Marine Fresh Water Generator: How Ships Make Fresh Water

A marine fresh water generator (FWG) produces fresh water from seawater while the ship is at sea. The dominant technology is vacuum distillation: the main engine jacket cooling water, typically circulating at 70 to 85 degrees C, heats a seawater feed inside a shell held under vacuum at roughly 60 to 90 mbar absolute. At that pressure, seawater boils at 40 to 55 degrees C rather than 100 degrees C, so the modest jacket-water temperature is enough to drive evaporation. The resulting vapour condenses on a seawater-cooled condenser and drains to the fresh water storage tanks. A salinometer continuously monitors distillate quality and a dump valve automatically rejects any batch that exceeds the conductivity set point. Reverse osmosis (RO) is the competing technology, using high-pressure membrane separation at 55 to 70 bar without any heat source. Both technologies require additional treatment before the output enters the potable water circuit. Production must stop whenever the ship is within 20 to 25 nautical miles of the nearest land or in port.

Why ships need onboard fresh water production

A fully crewed general cargo vessel with 25 seafarers consumes roughly 100 to 150 litres per person per day for drinking, cooking, sanitation, and laundry combined, totalling 2.5 to 3.75 cubic metres per day. A cruise ship carrying 3,500 passengers and 1,500 crew can consume 400 litres per person per day across all services, reaching 2,000 cubic metres daily. Carrying that volume in tanks for a 30-day transpacific voyage would require 60,000 cubic metres of tank space, which is not feasible.

Producing water at sea changes the economics completely. The main engine jacket cooling water circuit already dissipates around 25 to 35 percent of fuel energy as heat that must be rejected to seawater anyway. Using a fraction of that heat to drive an evaporator costs nothing in marginal fuel, because the heat would otherwise pass through the sea water cooling systems and go overboard. Alfa Laval reports that a single JWP-26 evaporator with a rated output of 26 tonnes per day consumes essentially zero additional fuel when powered by jacket water.

The Maritime Labour Convention 2006, Regulation 3.2 and Standard A3.2, requires flag states to ensure ships carry an adequate supply of potable water of suitable quality. IMO Circular MSC/Circ.1390 (2011) provides guidelines on the provision of potable water on passenger ships. Those instruments do not specify a minimum daily output per person in absolute terms, but they effectively mandate FWG operation on any deep-sea vessel too small to bunker adequate water for its full intended voyage duration.

The vacuum distillation evaporator

Why vacuum is necessary

At atmospheric pressure (1,013 mbar), seawater boils at approximately 100.6 degrees C when its salinity is around 35 parts per thousand. The main engine jacket cooling water leaves the engine at 75 to 85 degrees C on a typical slow-speed two-stroke engine and returns to the engine at 60 to 70 degrees C. There is no way to use that heat to boil seawater at atmospheric pressure because the heat source is cooler than the required boiling point.

Reducing the pressure inside the evaporator shell drops the boiling point below the available heat-source temperature. The relationship between absolute pressure and boiling temperature follows the Clausius-Clapeyron relation; the approximate boiling points at the pressures common in marine FWGs are:

A typical shell operating at 70 to 90 mbar absolute boils its seawater feed at 40 to 45 degrees C, leaving a comfortable 25 to 35 degree temperature difference versus the 70 to 80 degree C jacket water on the heating side. That temperature difference (the driving delta-T, expressed formally as $\Delta T = T_{hw} - T_{brine}$) determines the heat transfer rate and therefore the evaporation rate. Increasing load on the main engine raises jacket water temperature and increases FWG output; light-load port manoeuvring at 40 percent power can drop jacket water outlet temperature to 65 degrees C and cut output by 30 to 40 percent.

Single-effect vs multi-effect operation

In a single-effect evaporator, the jacket water passes through the heating circuit once, gives up heat to evaporate the seawater feed, and then rejoins the jacket water cooling loop. The vapour produced condenses on a separate condenser surface cooled by a seawater stream drawn directly from the sea. The condensate flows to the fresh water tank. The concentrated brine remaining in the shell is discharged continuously by the brine ejector, which also sustains the vacuum.

