Reverse Osmosis water makers are really rather simple in principle. Pressurize sea water to around 800 psi, apply that to one side of an RO membrane and about 10% of pure water will 'sneak through' the membrane leaving salt and other undesirables behind. Add pre-filters, gauges and valving and one can produce all the water they need right on board ship. Complexity increases with the addition of Automation as well as any post-conditioning of the produced fresh water.
On Viking Star we use a Kubota EB-300 (EA-300??) single cylinder Diesel motor rated at 5Hp continuous. It sips fuel and seems to be very reliable. This drives three items:
- A 130A Leece Neville Alternator
- A CAT high pressure pump
- A Jabsco raw water impeller pump
Completing the conversion is a purchased heat exchanger and an assembled exhaust pipe/mixer made up of simple galvanized steel pipe followed by a conventional water lift muffler.
Our setup is like several others with three exceptions: use of the Jabsco raw water pump to both cool the motor and act as the boost pump for the pre-filters. I did this to eliminate the commonly used electric boost pump for increased reliability. I also have added an extra pre-filter not often mentioned but that I think is a good idea: A Petrochemical filter. A UV sterilizer on the produced-water output rounds out the differences in our water maker.
A Kubota EB-300 procured from E-bay. Stripped off all the extra items: Fuel tank, air-cooling fan / radiator, and exhaust. I cut the flange off from the exhaust and had a 3/4” MPT pipe nipple welded to it. The rest of the new exhaust was assembled using common 3/4" galvanized piping, making sure to have a very high loop before using a simple 'T' to inject the cooling water. From there on it is a classic wet exhaust system with a lift muffler again to a high goose neck before exiting out the transom just above the water level.
The frame was welded up using aluminum sourced from E-bay / Craigslist. I cut all the materials on the table saw and had a local welder MIG it up for me.
Using a Kubota-sourced spindle I was able to attach the appropriately sized sheaves to drive the alternator as well as the water pumps. The heat exchanger was purchased new from Next Generation Power. They themselves manufacture complete Kubota based generators and were willing to sell just the heat exchanger for a reasonable price.
One thing I wish I had not done was discard ALL the original Kubota fuel system. Specifically the Kubota engine mounted filter / shut off valve. The return nipple on the injector is VERY small, smaller than the smallest marine fuel hose I can find and I am having issues keeping it leak-free. The original Kubota routed this back to the filter assembly which then bridged to a larger fuel return nipple.
Note 4/2/2012: I changed out the return line from 1/4" Marine Hose to good quality (Gates) 3/16" hose and this fits the nipple well. No more leaks.
Many of the commercial examples advertise using a 200A alternator. I tried this as well and quickly found I can stall the Kubota pulling anything above 140A. They say a rule of thumb is 5HP for every 100A of alternator, and that does seem to be true. So I have no idea how one can get 200A out of a 5HP motor using a common alternator; even if using the 7HP intermittent rated engine... Currently I am using a Leese Neville 130A Alternator which seems just right along with an external 3-stage regulator.
|Viking Star Water Maker and Generator|
(Click for larger view)
Look at the attached diagram (click on it for a larger view) and you can see the setup. Water is drawn in through the sea-strainer, then routed to the 'Diversion' Valve which when closed will divert water into the pre-filters. When used as a generator this valve is left open and water flows past into the heat exchanger and out the exhaust. In Water Maker Mode, this valve is slowly closed diverting some water into the pre-filters. This valve is not fully closed but just enough to get about 20 psi on the pre-filters. The diverted water is pre-filtered, pressurized and then fed into the membranes where about 10% per membrane is separated out as 'fresh' water. The remainder is then retuned back into the raw-water stream to continue its path on to cool the motor and exhaust. Produced water is routed through a UV sterilization light and can then be tested, dumped overboard, or used to fill the water tanks.
|Pre Filters in Action|
Boost Pump: Here I did things differently and it has worked out well so far. Most designs use an impeller pump for cooling of the engine and a separate DC or AC motor driven pump for the pre-filters. I felt this introduced an extra element of unreliability. Those small DC pumps are OK, but they do seem to be a common breaking point. Plus, due to Viking Star's hull design, the aft of the boat is not very deep in the water. Most pumps used for boosting require a flooded inlet, and that would be for us hard to achieve. So instead I use a slightly larger - very reliable- impeller pump for cooling AND pre-filter feed.
UV Sterilization: This is another not-so-common item one sees on Water Makers. In theory, water passed through an RO membrane will be VERY safe. Most, if not all, dangerous things are larger than H2O molecules, and as such will be strained out. This includes bacteria as well as viruses. However, the danger is if there is a damaged part of the RO membrane everything can pass though at that point. Now a small hole will pass a small amount of salt water and one will unlikely be able to tell, however even a small amount of Bacteria or Viruses can multiply and grow in your fresh water tank. So, adding the UV sterilizer is like Belts and suspenders.
In operation the unit is used either as a Generator or a Water Maker. A future enhancement is to put on a smarter regulator which will allow for about 50A to be generated while making water. But for now it is one or the other. As a generator we currently get around 100-110A max. At the peak regulated voltage of 14.45v this is almost 1600w. Anything more and we'll start to overload the Kubota as indicated by either Black exhaust, excess exhaust temperature, and/or inability to respond to small changes in the 'throttle' - acting Lugged Down.
