The purpose of this project is to design and mass-produce kits for a floating tiny house that can sail. It combines high-tech modeling and fabrication and low-tech assembly that can be carried out DIY-style on a riverbank or a beach. This boat is a 3-bedroom with a kitchen, a sauna and a dining room. The deck is big enough to throw dance parties. It can be used as a river boat, a canal boat or even a beach house. Oh, and it's rugged and stable enough to take out on the ocean. Kits will start at around $50k (USD). The design has been tested in simulation and prototype; full-scale production will begin next year.

Tuesday, February 27, 2018

Marine Russian Stove, Take 2

[Update: I added a schematic of the hot water plumbing system and improved the description of how it will operate.]

Thanks to all the feedback I received for the previous iteration of the design, it is much improved.

The basic goals of this design are as follows:

• The stove has to be cheap and easy to fabricate as a kit out of sheet metal stock using a plasma cutter and a brake.

• It should be easy to assemble the kit using a few hand tools: wrenches, screwdrivers and a pop riveter.

• It should be able to burn either wood or charcoal.

• It should also make it possible to fit a propane burner as an option. (Some marinas do not allow solid fuel stoves.)

• It should provide heat to a cooktop, a water heater and an air heater. Each of these may or may not be in use at any given time.

• It should produce negligible amounts of radiant heat (except from the cooktop, and only when it is in use)

A lot of people commented in favor of heating a thermal mass, and this is indeed a good idea. Thermal mass heaters, such as the traditional Russian Stove, use masonry to hold heat. But masonry doesn’t have a place aboard a boat, and hiding a few bricks inside a sheet metal stove won’t make much of a difference. However, there is one substance that is better than masonry at storing heat, and it’s free. It’s seawater. It takes 4.18 J/gK to heat seawater, while soapstone—the preferred material for solid mass storage—is only 0.98 J/gK, so its more than four times worse at holding heat.

Quidnon holds 5 tons of water ballast, and the best way to store heat aboard a Quidnon is to heat the ballast water to some reasonable temperature, such as 15ºC (60ºF). Running salt water through the water heater will cause it to crust up with salt deposits that can only be removed mechanically, using a grinder. It is much better to reinject heated fresh water from hot water tanks into the fresh water bladders floating inside the ballast tanks. This would also heat the surrounding ballast water, producing the same effect.

But this was part of the plan all along. The following realizations, however, are new, and demonstrate the power of the design process we’ve been following on this blog, where experienced people contribute many excellent ideas.


The first very important realization, thanks to Rhisiart, is that the stovetop needs to be directly above the firebox in order to produce sufficient temperatures for applications such as stir-frying and generating steam for the sauna (via sauna stones). But the stovetop doesn’t need to always be hot. When the stove is being used to heat the interior of the boat (via hot air) and/or to heat water, it should be possible to close off the stovetop using a sliding baffle.

The second very important detail, contributed by Jef, is that it is very important to be able to completely seal off the firebox from the interior airspace and to feed the fire by taking in outside air. I already knew how important this is: lighting a charcoal stove on board on a stormy night would sometimes send ash and lighter fluid fumes flying into the cabin. The trick was to catch lulls to light the flame, because once the draft was established wind gusts no longer mattered much.

This has prompted me to follow Jef’s advice and to add two air injectors. The bottom air injector is used for stoking the flame and can be driven by a small fan. Once the fire is burning, the top injector is used to produce combustion of the flammable gases emitted by the wood at just the right spot. The flow rate for the injector can be turned down, to keep a couple of hardwood logs burning all night, or turned up, to rapidly heat the stovetop and/or a lot of water.

