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Old 15-06-2011, 07:25   #1
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Free Fresh Water

I'm planning to install a heat exchanger / boiler into the exhaust system of my Perkins 6-354 to evaporate sea water at lowered pressure. Has anyone done it before on a sailboat? If yes, did you boil the water directly from the exhaust or used some heat transfer media (glycol, etc)? In theory a vacuum pump should be used only in the beginning until condensation starts lowering the pressure - does it really work or the pump has to be run all the time? Would you use copper-nickel or 316 SS to build the boiler?
I would appreciate any opinions and suggestions
T.
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Old 15-06-2011, 18:20   #2
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Re: Free fresh water

My thread hasn't gotten a lot of attention. I think I didn't explain clear enough what I'm working on. The idea is to use exhaust gases to produce fresh water. An ideal diesel efficiency is 56%. Practical efficiency is well below 50%. It means we dump ~50% of the energy by exhaust and cooling water. Why not use this energy to produce fresh water.
It has been implemented on cruise ships for a long time. The problem with small diesels presently on sailboats and smaller power boats is that the exhaust temperature is too low to boil water (212F). But if we lower the pressure we can lower the boiling point to, let's say, 170F- 150F, which is an acceptable range. The system is taking warm cooling water">engine cooling water and boiling it in a low pressure chamber using exhaust gases. The steam produced is then condensed using cool water coming to the engine. The condensation process lowers the pressure even farther which in turn evaporates a larger amount of water, until the system accomplishes equilibrium. The quantity of water produced this way could be really large without wasting any useful energy. After evaporating ~25% of the water, the chamber is emptied of the brine, and the new sea water is loaded. It may sound complex but the device is pretty simple. The only problem I see is corrosion protection of the boiler.
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Old 15-06-2011, 18:48   #3
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Re: Free fresh water

Well it sounds do-able, but unless you motor quite a bit, I would say that it might be a bit on the expensive side to produce water in that fashion. One should figure out how much water you can produce in that fashion versus a stove top or counter top still, and what the cost difference is in the energy consumed. I personally don't motor enough to make it worth while to install all the extra plumbing that would be needed.
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Old 15-06-2011, 19:02   #4
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Re: Free fresh water

And there would be a fair amount of dial reading and valve turning for each run of fresh water which might be distracting to the operation of a sailboat...
Otherwise, the stainless steel should last you about 10 years, mas o minus.

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Old 15-06-2011, 19:16   #5
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Re: Free fresh water

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Originally Posted by svcambria View Post
And there would be a fair amount of dial reading and valve turning for each run of fresh water which might be distracting to the operation of a sailboat...
Otherwise, the stainless steel should last you about 10 years, mas o minus.

Michael
The device is electronically controlled. The total cost of the controller was less then $75 plus two sleepless nights
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Old 15-06-2011, 21:06   #6
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Re: Free fresh water

Just thinking here and I'm blond BUT the seawater when its discharged into the exhaust elbow is not at 180 or even 150 degrees, but more like 80-85 degrees F for a fresh water cooled engine and maybe 130 degrees F. in a direct salt water cooled system. The fresh water tank is at 180 but that's a closed system at 13 psig ish.

You could, I guess build a double wall chamber and use the straight uncooled engine exhaust (air temp 400C+/-) to heat the inner chamber up as a straight up still. Yes probably 316 stainless. Then inject the engine cooling water down stream of the still. Rather then a batch process maybe do a continuous bleed of the blowdown based on PPM calc's.
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Old 15-06-2011, 21:18   #7
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Re: Free fresh water

Neat idea. I'd like to watch this thread to see how it pans out.
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Old 15-06-2011, 23:30   #8
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Re: Free fresh water

The idea is brilliant BUT try to do some math first. The energy of 1 gal of diesel in known, the consumption rate is known, efficency is ~50% the rest goes to boil salt water, energy needed to boil given amount of water is known as well. Just put it all together and if it turns out that you need to run your engine for 3 hours to get a glass of fresh water.... well... do it.
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Old 16-06-2011, 00:11   #9
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Re: Free fresh water

