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Marine Fresh Water Generator (FWG)

Marine fresh water generators (FWG) produce potable and process fresh water from seawater on board ships, eliminating the need to carry large fresh water tanks for long voyages. The dominant technology is single-effect or multi-effect evaporation using waste heat from the main engine jacket cooling water (typically 70-85 degrees Celsius) at vacuum conditions (typically 60-90 mbar absolute) to evaporate seawater at 40-60 degrees Celsius. The vapour passes through a demister to remove entrained brine droplets, then condenses on the cooling-water condenser, producing fresh water with TDS typically below 2 ppm. The principal manufacturers are Alfa Laval (formerly Aqua-Chem) with the JWP series, GEA Westfalia, Sasakura, and others. Output is typically 5 to 50 cubic metres per day on cargo ships, much higher on cruise ships (where reverse osmosis is increasingly used as alternative or supplement). Operational considerations include heat source temperature (jacket water above 70 C minimum for effective operation), seawater feed quality, vacuum maintenance, sterilisation (UV or chemical) for potable water, and the comprehensive maintenance regime required for continuous reliable operation. ShipCalculators.com hosts the principal computational tools: the FW generator output calculator, the system FW generator article, the plate-type evaporator article, and the shell-and-tube evaporator article.

Contents

Background

Why ships need fresh water generators

Ships need substantial fresh water for:

  • Drinking water: crew and passenger consumption (typically 4 to 6 L/person/day for drinking, plus cooking).
  • Sanitary: showers, hand-washing, toilet flushing (typically 100 to 200 L/person/day on cargo ships, 200 to 400 L/person/day on cruise ships).
  • Galley: cooking, dishwashing.
  • Laundry: washing, rinsing.
  • Engine cooling makeup: replacing closed-loop coolant losses.
  • Boiler feed makeup: replacing blowdown.
  • Process water: various ancillary uses.
  • Fire main: emergency reserve.

Total daily fresh water demand on a cargo ship: 1.5 to 3 cubic metres per day for crew of 25; on a cruise ship: 50 to 800 cubic metres per day depending on size.

For long voyages (transpacific 30 days, Asia-Europe via Cape 45 days), carrying all needed fresh water in tanks would consume substantial cargo capacity. Onboard fresh water generation eliminates this constraint.

Technology evolution

Fresh water generation evolution:

  • Pre-1950s: ships carried fresh water in tanks; very limited operations.
  • 1950s onward: vacuum distillation (single-effect) became standard.
  • 1970s-1980s: multi-effect evaporation for cruise ships.
  • 2000s onward: reverse osmosis increasingly competitive, especially for cruise ships.
  • Modern: hybrid configurations with both technologies.

Major manufacturers

Marine FWG manufacturers:

  • Alfa Laval (formerly Aqua-Chem): JWP series single-effect plate evaporators dominate.
  • GEA Westfalia: similar plate-type technology.
  • Sasakura (Japan): SCB series shell-and-tube.
  • Salt Separation Process Hisaka: Japanese plate-type.
  • HEM (Helsinki Engineering): Finnish FWG specialist.

Plus various reverse osmosis specialists (Eptra, RWO, Wartsila Hamworthy).

Vacuum evaporation principle

Why vacuum evaporation

At atmospheric pressure, water boils at 100 C. At reduced pressure (vacuum), water boils at lower temperature:

  • 100 mbar absolute: ~46 C boiling point.
  • 60 mbar absolute: ~36 C boiling point.

Marine FWGs use vacuum to bring the boiling point below the available heat source (jacket cooling water at 70-85 C). The temperature difference between heat source and boiling water drives the evaporation.

Single-effect evaporator

Single-effect operation:

  1. Heat source (jacket water typically) flows through the evaporator heating coil/plates.
  2. Seawater feed enters the shell at vacuum, partially evaporates as it contacts the heating surface.
  3. Vapour rises through a demister (removing brine droplets).
  4. Demisted vapour condenses on the condenser (cooled by seawater).
  5. Fresh water is collected from the condenser.
  6. Concentrated brine is discharged via brine ejector.
  7. Vacuum is maintained by a brine/air ejector or vacuum pump.

The single-effect cycle is simple and reliable but has lower efficiency than multi-effect.

Multi-effect evaporator

Multi-effect (typically 2 to 4 effects):

  • First effect: heated by jacket water, produces vapour at moderate vacuum.
  • Subsequent effects: heated by vapour from previous effect, at progressively lower vacuum and lower boiling temperature.
  • Final effect: condenser at lowest pressure.

The multi-effect cycle produces more fresh water per unit heat input. Multi-effect is favoured on cruise ships where high output is needed.

Heat source

Heat sources for marine FWG:

  • Main engine jacket cooling water: most common, with temperatures 70-85 C from main engine.
  • Auxiliary boiler steam: secondary source when main engine stopped.
  • Diesel generator jacket cooling water: for diesel-electric ships.
  • Exhaust gas economiser: indirect via produced steam.

The waste heat from main engines is essentially free (the heat would be rejected to seawater anyway), making FWG energy-efficient.

Reverse osmosis (alternative)

Reverse osmosis principle

Reverse osmosis (RO):

  • High-pressure pump: pressurises seawater feed to typically 60-80 bar.
  • Membrane: salt molecules and contaminants filtered out by semi-permeable membrane.
  • Permeate (fresh water): passes through membrane.
  • Concentrate (brine): discharged.
  • Energy recovery: typical modern RO uses pressure exchanger to recover energy from concentrate stream.

