Heating and ventilation in a small cruiser
This thread is essentially an off-shoot of my light-cruiser thread. I wanted to split this off to another discussion though so things don’t become all mixed up… So here are my thoughts on heating a small cruiser. First we start with the requirements:
(1) The targetted maximum airflow is 8l/s per person, above the minimum of 7.1l/s (15 CFM) per person which is recommended for homes, and well above the 5-6.7 l/s per person recommended in “Principles of Yacht Design”. You can always just turn the fans down a little.
(2) With that value in hand, I made a spreadsheet to calculate the heating requirements: 480W with a 20°C temperature difference between inside and outside, provided 50mm insulation.
(3) I want a very safe system, with next to no fire risk, and no risk of carbon monoxide buildup in the cabin
Now I propose the following:
Draw air in on one side of the cabin, warm it up, and vent it into the cabin near the floor (in the diagram it’s placed a little higher because I needed space to sketch in the heater). On the other end of the cabin, near the ceiling, there is another fan which draws air out. The fans would be 120mm PC-chassis fans, possibly running at reduced voltage, so they would for all intents and purposes be noiseless. The inlet / exhaust vents naturally have to have water-traps, but other than that they can be very low profile, no need for a bulky dorade.
To heat the incoming air, I would place 4 good CPU coolers (without their fans) back to back into the air inlet pipe (something like this placed inside the pipe for incoming air, but made of aluminium so there’s no corrosion problems in case a little bit of salt-water does come through). The CPU coolers themselves are fed with warmth via a very simple convective closed-loop water system; a kerosene burner warms the water on one side, causing the water to circulate, so you don’t need a pump for the water. (Though I’m unsure of whether that would still work in a good chop!)
The pipe with the warm water doesn’t have to be insulated, if it radiates some heat into the cabin that’s fine, and you could use it to dry your stuff as well, just give the pipe an S-bend and you have a bit of area to hang some wet stuff on, like those towel-drying racks run off the warm water from the heating system in some homes.
For the kerosene heater, I want a completely separate and sealed system. With that I mean a separate small air inlet and exhaust to ensure that there’s little to no risk of carbon-monoxide / carbon dioxide buildup in the cabin, and the burner itself, other than the inlet and outlet pipes, should be completely sealed when in operation, to ensure the fire-hazard is close to zero.
All this is off the shelf hardware for maybe ~300€ tops, plus the small closed loop kerosene burner, a bit of aluminium and plastic tubing, and 2-3 water traps.
What do you guys think?
Cheers,
Marco
P.S. - The fans themselves don’t need a lot of power, but if power proves to be an issue I can just buy a small thermoelectric element and place it between the first heat exchanger and the water-water pipe and run the fans from that.
I like the concept, especially the bit about using a TEC to power the fans (very elegant), but I’ve got a few suggestions on how to improve the price and efficiency of your proposed system.
For the air-water heat exchanger, why reinvent the wheel? instead of plumbing the hot water line into a waterblock attached to a bunch of heatsinks, just use a pc watercooling radiator: Since there is only a very thin sheet of Al or maybe Cu between the hot water and the cold air it will be much more efficient. They’re already set up for 120mm and/or 140mm fans (they screw right on) and you can even get ones where the fin spacing optimized for quiet low speed fans. It would be much cheaper and easier than a custom built solution, you can buy a good quality radiator sized for a single 120mm fan for about the same price you’d pay for a single aftermarket heatsink.
The TEC would be much more effective if you mounted it directly to the burner enclosure and put the heatsink in the airstream of the burner air inlet duct (which would be cold outside air). You’d have, idk, probably a 100+˚C temperature difference between the hot side and the cold side, and with one of those big “tower” heatsinks you’d be transferring a heck of a lot of heat, 90% of which would go right back into the burner. The added benefit is that the convection caused by the kerosene burner will mean that you don’t need to power an additional fan on the heatsink, you’ll get airflow for free. I think ~1A @12v is totally doable, hell, you could probably run some LED interior lights with the excess power and then top up your house batteries while you sleep.
Don’t bother undervolting the fans, it’s a waste of electricity since you’ll probably be using a resistor to do it. If you want quiet fans, just buy low speed fans to begin with. When you get around to shopping for some, look at the comparison tests on silentpcreview.com, they take this stuff very seriously.
