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Old 17-06-2011, 07:56   #31
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Re: Free fresh water



Why can't I go below 150F ?
Would the sea water diagram be much different?
T.
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Old 17-06-2011, 08:35   #32
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Re: Free fresh water

I can't see your immage.

Your going to make me break out the steam tables. My first thought is that it is possible, but your going to reach a maximum limit on the TDS in the resulting brine before you need to flush it out.

Basicly, salt lowers the vapor pressure of the solution it's in. IIRC, it lowers the vapor pressure by a molar relationship. If the vapor pressure of water is 5 psi at 150F, and you start with 3.4% NaCl salt by weight, it should lower the vapor pressure of that water by about 2.2%. The more water you evaporate from the solution, the higher the salt content, and the lower the resulting vapor pressure. IIRC, about the maximum for a salt solution is a 30% brine, above that the salt starts to fall out of solution. A 30% brine would lower the vapor pressure of the solution by about 21%. So at 150F, the vapor pressure of water is around 3.7 psi. If you start droping the vapor pressure with salt, you can easily see that you'll need to be around 3 psi abolute pressure to get the water to boil.

Depending on the constraints in your design, your going to have a minimum pressure. This minimum pressure for the unit is going to basicly be set to how fast your condensor can remove heat from the unit. The lower the pressure of the water vapor goes, the more surface area is required to remove a given quantity of heat. Also the lower the pressure is the less tempreture diffrental there is over that heat exchanger, slowing the rate of transfer of the heat. Since there is a maximum possible size to your condensor, and a maximum temperture for heat sink that your going to dump the heat into, there is a minimum pressure for the design.

This minimum pressure will then be used with the concentration of the brine to tell you when you have to dump the brine, and get more sea water. Dumping that hot brine will remove heat from the system, which will lower it's production of water. I belive that most units use the water dumped here in a sort of feed water heater, that is used to preheat the freash sea water that is being used to replace the brine. This is to concerve the heat, and minimize the loss of water production.

Another thing is that you can't use the exaust gas to directly heat the sea water. The exaust gas is too hot, and will cause the first part of the heat exchanger to heat up above the scaling tempreture. This will cause scale to form direct on the heat exchanger, lowering the efficency of the unit, and pushing the part the area that gets above the scaling limit to a larger part of the heat exchanger. It's hard for me to describe this effect, but it's basicly like trying to melt chocolate in a pan. The part of the pan directly under the fire gets too hot, and burns it. The solution is to use a liquid medium that you can control the tempreture of to make sure that the heat exchanger's surface never goes over the scaling tempreture. Basicly, you have to make a "double boiler" of some sort.

Again, all of this can be solved, but it typicly takes a unit that is rather large to make the economics of scale work out for the unit. As you can see, it will likly need several control units for the air removal system, the brine removal system, level controls in the unit, freashwater removal, and tempreture control. All of these issues have been solved, but they are why there are no mass produced, small size waste heat vacuum distilation units on the market. Instead everyone uses RO systems that don't have the size problems of the vacuum units, but instead work more efficently on a small scale with minimul control.
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Old 17-06-2011, 10:36   #33
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Re: Free fresh water

so what about using a corrugated steel inner still, Sort of like a steel bellows expansion joint thingy. Orientated vertically. First it would increase the wetted surface area lowering the corrugated steel temperature I'm thinking. Plus the corrigations would flex some causing lime buildup to break off and fall to the bottom. Use a 1" to 1-1/2" stainless steel bellows flexible hose, to use as the basis of the inner still. Outer casing is a 3" or 4" pipe

The attached sketch is just a basic idea. You would need additional ports for water level control, etc, etc. Plus you might be able to fabricate it in such a way that you can remove the inner still for cleaning / maintance etc.

Heres a link to the corrugated steel thingy

Metal Hose Assemblies - DME Flexible Metal Hose
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Old 17-06-2011, 11:28   #34
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Re: Free fresh water

I would be worried about material corroding and metal fatigue. Engine exaust contains acidic compounds, that should react with a flexable steel. If the flexable steel is moving with each vibration on the engine, it should rapidly reach it's fatigue life. Basicly, I'd expect the flexable metal to have a rather short life.

Some sort of stainless steel might work for the corrosion, but I'm not that fimilar with it's fatigue life.

Honestly, to make something like this work, you'll have to do something very very inovative. This has occured to just about every enginer that's ever thought about going cruising, including myself. None of them have been able to make something that is cost effective compared to RO units, including myself.
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Old 17-06-2011, 11:47   #35
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Re: Free fresh water

The folks that make the stainless steel flex hose use the exact same process for flex connectors for engine exhaust (it's on their website). The flex hose and engine exhaust connections are designed to flex, Plus they are stainless. They are very deep convolutions. So I'm thinking It would solve or greatly reduce the liming issues (installed vertically that is).

Put all the connections at the end caps with a level control outside. Sizing it to match the mass flows would require a little thought. But its doable.

Besides, you know you should know never tell an engineer it can't be done.
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Old 17-06-2011, 11:56   #36
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Re: Free fresh water

DOH. Use a stainless steel flex engine exhaust hose with a stainless water jacket around it. Then all the water jacket connections are simple
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Old 17-06-2011, 12:11   #37
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Re: Free fresh water

Fair enough.

I don't think it will work well. I still think your exchanger will scale up, and the whole deal will stop working. But I'd love to see someone try it out.

