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Old 16-06-2011, 15:38   #16
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Re: Free fresh water

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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
Tristan brilliant idea to use otherwise waste energy for water making. You seem to imply that you already have done this; was that as a prototype? Sailochic34, thanks for breaking it down, saves a lot of looking it up on the internet and probably getting the wrong answer anyways.

Tristan, can you post pictures?
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Old 16-06-2011, 15:55   #17
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Re: Free fresh water

When I was in the Navy on submarine duty we had a little watermaker as a back-up to our main evaporator. This unit in a way was like what you are describing in that it used a lot of heat recovery for effieciency and ran at a vaccum to allow the water to flash at a lower temperature. In the system the heat source was a vaccum pump (compressor) instead of the engine. The overall principle I feel is pretty close.

The system worked good once you got it all balanced. BUT it took a long time to get it balanced. And any little change would mess it up.

But if you build it and it works I'm sure we all look forward to see it.
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Old 16-06-2011, 16:50   #18
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Re: Free fresh water

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Tristan brilliant idea to use otherwise waste energy for water making. You seem to imply that you already have done this; was that as a prototype? Sailochic34, thanks for breaking it down, saves a lot of looking it up on the internet and probably getting the wrong answer anyways.

Tristan, can you post pictures?
No, it's not ready yet. I'm working now on the main boiler /heat exchanger . My engine has a turbo and it is a little bit difficult to design this part without creating too much restriction in the exhaust system. When it's ready I'll post the pictures.
T.
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Old 16-06-2011, 17:17   #19
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Re: Free fresh water

Interesting ideas. But my guess is the exhaust gas energy is too low of a temperture to work with easily. And the whole brine, sediment, precipitates problem, too. You've already recovered a large amount of the exhaust gas energy with your turbo. I bet it's simpler and more efficient to use a Spectra type watermaker. They are very efficient. Maybe work on more effieiently getting electricity from the engine....that is something better than the detroit-trash-type inefficient alternators we're using. That and only run your diesel at it's maximum efficiency power point.
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Old 16-06-2011, 19:49   #20
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Re: Free fresh water

Actually the exhaust mass air flow and temperature would be usable heat to flash steam at about 4 gph, per gallon of diesel used by the engine. Provided the discharge elbow/ water injection port is moved to after the vapor still. The beauty of the idea is its free energy. It's the waste energy being thrown down the drain, well out the exhaust. Sort of a mini cogen system. No additional energy/ fuel is used to make the water

With a high output alternator or engine driven RO pump your using engine horse power/diesel to produce the water. Potently the still, in theory anyway, would not increase fuel flow.

Yes the big challenge(well one anyway) is the total dissolved solids (TDS) or salts that would form. So a blow down system would be a must.

Oh, OK yes, I also hung out with my dad growing up when he was working on the cars, well car anyway. Plus OK I'm a little bookish in the same way that a fish swins a little.
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Old 16-06-2011, 19:52   #21
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Re: Free fresh water

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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 .

SailorChic--- First, your hair looks just fine. Better to enjoy that youthful look as long as possible.

Engineering! Yeah.... you addressed your analysis correctly. But now want to put engineering aside after doing engineering work??? Heck, I have been out of engineering for longer than I want to think about but most of what I think about is engineering related. There are pot holes along the rode (make that road) but my bet is you will continue to use your engineering background as you did in your posts.

And thermodynamics! I wish more politicians had some exposure to basic thermo.

Foggy
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Old 16-06-2011, 20:00   #22
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Re: Free fresh water

Would an oversized exhaust manifold assist in relieving the increased pressure from the internal fittings?
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Old 16-06-2011, 20:07   #23
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Re: Free fresh water

well I've designed a lot of really cool things, in the last 30 year. Large Bio-Pharmaceutics , semiconductor plants (well parts of them anyway) Those large hotel casinos in Las Vagas (well 3 or 4 anyway), and the normal commercial things, hospitals and the like. I tend to get a little bored after about the third building. Thing is I started out as a draftsperson, Never went to collage, but did pass the PE, wow 16 years ago. Oh I've been doing it too long to ever loose the knowledge.

Why I love sailing so much, as there is always something to learn
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Old 16-06-2011, 20:12   #24
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Re: Free fresh water

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Would an oversized exhaust manifold assist in relieving the increased pressure from the internal fittings?
Yes, it would but ... -simply I don't have it . It would have to be custom made.

And to sailorchic
The TDS problem can be solved by periodic acid wash and also if the boiler can be accessed for a mechanical scrubbing.


