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Old 03-07-2009, 22:54   #1
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Waste Heat Distillation System?

Is anyone aware of a distillation system designed to be used on a yacht under say 50 foot, to be powered mainly be waste heat from an engine?

It seems to be a simple concept, but I can't seem to find any examples.

If no one knows of one, any one know why they're not common?

Thanks!
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Old 04-07-2009, 00:29   #2
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'Interesting idea. Isn't most of the system already inplace? The raw water heating system has the heated seawater from the heat exchanger added to the exhaust at the mixing elbow. Couldn't some water vapor at a point before the mixing elbow be diverted into a condensation tube from which desalinated water would collect?
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Old 04-07-2009, 02:06   #3
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They are commercially available but only for big ships. Not very common nowadays because RO have totally supplanted them. The waste heat distillation units were big, bulky and very maintenance intensive.
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Old 04-07-2009, 02:09   #4
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The only area where there is enough heat of sufficient temperature to boil water at room pressure is in the block or at the exhaust manifold. This is not a good idea in the engine block and not in the exhaust manifold either.
Vacuum distillation would be the only way to do it. Let’s assume we use the seawater exiting the heat exchanger. It is around 160-190 F. You would need to pull a vacuum so it would boil at 160-190 F. Say we boil off a third of the water. Any more may jeopardise the cooling of the exhaust.

To do this we need a metering valve or equivalent at the entrance to the boiler (or evaporator). A pump to evacuate the concentrate in the boiler and inject it into the exhaust. It also doubles as the vacuum pump. A air or liquid cooled condenser with a fan or pump. Some type of control system.
Adding all the cost of the above parts and the modifications required on the engine along with any hidden costs could buy a good sized watermaker.
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Old 04-07-2009, 02:58   #5
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Here’s a patent for a “Water distillation using waste engine heat from an internal combustion engine”, producing a steady state distillation rate of 2 gallons per hour.

DSpace@MIT : Water distillation using waste engine heat from an internal combustion engine
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Old 04-07-2009, 03:17   #6
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Thanks Gord,
for those of us that have dry exhausts, this will prove invaluable.
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Old 04-07-2009, 09:31   #7
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I have found there ar pattents for almost everything. That doesn't mean the pattent works, or is a comershaly viable device. I take it no one knows of such a decive already in mass production?

The goal would be to replace the normal water maker. A RO water marker seems to be one of the higher draw electrical appliances on a boat. Something running off of waste engine, or stove heat, would draw very little electricy, and SHOULD make water any time you fire up the engine or stove.

But like you say, vacuum distillation would be the only way to go, and you'd have to use the exaust, cause the 190F coolent in a FW cooling system wouldn't be hot enough.
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Old 04-07-2009, 12:03   #8
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well.... how does one of those solar emergency distillation units work then? it's not boiling..... could it be as simple as pumping cool seawater through a copper coil wrapped around a hot exhaust and catching the drips?
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Old 04-07-2009, 12:47   #9
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Very Very Slowly
Don't expect to be showering!
Those units work in the same way as dew forms at night. It is a physically closed system, one part warm and one part cool. Water slowly evaporates in one area and condenses in the other.
Wrapping a coil around a hot exhaust would not work very well. Very little of the coil would be touching the exhaust pipe.
The problem is you need a lot of surface area and a sizeable temperature gradient for effective heat transfer. You need a lot of heat to boil 1 gallon of water, 2,000 Btu per lb. Depending on the size of your gallon you need 8,000 or 10,000 BTU for 1 gallon of water. The exhaust after the water cooled exhaust manifold is around 300-400 F and few inches later the raw cooling water is being injected rapidly reducing the temperature.
A pot on a gas stove takes how long to vaporize 1 gallon of water. It has a heated surface area of 65 sq inches (9 inch pot) and a temp difference of 400 F.
A coil wrapped around 2 inch exhaust for a length of 6 inches of exhaust would have an effective area of perhaps 15 sq inches, temp difference of 150 F.
It would take over 10 times as long as the pot on the stove. If a special purpose fitting where to be made up you would increase the rate. You may get a half gallon or so in an hour.
Remember you will be cooling the exhaust by boiling the water, thereby reducing the temperature gradient.
Approaching the problem from a different angle a diesel engine using a half gallon of diesel an hour is producing 60,000 to 70,000 Btu per hour of energy. Deducting the mechanical energy delivered to the shaft, heat looses through the block, oil cooler, etc, you may have 10,000 BTU per hour available in the exhaust or the capability to boil 1 gallon of water an hour in a perfect world. In our imperfect world you could produce a quarter of a gallon an hour.
There is no free lunch.
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Old 04-07-2009, 13:28   #10
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In my mind, you would use the heat from the cooling system, and the exauste, with a heat exchanger system. I would expect that you would get up to half the energy released by the fuel.