The performance ratio of a single-effect unit is approximately 0.8 to 1.0: for every kilogram of heating steam equivalent, around 0.8 to 1.0 kg of distillate is produced. Multi-effect evaporators improve on this by using the vapour from one effect as the heat source for a second effect operating at a lower pressure and lower boiling temperature, then repeating for a third or fourth effect. Each additional effect increases the performance ratio toward a theoretical ceiling of n kg of distillate per kg of heating steam in an n-effect unit.

On cargo ships, the single-effect plate evaporator is standard because jacket water heat availability limits output anyway, and the added complexity of multi-effect is not justified. Cruise ships, which run large main engines at sustained power for days at a time and have very high water demand, use multi-effect evaporators or large RO plants. Alfa Laval’s AQUA Blue multi-effect units for cruise applications are rated up to 2,000 tonnes per day in combined configurations.

The brine ejector and vacuum system

Vacuum in the FWG shell is created and maintained by a combined brine-air ejector. The ejector uses a jet of seawater at modest pressure (around 2 to 3 bar) to entrain both the concentrated brine from the shell bottom and the small quantities of non-condensable gases (air leaking through gaskets, dissolved gases from the seawater feed) that would otherwise accumulate and destroy the vacuum. The two-fluid jet mixes in the ejector throat and discharges the combined stream overboard.

Loss of vacuum is one of the most common FWG faults. Causes include ejector wear, pump underperformance, gasket leakage on the shell flanges, and high seawater temperature reducing the condenser’s ability to absorb the vapour. When the condenser seawater temperature rises above about 30 to 32 degrees C (common in the Red Sea and Persian Gulf), single-effect evaporator output drops sharply because the condenser can no longer sustain the design vacuum.

Plate-type vs submerged-tube evaporators

Plate-type evaporators

The plate-type evaporator, typified by the Alfa Laval JWP series (JWP-16 to JWP-200, rated 16 to 200 tonnes per day) and similar units from GEA, uses a gasketted plate-and-frame arrangement inside a pressure shell. Corrugated titanium plates form alternating channels: the hot jacket water flows down one set of channels while the seawater feed flows up the adjacent set. The large surface area packed into a small volume gives high heat transfer coefficients of around 2,000 to 4,000 W per square metre per Kelvin, which is 3 to 5 times better than a plain tube bundle at equivalent velocities.

Advantages of the plate type are compact footprint, relatively easy disassembly for descaling, and good performance at low temperature differences. The corrugated plate geometry promotes turbulence even at low flow rates, keeping the local heat transfer coefficient high. Disadvantage is sensitivity to scaling: hard, brackish seawater in the Baltic or Adriatic can deposit calcium carbonate and calcium sulphate on the plate surfaces within a few weeks of operation without scale-prevention dosing.

The Alfa Laval JWP-26C, the most widely fitted single cargo-ship unit, weighs about 2,200 kg dry and occupies roughly 2.5 by 1.2 by 1.8 metres. Its rated output of 26 tonnes per day at 83 degrees C jacket water and 29 degrees C seawater drops to around 18 tonnes per day if jacket water temperature falls to 70 degrees C.

Submerged-tube evaporators

The submerged-tube (or shell-and-tube) evaporator used in Sasakura’s SCB series and older designs from Atlas and other makers consists of a cylindrical horizontal shell. Inside the shell, a bundle of small-diameter tubes carries the heating medium, usually jacket water or low-pressure steam. The shell is flooded with seawater feed to a controlled level above the tube bundle, and boiling occurs at the outer tube surface under the vacuum maintained by the ejector system.

Submerged-tube units handle feed water with higher suspended solids more tolerantly than plate types, because there is no narrow plate channel to block. They are also more suited to steam heating at variable pressures, so they were the standard choice when auxiliary boiler steam was the primary heat source on older motor ships and on steam-turbine ships. The trade-off is a larger footprint per tonne of daily output and more difficult cleaning: the tube bundle must be removed or chemically descaled in situ, and the process takes 6 to 8 hours compared with 2 to 4 hours for a plate type.