T.
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Old 16-06-2011, 20:19   #25
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Re: Free fresh water

SailorChic--

Good for you! I thought about taking the PE (power) a couple of years ago but did not know what I could use it for. My work did not require a PE. I spent years doing radar transmitter designs (military), power supplies, high voltage circuits, the list is long and engineering management.

You will be a great technical contributor to the site with your background! OH--- I moved from sailing after 25 or so years to power. Needed the better creature comforts.

Foggy
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Old 16-06-2011, 20:19   #26
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Re: Free fresh water

Assuming a 2" exhaust, I might use a 2-1/2" exhaust pipe with a 1 to 1.4 inch dia interior still chamber so that the cross section area . Ideally with fins or ridges to increase the surface area. as the GPM flows / pounds per hour of steam is low you don't want alot of water mass. Ideally a fire tube heat exchanger to increase water contact. But not too much, as you need an area at the bottom of the interior sill to collect sediment.

Ah a two pass exchanger with the steaming chamber top and bottom fire tube/ water jacket thingy. This is a pretty small still as you need to balance the mass flow Going to need some type of water level control maybe. Preventing TDS buildup IS the big issue I think.
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Old 16-06-2011, 20:47   #27
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Re: Free fresh water

Right, because of the small capacity, Your going to need a steaming chamber with top 1/3 for steam disengagement area. Really its a simple two pass heat exchanger/steam generator, made of 316 stainless. Probably 18" ish long x 4 to 6 inch dia as the steam rate is low ( .06 pound per minute) per gallon of diesel per hour. hot gas in the tubes, water in the shell
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Old 16-06-2011, 21:03   #28
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Re: Free fresh water

On the TDS, your looking at 3.5 pounds of crud for each 13 gallons of fresh water made from sea water steam. salt being about 3.5% of seawater. Its a problem in industrial steam boilers using fresh water. With salt water its going to be just a little more exciting.

It can be solved as the Navy does it.
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Old 16-06-2011, 22:57   #29
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Re: Free fresh water

Your going to need to use a pump to extract the concentrated brine from the unit, and flush it overboard before it turns into a solid. This will have to be monitored, and set to a point where the scale doesn't form an insulator around the heat exchangers in the unit. The brine that is drawn off of the unit is as hot as the unit is, and there has to be sigificant heat exchangers to try to concerve that energy. Typical distilation plants adapt to this by using multipul chambers with steadly reducing tempretures, and pressures. They also use recovery heat exchangers to help keep the heat loss to a minumum.

This however isn't your primary problem. Around 150F, sea water will start to form a scale that will deposit on any solid surface. This scale is a sigificant insulator. This is why the heat exchangers for marine engines are set to remain cooler than that. It's difficult to maintain enough of a vacuum to keep the boiling point of the water below 150F. (Actualy, right now I can't even recall if that's possible. It's certainly possible with freash water, but I don't recall how high a precentage you can go with a brine before it's a problem. I really don't want to pull out my old steam tables right now.) And the great thing is that the scale is a great insulator, so the more the scale builds up, the less heat transfer you get.

Also, since we're talking about a vacuum distilation unit, your probably going to be planning to use the condensor to maintain the vacuum. From a theoretical stand point this is great. However, from a practical stand point there are several problems with this. In order to keep the water boiling, the vacuum must be maintained. Any sort of air leak will slowly decrease the vacuum in the unit untill the boiling stops. Also, water as air disolved in it. When you pull a vacuum on the water, some of that air will come out of solution, again, ruining your vacuum. As such, you'll basily need a constantly running vacuum pump to keep the air from ruining the whole system.

I'd like to add that your also going to have a sooting issue from the engine. Diesel engines make soot, and it will tend to collect on things that have cool tempretures, like your heat exchangers in the exaust. The more that soot collects on the exchangers, the higher the back pressure, and the less heat is drawn into the unit.

Not to mention, even the best insulated unit of this size is going to lose a sigificant amount of heat.

As you can see, each of these problems is solveable, but requires weight, engineering, and control. These problems are easier to solve, the bigger the scale of the unit. As a result, most of the avalible distilation units are rather large sized, in order to use the economics of scale in their favor. For such a small unit, using such a small source of waste heat, you've got issues as all of the issues with the scale of the unit work aginst you.
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Old 16-06-2011, 23:09   #30
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Re: Free fresh water

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Ah a two pass exchanger with the steaming chamber top and bottom fire tube/ water jacket thingy. This is a pretty small still as you need to balance the mass flow Going to need some type of water level control maybe. Preventing TDS buildup IS the big issue I think.
Would an adaptation of this, a kelly kettle work?


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