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CaptTom 01-01-2013 07:37 AM

Flash Evaporation?
The water filtration thread got me thinking:

I see a lot of reverse osmosis watermakers, but it's a very energy-intensive process. Why hasn't anyone invented a flash evaporation process for small boats?

That's what the cruise ships use. Waste heat from the engines raises the temperature of the water, and the pressure is reduced until it "flashes" to steam, which is condensed back into pure water. I tried researching how much vacuum would be required to boil, say, 140 degree water. But it's hard to convert the ratings of vacuum pumps to figure out whether this would be more, or less, efficient than reverse osmosis.

If it worked, you could use your waste engine heat while motoring to make pure water.

bigdogandy 01-01-2013 09:19 AM

Re: Flash Evaporation?
I think you'd need to reduce the pressure to about 3 psi if the water is heated to 140 degrees F by the engine. A small refrigeration compressor should handle that quite easily. Inject the warmed water into an expansion tank, use the compressor as a vacuum pump to create the 3 psi environment, and then collect the vapor that is released.

Not sure how much capacity you could generate doing this with a sailboat engine, but this would be an interesting exercise for someone that had access to some machine tools, an expansion tank from a fresh water system, an old refrigeration compressor, and some piping and valves.

CaptTom 01-01-2013 09:39 AM

Re: Flash Evaporation?
I hadn't thought of an A/C compressor, that makes sense.

But what if you used a compressor out of a car, instead of a fridge? You could run it right off the engine, no 120V supply needed. They put A/C in some pretty small cars, so there must be belt-driven compressors that consume very little HP. I wonder if they could pull enough vacuum.

I also like the idea of using a common potable water expansion tank, at least for the experiment. Now we just have to come up with the condenser side.

SansaBoat 01-01-2013 11:38 AM

Re: Flash Evaporation?
I think the controls and equipment do accomplish this are far more complicated than is reasonable for anything smaller than a 60' yacht. I work with evaporators in industrial plants, and they are not easy to operate, and require sophisticated controls and maintenance.

That being said, an RO is a fairly high-maintenance item as well!

I found that there are companies making them for marine applications. Interesting....

HJ Series - Heat Recovery Evaporators

sidney777 01-01-2013 11:53 AM

Re: Flash Evaporation?
I met 36 ft trawler owner who used water from his -roof top air conditioner -(like RV A/C)- run by generator , while in the Bahamas. You may want to review safety opinions for drinking this water, but he & wife are still alive. He didn't mention any treatment to this water.

CaptTom 01-01-2013 12:08 PM

Re: Flash Evaporation?
SansaBoat, I've read the web sites of a number of manufacturers of large units for commercial ships. The idea is pretty simple but yes, the details get complicated. This is just a thought experiment at this point, but I suspect if we ever got it working, things could be scaled down and a controller programmed to make it at least as simple as an RO system.

Sidney777, condensed A/C water is pure, but I suspect it could harbor bugs (remember Legionnaire's disease?) if the surfaces it condenses on aren't cleaned regularly. Still, it gives a good example of how much water one can collect just from condensation.

RichH 01-01-2013 01:13 PM

Re: Flash Evaporation?
RO is much less energy intensive than 'flash evaporation' .... takes a whopping amount of energy to change water from liquid phase to vapor phase and to bring the 'pressure' down so that the vapor pressure of the water is suitable for subsequent condensation back to liquid phase (+ energy for the vacuum pump & the pump to drive the needed Hx,condenser').
Normal RO boat systems consume ~4HP to deliver ~10-20 gallons per hr.; a flash distiller will need a high vacuum pump plus independent Hx ... just a wild assed guess but probably in the range of 10-12 HP per that 10-20 gallons per hour from a flash distilling column. The only 'economizer' possible would be to preheat the infeed water to the flash distiller by engine exhaust (another Hx) .... the BIG energy consumer is going to be the needed vacuum pump.

On boats with 'large' engines (or 'boilers'), yes its possible and is done that way; on small boats you dont have that much 'waste heat' to do the job very efficiently. RO is much more energy efficient than 'distillation', simply because the 'water' doesnt have to go through TWO 'changes of state'.
The 'change of state' ... liquid to vapor & vapor back to liquid is the 'energy consumer'; there is no 'change of state' with RO.

Here's a fairly good 'write up' on cobbling together your own DIY, 'manually controlled', RO ... especially if you can get the hardware components from ebay, Craigs List, etc.

The problem with AC condensate drainage is its quite high propensity to grow some VERY pathologic bacterial species - "legionella" - (Legionnaires disease) and other similar potent 'toxics' is fairly common. But this of course is in an 'open to atmosphere' system and one can contain/control then 'filter' the input and output air to remove such 'spores', just takes energy and totally 'hygienic' regimen. Such 'toxics' are transmitted via the 'mists' that are created across the 'cooling plates' and the bacteria aspirated into the lungs. Its a quite rare occurrence, but 'can' happen under the 'right' conditions. (A good reason to clean your boat AC evap. plates on a routine basis.)

Boat RO is also be fairly cheap if you DIY an engine driven, totally 'manual' system: pressure washer pump, RO membrane + housing, and a high pressure control valve ... some gages, filters, and strainers, 'hydraulic' hosing, etc.

Here's a fairly good 'write up' on cobbling together your own RO ... especially if you can get the hardware components from ebay, etc.

CaptTom 01-01-2013 03:12 PM

Re: Flash Evaporation?
Rich, thanks for the analysis. The HP required to pull sufficient vacuum was always the unknown in my calculations. Still, 10-12 HP? I know a car air conditioner compressor doesn't pull that much.

Also, thank you for the link. Reading it, I'm struck by how complex the RO process is. Even if energy-wise the flash process was a wash, it may be simpler in the long run, not only while running, but in routine maintenance, lay-up and pickling.

And yes, the idea was to preheat the feed water from engine heat. You probably wouldn't use jacket water directly, on the off chance it had some lubricant or fuel residue in it. Although that should never happen. If you had a fresh water cooled engine, you could probably use raw water after it's been through the heat exchanger. Or use a dedicated heat exchanger the way a water or cabin heater does. And of course it would depend on how often you run your main. Just as an engine-drive RO system does.

TwoD83 01-31-2013 04:10 AM

Flash Evaporation?
Flash evaporation is more efficient by far. That is why ships use it. The vacuum pump's purpose is not to 'create vacuum', but to remove non-condensible gasses (mainly air). The vacuum is created and maintained by the steam vapor collapsing into liquid. The huge reduction in volume from the gas becoming a liquid is the main contributor to the vacuum.
As for the heat requirement, in a properly designed flash evaporator, most of the heat is reused back into the cycle. The cooling water used to condense and cool the distillate is at the same time preheated and then introduced into the flash chamber with very little additional heat added. I've worked with flash evaps and RO's for several years. Both have their advantages and disadvantages, but in terms of energy efficiency, evaporators win with ease!

CaptTom 01-31-2013 06:04 AM

Re: Flash Evaporation?
Thanks TwoD!

I was assuming most of the energy went into getting the pressure low enough that warm, engine-heated water would flash to steam. I'm taking a weather course, and right now we're studying the gas law. So it makes sense that the pressure is reduced when you cool the steam.

Which changes my whole understanding about how the system works. Sounds like you need steam to begin with, something the small mains on our boats aren't good at producing.

Still, I'd like to hear your opinion on whether ANY form of flash evaporation would make sense if scaled down to the point where it was comparable to off-the-shelf recreational RO systems. Even if we had to put energy in to make small amounts of steam, it might end up simpler than an RO system.

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