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= Watermakers Produce Fresh Water Every Day =
Before the invention of devices to desalinate seawater, sailors had to carry enough water on board for an entire voyage between land-falls. Water stored for long periods attracts bacteria, making it taste sour. Modern watermakers remove this burden, producing fresh water every day. Although there are several means of desalinating seawater, the only practical marine application is a [https://en.wikipedia.org/wiki/Reverse_osmosis reverse-osmosis ] watermaker.
For example, a relatively new watermaker technology uses a type of dehumidifier to condense water from the air. Manufacturers include companies such as Air Eater Proprietary Limited, Hyflux and Vapaire Water Purifier. But the capacity of such systems is limited, around 1 litre/hour (6.25 gal/day), in conditions of 45 to 60% humidity. These systems, while interesting, clearly do not meet the daily output requirements of a boat.
Reverse osmosis is the process of causing pure water to move out of the solution and through the membrane to the other side. To accomplish this, a high-pressure pump must be used, around 800 psi. The process is not very efficient, either. About 90% of the seawater and energy are wasted. In some designs, notably Spectra and Livol, this energy is partially recovered by applying it on the backside of the piston in the pump.
=== Pre-Filtration ===
To prevent clogging the membranes, a pre-filtration system can remove particles of sand and organic matter down to 1 micron in size. Install a back-washed sand filter at the inlet. Use a series of filters (e.g., 50-25-5-1 micron) to remove other particles. If you put the finer filter first, it will clog immediately. Install an oil-water separator to remove oil. An absorption/reactive carbon filter will remove taste, odour, chlorine and organic chemicals.
=== Post-Filtration ===
A second carbon filter can capture chemicals not removed by the reverse osmosis membrane. An ultraviolet lamp can sterilize any microbes that may escape filtering by the reverse osmosis membrane.
== Right Sizing the Capacity ==
The small unit would cost a lot less, too. So, the immediate temptation is to go with a smaller unit running 24 hours/day. But most watermakers are not rated for continuous duty, and even if they are, running any equipment continuously will wear it out sooner. Plus there is the factor of noise.
At the other extreme, very large watermakers are not available in DC models and require AC to keep the amperage and wire size manageable. but But on a boat AC is not as efficient as DC because DC electricity from the house bank has to be converted to AC using an inverter. The inverter also has to be larger, and more expensive, if we run the watermaker on AC.
Alternatively, a large watermaker could be run directly off the engine during its daily run to charge the batteries. The drive for the watermaker would have to be clutched, to disengage it on long engine runs.
Somewhere between these two extremes is probably best, but exactly where? A lower duty cycle increases longevity; while we would expect larger capacity gives greater efficiency, but this doesn’t hold true (see Table below). So within reason bigger is better.
For boats other than superyachts, the optimum choice is a large DC watermaker running for several hours a day, rather than an AC model running for a shorter time but requiring an over-sized inverter. The optimum, considering efficiency, backwashing, wear and tear, etc., could be running the a medium-size watermaker for two or three hours every three days.
This will probably necessitate designing the [[ElectricalCapacityDC|electrical system]] for 24 VDC; although two 12-VDC units could be put in parallel.
Once you have determined the capacity required in GPH, pick a model close to this which has the best GPHA (GPH/AH) rating. Next, analyse the impact on the electrical system. If that doesn’t calculate-out the way you would like, repeat the exercise with a modified scenario.
== Installation and Maintenance == For cold water expeditions, the water intake to the watermaker should be preheated.
See [[WatermakerInstallation]] and [[WatermakeMaintenance]].
[[Category:FreshwaterSystems]]