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Go Back   SailNet Community > Out There > Cruising & Liveaboard Forum > Flash Evaporation?
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Topic Review (Newest First)
02-06-2013 08:03 PM
TwoD83
Flash Evaporation?

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!
02-05-2013 11:52 PM
RichH
Re: Flash Evaporation?

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.
02-05-2013 11:42 PM
TwoD83
Flash Evaporation?

@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!
02-05-2013 12:25 PM
sd1953
Re: Flash Evaporation?

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.
02-05-2013 11:26 AM
RichH
Re: Flash Evaporation?

A significant problem with evaporators is constant scaling of and fouling by carbonates .... anytime seawater is raised above ~153įF 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.
02-05-2013 09:35 AM
CaptTom
Re: Flash Evaporation?

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.
02-05-2013 08:57 AM
sd1953
Re: Flash Evaporation?

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.
02-05-2013 06:14 AM
CaptTom
Re: Flash Evaporation?

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.
02-05-2013 12:06 AM
TwoD83
Flash Evaporation?

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
01-31-2013 06:04 AM
CaptTom
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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