Other parameters of the design were relatively easy to derive. A 3-inch flue is reasonable for a boat-based stove (I’ve used it successfully before). The cross-section of the flue has to remain the same both inside and above the stove. There is enough space available on Quidnon for a 16-inch-wide stove (plus a couple of inches all around for rock wool insulation and an exterior jacket made of either aluminum or stainless steel sheets pop-riveted together). Thus, the width (w) of the flue box inside the stove is 16 inches, and its depth works out to

πr2/w = 3.1415 * (3 / 2)2 / 16 = 0.44 = 7/16 inches

The most common size of firewood is 16 inches long, so the firebox is 17 inches deep. It is reasonable to keep the profile of the firebox reasonably square, so it is 16 inches wide by 16 inches tall. The ash box at the bottom and the stovetop compartment at the top, above a baffle, bring the total height to around 30-32 inches.

The reason the section below the stovetop is taller than at first appears necessary is because of the threshold. Thresholds are commonly used inside stoves to keep in the hot gases, raising the combustion temperature and making for a more efficient burn. There are two thresholds: one in the firebox, and one below the stovetop. The front opening of the stove (which is fitted with a door that seats tightly against a gasket) is sized so that the top of the opening is just below the bottom of the threshold, to keep combustion gases from escaping into the cabin while loading firewood. The ash box is similarly sealed from the inside air.

There are two heat exchangers: air and water.

The simplest and cheapest heat exchanger design is one with hot flue gases on one side of a metal plate and the medium to be heated on the other. For the air heat exchanger, just as with the flue, it is important to maintain the same cross-section both outside and inside the stove, and a 3-inch duct is reasonable for piping hot air under the cabin sole and distributing it throughout the boat. This translates to the depth of the air heat exchanger also being 7/16 inches. It may at first seem strange that the cold air is pumped in at the top, since hot air rises, but this makes no difference since it then has to be forced under the cabin sole anyway.

For the water heat exchanger, the flow is much slower because of the very high heat capacitance of water, and matching the cross-section of the input and output pipes to the cross-section of the heat exchanger is unnecessary. Above the stovetop, the flue forms a square box, insulated on the front. It shares its back wall with the water heat exchanger box, which has a couple of nipples welded to it for letting water in and out.

It is a given that the water heater will generate some amount of steam, and this steam has to be released. It is also a given that quite a lot of the time the water heater will not be used, and the stove’s sole job will be to heat air. This will make it necessary to vent the steam and to let the heat exchanger stand empty.

For this, I am thinking of doing the following.


A demand pump set to 16 psi injects cold water from the fresh water tanks directly into the hot water tank until the air release valve located near the top of the tank starts spitting water and is manually closed. The air release valve is connected to a nipple located a couple of inches from the top of the hot water tank, leaving an air pocket at the top that allows it to operate as an expansion tank, absorbing excess pressure periodically generated by steam. The demand pump keeps the tank permanently near full and pressurized as water is drawn from it. So far, this is just a pressurized cold water system with a reserve tank.

When the stove temperature sensor reads above 80ºC, a second pump starts injecting water from the hot water tank into the top of the heat exchanger. This is not a demand pump but a simple circulator pump, and it doesn’t particularly care what pressure it’s generating (up to a point). The heated water drains out of the bottom of the heat exchanger and back into the hot water tank through a check valve. It keeps running until a temperature sensor in the hot water tank reads above 80ºC.

It is at that point—when the circulator pump stops running—that an interesting series of events has to unfold, because the remaining water trapped inside the heat exchanger starts to boil and generate steam. Steam pressure forces most of the water remaining in the heat exchanger down into the hot water tank. As steam pressure in the heat exchanger continues to increase, the pressure relief valve opens and vents the steam.

As steam is being vented, pressure in the heat exchanger falls below 16 psi but the check valve keeps water from being forced back up into the heat exchanger. The heat exchanger then stands mostly empty (if the check valve leaks a bit, the pressure relief valve will periodically produce puffs of steam) until the hot water tank cools down below 80ºC, and if at that point the stove is still above 80ºC the circulator pump starts squirting water into the heat exchanger again. After some amount of hissing, during which the heat exchanger generates steam, the hissing stops and water starts being heated again.