Instead of using the exaust gas to directly heat the seawater, you might think about running the freash water cooling fluid through the heat exchanger with the exaust after running it through the engine. After that, use the resulting hot fluid in some sort of a controled way to run your system.

This should allow you to use the cooling heat, as well as the exaust heat. I'm willing to bet the cooling system has roughly half the waste heat from the engine.
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Old 17-06-2011, 12:23   #38
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Re: Free fresh water

Oh Scaling is the big issue for sure. I'm thinking the flex joint might vibrate enough with the engine pulses that the lime/scale will break off, or through tempurature swings from cold to hot. Plus a lot more surface area so scale could be better tolerated. I'll let Tristen do the calc's as I'm just a big picture girl, and its his thread.
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Old 17-06-2011, 19:14   #39
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Re: Free fresh water

First, I would like to thank Sailorchic34 and ViribusUnitis for their involvement in this project. I've received a lot of valuable information from you - Thank You.
Getting on to the technical issues... I would prefer to have physical access to the inside of the boiler to be able to once in a while mechanically remove the solid products. On a regular basis an acid wash should take care of it, but I believe it wouldn't remove 100% of the accumulation.
The melt chocolate problem has been solved already where you melt chocolate - in the kitchen - and there they call it aluminum clad. I think if the thermal inertia of the whole boiler is increased by adding a thick layer of aluminum or copper (better even copper-nickel which has 75% of copper and good corrosion resistance ) it would keep a uniform temperature in the whole volume.
A similar effect can be accomplished by using inter-media substance like pressurized glycol or some high temperature mineral oils. This method has an additional positive feature - it makes the whole system more corrosion-tolerant but unfortunately add extra components to it.
If the system was working in continuous mode vs. batch mode (as sailorchic suggested) there wouldn't be problems with TDS because it would be flashed continuously like an RO membrane, but I do not know how the vacuum can be controlled in this case.
T.
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Old 17-06-2011, 20:02   #40
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Re: Free fresh water

Quote:
Originally Posted by Tristan View Post
First, I would like to thank Sailorchic34 and ViribusUnitis for their involvement in this project. I've received a lot of valuable information from you - Thank You.
Getting on to the technical issues... I would prefer to have physical access to the inside of the boiler to be able to once in a while mechanically remove the solid products. On a regular basis an acid wash should take care of it, but I believe it wouldn't remove 100% of the accumulation.
The melt chocolate problem has been solved already where you melt chocolate - in the kitchen - and there they call it aluminum clad. I think if the thermal inertia of the whole boiler is increased by adding a thick layer of aluminum or copper (better even copper-nickel which has 75% of copper and good corrosion resistance ) it would keep a uniform temperature in the whole volume.
A similar effect can be accomplished by using inter-media substance like pressurized glycol or some high temperature mineral oils. This method has an additional positive feature - it makes the whole system more corrosion-tolerant but unfortunately add extra components to it.
If the system was working in continuous mode vs. batch mode (as sailorchic suggested) there wouldn't be problems with TDS because it would be flashed continuously like an RO membrane, but I do not know how the vacuum can be controlled in this case.
T.
1. Control the vacuum through your condensor. Just let the condensor run as fast as it can. I'd figure that the raw water pump for the engine could send it's water through the condensor for your unit, before pumping it through the engines heat exchanger. The more steam it condenses, the lower the pressure will be. The lower the pressure is the faster it will make water, so lower is better. You'll just be limited by how fast the condensor can pull the vapor out, and turn it into water to be removed. You'll need some sort of pump to remove the water from the unit, when it reachs a high enough level.

2. Set the vacuum pump to a maximum pressure and tempreture switch. When the pressure is over say 3 psi, and the tempreture is over 150F then the vacuum pump is switched on, and removes gas from the area at the end of the condensor coil. This should be the highest concentration of air you can find in the system.

Level controls should be simple enough. A version of a toilet float to keep the level up to a specific level, and then a density switch (weighted toilet float) to turn on a pump to pump out the brine when it reachs too high of a level. Some sort of heat exchanger will be required on the brine line both to recover the heat that would be lost with the brine, and to keep the vapor pressure of the water going through the pump low enough to avoid cavitation in the pump.

I wouldn't try for a continuous operation, but more of a off and on operation. Continuously varible pumps and valves are expensive to buy, and difficult to design controlers for. Small diaphram pumps that are capible of pulling 10-13 psi vacuum would seem ideal.

I did alought of thinking, and working out the rough design for such. Once I started to price out materials, fabraction, and devinces to make it work, I got discouraged as the price ended up being well more than I origionaly was hoping.
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Old 17-06-2011, 20:23   #41
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Re: Free fresh water

after pondering it, I agree, batch blowdown will be easier with better overall control.

one other thought, watch out on the pressure drop/load on the raw water pump. too much friction loss downstream of the engine would reduce raw water flow which could cause issues in the fresh water tank, etc.
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Old 17-06-2011, 21:47   #42
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Re: Free fresh water

Okay, I don't understand 1/2 of what you are discussing, but I love the on-line collaboration of thinking this through, and it sounds like a solution may be found "on paper" before actually spending $$$.

Now if you need someone to run a cost/benefit analysis, I am pretty good with that stuff... grin.
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Old 18-06-2011, 02:42   #43
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Re: Free fresh water

Did the title say "free" ... this is getting complicated ... interesting though .... there's some real brainiacs on here ....
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