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High pressure at low power

Oct 21, 2009

Voyagers are surrounded by water, but drinking seawater is not an option. Fortunately, osmosis, the natural physical process that makes drinking seawater fatal, can be reversed and used to extract potable water from seawater. However, reversing what nature has created always comes at a cost; in this case we need a substantial amount of energy to overcome the natural osmotic pressure that would move fresh water into saltwater.

Although the devices used to provide drinkable water on board our boats are commonly called “watermakers,” they don’t make water, they remove enough of the dissolved solutes in seawater to produce water we can drink. While there are a number of ways to produce drinkable water from seawater: distillation, electro dialysis, ion exchange and reverse osmosis, the latter, commonly called R/O, is the system of choice on yachts. The water produced by an R/O system will generally be entirely suitable for human consumption, with the possible exception of viral contamination, a problem that can be eliminated by treating the product water with a UV sterilizer.

We can reverse the osmotic flow by increasing the pressure applied to a high solute solution (seawater), forcing it through a membrane whose pores are too small to allow the relatively large solute molecules to pass through. The pressure required to separate out the undesirable materials dissolved in the intake water varies with the concentration of the solutes and the temperature of the water.



High pressure water

For seawater, the required reverse osmotic pressure ranges between about 800 and 1,180 pounds per square inch. The fact that a substantial amount of energy is required will be attested to by anyone who has used a hand-powered reverse osmosis device to produce a liter of drinkable water from seawater. The theoretical minimum amount of energy required to produce a liter of drinkable water using R/O is 0.767 watt-hours/liter, accounting only for the work required to overcome the osmotic pressures in the system.

Actual energy consumption for small, 12-volt-powered R/O systems ranges from about six to 12 watt-hours per liter, with the exception of systems that recover energy from the pressurized brine flow. The Clark-pump-equipped units from Spectra Watermakers can produce drinking water at an energy cost as low as 3.8 watt-hours per liter. (These energy consumption estimates do not include the power consumed by feed water pumps used to ensure a constant flow of seawater to the R/O system).

Selecting an R/O system for your boat will largely be dictated by the boat’s electrical power system capability. Boats equipped with AC gensets can choose among a wide range of AC motor-powered R/O systems. Those limited to using only 12- or 24-volt DC power are necessarily more limited in their choices.

Reverse osmosis desalinators suitable for use on boats differ primarily in the type of pump used to create the water pressure required to overcome the osmotic pressure of the seawater and the pressure of the brine — the seawater that contains the solutes rejected by the R/O membrane, the waste product of the desalination process.



Multi-piston pumps

Typical systems deliver between one and two liters of fresh water for each 10 liters of intake water. The most common pumps are multi-piston units similar to those used in the pressure washers sold at hardware stores. For marine R/O use, however, they must be made of metals that can withstand constant exposure to highly corrosive, high-pressure seawater. Many of the pumps are made of 316L stainless steel and, depending on size, can pump between 0.5 and 10 gallons of water per minute at a pressure of 1,000 pounds per square inch. Some manufacturers use titanium, a metal highly resistant to corrosion, for the pump heads.

The piston pumps in the smallest 12-volt DC powered R/O systems are usually belt-driven from a fractional horsepower DC motor. The Offshore Marine Laboratories Sea Quencher can produce up to eight gallons of drinking water per hour (depending on seawater temperature) operating from 12-volt DC power with a current drain of 17 amps, 6.62 watt-hours per liter. Larger R/O systems are usually powered by direct-coupled AC motors and therefore can be operated only when a supply of line voltage power is available (although it is possible to power some of the smaller AC motor units from a boat’s DC/AC inverter).



Clark pumps

An alternative method for supplying seawater at the pressure required for operation of a boat’s R/O system employs a Clark pump, a type of hydraulic amplifier in place of a conventional positive displacement motor-driven pump.

A Clark pump can be considered to consist of two pistons of different surface areas connected to a common piston shaft. A high volume supply of relatively low-pressure (typically less than 100 psi) seawater is supplied to the larger piston from an external motor driven pump. The resulting displacement of the large piston moves a rod connected to the small diameter piston, creating water pressure in the cylinder bore that houses the small piston in the ratio of the difference in the diameters of the two pistons.

