Freshwater Generator (Evaporator)
A freshwater generator makes drinking and boiler water from seawater by boiling it under vacuum at a low temperature, low enough that waste heat from the main engine's jacket cooling water alone is normally sufficient to do the evaporating, without needing a dedicated heat source.
Read more — Freshwater Generator (Evaporator) explained ▾
What defines this type
A freshwater generator, commonly called an evaporator, produces freshwater from seawater by distillation under vacuum, and the vacuum is the defining feature: it lowers the boiling point of seawater to roughly 40-60°C, low enough that the main engine's jacket cooling water, which would otherwise just be rejected overboard through the heat exchanger, can supply all the heat needed. This is what separates it from a reverse osmosis plant, which uses membrane filtration under high pressure rather than a phase change, needs no heat source but does need significant electrical power for the high-pressure pump.
Main components
Evaporator shell
A vessel where feed seawater flows over a heating element carrying jacket cooling water and boils under the vacuum maintained inside the shell.
Heating element
A plate or tube bundle transferring heat from the jacket cooling water to the boiling seawater, without the two streams mixing.
Demister and separator
A mesh or vane arrangement that strips entrained seawater droplets out of the rising vapour before it reaches the condenser, since carried-over droplets would salt-contaminate the product water.
Condenser
Cools the vapour back to liquid freshwater using a separate cooling seawater stream, and its outlet feeds the distillate pump that delivers product water to the tank.
Ejector or vacuum pump
Maintains the shell's vacuum, either a seawater-driven ejector or an electrically driven vacuum pump, and also extracts non-condensable gases that would otherwise reduce heat transfer efficiency.
Salinometer
Continuously monitors product water conductivity and automatically dumps the output to bilge or feed tank rather than the freshwater tank if salinity exceeds the set limit.
Selection and sizing
Capacity is set against the ship's daily freshwater consumption for the crew complement and any process use such as boiler make-up, with margin for the fact that output falls when jacket water temperature or engine load drops, for instance during manoeuvring or slow steaming. Single-effect units are simplest and match most cargo ships' needs; multi-effect or vapour-compression designs recover more heat per unit of seawater processed but add complexity that is only worth it where freshwater demand is high relative to available waste heat, such as on large passenger ships.
Regulations and class
Freshwater intended for drinking falls under the ship's potable water quality requirements, which in practice means the evaporator's salinometer alarm and automatic diversion function are checked at survey, along with the general condition of the pressure and vacuum parts as heat exchange equipment. There is no dedicated SOLAS chapter for evaporators as such; they are covered under the general machinery and potable water provisions the flag state applies.
Typical faults
| Fault | Cause | Consequence |
|---|---|---|
| Falling freshwater output | Scale build-up on the heating element from seawater minerals precipitating at the boiling surface | Reduced heat transfer and lower daily production, worsening progressively if not descaled |
| Product water intermittently salty | Demister fouled or damaged, allowing seawater carry-over into the vapour stream | Repeated automatic dumps of product water, reducing net output even when the salinometer is working correctly |
| Loss of vacuum | Air ingress through a worn gasket or a failing ejector/vacuum pump | Boiling point rises, and available jacket water heat may no longer be sufficient to sustain production |
| Sudden drop in output during manoeuvring | Reduced main engine load lowering jacket cooling water temperature | Expected behaviour rather than a fault, but often reported as one by crew unfamiliar with the dependency |
What to look for in a supplier
- Heating element material suited to the ship's typical seawater conditions, since scaling rates vary significantly with water temperature and salinity on the trading routes involved
- Rated output specified at a realistic jacket water temperature and engine load, not only at maximum continuous rating
- Salinometer and control system compatible with the ship's existing potable water monitoring, or supplied as a complete matched set
- Availability of gaskets, demister pads and ejector nozzles as consumable spares, since these wear faster than the shell or heating element itself
Track daily output against jacket water temperature, not against a fixed target — a genuine efficiency loss from scaling shows up as reduced output at the same jacket temperature, which a simple daily log makes obvious well before the unit fails to meet demand.
Technical drawings & plates
Historical engineering archive — public domain sources, cited per plate. Principles shown remain valid; always consult the OEM manual for model-specific data.