The heat source in detail

Main engine jacket cooling water circuit

The main engine jacket cooling water circuit on a marine diesel engine circulates fresh water through the cylinder liner and cylinder head water jackets at a controlled temperature to keep the liner walls within the design temperature band (typically liner wall temperature 120 to 180 degrees C, coolant outlet 75 to 85 degrees C on a slow-speed two-stroke). A thermostatic valve maintains the supply temperature to the engine at 60 to 70 degrees C by mixing the hot return with cooler water from the jacket cooler.

The FWG taps heat from the high-temperature side of this circuit, before the jacket cooler. On a MAN B&W S60ME-C10.5 rated at 14,280 kW MCR, the jacket water heat rejection at 100 percent MCR is approximately 1,100 kW. An Alfa Laval JWP-26C evaporator needs around 330 kW of heat input for its rated 26-tonne-per-day output (based on the latent heat of vaporisation at 42 degrees C, approximately 2,400 kJ/kg). The engine can easily supply this without any temperature drop outside the normal operating range.

When the main engine runs at reduced power, say 50 percent MCR, jacket heat rejection falls to roughly 550 kW, still enough for the JWP-26C. At 25 percent MCR or below, or during port manoeuvring where the engine may be stopped entirely, the heat available drops below what the evaporator needs and production stops. Ships therefore maintain a fresh water reserve that covers the expected time in port plus a safety margin of at least 2 to 3 days at sea consumption.

Auxiliary heat sources

Some ships equip the FWG for dual-source operation: jacket water during sea passage and low-pressure steam from the auxiliary boiler during port stays or when the main engine is on standby. The submerged-tube evaporator adapts to steam more readily than the plate type, because steam at 1.0 to 2.0 bar (saturation temperature 120 to 133 degrees C) gives a larger driving delta-T and higher evaporation rate per square metre of tube surface. On diesel-electric ships, each running generator engine has its own jacket cooling water circuit, and the FWG draws heat from whichever engines are on load, giving continuous production independent of the propulsion mode. This is discussed further in the waste-heat recovery system article.

Reverse osmosis as an alternative

Membrane separation principle

Reverse osmosis forces seawater through semi-permeable membranes at a pressure that overcomes the natural osmotic pressure of the saltwater. The osmotic pressure of 35 g/kg seawater is about 27 bar. To drive a net flux of water from the salty side to the fresh side, the applied pressure must exceed this figure; in practice, marine RO systems operate at 55 to 70 bar.

At those pressures, water molecules pass through the membrane while dissolved sodium chloride, magnesium, sulphate, and other ions are retained on the concentrate (reject) side. The permeate (product water) flows out at near-zero salt concentration, typically 50 to 200 milligrams per litre (mg/L) or 0.05 to 0.2 parts per thousand, depending on membrane selectivity and the number of passes. The concentrate stream, at about 1.5 to 2 times the feed salinity, goes overboard.

Energy recovery is essential in modern RO. Without it, the high-pressure pump energy per cubic metre of permeate is around 8 to 12 kWh. With a pressure exchanger, which takes the residual pressure in the concentrate stream and uses it to pre-pressurise part of the feed, net energy consumption drops to 3 to 5 kWh per cubic metre of permeate. That is still 3 to 5 times the marginal energy cost of running a vacuum evaporator on free jacket water heat, but for ships with surplus electrical generating capacity (cruise ships, LNG carriers with large power plants, offshore vessels) the difference in running cost is manageable.

RO vs vacuum evaporation: direct comparison

The choice between RO and thermal evaporation depends on the operational profile of the ship rather than on any single technical superiority of one technology.

RO is better suited to ships that spend significant time in port or at anchor without running the main engine, or to ships operating in warm tropical waters where condenser limitations cut thermal evaporator output. Thermal distillation remains preferred where waste heat is freely available and the ship operates mainly at sea on sustained main-engine power.

Salinity monitoring and the dump valve

The salinometer

Every FWG distillate line includes a salinometer: a cell that measures the electrical conductivity of the distillate, typically in microsiemens per centimetre (µS/cm). Pure fresh water at 25 degrees C has a conductivity around 0.05 µS/cm; WHO-acceptable drinking water is typically below 2,500 µS/cm (corresponding to about 1,500 mg/L TDS); distillate from a well-operating vacuum evaporator reads 2 to 10 µS/cm (roughly 1 to 5 mg/L TDS). The salinometer set point for the dump valve is typically 10 to 15 µS/cm for potable-water service, well below the WHO limit but high enough to avoid nuisance tripping on normal start-up transients.