So Diesel has about 140,000 btu's per gallon, Say you burn 1/2 gallon per hour, thats 70k btu's X 60% diesel eff = 42,000 btus out the exhaust split between hot gas and raw water. Rough ballpark anyway

Lets assume 30K btus at out the engine exhaust just for fun. Assuming a heat exchanger overall eff of 50% (conservative I know) you'll have 15,000 btus uses to convert water to steam. Think from memory steam at 212 F is something like 980 btu's per pound, it's in the ball park anyway so divide 15,000 by 980 gives you 15.3 pounds of steam per hour (roughly anyway) or a little less then 2 gallons per hour.

A vapor compression still would improve that a bit but you need a vacpump or condenser. Bet the design would be tricky for so small a load

So sort of doable, Issues would be blowdown (HIGH TDS). Keeping the exhaust back pressure low to keep the engine happy. Need to divert small raw water flow to still, comes to mind.

Oh, Ok I might have been an engineer in a past life.
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Old 16-06-2011, 05:56   #10
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Re: Free fresh water

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Originally Posted by sailorchic34 View Post
So Diesel has about 140,000 btu's per gallon, Say you burn 1/2 gallon per hour, thats 70k btu's X 60% diesel eff = 42,000 btus out the exhaust split between hot gas and raw water. Rough ballpark anyway

Lets assume 30K btus at out the engine exhaust just for fun. Assuming a heat exchanger overall eff of 50% (conservative I know) you'll have 15,000 btus uses to convert water to steam. Think from memory steam at 212 F is something like 980 btu's per pound, it's in the ball park anyway so divide 15,000 by 980 gives you 15.3 pounds of steam per hour (roughly anyway) or a little less then 2 gallons per hour.

A vapor compression still would improve that a bit but you need a vacpump or condenser. Bet the design would be tricky for so small a load

So sort of doable, Issues would be blowdown (HIGH TDS). Keeping the exhaust back pressure low to keep the engine happy. Need to divert small raw water flow to still, comes to mind.

Oh, Ok I might have been an engineer in a past life.
It is very close to my calculation except my engine burns over 2 gal/hr and I've gotten 10 gal of fresh water per hour. There are a lot of assumptions so I might be wrong by +/- 25% I've been thinking about continuously fed system instead batch mode but it would require running the vacuum pump most of time
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Old 16-06-2011, 11:17   #11
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Re: Free fresh water

You can make fresh water from ocean water or urine using the sun, not fossil fuels AND it works on a boat.
Watercone Solar Still Mage Water Management
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Old 16-06-2011, 12:00   #12
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Re: Free fresh water

And if you still want to burn diesel a genset and a Horizon water maker Horizon Reverse Osmosis Watermakers - Fresh water for your home away from home or a belt driven Village water maker would probably be more efficient.
Little Wonder series watermakers
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Old 16-06-2011, 12:21   #13
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Re: Free fresh water

sailorchic34 there is no way you are a true blond.You had to use a bleach bottle.
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Old 16-06-2011, 12:33   #14
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Re: Free fresh water

Quote:
And if you still want to burn diesel a genset and a Horizon water maker Horizon Reverse Osmosis Watermakers - Fresh water for your home away from home or a belt driven Village water maker would probably be more efficient.
Little Wonder series watermakers
That would be the obvious thing to do--use some sort of watermaker or counter top distiller while the engine is running. On the other hand, I believe what Tristan is thinking, is how to use the energy being wasted by going out the exhaust after combustion has taken place.
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Old 16-06-2011, 15:24   #15
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Re: Free fresh water

Ha.... More that I've spend too much time working for engineers and know way too much about steam and btus, etc. Doing my best to forget it all now and bring out my inner sailing bimbo. Besides gray is far more of an issue nowadays for me .
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