35 comments:

  1. Instead of a multi-slot baffle, I'd be inclined to try a single exit hole for the gases, from the fire-box to the top box, placed at the front end of the baffle plate, so that gases go from the burning logs to the forward end of the baffle, up through the open gap - sized to equal the standard x-sectional area of the flow path, and with a liftable cast-iron hot-plate directly above it.

    As long as there's enough draw in the exit chimney from the top box, you should be able to lift the plate off, to cook directly over the flame, without gases escaping into the cabin. When I do this, I can see in-going air curling the small amount of visible smoke back down from the opening and carrying it away towards the exhaust flue - which is hot and drawing by this time. Necessary to keep the exhaust flue well cleared of soot/creosote build-up, though, to maintain a good strong draw like this. I have an easy-access exhaust chimney that just needs the rain-hood lifting off and a quick scrape with a long-handled tool about once a month at the moment, when I'm running continuous burning through the Winter. 3" diameter flue pipe is about the smallest practical size. 4" has always served me better; 5" is often recommended for a 'family-sized' stove.

    I've tried wide-faced, narrow-gapped heat exchangers before. They work, but it's important to be able to get a scraper inside them easily, without any awkward dismantling, to keep them clear. Soot can build up easily in this x-sectional shape.

    ReplyDelete
    Replies
    1. I can see how a round hole in the baffle that can accommodate a Wok would be useful for stir frying. But I think it's a more important requirement to block the heat from reaching the cooktop when it isn't being used, so a slotted design makes more sense.

      I do see the need to be able to scrape the crud off the walls of the flue and have it fall down into the ash box. There will need to be a component that changes from a slot at the bottom to a 3" pipe at the top, and it can be made to be easily removable.

      Above deck, the entire chimney will be removable for cleaning, so swabbing it out won't require acrobatics; just hang one end overboard and swab away.

      Delete
  2. For many years I preheated hot water in an uninsulated tank by a thermo-syphon from a loop passing through my wood stove. While the tank (just a stripped electric HW tank) had a pressure relief valve as any HW tank, it never had a need to release any pressure. The uninsulated tank let go enough heat to preclude ever boiling. I wonder if you might use this principle to both store heat and move it to another part of the boat you want warmed, the rules of thermo-syphons permitting. You gotta love thermo-syphons, the only moving part is the water! By the way, the water loop passing through the bottom of the fire box always had creosote on it, it acts as a condenser there.

    ReplyDelete
    Replies
    1. There is a sauna (banya) hut near our family's village house. We use it at least once a week while we are there, which is 3 times a month. It has the exact design you describe: a thermo-syphon and an uninsulated tank above a wood stove. The guys who put it together (while I was away) didn't realize that the different sides of the thermo-syphon have to enter the tank at different heights (both below the water level). I fixed that problem by screwing in a length of pipe inside the tank, and all has been well since then. These guys were local, experienced stove builders, and my stock definitely went up with them after that.

      Delete
  3. Dmitry, thermosyphon principles dictate you should bring cold water into the bottom pipe of the manifold that wraps the flue, and exit the hot water from the top pipe. If you will use electric pumps to move water, you should still plan for water to flow up as it heats.
    You should never let a "water jacket" be empty of water, or lock in water not able to thermosyphon. If the manifold will be shut down with water trapped in it, this water will boil off (needing a steam valve) until the jacket is empty. At which point the metal itself could overheat. The instructions from the manufacturer of my metal wood stove with water jacket are to never fire the stove without flowing water in the water jacket to avoid damage and failure.
    If you release water as steam into a flue, you may cool the top of that flue to the point where it doesn't draw and you fill your vessel with smoke escaping past the firebox seals. And steam in a flue sounds like a recipe for corrosion too. I'd suggest a separate larger tank for storing hot water that is away from the heat source, and allowing the water to freely circulate through the flue jacket at all times. The bottom of this main tank should be higher than the top of the flue water jacket to prevent reverse thermosyphon when the stove isn't lit, which would act to cool down your hot water supply.