On start-up, the evaporator produces slightly saltier distillate for the first 10 to 20 minutes as the shell temperature and vacuum stabilise. The dump valve (also called the three-way valve or divert valve) automatically routes this initial distillate back to the brine discharge or overboard rather than to the fresh water tank. Once the salinometer reads below the set point, the valve shifts to the “produce” position and distillate enters the storage tanks.

A salinometer failure or a stuck dump valve is a contamination risk. Best practice, and the requirement in many class society rules (DNV Pt.4 Ch.6 is one example), is to fit an independent high-salinity alarm at the tank inlet line as a second layer of protection. Some operators also carry a handheld conductivity meter for manual spot checks during routine rounds.

What causes high distillate salinity

Several failure modes produce out-of-spec distillate without an obvious external cause. The most common are:

The demister becoming damaged or partially blocked with scale. The demister, a mesh pad or chevron separator above the boiling surface, strips entrained brine droplets from the rising vapour. If it fails, droplets carry directly into the condenser and the distillate. Conductivity spikes from this cause can be sharp and intermittent.

Loss of vacuum pulling in seawater through a leaking gasket or flange. If shell pressure rises above about 150 mbar absolute, the boiling seawater can splash into areas from which it reaches the condenser without adequate demisting.

Excessive seawater feed rate flooding the shell above the demister level. Most plate-type units have a float-controlled feed valve, but sticking or misadjusted feed regulators can cause intermittent flooding.

Condenser leakage: a cracked plate or tube allows high-salinity seawater cooling water to mix directly with the distillate. This is rare but the conductivity reading can be very high (thousands of µS/cm) and the dump valve will respond.

Potable water treatment after production

Why distillate is not immediately drinkable

Distillate from a vacuum evaporator is very pure in the sense of being almost salt-free, but it is not safe to drink without further treatment for two reasons.

First, if the seawater feed is contaminated, some volatile organic compounds and heat-stable toxins can carry over with the vapour. Bacterial contamination of the seawater feed (sewage outfall near the intake, harbour pollution) is the main concern. Distillation at 40 to 55 degrees C does not reliably kill all pathogens. At 100 degrees C, pasteurisation is rapid, but FWG evaporation temperatures are well below this.

Second, once distillate enters the fresh water tanks and distribution pipework, it can become contaminated through biofilm growth in tanks, Legionella growth in hot-water calorifiers, or contaminated hose connections at bunker points. The treatment chain must therefore address both the immediate output of the FWG and the integrity of the downstream distribution system.

Mineralisation

RO permeate in particular is aggressive to metal piping because of its very low buffering capacity. The permeate pH is typically 5.5 to 6.5 (slightly acidic from dissolved CO₂), and its low calcium and bicarbonate content means it cannot form a protective calcite layer on the inner wall of copper or mild steel pipes. Mineralisation corrects this by passing the permeate through a calcite (calcium carbonate) contact bed, which dissolves into the permeate and raises pH toward 7.5 to 8.0 while adding hardness of about 50 to 100 mg/L as CaCO₃. Some operators add a small dose of calcium chloride and sodium bicarbonate as an alternative to calcite beds.

Disinfection: UV and chlorination

UV disinfection at a wavelength of 254 nm disrupts the DNA of bacteria and viruses without adding any chemical to the water. A UV dose of 40 mJ per square centimetre is the minimum target for potable water under WHO guidance; most marine UV units are rated to deliver 60 to 100 mJ/cm² at the end of lamp life. UV is effective against Legionella, E. coli, norovirus, and most other pathogens, but it provides no residual protection: water that passes through the UV unit and then sits in a tank for days can become re-contaminated.

Residual chlorination adds a sustained protective concentration of free chlorine, typically 0.2 to 0.5 mg/L at the point of use, throughout the distribution system. The disinfectant is usually sodium hypochlorite solution dosed by a small pump into the supply line downstream of the UV unit. WHO guidelines recommend maintaining 0.2 mg/L free chlorine residual in the distribution system as a minimum, with higher concentrations (up to 5 mg/L) for system shock treatment after tank cleaning or after a suspected contamination event.

