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More on watermakers

Jun 7, 2012

To the editor: In the recent article by Mark Nave addressing watermakers (Fresh benefits, Ocean Voyager 2012), I couldn’t help but notice that there are a few inaccuracies and misleading statements presented as facts.

Seawater desalination requires 800 psi of pressure in most systems. The actual pressure needed varies depending on a significant number of factors, however, most system manufacturers design their equipment to operate at 800 psi, not 100 psi. This may seem like a minute detail, but when you apply this towards an overall energy budget it makes a significant difference in the power required and consumed. For example: a system with energy recovery, such as our Ventura 200T, has a power consumption of 1.2 amp-hours/gallon, with a net draw of roughly 9 or 10 amps on 12 VDC. A traditional system with no energy recovery could require as much as 120-plus amps at start up, and 35 amps in normal operation to produce the same amount of water on 12 VDC. Certainly these factors need to be taken into account when evaluating the difference between a traditional RO system and integrated energy recovery systems. Also, a minor — but important — note, I believe Nave really means amp-hrs/gallon not gallons/amp-hour. The latter of which suggests much higher output for a given amount of energy consumed.

Nave also does your readership a disservice when he suggests that a five micron prefilter will last 60 running hours, or 3,840 gallons of water through-put. Pre-filtration is directly dependent on the feed water source in which the system is being operated. This may seem intuitive to most, but the disparity between the operating life of a cartridge filter in the Chesapeake Bay is significantly different from that of one on an offshore voyage halfway between Seattle and Maui. To suggest that the life of a filter is so defined ignores countless variables in system design, feed flows, the number, size and type of filters installed, as well as the location in which the system will be operating.

In reference to Nave’s line about desalinated water being “slightly acidic,” this is incorrect. The pH of the permeate water from any desalination system will be directly dependent on the composition of the incoming water. If the source water has a high CO2 content, then it is possible that the permeate from the watermaker will have slightly lower pH than the feed. Our experience has shown this circumstance to be extremely rare, and I do not believe we have ever been contacted regarding a watermaker’s permeate water having eaten through tanks or feed lines.

I am also concerned about his suggestion to cruisers that operating a high DC load, such as a watermaker drawing 35 amps on 12 VDC, while charging batteries will have no net effect on the duration and effectiveness of a battery charging cycle. Perhaps this issue is addressed later in his book, but his suggestion needs to be part of a much larger “load management” evaluation in which a vessel owner, at the time of outfitting, decides which loads are going to be operated on batteries alone, which ones can be powered from an AC generator, and which battery driven loads should only be operated with a charge source present. The DC refrigeration system, radar, autopilot, watermaker, and all other DC loads will add up, and can quickly overload a charging circuit, rendering it unable to actually charge the batteries.

Absent from Nave’s evaluation on the merits of low vs. high current DC systems is any reference to the ability to operate on renewable power sources, such as a wind generator or solar array. As fuel prices rise and cruisers reach out to farther and more remote destinations, it is becoming more and more prevalent that owners outfit their vessels with renewable energy generating devices. The use of a traditional non-energy recovery type watermaker would all but preclude an owner from being able to run their system while charging the batteries from a solar array or wind turbine, forcing them to install a diesel generator or run a main engine to charge the batteries.

Another point of contention comes up when Nave suggests that energy recovery systems are more complicated than traditional equipment. Energy recovery style systems operate on the simple principle of hydraulic pressure amplification. Most small-scale “energy recovery” type systems available today are in some way clones of our proprietary Clark Pump, which have the significant advantage of a much slower cycle rate. A traditional watermaker will often run at 3,600 rpm, whereas a Clark Pump will cycle anywhere from five to eight times per minute. This greatly increases seal life, can reduce pulsations at the membrane, eliminates operating adjustments in variable salinity waters, and in general introduces far less load on the pumps and motors, not to mention a significant difference in noise levels. Traditional systems have a high-pressure pump fed by a low-pressure boost pump, with cartridge filters between, and some system of controls. Energy recovery systems have the same basic components, with no more complication in the high-pressure intensifier than within a standard, off-the-shelf, triplex plunger pump.
    
—Kyle Stephan is general manager of Spectra Watermakers in San Rafael, Calif.

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