    ReplyDelete
    Replies
    1. Looks like you looked at the schematic but didn't read the text.

      Unfortunately, your suggestion won't work. It is a requirement that this stove can be used with the water heat exchanger empty.

      If there is enough combustion happening to actively make steam, then there is enough convection in the flue to quickly carry it away. And if there is not enough heat, there won't be much steam.

      Delete
  4. Hi Dmitry,

    I have a question about the forced air system and how it relates to the stove. I am assuming that you will draw the air from inside the cabin, correct? And then move it under the sole (perhaps forward) before venting it back into the cabin?

    If this is correct, would it be possible to add the ability to pump in air from the outside? So that instead of recirculating the same air, you could bring in fresh air (might be nice when a large dinner party with candles, etc... is in full swing).

    Perhaps the vents on the deck arch could provide the intake for the air? And then have these new 3 inch ducts connect at the stove (or the blower), so that you can choose which air source you want to use.

    This might be overly complicating things, but as someone born in Texas, my first thought is how are ya going to keep cool!

    And now that I think about it, you're already going to be bringing in outside air for the stove fire, maybe incorporate the forced air source into that system?

    ReplyDelete
    Replies
    1. The air input to the hot air heating system will be a mix of inside and outside air. A fancy design would use a CO meter and regulate outside air input to keep it below a programmed threshold.

      Delete
    2. You could probably also use the oxygen level in your exhaust stack as a good guide. Boilers typically run about 3% oxygen in the stack, 2% or lower risks you generating CO and above 3% you're wasting BTUs. I think sensors will trash the robustness of your design though ...

      Delete
  5. I’ve been ruminating on this stove the last couple weeks and had the same realization that the ballast water should be the heated thermal mass. Just getting back to internet land after being away from modernity and I very much enjoyed reading this revelation. I have an alternate idea on how to heat the water.
    Many wood cookstoves have an attached water reservoir fitting snugly to one side. The reservoir is often several gallons or more in capacity and is non pressurized with a hinged lid on top intended for occasional manual top off with fresh water. The marine stove for Quidnon could have a small non pressurized reservoir next to the stove. This water could be warmed up passively from the side or from a stainless loop in the stove. A small copper pipe leaving the fresh water supply in the bladders could penetrate the side of the reservoir, make some coils inside the hot water bath, then exit and head toward the water bladders again forming a loop. This idea comes from Geoff Lawton’s mass water heater design. It is a gentler heating of the water and prevents high pressures from building up. An air bubble in the line or loss of flow would be no big deal.
    There are a few issues. I don’t believe a thermosyphon would work with this example. A very small, simple, circulating pump could be used. It could run continuously or set to a timer relative to heating needs.
    The unsealed lid on the reservoir would spill over while underway or in rolly anchorages. The lid could be clamped down with a gasket seal and vented out of the cabin top with a 2 inch pipe (to remain unpressurized). There might be a gurgle noise in this pipe as water sloshes or occasionally comes to a boil. I imagine one might get attuned to the way the gurgle sounds when the reservoir needs a fresh top off.
    This plan seems attractive, but I acknowledge it might not work well if the heat output of the stove is on a low setting... or the reservoir is required to be too small due to space issues.

    ReplyDelete
    Replies
    1. You just described a samovar. Adding a fountain pump just adds complexity, because you would need at least a float valve to keep the pump from overflowing the tank.

      Delete
    2. There is a Russian saying for a ridiculous undertaking: "Going to Tula with your own samovar." (Tula was a big samovar-making town.)

      Delete
  6. Just read your response to Nathan. I also didn't quite understand your water heater after the first read. Read it again and now get it. Its sounds more resilient than I first thought.