The combination of UV followed by low-level chlorination is the most widely adopted approach on commercial ships because it provides both instantaneous kill and ongoing residual protection. The marine domestic water systems article covers the distribution and storage side of the potable water circuit in detail, including tank sizing, coating specifications, and the hot-water calorifier requirements that apply to Legionella control under flag-state health regulations.

Silver ion disinfection

Some manufacturers offer silver ion disinfection as an alternative to chlorine for operators who prefer not to add halogenated compounds to the water. Silver ions at concentrations of 0.05 to 0.1 mg/L suppress bacterial growth. The disadvantage is that silver ion systems are more expensive to operate, and several national health authorities (including the US EPA) do not accept silver ion as the sole residual disinfectant for potable water systems. Silver disinfection is more common in European flag-state registries and on specialised passenger vessels.

The coastal and harbour production restriction

Why production must stop near land

The rule prohibiting FWG operation within a defined distance of the nearest land is one of the most operationally important restrictions on the equipment. The prohibition exists because coastal and harbour waters carry microbial, chemical, and particulate contamination loads that open-ocean water does not.

Within port limits, the water column receives sewage outfall, industrial discharge, antifouling paint leachate from vessels, lubricating oil and fuel micro-spills from vessel operations, and agricultural runoff from nearby river mouths. Several of these contaminants are either heat-stable at FWG operating temperatures or carry in the vapour phase. Thermal distillation at 40 to 55 degrees C is not equivalent to pasteurisation at 72 degrees C or boiling at 100 degrees C; some enteric bacteria and protozoan cysts can survive the process under the conditions prevailing in a single-effect marine evaporator.

Beyond the biological risk, some chemical contaminants, notably low-molecular-weight volatile organic compounds such as benzene, toluene, and light fuel fractions from harbour surface sheens, are more volatile than water and concentrate in the vapour phase during distillation. This means the distillate could actually be more contaminated with such compounds than the seawater feed.

The 20-25 nautical mile rule

No single universally binding international instrument specifies an exact minimum distance. The restriction is instead found in class society rules, flag-state regulations, and manufacturers’ operating manuals, which converge on a range of 20 to 25 nautical miles from the nearest land as the minimum safe operating distance. DNV Rules for Classification of Ships, Pt.4 Ch.6, reference this threshold. Alfa Laval’s JWP operating manuals state that the unit must not be operated within 20 nautical miles from the nearest land, and Sasakura’s SCB series manuals are consistent with this figure.

Some flag states specify stricter requirements. The US Centers for Disease Control and Prevention Vessel Sanitation Program (VSP) guidelines for cruise ships require that potable water not be produced in ports or near shore without specific approval of the water quality. The IMO MEPC Guidelines on Ballast Water Sampling, while not directly applicable to FWGs, reflect the same principle that harbour water quality is fundamentally different from open-ocean water.

In practice, chief engineers note the ship’s position and local time when the FWG is started and stopped at each voyage leg. Many ships’ planned maintenance system (PMS) records include FWG start/stop with position coordinates as a compliance record for port state control inspection.

Turbid or algae-bloom conditions

Even outside the 20-25 nautical mile limit, some sea areas have poor water quality due to seasonal algae blooms, river plume outflow (the Amazon plume extends over 2 million km², measurably diluting the salinity of the South Atlantic over a wide area), or upwelling of nutrient-rich water. Operators are advised to monitor seawater inlet turbidity and to stop FWG production if the intake filter becomes blocked rapidly, indicating high biological content in the feed. Sasakura and Alfa Laval both recommend periodic inspection of the seawater intake strainer and shell internals for biological fouling during voyages through algae-dense water.

Scale formation and its control

Why scale forms

Heating seawater deposits mineral scale on heat transfer surfaces for the same reason that a domestic kettle furs up: as temperature rises and water evaporates, the concentration of dissolved calcium carbonate and calcium sulphate exceeds their solubility limits and they precipitate. In an FWG, the hottest surface is the heating plate or tube wall, and scale forms preferentially there.