And I don’t know that an exhaust fan is strictly necessary. You’ll get almost as much airflow with just the intake fan at only half the amp draw. Just pull air in and let positive pressure push it out.
To maximize the amount of heat transferred from the kerosene burner into the cabin, consider running the flue up through the cabin air inlet. You could even solder on a bunch fins running lengthwise up the outside of the flue to increase its surface area. By pulling cold outside air past the flue you’ll get a lot more heat out of the exhaust gasses than you would by just having the flue inside the cabin, because not only will there be a larger temperature difference but also forced convection produces much better heat transfer coefficients than natural convection.
Here’s a quick sketch of what I had in mind:
I really like your improvements Greg! 😊
One thing to keep in mind are the thermal envelopes of the various components though. That was the reason I initially ignored the possibility of using an off-the-shelf water PC water cooling unit; I automatically assumed I was going to have the water at a relatively high temperature, say 60-80°C, which could well melt some of the seals / components. I didn’t really think that through though: there’s no need to let the water in the closed loop get that warm, and maybe off-the-shelf units can handle a bit of temperature too. I’ll have to check what the limits are there.
That said, the heat-exchanger which gets the heat from the burner into the water, could be an issue, because that heat exchanger would be exposed to essentially the combustion temperature of kerosene. While copper can take that, aluminium definitely can’t, and any kind of synthetic seals can’t either. So that may be a tricky spot. A similar story may be the case for the Peltier element, I don’t know what kind of temperatures those can take without being damaged. I’ll have to look into that too.
Preheating the combustor inlet air stream with the heat which passes through the Peltier element would definitely increase the temperature in the combustion chamber quite a bit, at least if that that heatsink and peltier are designed to really put out a bit of power. That temperature increase might be desirable (higher temperature difference for the peltier to work with, if it can take it), or if the temperatures are already problematic, then that would make the problem worse. Should the latter be the case, then you could place the peltier heatsink into the cabin air inlet stream instead, to help keep the combustion chamber temperature a little lower.
—This whole thing reminds me a bit of staged-combustion rocket engines to be honest. xD
The positioning of the fan in your setup makes fan maintenance / replacement, nice and easy. I also like your idea of using the combustor flue to preheat the cabin inlet air, very clever! 😉 We’ll have to keep an eye on how easy it is to disassemble and clean the whole thing though. If carbon deposits build up inside the combustor exhaust tube or the combustion chamber, then the thermal conductivity of those surfaces can be more than halved, so making sure the combustion process itself is very clean and very complete will be important (needs enough fresh air!), but there’s no getting around having to clean the whole thing from time to time, so the setup needs to be designed with that in mind.
Not running the warm water-pipe through a bit of the cabin, eliminates the possibility of using it to dry your stuff, but I think that’s not a major loss, given that you could just as well use place stuff which you want to dry, infront of the fan which is blowing warm air into the cabin. If anything, it’s the combinations of warm air and plenty of airflow which will get stuff dry quickly.
There is one other thing which hadn’t really gone through my head before; is if it’s cold enough that the heaters are running, then chances are there’s not a lot of sunshine to keep the batteries topped up with solar cells, making any power that could come out of the TEC potentially quite valuable. The TEC is silent, and when coupled with the heater which is running anyway, is 100% efficient, so it would be a good way to get a bit of power into the battery, even a couple of watts surplus would be enough to power a cabin LED lamp, or keep the handheld VHF charged. I think designing the TEC-setup to put out as much net electrical power as is reasonably possible would be a good idea (no exhaust fan!). If the sun isn’t shining, electrical power becomes a very valuable commodity, and the TEC could provide at least some, if not all of it.
Cheers,
Marco
I think you’ll be surprised by just how much heat some synthetics can handle. 😊
This RTV silicone gasket maker stuff is good up to 350°C, and these silicone heater hoses are rated to carry coolant up to 177°C, and PTFE hoses can be good up to 250°C.
Since Jet-A (basically kerosene) supposedly burns at 260-315°C in open air, you could theoretically make the combustor in 3 sections (for full access to the burner and the heat exchanger), hold them together with tension springs (much like a motorcycle exhaust) and seal the thing up with that RTV silicone.
The combustor and heat exchanger will need to be copper and connections will need to be brazed, there’s really no way around it, but it should be pretty much bullet-proof at that point. Melting temps for brazing alloys start at ~450°C and go up from there.
