# Flash Evaporation?



## CaptTom (Aug 27, 2012)

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.


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## bigdogandy (Jun 21, 2008)

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.


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## CaptTom (Aug 27, 2012)

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.


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## SansaBoat (Feb 23, 2010)

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


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## sidney777 (Jul 14, 2001)

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.


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## CaptTom (Aug 27, 2012)

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.


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## RichH (Jul 10, 2000)

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.

DIY 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. https://sites.google.com/site/llichtveld/newpage

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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. https://sites.google.com/site/llichtveld/newpage


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## CaptTom (Aug 27, 2012)

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.


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## TwoD83 (Apr 6, 2011)

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!


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## CaptTom (Aug 27, 2012)

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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## TwoD83 (Apr 6, 2011)

This thread caught my eye because I was contemplating building one myself. I have not seen or heard of any small enough for a boat. They are quite simple to design (even the large industrial units have three moving parts!).

Once the vacuum pump has initially evacuated the shell of air, introducing water into that shell will immediately produce some vapor to try to equalize the pressure (nature abhors a vacuum). This provides the starting steam. By running cooling water through the condenser portion, you will condense some of this vapor back into clean, desalinated water. The process will be very slow at this point, you need to add heat to the feed water. Almost any heat source will work (ships do, in fact, use diesel engine jacket water for this purpose, or steam from a boiler, or even electric if necessary). The more heat you add, the more water you'll produce. The heat you can recover within the cycle, the more efficient your system will be!

This all applies to most common flash evaporators used in industry. A type that I've not directly dealt with, but to me seems more user friendly, efficient, and better suited for boats is the vapor compression type. It uses a similar cycle to a refrigerator to recycle energy while using very little. If you find any commercially available evaps for boats, let me know! Good luck!

Here's a site I found after a quick search with some good diagrams: http://maximevaporators.com/PageDisplay.asp?p1=6097


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## CaptTom (Aug 27, 2012)

Yes, that's one of the sites I looked at when researching this. It seems quite feasible, but I'm not sure how much energy it would take to get enough vacuum to boil water heated by the engine. I know you can get water to 150 degrees or so pretty easily. You can look up the pressure at which that would turn to steam. What I haven't found is a way to estimate how "big" a compressor you'd need. They seem to rate them by flow. We need to know how fast they can take a given volume of air down to a certain pressure, and how much power they'd draw doing that.


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## sd1953 (Mar 21, 2010)

The larger ships have dry exhaust up a stack. Your sailboat will not. The ships run the engine all the time (or at least a large diesel electric gen set), your sailboat will not (hopefully!!). The ships have acreage to spare for an engine room, your sailboat does not (or if it does, we NEED pictures!!!!). 
The biggest problems I see are redesign of the diesel exhaust system and real estate dedication in the engine room. 
The biggest advantage to an RO watermaker is that you can add it to an existing cabinet space without a complete mechanical redesign of the existing systems and pipe the water to and from it. It is an easier add on. If you are starting with an empty shell of a hull and designing the mechanical systems first, then you have a shot at making this work.


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## CaptTom (Aug 27, 2012)

I was thinking of using heat from the jacket water, not the exhaust. You'd use the same two hoses that are used for an engine-sourced cabin or water heater.

Given the current draw of an RO system, you'd probably only be running it when you're running the engine anyway, so this would be no different.

I wasn't looking for a ship-sized unit. In fact, my question was basically about whether flash evaporation COULD be scaled down to fit in the engine compartment or under a cabinet.


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## RichH (Jul 10, 2000)

A significant problem with evaporators is constant scaling of and fouling by carbonates .... anytime seawater is raised above ~153°F carbonates precipitate. Usually in 'industry' flash evaporation for the production of 'purity' water is preceded by demineralization / RO, etc. and thats for 'fresh' water infeed.


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## sd1953 (Mar 21, 2010)

Capt. Tom,
I am not trying to be difficult, I am trying to get educated. I just bought a sailboat. It has a seawater cooling system that cools a fresh water loop which in turn cools the diesel. I thought I understood your intent was to flash evaporate the seawater to condense out fresh water to add to the potable water supply. You would not flash all the seawater and you would get two streams out of the system, one fresh (no salt) the other brine (higher salt content). Would you then run the brine into the exhaust? Would the flow be enough to cool the exhaust gasses and to muffle the sound? Would the hot brine not eat your mixing elbow LOTS faster than just the seawater does?
It seems to me that the fundamental technology exists, you have three problems to solve; one, design specs for a system that can work off the amount of waste heat from YOUR diesel installation; two, real estate allocation in your machine spaces; and three materials choices for the environment you will create by increasing the salinity of the waste stream.
Again, please understand, I am not trying to be argumentative, trying to get educated. It seems like a good idea.


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## TwoD83 (Apr 6, 2011)

@Capt. Tom
It is true there would be many thermodynamic equations needed to actually design a working system, but I do know that if properly designed, the energy required to run the compressor would definitely be less than that required by the high pressure pump of an RO system. Also, the energy used by a compressor to compress the gas manifests itself as added heat in the gas. Normally this is considered a loss, but in this case it's the heat that you want, so it's actually helping you.

@RichH
It is true that scaling is a problem, but I've never heard of an RO being used to pretreat evaporator feed water. Evaporators produce cleaner water than RO out of dirtier water than RO's can handle. The standard pretreatment is a dosing pump with a chemical anti-scaling and anti-foaming agent, though this may not be necessary for the size of unit we are talking about.

@sd1953
We're all here to be educated! An evaporator is roughly box shaped, whereas most boat RO's are long and narrow. The evaporator would not necessarily be 'bigger', just differently shaped and it need not be in the engine room. It can be anywhere you can run power to. The primary heat source would ideally be engine jacket water, the engine's saltwater cooling loop would not be interfered with (if you want to chance it, go ahead, but for all the reasons you listed, I wouldn't!). The evaporator is in a vacuum while operating, so it can draw in feed water with just a float regulator and not a feed pump, but it will need a brine pump to discharge the brine overboard. A nice addition is that the hot brine can also be used to preheat the feed water, saving more energy!


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## RichH (Jul 10, 2000)

TwoD ... if you dont thoroughly demineralize and pretreat, the Coefficient of Ht transfer drops from an idealized 400 U(f) to about 50-80 U(f) due to fouling ... that drop in Heat transfer coefficient is pretty much indirectly proportional to the needed surface area of transfer, to cover the expected fouling. Flash evaporator 'may' produce equal purity or even better purity ... but not when fouled, especially when operated in sea water and the coefficient of heat transfer becomes severely limited by the high fouling.


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## TwoD83 (Apr 6, 2011)

Scaling only occurs on surfaces where the seawater is flashing, leaving behind scale if not treated. In a system that is being operated correctly, flashing only occurs in the flash chamber and not in any of the heat exchangers. This eliminates the possibility of scaling on the heat exchanger surfaces. If excessive scaling occurs in the flash chamber, you need to increase the seawater flow to reduce brine concentration.

An inlet strainer (which should be on anything that uses seawater) will remove most marine life/flotsam and the chemical dosing combined with the conditions of the evaporator will kill anything else and keep it from building up.

I ran an evap for 6 months continuously on my ship with only a strainer and a small chemical dosing pump for pretreatment. Worked perfectly fine and upon shutdown, there was very little fouling on some parts, none on most. The zincs needed to be replaced, but that was pretty much it!


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