    ReplyDelete
    Replies
    1. The trickiest part of the design is that steam pressure is used to purge remaining water out of the heat exchanger once the hot water tank is hot enough, then the steam is vented and the heat exchanger sits empty. I haven't worked out all the details of this yet.

      Delete
    2. Dmitry, I know it's not a fully DIY option but have you looked at a steam trap? Lots of different options out there. Vendor websites may have the right model for you. Very robust, few moving parts and typically inexpensive. Hard to know what style to go with given the details you've got here but I think that may be an option.

      For the DIY option water is about .434 psi per vertical foot of head. So if you have enough steam pressure you should be able to force 'slugs' of water out. The key is designing the P trap so that the steam pressure builds to a high enough level to push the water out but doesn't vent before that. It's not like a sink drain, you need a orifice plate and a down-leg to allow the pressure to build without venting the steam constantly. That being said with the DIY option I don't know how to fully clear the exchanger as that steam you're using to force the water out will then condense and still need to be drained. Hopefully there's a good trap model/style out there for you. Maybe in a future post you could diagram out the exchanger system and we could crowdsource you some ideas?

      Delete
    3. Yes, I realized that a diagram and some more description is needed to move the discussion forward. I am about to add one.

      Delete
  7. This comment has been removed by a blog administrator.

    ReplyDelete
    Replies
    1. NewsMyLife are a bunch of crooked spam artists.

      Delete
  8. My only concern is the over-pressure relief valve being used as a process function valve. These things aren't really designed to be used repeatedly, and will eventually fail. In the interest of offering a possible solution, perhaps a second over-pressure relief value set to a slightly higher pressure, but with a small steam whistle attached to it. So when the primary valve fails, *if* it fails in a closed state, the secondary valve will open before a "boom-squish" condition can occur, while at the same time it will create a notification that something is amiss.

    ReplyDelete
    Replies
    1. The type of pressure release valve I am thinking of is the type used in pressure cookers, and pressure cookers are not known to blow up at all. It's basically just a weighted needle valve.

      Delete
  9. This comment has been removed by the author.

    ReplyDelete
    Replies
    1. Nevermind that idea, I re-read the post, and my second idea doesn't make anything better.

      Delete
  10. If I am reading this correctly, the circulation pump takes water from the hot water storage tank, pushes it to the *top* of the heat exchanger, and then the water just trickle drains back into the hot water storage tank. This being the case, a tiny weep hole just after the circulation pump will permit the trapped water to drain from the supply line, and there won't be any water trapped inside the heat exchanger to turn into steam anyway. The heat exchanger could be as simple as 1/4" copper tubing bonded to the back of the flue plate. There would be a small amount of steam produced when the circulation pump starts up again, but the volume should be trivial. The over-pressure valve remains a good idea as a safety backup, but I'd say a steam trap is overthinking this.

    ReplyDelete
  11. Dmitry,

    If your installation was on land I would humbly recommend changing your 'float plug' to a bimetallic steam trap and using a poppet check valve (if mounted vertically) or a spring loaded disc check (horizontal) as the backflow preventer.

    You might even be able to get away with just the steam trap if you had the reverse seating style of trap where backflow help re-seat the valve disc.

    I'm trying to think of how being underway would affect that recommendation but don't have the experience to suggest anything. I have been told by qualified people who've run boilers on land and on a ship (Cdn Navy) that it's much harder on a ship compared to my land-based experience. But this isn't strictly speaking a boiler more of a hot water heater that needs to handle steam.

    Hope this helps ...

    ReplyDelete
    Replies
    1. These are useful suggestions. I don't think that being afloat changes the picture all that much, except for the boat rocking through 20-25º. Can you suggest some industry-standard components for these three elements—pressure release valve, steam trap and check valve—that would fit on 1/2" pipe? The cheaper and more standard the better.