For example; a piston area ratio of 10:1 will produce a high side pressure of 800 psi when the pump is operated from a low side supply pressure of 80 psi. The volume of 80 psi water will have to be 10 times the volume of 800 psi water required by the R/O system. While the amount of work that must be done by the low pressure water pump might be somewhat less than that required by an equivalent high pressure piston pump, the efficiency gain would not in itself be sufficient to justify the use of the Clark pump. However, the Clark pump system makes use of the pressurized “reject” water or brine that exists on the seawater side of the R/O membrane to improve the overall mechanical efficiency of the water pressurization process. This energy recovery technique allows the Spectra Catlina 300 R/O system to be specified by the manufacturer to produce 12.5 gallons of product water per hour with a 12-volt DC current drain of 15 amps, 180 watt-hours, 3.74 watt-hours per liter. (An analysis of the operation of the Clark pump can be found at the Spectra Web site: www.spectrawatermakers.com/articles/CREST_Clark_Pump.pdf.)

A number of additional considerations must be accounted for when installing an R/O system. A dedicated seawater intake with a seawater strainer will be needed and a discharge port for the reject water (brine). One or more prefilters are necessary to ensure that the most debris-free water is presented to the system’s pump. A raw water feed pump may be required to ensure positive water pressure at the system’s high pressure pump. Although R/O systems should normally be operated only in clean water there may be times when it becomes necessary to use the system in areas where oil contamination may exist. Special oil excluding filters will be needed in such circumstances. Spare filters will be a necessary part of the boat’s stores.

The joy in having a watermaker on a boat can extend beyond being able to take as many showers as you wish. For some mariners the ultimate R/O system experience occurs when they are able to use fresh water to wash down the anchor chain before sending it below into the chain locker.
 

Osmosis and reverse osmosis



Osmosis is the movement of water molecules through a membrane (the walls of our cells) from a region of low solute concentration to one of higher solute concentration. Simply put, if you separate two containers of water with a permeable membrane and add salt to one of the containers, osmotic pressure will force water from the no-salt side to flow into the water containing the salt.

The fluid in our bodies is in an isotonic state (equal amounts of solutes — salts — throughout. Seawater typically contains about 35 grams of various dissolved elements per liter or kilogram; Chloride (54 percent), Sodium (30 percent), Sulphate (7 percent) Magnesium (3 percent) and about 1 percent of Calcium and Potassium. Drink seawater and it will create a hypertonic (high solute) solution in the stomach and gut. The lower solute concentration fluid in the surrounding cells will move through the cell membrane into the high solute fluid, dehydrating the cells, damaging them or at the extreme causing them to collapse and die. Rather than quenching our thirst and hydrating our bodies, the ingestion of seawater or any other high solute solution will add to our thirst and dehydrate us. This effect is often demonstrated in school biology labs where a small amount of salt applied to the body of a common garden slug (a snail without a shell) will draw water from its cells, killing it.

Chuck Husick
 


Watermaker case study



We have a small watermaker on our boat Bahati, a Katadyn PowerSurvivor 80E. It was the only one that would fit where we needed to put it. It has served us well to-date.

We use it when we are offshore only to help augment what we carry in tanks (60 gals) and in jerry cans (36 gals). We tend to run the watermaker when we are running the engine as it draws quite a lot of amperage for our power supply (500+ amps). We run the engine about one to two hours per day, typically for the fridge etc., so we can make about six to eight gals/day which is enough to top-off the port aft tank which we draw on first and into which the watermaker fills.

So far (knock on wood!), we have had no major issues with the Katadyn. We bought it before we left Maine in 2006. We clean the filters regularly — whenever they look like they need it — and we pickle it when we are not using it for long periods of time. Both those processes are relatively simple. It runs quietly and requires little maintenance, just a little grease on the pump piston and cleaning only.

I would’ve bought something with higher output had we been able to find a place to put it. I am always jealous when I hear the capacity of some other boats’ watermakers. That said, this one has done well by us so far and we have yet to have a water quantity problem even with five or six people on board for 10+ day passages (knock on wood again!). We do not shower with fresh water on a passage and are careful with consumption, even washing dishes in saltwater.

Boat name: Bahati
Make and length: Montevideo 43
Total water tank capacity: approx. 100 gals
Watermaker brand and model: Katadyn PowerSurvivor 80E
Watermaker maximum output capacity in gallons per hour: 3.4
Your typical watermaker output per day: 3-6 gals on passage

Nat Warren-White
 


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