Calcium carbonate (CaCO₃) is the dominant scale compound at temperatures below about 60 degrees C and relatively low concentration factors. At higher temperatures or higher brine concentration factors, calcium sulphate (CaSO₄) deposition increases. Magnesium hydroxide (Mg(OH)₂) deposits at higher pH values that can develop locally on the heating surface.

Scale is the main cause of reduced FWG output over time. A 1 mm layer of calcium carbonate on a plate evaporator surface, with a thermal conductivity around 2 W/mK, adds a conductive resistance equivalent to reducing the effective heat transfer coefficient by roughly 30 to 50 percent, depending on flow conditions. This manifests as reduced distillate output at the same heat input, before any visible fouling is apparent from outside.

Scale control methods

The brine concentration ratio (the ratio of brine salinity to feed salinity) is the primary operating parameter for scale control. Operating the FWG at a lower concentration ratio, meaning a higher brine blowdown rate relative to the feed, reduces the concentration of scaling ions in the shell. Most plate-type FWG manuals recommend a maximum concentration ratio of 1.5 to 2.0: the brine salinity should not exceed 1.5 to 2.0 times the feed salinity.

Antiscalant (scale inhibitor) dosing suppresses the crystallisation of calcium carbonate and sulphate by adsorbing onto the crystal nuclei and disrupting their growth. Polycarboxylate and polyphosphonate antiscalants are the most common types used in marine FWGs. They are dosed into the seawater feed at concentrations of 2 to 10 mg/L. The dosing pump is typically interlocked with the FWG feed pump so that it cannot run without antiscalant.

Descaling (acid cleaning) is the corrective treatment once scale has formed. Citric acid solution at 3 to 5 percent concentration, recirculated through the evaporator at 40 to 50 degrees C for 2 to 4 hours, dissolves calcium carbonate deposits effectively. For heavier scale or calcium sulphate deposits, hydrochloric acid (5 to 10 percent) is used, but this requires more rigorous corrosion inhibitor dosing and is avoided on titanium-plate units where contamination of the cooling-water side could occur. Descaling is typically required every 6 to 24 months depending on feed water hardness and antiscalant effectiveness.

Maintenance requirements

Routine operational checks

The chief engineer or designated watchkeeper carries out daily checks that include verifying the salinometer reading, confirming the vacuum reading on the shell gauge (a degrading vacuum is the earliest warning of ejector wear or gasket leakage), checking the distillate flow rate against the expected output for the current heat-source temperature, and inspecting the ejector pump seal for leakage. Output variations of more than 10 percent from the expected figure at a given jacket water temperature warrant investigation before the variation reaches 20 to 25 percent, at which point output has usually fallen enough to affect tank levels on a long voyage.

Plate cleaning and inspection

Plate-type units require removal and inspection of the plate pack every 6 to 12 months on an intensive-service cargo ship route, or annually on ships with well-controlled feed water and consistent antiscalant dosing. The plate pack is disassembled, each plate examined for pitting, cracked gaskets, or blocked channels, and the plates are either acid-washed individually or cleaned in a tank of citric acid solution. Re-gasketing the full plate pack is good practice at every second disassembly, because the nitrile or EPDM gaskets harden and shrink over time and become a source of leakage and vacuum loss.

Ejector and vacuum system maintenance

The ejector nozzle and throat wear over time from the abrasive action of the seawater jet. On most units this is detectable as a gradual rise in shell vacuum reading (i.e., the absolute pressure creeping up from the design 70 to 90 mbar toward 120 to 150 mbar) coupled with falling output. Nozzle and throat replacement is typically a 2 to 4 hour job and is the most common corrective maintenance action on vacuum distillation FWGs.

The ejector seawater pump (or air ejector compressor on units that use a vacuum pump rather than a jet ejector) should have its impeller clearances checked and bearings inspected at each class survey. A worn impeller that cannot achieve the design discharge pressure reduces the vacuum available and limits evaporator output just as effectively as a worn nozzle.

