would go this route. Still, using a monotube with wood fuel would be difficult for anything with a highly variable load (hence the use of large diameter tubing). The approach that I would try first would be a system that uses regularly sized small wood splits and stick wood loaded onto a grate contained in hopper with an insulated base. The hopper is closed at the top, and air enters the base of the grate. The steam generator/boiler unit is connected on the side of the hopper such that the pyrolysis gases generated in the insulated base of the hopper are pulled into the base of the furnace and mixed with preheated air drawn in from the annular space surrounding the flue (discussed earlier). The reason why I like this approach is it avoids having to chip wood. I like a system that doesn't need anything more than an axe and perhaps a saw to process fuel. As far as the steam generator/boiler goes, the first thing I would try would be to place a simple coil of large diameter tubing in a cylindrical combustion chamber such that the tubing provides a water wall for radiant heating. Smaller tubing is placed in the flue for efficient convective heat transfer (counterflow), and the small tube connects to the base of the large tubing coil. Steam is taken off the top of the large tubing coil through a penetration in the shell. The combustion chamber and flue is surrounded by an insulated shroud that directs combustion air down to the base of the unit where it mixes with the pyrolysis gases pulled in from the gasifier.

As I recall, I did consider a small steam bleed valve on the boiler that is actuated when the engine throttle is closed. The purpose here was to provide a constant load on the furnace. I mention this because you considered diverting furnace gases away from the boiler on engine standby. It seems powering the feed pump with an electric motor would be preferable, then dump steam through the bleed valve when the engine shows no steam consumption. Most steam boat systems I've seen have 2 or 3 different ways to feed the boiler, so this seems reasonable to me. Besides, if you're going to have a steam boat with a good size, then you really should have an electric system for other systems (lights, etc.)... using a small electric motor for the feed pump makes sense.

I also considered a means to control automatically the dampers for the system by using a copper tubing heat exchanger filled with thermal oil to position a small hydraulic actuator against spring tension. The idea here is that reducing steam demand will increase the temperature of the flue gases, and this can be used in a feedback control system. Things sure are simpler with a large fire tube boiler, but these are heavy, expensive, and inefficient... and dangerous.

There are other problems (like boiler water level) that are problems. The whole thing would be a serious challenge.

When running a steam engine at realitively constant demand, water lever (feed rate in a monotube) can be propperly adjusted by a variable stroke feed pump.
I quite understand about your desire for simplicity. that and safety are my highest prioritys. I do believe, though, that you would experience better results by winding a coneacle configuration above the riser tube thus keeping the gasses flowing at the highest possible velocity. Such a configuration would allow for the greatest radiant heating surface to be achieved.
An electric pump does have the advantag of being run independent of weather the steam engine is running. Understand though that when speaking of marine use in a realativly smaller boat that horse power is usually marginal and there is a percentage of power lost by converting mechanicle to electrical. It is also a wetter invironment to opperate on. I personally would only use it for a stand-by mode.

richard orr wrote:When running a steam engine at realitively constant demand, water lever (feed rate in a monotube) can be propperly adjusted by a variable stroke feed pump.
I quite understand about your desire for simplicity. that and safety are my highest prioritys. I do believe, though, that you would experience better results by winding a coneacle configuration above the riser tube thus keeping the gasses flowing at the highest possible velocity. Such a configuration would allow for the greatest radiant heating surface to be achieved.
An electric pump does have the advantag of being run independent of weather the steam engine is running. Understand though that when speaking of marine use in a realativly smaller boat that horse power is usually marginal and there is a percentage of power lost by converting mechanicle to electrical. It is also a wetter invironment to opperate on. I personally would only use it for a stand-by mode.

Makes sense. In particular, achieving a conical configuration above the riser should be easy to accomplish, and therefore justified by my "KISS" philosophy (i.e. Keep It Simple, Stupid). As far as water feed goes, since I'll be operating within a very narrow power range, I will be using a simple plunger feed pump driven off the cross head of the engine. With the bulk of the water volume in the system contained by the steam generator, this will keep water level there constant as long as the pump output is slightly higher than required. BTW, I will be emphasizing a very tight system with high temperature sealing on all joints, and there will be no oil lubrication (so no blowdown required). I have a simple way to verify boiler level, and there will be a simple means to replenish water if necessary. Sorry if I seem cryptic, but I've worked out a lot of details on the engine that I'm not willing to share. BTW, check out my most recent post as I mentioned two micro gasifiers that will be coming to market soon. The design on these is unique, and I'm optimistic about their being able to generate a tar free fuel gas at very low outputs.

What kind of steam valve do you have for your steam engine? If it is a D valve, they don't want very much super heat....perhaps + 50 degrees Also what r.p.m. are you going to run at? If high r.p.m.'s then make sure your check valves are plenty large or you may experience hammering in a monotube.

richard orr wrote:What kind of steam valve do you have for your steam engine? If it is a D valve, they don't want very much super heat....perhaps + 50 degrees Also what r.p.m. are you going to run at? If high r.p.m.'s then make sure your check valves are plenty large or you may experience hammering in a monotube.

Thanks for the advice. I can't discuss specifics of the valve I'm using, but it is a bash/bump valve (piston operated). My system is designed to operate normally at 6 hertz, and will never exceed 500 rpm (8.3 hertz). Yeah, those D valves don't do good with superheat (not without generous lubrication)... so good question - thanks for watching out for me. I have a lot of specifics for my system worked out, but I expect problems to pop up as they always do in the real world. I've pussyfooted around with this design for a few years now, and I'm still not in a position to start fabrication. However, I've done enough research to git r done fairly quickly once I start. If it doesn't prove viable, then I'll be going the gasification route which I am 100% confident I can do. The reason I consider steam at all is because I believe it can be superior under the conditions I've described. One thing I've discovered first hand is even seemingly simple engineering projects like this often get very expensive very quickly, so a lot of research is necessary for people like me who are unwilling to risk a lot of money. I accept any advice I can get.