Comparison with evaporation

RO vs evaporation:

  • Energy consumption: RO uses 3-5 kWh per cubic metre vs evaporation uses essentially free waste heat.
  • Operating cost: RO has membrane replacement (typically every 3-5 years); evaporation has minimal consumables.
  • Output flexibility: RO can be run independently of main engine load; evaporation depends on main engine running at sufficient load.
  • Water quality: RO produces fresher water (TDS typically <0.5 ppm) than evaporation (TDS 1-2 ppm).
  • Footprint: RO is more compact than equivalent capacity evaporation.

Hybrid systems

Modern cruise ships often have both:

  • Evaporation when main engines running on long voyage.
  • RO when in port, anchored, or main engines stopped.
  • Capacity overlap: providing redundancy and flexibility.

Operational considerations

Output rate

FWG output varies with conditions:

  • Designed output: at design conditions (specific heat source temperature, seawater temperature, feed water salinity).
  • Actual output: typically 80 to 110 percent of design depending on conditions.
  • Reduced output: at high seawater temperature (reduced condenser performance), low engine load (insufficient heat), high feed salinity.

The FW generator output calculator computes output for various conditions.

Water quality

FWG water quality requirements:

  • TDS: typically below 2 ppm for boiler feed, below 5 ppm for drinking after sterilisation.
  • Salinity controller: continuous TDS monitoring with auto-discharge if above set point.
  • Three-way valve: returning out-of-spec water to overboard or to feed system.
  • Sterilisation: UV or chlorination for potable water.

Sterilisation

Sterilisation methods:

  • UV sterilisation: ultraviolet light treating water at 254 nm wavelength, killing bacteria and viruses.
  • Chlorination: chemical sterilisation with sodium hypochlorite.
  • Combined: UV + residual chlorine for both immediate and ongoing protection.
  • Mineralisation: adding minerals to RO water for taste and corrosion management.

Potable water from FWG must meet WHO drinking water guidelines and any local regulations.

Maintenance

Routine maintenance:

  • Daily: visual inspection, output monitoring, salinity check.
  • Weekly: more detailed inspection, manual TDS verification.
  • Monthly: vacuum check, condenser performance.
  • Quarterly: opening for inspection, plate cleaning (where accessible), demister inspection.
  • Annual: comprehensive overhaul, scale removal.

Common operational issues

Common FWG issues:

  • Reduced output: due to scaling, vacuum loss, low heat input, condenser fouling.
  • High TDS: due to inadequate brine separation, demister damage, vacuum loss causing carryover.
  • Vacuum loss: due to ejector failure, leaks, condenser inadequate.
  • Scaling: from feed water hardness, addressed by scale-prevention chemicals or descaling.

Regulatory context

MLC 2006 and fresh water

MLC 2006 requires:

  • Adequate quantity of potable water for crew.
  • Acceptable quality meeting WHO standards.
  • Separate provisions for drinking water (apart from sanitary).
  • Cold and hot water at washbasins.
  • Storage capacity for emergency reserve.

The FWG capacity is sized to ensure compliance.

Class society oversight

Class society oversight:

  • Type approval for new FWG installations.
  • Annual surveys verifying operational status.
  • Detailed inspection at periodic surveys.
  • Tank inspections of fresh water storage.

Fresh water tank requirements

Fresh water tank requirements:

  • Coating: epoxy or other potable-water-grade coating.
  • Periodic cleaning: typically annually.
  • Inspection access: for class survey.
  • Protection from contamination: separate from other tank systems.

Specific applications

Cargo ship FWG

Cargo ship FWG:

  • Capacity: 5 to 25 m³/day.
  • Single-effect evaporator typical.
  • Operation: continuous when main engine running.
  • Storage: typically 50 to 200 m³ fresh water tank capacity.

Cruise ship FWG

Cruise ship FWG:

  • Capacity: 100 to 1000 m³/day.
  • Multi-effect evaporator plus RO.
  • Operation: continuous with cycling.
  • Storage: large capacity for peak demand.

Polar service FWG

Polar service:

  • Reduced effectiveness: cold seawater reduces condenser output.
  • Steam-only operation: when main engines stopped in ice.
  • Higher tank capacity: for periods without FWG operation.
  • Tank heating: preventing freezing.

Offshore vessel FWG

Offshore vessels (PSV, AHTS):

  • Smaller capacity: 5 to 15 m³/day typical.
  • Variable operation: depending on mission profile.
  • Provision for shore-side fresh water: when alongside platforms.

Future developments

Decarbonisation impact

Decarbonisation and FWG:

  • Lower main engine waste heat if engines run at lower power for fuel efficiency.
  • Heat pump augmentation: using waste heat with heat pump to upgrade temperature.
  • More RO use: where electrical power is increasingly green (electric ships).
  • Combined systems: optimised for varying operating profiles.

Water efficiency

Water efficiency improvements:

  • Low-flow showers and faucets: reducing demand.
  • Rainwater capture: on cruise ships.
  • Greywater reuse: with treatment for non-potable uses.
  • Smart monitoring: of consumption per cabin/zone.

Improved technology

Technology improvements:

  • Higher-efficiency multi-effect: more effects with thermodynamic optimisation.
  • Membrane improvements in RO: lower energy per cubic metre.
  • Predictive maintenance through digital monitoring.
  • Modular designs enabling capacity scaling.

See also

References

  • WHO Guidelines for Drinking-Water Quality, current edition.
  • Maritime Labour Convention 2006 Part 4.5.
  • IACS Common Structural Rules.
  • Class society marine FWG rules.
  • Alfa Laval, GEA, Sasakura technical documentation.