Energy and the waste-heat recovery angle

Free heat and its limits

The main engine jacket cooling water circuit is a waste-heat stream: it removes heat that the engine cannot convert to shaft work, and this heat must be rejected somewhere. On a ship at sea, the only economical heat sink is the ocean, via the jacket cooler and the sea water cooling system. Using this heat in the FWG rather than the jacket cooler reduces the load on the jacket cooler and on the sea water cooling systems, slightly reducing the seawater cooling pump power. The net energy cost of running a jacket-water-heated FWG is therefore close to zero: no additional fuel is needed and the cooling system workload is marginally reduced.

The limit of this argument is the available heat. A main engine provides only so much jacket heat at a given MCR, and other consumers compete for it: the jacket cooler itself, the lubricating oil cooler (which is sometimes integrated into the jacket water circuit on medium-speed engines), and the auxiliary fresh water heater for accommodation hot water. On a ship with a modest-size main engine and a relatively large FWG, running both at full capacity can pull the jacket water return temperature below the engine’s minimum recommended value of 60 to 65 degrees C. Class society rules and engine maker instructions require that the jacket cooling water inlet to the engine does not fall below a certain temperature (typically 60 degrees C) to prevent cold corrosion of the liner surface. Designers ensure the FWG heat extraction is within the available budget before specifying the unit size.

Integration with waste heat recovery systems

More sophisticated waste-heat schemes are described in the waste-heat recovery system article. On large container ships and tankers that operate a waste heat recovery (WHR) system, the WHR takes priority claim on jacket water heat (and exhaust gas heat through an economiser) to generate steam or shaft power. The FWG on such ships may be supplied with low-grade steam from a steam-dump valve rather than direct jacket water when the WHR system is operating at full capacity. The priority hierarchy is: engine jacket water temperature control first, WHR second, FWG third.

Operational limits and failure modes

Limitations

Vacuum distillation FWGs have several hard operational boundaries.

Production stops when the main engine is shut down or operating below roughly 25 percent MCR, because jacket water temperature and heat flow are insufficient. Ships must carry adequate reserve to cover port time plus at least one full day at sea as contingency.

Very warm seawater (above 32 to 33 degrees C, encountered in the Persian Gulf and Red Sea in summer) limits condenser performance and can reduce single-effect output by 20 to 40 percent relative to rated capacity in temperate water. This is the operating season when fresh water demand is also highest (more crew drinking), and the combination requires careful tank-level management.

Highly saline feed water, such as the Red Sea with salinity up to 43 g/kg versus Atlantic open-ocean salinity of 35 g/kg, increases scale formation rate and may require more frequent antiscalant dosing adjustment.

The coastal production restriction means that FWG cannot run during coastal passages or port stays, which on short-sea trades can account for 40 to 60 percent of total operating time. Short-sea vessels are consequently more dependent on shore bunkering than deep-sea vessels.

Reverse osmosis systems face different limits: membrane fouling from biological matter requires pre-treatment, and the high-pressure pump and energy recovery device have maintenance intervals that must be followed strictly to avoid sudden loss of production.

Regulatory failure risk

A failing salinometer or stuck dump valve is a compliance risk under MLC 2006, flag-state health regulations, and port state control inspection. Port state control officers (PSCOs) check fresh water records on passenger ships in particular. A ship arriving in port with fresh water tanks contaminated by high-salinity distillate, or with no evidence of operational salinometer readings in the planned maintenance records, faces the prospect of deficiency notices and potentially a requirement to bunker all water from shore at the officer’s direction before departure.

Related calculators

The FW generator output calculator estimates production rate from jacket water temperature, seawater temperature, and unit heat transfer area. The system fresh water demand estimator calculates daily demand from crew number and consumption rates, which can be used to size the FWG against the anticipated production.

Additional companion tools: the system FW generator calculator models the heat balance for the complete evaporator circuit, and the separate plate-type and shell-and-tube variants (plate-type evaporator and shell-and-tube evaporator) let engineers compare heat transfer area, log mean temperature difference (LMTD), and output for either configuration.

See also

• Marine Domestic Water Systems
• Marine Sea Water Cooling Systems
• Waste Heat Recovery System
• Marine Sewage and Grey Water Treatment Systems
• Marine Diesel Engine
• Marine Boilers and Steam Systems
• MLC 2006