Most of the diesel engines will be dumping over 2/3 of their heat between the cooling system, and the exaust.

So .5 gal/hr = 70,000 BTU/hr. If you can use half of that, we're looking at 35,000 BTU/hr. Water is what, 9000 btu/gal to boil, so your talking about maybe 4 gal/hr. That's still not much.

But then with the lowest end of the RO units being around 8 gal/hr, that's not impossibly terrible. Also .5 gal/hr is pretty low for even a sailing aux engine. 1 gal/hr is pretty common, which if the above is workable, we might get 8 gal/hr.

The big trick would be how large and expensive would a set of heat exchangers, and the distilation container.
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Old 04-07-2009, 14:51   #11
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You could build a very large and heavy corrosion resistant boiler and bolt it directly to the exhaust ports. The raw water coming from the engines heat exchangers (water and oil) would be piped to it. They would preheat the water for us. There would not be much in the way of energy losses.
The boiler would be arrange it in such a way the passageways conducting the exhaust out are completely flooded and of sufficient surface area to cool the exhaust gasses down to lets say 250 F. The excess raw water would be injected into the exhaust or dumped over board. Any water vapour being produced would be drawn out to a raw water cooled heat exchanger you would need a pump here.
For a US gallon you would produce maybe 2 gallons an hour. The boiler would probably cost US$2000 or more if mass produced. It would be 2 to 3 times the size of a Bowman heat exchanger-exhaust manifold for a Perkins engine. It would be made of cast aluminium. The raw water pump and connecting hoses etc would add $500. Then we must add the labour costs.
It would be fun to do but….

The actual rate of production of water could be calculated knowing the exhaust temperature of the engine at the exhaust ports. Knowing the rate of fuel consumption and air to fuel ratio we can calculate the available heat in the exhaust gasses. Knowing the temperature of the water being delivered to the boiler and the flow rate we would precisely know how much water we could produce. It is a simple calculation.

What do you think?
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Old 04-07-2009, 15:04   #12
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That sounds about like what I was envisioning.

I'm a chemE by training, so your right, the thermo is a simple calculation, but the kenetics would be a pain.

In anycase, it sounds doable, but no one had done it yet.

I guess when/if I buy a damn boat, it'll be on my list of things to build.
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Old 04-07-2009, 16:50   #13
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I am a ChemEng by training also.
It would be a fun project to play with. A prototype boiler could be cheaply made of welded steel plate and pipe.

There are a whole pile of things to play with on a boat. One of which is right up a ChemEngs alley, phase changes or refrigeration. I spent some time and $ to design and build the most efficient system possible. It is so very simple.
Commercially available refrigeration systems are designed to be inexpensive to build and easy to install for the do it your self boat owner. It is easy to knock off 10-20% of the energy use without any trouble at all. Relocating or enlarging certain other parts you can gain another 20% to 30%.

Then there is the whole electric power issue which is continually talked about. Battery capacities, charge rates etc. all once again in a ChemEngs field.
Get a damn boat and drive everyone crazy. And your self too. Best of luck.
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Old 04-07-2009, 17:05   #14
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The exhaust temperatures are significantly higher on a dry exhaust, if fitted with a turbo charger they will run up in the 900*F range, which I believe will be plenty of BTUs for the project, the hardest part is going to be designing a heat exchanger that does not block the exhaust flow. I guess you could create a section of exhaust pipe that has a water jacket incorporated into it. The rest would be calculating the flow of water so that you could capture the steam and condense it back to fresh water. Of course this only works when you are running your engine. For those of us in the north, we could also put a coil in the firebox of the diesel stove, I already do this for hot water to shower with, I am going to have to cogitate on this a bit and see if I can run salt water through the coil slow enough to generate steam.
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Old 04-07-2009, 18:42   #15
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You don't want to be running a tight coil with salt water in it to generate steam. It will not take long and the coil will be pluged with scale. The scale will be almost impossible to get out. Much the same could be said about using the super high temp exauste to generate steam directly.

My thoughts are that vacuum distillation is the only way to go to try and keep the scales down. Also need to use a heat exchanger shape that would be easy to clean.
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