Typical Manufacturers
3 manufacturers · 3 models
Alfa Laval
1- Design Pressure M-Line
- >10 bar (145 psi)
- Marine Engine Room Modelle
- T6 (DN60, 800 kW) bis T35 (DN350, 35.000 kW)
- Marine FSRU Modelle
- TS45, T45, T50 (DN450-500, 45.000-55.000 kW)
- Temperaturbereich Standard
- 0–180°C (depending on model)
- FSRU Temperaturbereich
- -50 bis 150°C
- Plattentypen
- Chevron, Gemini Doppelwand, FlexFlow™
- M-Line (High Pressure Series: M3, M6, M10, M15)
- T-Series (Industrial: T2, T5, T6, T8, T10, T20, T25)
- AQ-Series (HVAC: AQ1, AQ2, AQ3, AQ4, AQ10)
- Marine Line (Engine Room: T6-T35; FSRU: TS45, T45, T50)
- Baseline (Competitive Line)
- Hygienic Line (Premium)
- Area: Gasket wear and embrittlementCheck: Inspect seals visually for cracks, hardening, swelling and discoloration; verify material compatibility with process fluids; check ClipGrip™ fastening. Aging symptoms and loss of elasticity are replacement indicators.
- Area: Clamping dimension and tightness (Tightening Dimension A)Check: Verify A-dimension per manufacturer specifications with caliper (metallic contact required). Keep bolts clean and lubricated. Avoid over-tightening. Perform regular re-tightening after operating hours. At too low A-dimension, pressure loss and leakage are to be expected.
- Area: Fouling and scale formation on plate surfacesCheck: Monitor pressure differential across heat exchanger (increase indicates deposits). Check heat duty versus design value. Optimize CIP intervals after performance audits. Deposits generate higher pump load (energy waste up to 8,000 kWh/year possible).
- Area: Plate corrosion and cracks / breakthroughsCheck: After disassembly, inspect plate surfaces for local measurements (pitting), cracks and perforation corrosion. Verify material compatibility with process fluid. Observe fatigue cracks especially at corners and transitions. When perforation occurs, plate is unrecoverable and must be replaced.
- Area: Pressure test and tightness inspectionCheck: After disassembly and cleaning, perform hydrostatic test per data sheet. Test pressure typically 1.5× operating pressure. Check external and internal leakage at all connections and plate edges. Document findings.
- Area: Plate sequence and alignment (5-point alignment)Check: After opening, verify plate sequence and honeycomb pattern (corrected chevron patterns must alternate). Activate/validate 5-point alignment system for larger units. Offset or misplacement leads to short-circuiting and reduced heat duty.
Type-universal inspection points for Alfa Laval Gasketed Plate Heat Exchangers (GPHE) (Alfa Laval, 2026-06). Per-model specs not auto-populated.
- High heat‑transfer coefficient due to gasketed plate design, delivering efficient evaporation at modest pressure (≥10 bar).
- Compact and lightweight compared with shell‑and‑tube evaporators, saving valuable engine‑room space.
- Modular plate stack allows capacity scaling by adding or removing plates without major redesign.
- Easy access for cleaning and gasket replacement, supporting routine maintenance schedules.
- Proven Alfa Laval corrosion‑resistant materials suitable for seawater service.
- Gasket wear and hardening require regular inspection; failure can lead to leaks and loss of performance.
- Susceptible to fouling and scaling if seawater pretreatment is inadequate, increasing pressure drop.
- Maximum design pressure limited to ~10 bar, unsuitable for very high‑pressure applications.
- Not intended for continuous operation while in port due to discharge regulations (service note).
- Plate corrosion or pitting can necessitate costly plate replacement.
Nirex
1
- Evaporator plate fouling
- Vacuum pump failure
- Salinity sensor drift
- Small footprint – fits in limited engine room spaces
- Low steam/heat demand makes it suitable for auxiliary boilers
- Quick start‑up and shutdown compared with larger multi‑effect plants
- Simple mechanical design facilitates routine inspection
- Plate fouling can reduce heat transfer efficiency if not cleaned regularly
- Vacuum pump reliability is a known weak point, requiring spare parts on board
- Salinity sensor drift may affect water quality monitoring and needs frequent calibration
- Limited production capacity – not intended for primary freshwater supply
Sasakura
1
- Scale buildup on evaporator tubes
- Brine ejector failure
- Distillate pump seal leak
- Compact footprint ideal for space‑constrained installations
- Rapid start‑up and shutdown compared to multi‑stage systems
- Relatively low power consumption for its capacity range
- Simple mechanical layout facilitating routine maintenance
- Susceptible to scale buildup on evaporator tubes if water treatment is inadequate
- Brine ejector can fail under high‑load or corrosive conditions
- Distillate pump seal leaks reported in field service
- Requires regular descaling with citric acid, adding operational overhead