As for the temperature capacity of pc watercooling gear, if we were talking about pumps, waterblocks or hoses I’d worry too. But the radiator is just brass copper and brazing (or maybe solder), but even the lowest melting point solder only melts at ~180°C, so I wouldn’t be concerned at all. Just screw in some brass hose barbs with teflon tape and run silicon heater hoses down to a pair of brazed on brass hose barbs on the combustor heat exchanger and you’re golden IMO.
With the peltier/heatsink preheating the air in the combustor, don’t forget that the energy to do so comes from the combustor in the first place. It should actually lower the combustor temps by a little bit, basically the energy that’s being converted to electricity and a little bit of leakage.
And for drying wet clothes, there should be plenty of waste heat coming off the surface of the combustor. I’m sure there’s a way of directing that rising heat to achieve the same ends.
A friend of mine pointed out that TECs don’t put out 12V, and that means you either have to use a step-up converter which is about 65% efficient, or you have to stack ~3 TECs to get up to 12V. Either way, you’d likely be paying quite a bit more per watt of installed capacity than for a solar cell (which I intend to have anyway) or a wind generator.
TECs also have the disadvantage that the ceramics they are made of are very brittle, and sensitive to moisture, so they have to be packaged well.
The solar cell / wind generator solution on the other hand is not as silent as the solar cell / TEC solution, and placement of the wind generator could be an issue on the proa (I guess on the akas is the only available spot).
As I said before, the wind generator costs less per installed watt though, and produces electricity even if you aren’t running the heating (for instance in relatively warm weather), so all in all you’d probably get quite a bit more bang for your buck.
I’ll have to look at the numbers in some more detail and see just how big the price difference is, and think about where and how a wind generator could sensibly be installed on a cruising proa, before I make a decision there though.
That said, even without the TECs, I think this heating setup (with greg’s flue improvement) would be really good. Active high airflow ventilation, a closed-loop heater for minimal risk of fire and carbon monoxide poisoning, and a place to dry your cloths, all in one simple, cheap, robust unit would be great as it is!
Marco
I haven’t read through every word posted earlier, but it appears that both solutions posted above waste heat going through the outgoing vent. What if you use the outgoing vent to pre-heat the incoming vent. And the exhaust to pre-heat the incoming heater air. You could also make your exhaust go up in a less linear fashion. Lead it up in a series of turns, could even be through a heat storing material. That way again less heat goes up the exhaust.
(I haven’t really thought this through in detail so I could be missing something).
-Thomas
The solar cell / wind generator solution on the other hand is not as silent as the solar cell / TEC solution, and placement of the wind generator could be an issue on the proa (I guess on the akas is the only available spot).
A wind generator also works on dim cloudy days and at night….. You can mount the wind generator on a cantilever bracket off the windward side of the mast above head height or even on the mast head to get it out of the way.
Another possiblility is water generation (whilst sailing or whilst anchored or moored in a tidal stream) I am very interested in using or adapting an electrically operated retractable counter rotating thruster for auxiliary propulsion instead of noisy petrol driven outboards and somehow adapting or finding one which can work “in reverse” to generate power as well.
http://www.green-motion.com/site/
http://www.wattandsea.com/en
http://www.energymatters.com.au/ampair-100watt-12v-micro-hydro-water-powered-generator-p-679.html
http://www.duogen.co.uk/page14.html
https://www.sideshift.com/shop/product.php?id_product=23
I haven’t read through every word posted earlier, but it appears that both solutions posted above waste heat going through the outgoing vent. What if you use the outgoing vent to pre-heat the incoming vent. And the exhaust to pre-heat the incoming heater air. You could also make your exhaust go up in a less linear fashion. Lead it up in a series of turns, could even be through a heat storing material. That way again less heat goes up the exhaust.