      Delete
    2. http://www.spiraxsarco.com/global/us/Products/Pages/bimetallic-steam-traps.aspx

      https://www.swagelok.com/en/catalog/Product/Detail?part=SS-8C4-1

      Delete
  12. Hi Dmitry,

    I apologize if this is not the right post for this question, but I wasn't sure how else to get the question to you. I was thinking about the layout of the salon and I am curious how the settees are structurally related to the keelboard/water tank structures? I ask, because if you are removing one keelboard from the design, could someone also remove its corresponding settee to give more floor space in the salon? Or put another way, could a customer remove a settee without causing structural damage and/or harm?

    Cheers,

    Greg

    ReplyDelete
    Replies
    1. Hi Greg, good question! Yes, the settees should be nonstructural and therefore removable. One of the possible uses of a Quidnon is as a cargo vessel. The entire salon can be cleared and packed with boxes of wine or wheels of cheese or what have you, loaded and unloaded using a cargo net through the large deck hatch. The pilot berths can also be used as cargo, as well as the U-berth in the bow, although being up above the waterline they should be used to hold lighter stuff. Even with the boat packed in this manner, there would still be full accommodations for a crew of 4, including two twin berths, galley, heads and places to sit down for a meal.

      Delete
    2. An excellent, I hoping this was the case. I spent a lot of time squinting at the Quidnon youtube video trying to figure out how all the jigsaw pieces fit together ;) Glad to hear it.

      Delete
  13. Now that I'm thinking about it, another question. With the solid ballast no longer necessary, what will become of that space? More locker space? I was wondering where you would put the electrical system batteries. If you could access the space under the chain locker from inside the cabin, that would be a nice space to put them. Close to the inverters and controllers, but not in a high traffic area. Have you already figured out a use for that space?

    ReplyDelete
  14. Very cool. Love seeing how this project comes along!

    I would second the comment suggesting that a steam system should maybe be avoided, for corrosion and salt-deposit reasons. (and safety as well), in favor of a hot water system, with the heater outputting 85-90C water nominally. As far as the heat exchanger goes, it's a matter of higher flow vs dimensioning of the heat exchanger.

    The alternate solution to boiling of residual water when flow stops, is to divert the flow of the hot gas via a baffle or a heat shield. And keep the cold water flowing until the temperature of the hot surface is < 90C or so - in practice, when the measured temperature of the water coming out of the heater is < 60C or something like that.

    Homebrew exchangers making steam are an adventure. If anything goes wrong you won't want to be right next to it, and in a small cabin, you have no choice but to be near it.

    Ps- for a crude heat exchanger when there is plenty of heat available (as is typical with wood stoves), coils of metal tubing work fine. IMO managing the ash and hot exhaust should be the focus of attention - the components in the hot exhaust side are always what fail.

    ReplyDelete
  15. The steam trap linked to above is designed to remove things (like air and condensate) from a steam line, it's designed to close when hot, not open.
    What you seem to be describing is a pressure relief valve.

    ReplyDelete
    Replies
    1. The term "steam trap" was reader-contributed, and I did find some commercially available elements that seem, from their description, to do the job.

      And the job is:
      - Don't let steam through one way
      - Don't let water through the other way

      What do you want to call it, and what do you recommend we use?

      Delete
  16. 1. Please add a couple of small lugs with holes on either side of the body to enable bits of wire to be threaded through to form loops through which sticks can be run, to enable lifting and moving of the stove. Like a stretcher.
    2. Rather than trying to divert heat from the cook top (which, being at the top will get v hot regardless of baffles) create an insulated lid which hinges up to rest against the flue when the cook top is in use. Typically such lids are about 100 mm thick, depending on the quality of the insulation. Will save you messing about with dampers etc inside, which will not really work anyway to keep the top cold enough to be safe without a lid.
    http://www.everhot.co.nz/
    3. Somewhere nearby you need storage for the day's supply of dry wood, and a locker for bulk wood supply when underway which is ventilated to dry and keep dry the firewood.

    ReplyDelete
  17. I like your Blog, thanks for share this blog.

    ReplyDelete