In water maker mode we produce anywhere from 32-36GPH from the two membranes. One run in 51oF water produced 36GPH from 2.5 GPM byproduct outflow (corrects to 3.1 GPH input when adding back in the 'produced' water) and a bit over 800psi. In operation, I need to keep the outflow below 50 GPH (at 77oF ==> 30 GPH in 51oF water ) and 1,000psi. In operation it is really these two parameters (outflow and Max pressure) that are the limits to watch. So you can see at 36 GPH, I am actually running the system over capacity and SHOULD back down on the pressure.
Why not "40 GPH"? Well, the Dow membranes I am using are rated at 77oF and as water temperature changes their output is impacted. Here is a simple correction table, more details can be found at the DOW web site: http://www.dowwaterandprocess.com/
Temperature vs Rated Fresh Water Production
% of Rated Production
Our TDS meter quickly settles into the 110-140ppm range, seems anything under 500ppm is good, and under 1000ppm is acceptable. So, we must be doing something right!
When finished making water I open the HP Valve slowly and flush the system using fresh water from the house water system, the Charcoal filter removing any chlorine to protect the membranes. About 5 minutes seems sufficient. If we do not produce water every 4 days or so, I do another flush to sweep things out. I understand one can keep the system fresh indefinitely doing this 'flush' every 3 days or so, or one can pickle the system with preservative for longer layups. The issue as I understand it is Sea water actually has lots of small living things in it. These things die. And when they do they start to decompose, and start to stink. That is the purpose of Flushing, to flush out these 'dead' things. If you do not you will get the stink. This happened once to us but running the water maker for about 20 minutes cleared things up. Normally we run it for about 5 minutes or so before we start filling the tanks just to let things settle down. This, BTW, is another thing I like about the simple output hose, we always taste the water after the TDS meter says it is OK, and before putting it into the tank. Easy to spot the Stink!
RESULTS and WHAT WOULD I CHANGE?
Overall we are OK with this setup. The use of the common pump for cooling and pre-filters is working well. There has been no indication of 'stuff' feeding back into the membranes. That is what the Check Valves are for, but I am also careful to close the valves on both ends when not making water. Some specific things I would do differently and/or are looking to change:
- Stiffen up heat exchanger flanges to allow for better seal and stop coolant leaking
- Pre-heat water by routing through heat exchanger 1st.
- Better alternator regulator to allow for some generation while making water
A3rd membrane in series with the existing two would give us perhaps another 15 - 20 GPH. I have seen many commercial examples where they use 3 membranes in series, though with each additional one I understand the amount of produced water is lower due to it receiving feed water starting with a higher salt concentration due to the prior membranes.
Using the hose for water outlet has worked well, it is easy to select 'dump', test, or stuff it into the water fill. This also allows for me to easily fill someone else's tank or jug if the occasion arises. Using a simple hand held test meter works well, and we like the Taste Test to confirm. And while waiting for things to settle down I use the hose to rinse off salt accumulations from the cap rail!
The 20 and 5 micron pre-filters need to be cleaned every 10-15 hours or when dirty. Mostly the 20 micron one. I just remove them from their housing, tie them to a small rope and dunk them overboard for a few days (NOT while underway). The rocking motion of the boat agitates them clean, along with a whole host of small critters who come to feast on any algae in the filters. A few days out in the sun to 'bleach' them out and they are ready to go back into service. Next year will likely use two sets of pre-filters. One filtering and one 'cleaning'. The Oil filter does not need regular cleaning as long as the other pre-filters are doing their job.
I am a bit disappointed not being able to get 200A out of the charging system. I had read about the 5hp = 100A Rule of Thumb, but then noted all these commercial setups advertising their 200A ratings! We use about 200-300Ah / day, and being limited to 100A we have to run the generator about 2-3hr/day. Doing it again I might look into using the 2 cylinder / 13HP Kubota. THAT would have no issue driving the 200A alternator.
And overall, I might reconsider the whole DC generator and instead put in an AC generator. Why? I understand AC generator heads are more efficient then common DC alternators, might be able to get 2500W out of this 5HP motor instead of the current 1500-1600W. And in our actual usage I had not considered how often we run the clothes washer/dryer. We end up using it about 2-3 times a week. So, a possible scenario is to install say a 5Kw generator, run it 3 hours every other day to wash clothes and provide 200A charging via the two inverters we have on board. Combined with an AC driven water maker - we could just have a Busy / Noisy time and do it all at once!
Or, given the rise of those wonderful Honda 2,000w generators (we have one for a backup) - perhaps something with that. One person I ran across installed a dedicated 20A charger and direct wired it to an extension cord. He runs his Honda all day humming along at idle while quietly putting 20A per hour back into the batteries. That gives him almost all the power he needs for his very large boat. Perhaps that and some other solution for the water maker.
But we have what we have, and it is working well. Perhaps if I added some sound insulation around the system that will make me happier even if it is not really that noisy, less so then most other boats 'generators' I have listened to. Perhaps adding some Solar panels would cut down on the times we have to run it. (but, that is another topic I guess)
One hint: make sure you install all this in an area that can get wet. Changing and bleeding the pre-filters is a wet process, so make sure nothing around them will get damaged by sea water.