You’re quite right about the lost heat. If you look at the spreadsheet Marco linked to in the original post, his math shows that even with the minimum amount of ventilation he’d be losing twice as much heat through the air vents as through the hull. What you’re describing is a “heat recovery ventilator”, and a good one could cut his kerosene consumption by at least a third. There are two problems though. The first is space, commercially available units are comically oversized for this but even a DIY project (not that hard, actually) would still be fairly substantial box. In a bigger boat it would make perfect sense, but in a ~26’ proa it’s debatable at best. The second problem is flow restriction. Even though by itself it wouldn’t be very restrictive, once you add in the water trap and the ducting and the radiator it starts to add up, and axial flow fans don’t handle flow restriction particularly well, especially not low speed / low noise fans, and once the flow rate drops too low you risk losing heat out of the intake vent through convection. So you’d be trading even less interior space and a noisy fan for ~0.7L of kerosene / day. It’s kind of a toss-up IMO, but that didn’t stop me from drawing up a prototype in Solidworks yesterday. 😉
The exhaust is also an area where flow restrictions have a large impact. Since the current design relies on natural convection, flow restriction needs to be kept low to ensure that the burner receives enough oxygen. If the Air/Fuel ratio drops too low you’ll get very dirty combustion leading to a heat exchanger and flue coated with soot which would drastically reduce the efficiency of the system. You’re right in theory, but it would need to be done with great moderation and quite a bit of trial and error.
I am very interested in using or adapting an electrically operated retractable counter rotating thruster for auxiliary propulsion instead of noisy petrol driven outboards and somehow adapting or finding one which can work “in reverse” to generate power as well.
It’s not exactly what you’re looking for, but it’s close in spirit.
I haven’t read through every word posted earlier, but it appears that both solutions posted above waste heat going through the outgoing vent. What if you use the outgoing vent to pre-heat the incoming vent.
You’re right Thomas. Using the exhausted cabin air to preheat the inlet air could cut energy consumption quite a bit, but like Greg said, there’s some pratical arguments which speak against it. The bulkiness of the unit didn’t even occur to me, but I persoanlly wasn’t too happy with the necessity to run an air duct back (and forth) across the cabin. I think the cabin inlet and outlet vents have to be on opposite sides of the cabin, to avoid just blowing the fresh warm air directly out again. I admit, you could potentially run the pipes back and forth under the floorboard though, where they wouldn’t get in the way at all. Similarily the bulky box could be placed into one of the closed bow compartments where space wouldn’t be much of an issue, but all that would be extra labor, which like greg said, doesn’t save a whole lot of money in the end. :/
The other reason I chose to leave it out, is potential condensation issues in the heat recovery ventilator. Warm and potentially quite humid humid cabin air running through cold pipes will cause a lot of condentation, and then you can get algae / fungi growing in the unit pretty quickly.
Since the current design relies on natural convection, flow restriction needs to be kept low to ensure that the burner receives enough oxygen. If the Air/Fuel ratio drops too low you’ll get very dirty combustion leading to a heat exchanger and flue coated with soot which would drastically reduce the efficiency of the system.
Agreed. The airflow for the burner has to be good, I don’t want to have to install an additional fan for the kerosene burner, and for safety reasons (the carbon monoxide hazard) I don’t want to use the same air inlet for the cabin air as I do for the burner. If I could ensure the flow from the burner never goes backwards for any reason whatsoever (think single-point-failure-tolerance here), e.g. with a check-valve, then you could use the same inlet and fan for both, but I think the pressure differences in this system are just too small, even for lightweight reed valves, it’s only a quiet 120mm PC fan after all.
So you’d be trading even less interior space and a noisy fan for ~0.7L of kerosene / day. It’s kind of a toss-up IMO, but that didn’t stop me from drawing up a prototype in Solidworks yesterday. 😉
Pics or it didn’t happen! 😊
Cheers,
Marco
I want to throw something else into the room: I’ve been toying with the idea of having a sea-water / cabin-air heat exchanger, as a means to cool cabin air in the summer, without having to resort to energy-intensive air conditioning. You’d have the condensation issue there too though, and if you have to run a pump to get the sea-water through the system, then you would need a modest amount of power to do that, though if it’s hot enough that you want to run cabin cooling, then chances are you had pretty good solar output that day, so maybe the requirement to run a small water pump wouldn’t be that bad. I wonder if a cooling unit like this could sensibly be integrated into this whole setup…
On the other hand, simply relying on some fans to keep you cool in the cabin would be an easier solution though, and I doubt it’ll even get hot enough where I’m going for something more than fans to be a must.
Marco
Pics or it didn’t happen! 😊
Satisfied? 😊
I was inspired by this thing or, to be more specific, I was inspired by it’s crudeness to design something better. Proas barely even entered in to it, it was more of a mental challenge I set for myself.
It’s built up from 3mm plywood (probably laser cut) and household aluminum foil. The idea was to cram as much surface area into as small a volume as possible, while avoiding as much flow restriction as possible. Zero actual engineering went into this, so it’s probably longer than necessary (~30” overall) and thicker ply might work better for a low power fan, but I mostly just wanted to see if the concept could work.
