Vacuum freshwater generator
Seawater evaporation, condensation and distillate quality
Marine vacuum distillation: heating sources, ejectors, droplet separation, salinity monitoring and reduced-output diagnosis.
Introduction: purpose and applications
Water boils when its saturated vapour pressure equals the surrounding pressure. Under vacuum this occurs at a lower temperature than under atmospheric conditions. A freshwater generator can consequently use relatively low-grade engine jacket-water heat; some installations also accept a steam heating source. Reduced pressure makes available heat useful, but does not remove the energy requirement for evaporation.
Water vapour forms in the evaporator while most dissolved salts remain in the liquid brine. Only part of the incoming seawater is evaporated, with concentrated residue continuously removed. Evaporating too large a fraction increases salt concentration and deposition risk. Output therefore depends on coordinated feed and brine flows as well as heating-water temperature.
Construction and main components
- Evaporating surface — receives heat from water or the specified steam source, separated from seawater by metal.
- Condenser — cools generated vapour and turns it into liquid distillate.
- Demister or plate separation zone — returns salty droplets to the brine instead of allowing them to travel with vapour.
- Air and brine ejector — uses a motive-water jet to remove air and concentrated residue.
- Motive seawater pump — provides the flows required by the arrangement; the condenser also needs effective heat removal.
- Distillate pump — transfers product out of the vacuum space toward the designated tank or consumer.
- Salinometer and diversion valve — monitor product conductivity and prevent unacceptable water taking the normal outlet route.
Shell-type units enclose evaporation, droplet separation and condensation within a vacuum vessel. In the AQUA plate arrangement, the plate pack integrates these functions and contains the vacuum without a large outer shell. The two-stage AQUA E2 additionally reuses condensation heat from one stage to evaporate water in the next. That is a particular constructional feature, not a property of every plate freshwater generator.
Working principle, step by step
- Motive water reaches the ejector, which removes air. Reduced pressure develops, allowing water to boil at a lower temperature.
- The heating medium flows through separate passages and transfers energy to seawater. The fluids must not mix.
- Part of the feed evaporates. Most salt remains in the brine, which is removed to limit concentration.
- Vapour passes the droplet-separation zone, releases heat in the condenser and becomes liquid distillate.
- The pump transfers product through quality monitoring. Unacceptable water takes the specified diversion route, while its eventual use depends on the ship’s water arrangement.
Water vapour contains little dissolved salt, but it can entrain seawater droplets. A demister or purpose-designed plate separation zone returns those droplets to the brine before they reach the condenser. Excessive brine level, overloading or damaged separation elements may produce salty distillate while evaporation continues normally. Leakage between seawater and product passages provides another salt-contamination route.
A salinometer assesses product conductivity and operates the specified diversion arrangement for unacceptable water. It is not a complete chemical or microbiological analysis. Sensor condition, a representative sample flow and the diverting valve must be verified together. A low instrument reading without a credible sample does not establish the quality of water actually entering the storage tank.
Distillate can supply technical circuits after appropriate conditioning. In the AtSea machinery-space tour, the generator is discussed in connection with technical water and boiler makeup, while drinking water is bunkered separately. This specific installation illustrates why produced water cannot automatically be labelled drinking water.
Potable use depends on the complete chain: source-water quality, acceptable production location, subsequent treatment, storage-tank hygiene and distribution condition. Low mineral content may require adjustment for the intended use, while sanitary safety requires the specified disinfection and monitoring programme. Good product water can still become contaminated inside a tank. Generator capacity and fitness for drinking are therefore assessed through different evidence and must not be substituted for one another.
Key characteristics
Output depends on heating-source and cooling-water temperatures. Always identify the pressure scale: absolute pressure, gauge vacuum and percentage vacuum are not interchangeable readings.
| Parameter and unit | Meaning |
|---|---|
| Production, m³/day or L/h | Product volume at stated thermal conditions, rather than a constant output. |
| Absolute pressure, kPa or bar | Determines boiling temperature; gauge-vacuum readings require conversion. |
| Fluid temperatures, °C | Describe available heating and the condenser’s ability to reject heat. |
| Conductivity, µS/cm; salinity, ppm | Related by instrument calibration, but not a complete assessment of drinking-water quality. |
| Motive-water pressure, bar | One indication of ejector conditions; discharge back pressure also matters. |
Maintenance: planning the checks
The calendar below is a planning framework, not mandatory overhaul intervals. The ship’s planned maintenance system (PMS), running hours, condition and maker instructions determine the actual scope and timing.
| Planning horizon | Observation and work |
|---|---|
| Daily or each watch | record production, vacuum, temperatures and salinity, observing leakage and actual diversion of unacceptable product. Compare values with engine load and seawater temperature so that a natural reduction in output is not mistaken for fouling. |
| Monthly PMS review | examine heat-transfer trends, sample-flow condition, specified chemical consumption and quality alarms. Plan salinometer and valve verification as one functional chain. A low displayed number without a representative sample does not establish healthy monitoring. |
| Annual planning | coordinate surface cleaning and gasket, ejector or pump examination with condition and maker instructions. Chemical cleaning requires compatible products, fluid isolation and controlled waste handling. Following assembly, verify circuit separation and product quality; the calendar alone does not require opening a healthy plate pack. |
Troubleshooting: symptom, cause, action
| Symptom | Possible cause | Check or action |
|---|---|---|
| Output decreases | Less available heat, warm seawater, scale or disturbed flow. | Compare inlet conditions, vacuum and flows before increasing heat. |
| Vacuum cannot be maintained | Air ingress, inadequate condensation or ejector-flow problems. | Check tightness, cooling, motive-water supply and unrestricted ejector discharge. |
| Salinity rises | Droplet carryover, high brine level, surface leakage or measurement error. | Keep product diverted and check separation, representative sampling and integrity. |
| Distillate pump delivers no water | Insufficient suction liquid, air or an obstructed outlet. | Compare level, suction conditions and valve positions; rotation alone does not prove delivery. |
Frequently asked questions
Why operate under vacuum?
It lowers boiling temperature so engine cooling-water heat can be used. Evaporation still requires energy: vacuum alone does not produce freshwater without a heat source.
Can distillate be drunk immediately?
Not necessarily. The intended treatment and sanitary control of storage and distribution are still needed. A salinometer does not detect every chemical contaminant or microorganism.
How can salt reach the condenser?
Vapour may carry salty droplets if separation fails. Leakage through a surface provides another route, so a high reading requires investigation of several possibilities.
Is this the same as reverse osmosis?
No. This unit evaporates and condenses water. Reverse osmosis uses pressure and a membrane, with different pumps, limitations and maintenance requirements.
Conclusion
A vacuum generator combines heating, evaporation, droplet separation and condensation. Useful output must be accompanied by verified product quality and the correct route to its intended use.