It’s intended to be used in a counter-flow arrangement, and the intake and exhaust channels are all interleaved, so it’s intake-foil-exhaust-foil-intake-foil…etc. Version 2.0 would have S-shaped channels rather than U-shaped so that all of the laser cut wood pieces would be identical. But other than that little change for practicality’s sake, and tweaking of it’s dimensions, I think it has the potential to be very efficient as is.
Wind generators may have a lower cost per installed maximum watt than a TEG, but how many watts do you actually need? perhaps more importantly, how many watt-hours do you actually need? Leaving aside cost for a second, even small commercially available wind turbine is going to weigh ~5+kg, add in the mounting hardware and you’re looking at at least 10kg worth of stuff. ~10kg is about a 30-35 AH 12v sealed deep cycle battery that you can charge up on mains power while you’re at home. When you consider that you’ll need a battery with the wind/solar setup anyway you’d probably be looking at ~40+ AH total (~240 usable WH at 50% drain) with no real weight penalty over a setup with a wind generator. On top of that, batteries take up a lot less space than a wind generator and can be shoved into small akward unused spaces instead of taking up room on the deck/tramp. And of course, they are worlds apart in terms of cost.
If you’re just going to run some led interior lamps and some of small fans off and on for a few days, it might make more sense to just forget about both the wind and TEG and save yourself a lot of money and work by carrying some bigger batteries.
I want to throw something else into the room: I’ve been toying with the idea of having a sea-water / cabin-air heat exchanger, as a means to cool cabin air in the summer, without having to resort to energy-intensive air conditioning.
I’d just go with the fans. You’d be opening up a whole other kettle of fish with that seawater heat exchanger; Through hulls, seacocks, galvanic corrosion, biofouling in the tubes, etc. Not worth the headaches or potential for sinking, IMO. If you get too hot just jump in the ocean, and/or sleep on the tramp between the akas. It’s a lot simpler that way. 😉
I had an Amigo 23, a Swedish long keeled sailboat. (see picture)
We were out weekends with the boat early may and late September, and several nights the temperature dropped down to about 5 degrees C. There was no insulation of any kind. It is only a solid layer of fiberglass and polyester.
We could keep the cabin-temperature comfortable with only our single burner stove and a kerosene lamp. Usually we would only use the kerosene lamp to keep the temperature comfortable, but we used the stove when drying cloths and raising the temperature fast (someone cold and damp from jumping into the water).
Today I would build a small rocket-stove out of 120 mm steel pipe and some sand and gravel. I think that is the ultimate cruising heat source and stove. It is simple, safe and efficient and can burn almost anything. Oil, kerosene, alcohol, gasoline, wood, grass, leafs, dried seaweed, and manure. I would use a small chimney ontop of the stove when I only want the heat, and fold the steel-pipe-chimney to the side when cooking. (of course one needs ventilation when cooking. )
http://en.wikipedia.org/wiki/Rocket_stove
Cheers,
Johannes
http://www.richsoil.com/rocket-stove-mass-heater.jsp
One could build the rocket stove mass heater with the exhaust pipe going all the way through the hull under the floor, but it would get seriously hot when cooking food in the summer.
Cheers,
Johannes
One could build the rocket stove mass heater with the exhaust pipe going all the way through the hull under the floor, but it would get seriously hot when cooking food in the summer.
If one were to go that route, I’m sure it wouldn’t be too difficult to rig up a means of switching between the underfloor flue and a “shortcut” that would allow the stove to be used with minimal unwanted heat. With a butterfly valve or two and a bit of extra pipe, you could switch from stove/heater to just plain stove in a matter of seconds.
The only problem I see is that the “mass” part of the “mass heater” isn’t particularly proa friendly. A smaller continuously burning stove would have a similar effect without the need for extraneous mass to act as a thermal battery. The big benefit of a mass heater is that you can have a comparatively large fire burning for a couple of hours and then be kept warm all day/night by the stored up heat energy. It’s a great idea in a house but I’m not convinced of it’s practicality on a boat, much less a boat who’s sailing performance is so dependent on staying light weight.
But thanks for posting about rocket stoves. I had never heard of them before, and having looked into them I’m really impressed. They certainly do have a lot going for them; inexpensive, effective and reliable-as-a-hammer is a pretty compelling combination.