Fresh Water Generator — Vacuum Boiling, Operation & Faults
Fresh water generator working principle: vacuum evaporation with jacket water, FWG start-up procedure, plate vs flash evaporator, brine ejector, high salinity and low output causes, salinometer and sterilisation.
1. Why Vacuum Turns Waste Heat Into Fresh Water
At atmospheric pressure seawater boils near 100 °C and jacket water at ~80 °C could never touch it. Pull the shell down to ~92% vacuum and the boiling point collapses to ~50 °C — suddenly that same jacket water is a strong heating medium. Seawater evaporates, salt stays behind in the brine, and the vapour condenses on seawater-cooled tubes as distillate. Heating medium is jacket water or steam; the unit is a self-contained vacuum package.
- ~92%: Vacuum to build before feeding
- ~50 °C: Shell boiling temperature
- ≥ 3 bar: Ejector seawater pressure
- ~½ feed: Becomes distillate; rest is brine
What lives inside the package: coated mild-steel shell, vertical-tube evaporator bundle, shell-and-tube condenser, brine-and-air ejector with its pump, distillate (condensate) pump, vacuum gauge, distillate level gauge, salinometer, dosing connection and a shell relief valve. One saturation pressure rules both chambers — evaporation and condensation sit at the same boiling point for that vacuum.
2. Start-Up — Vacuum First, Heat Last
Ejector lines open, pump started, ≥ 3 bar proved — suction, discharge and overboard valves open first. Why: the air ejector is the vacuum pump; weak motive pressure means no vacuum ever builds.
Wait for ~92% shell vacuum on the gauge — feed nothing until it arrives. Why: no vacuum, no low-temperature boiling, no water.
Crack the feed valve, set feed pressure and flow — restricted feed, not full bore. Why: only about half the feed can flash; flooding the chamber drowns boiling and pushes brine toward the demisters.
Admit jacket water gradually, vent the evaporator top until solid — open inlet and outlet slowly, crack the air vent until water-filled. Why: sudden heat shocks tubes and trapped air blankets heat transfer.
Salinometer panel on, distillate pump primed, product to nominated tanks — watch the suction sight-glass for solid prime. Why: an air-locked distillate pump cavitates while off-spec water could already be heading for the tanks.
3. Running Watch — What Each Reading Means
| Check | Normal | Drift means |
|---|---|---|
| Flashing in shell glass | Lively boiling visible | Dead glass: vacuum lost or jacket heat gone |
| Brine level | Steady, neither high nor low | High: ejector nozzle clogged or overboard shut; low: feed starved |
| Shell temperature / vacuum | ~50 °C, vacuum above 90% | Temperature up with vacuum down: air leak — the single most important reading |
| Condenser seawater in/out | Sensible temperature rise | No rise: over-cooled evaporator or flooded condenser |
| Ejector, distillate, dosing, amps | Ejector > 3 bar; steady distillate flow; dosing flowing; normal currents | Sagging ejector pressure, swinging amps or empty doser precede the fault tables below |
4. Plate vs Flash — Where Every Stream Goes
Plate type in detail: hot jacket water crosses the heated nest while restricted seawater feed enters the low-pressure chamber and boils — about half flashes to steam, half stays as brine. Steam crosses a separator that knocks out water particles, sheds released air and non-condensable gases to the air ejector, passes baffles and demisters that stop saline carry-over, condenses on the cold bundle, and the distillate pump delivers it past the salinometer. Brine collecting at the bottom is drawn off continuously by the brine ejector — stop removing it and density climbs until priming carries salt into the product.
Scale chemistry: heating drives gas out of calcium bicarbonate, leaving calcium carbonate that plates tubes as soft scale. At ~60 °C coil duty it stays manageable — push temperatures up and it hardens into an insulating blanket that strangles output.
Flash type in detail: seawater heats in one chamber, then spills into a second chamber at substantially lower (sub-atmospheric) pressure — the sudden fall flashes part of it to steam without further heating. That steam condenses on much-cooler salt-water tubes exactly as above, and baffles plus demisters guard the distillate in both designs. Single or two-stage layouts exist.
5. Two Faults, Two Checklists
| High salinity — check in order | Why it salts the product |
|---|---|
| Capacity too high — reduce feed rate | Over-firing floods the separator; spray primes straight into the steam lane |
| Evaporation temperature too low — reduce seawater cooling | Over-cooled shell condenses poorly and drags brine droplets along |
| Brine level too high — ejector nozzle clogged or overboard valve shut | Rising brine drowns the vapour space; open the overboard path and clear the nozzle |
| Condenser tube leaking — find and plug | Seawater leaks directly into distillate inside the condenser |
| Salinometer probe defective or dirty — clean or renew | A fouled probe condemns good water or passes bad; prove the instrument before opening the plant |
| Low capacity — check in order | Why output falls |
|---|---|
| Heater tubes fouled with scale — clean | Scale insulates the bundle; jacket heat never reaches the seawater |
| Air leaks, low vacuum — find and seal | Air raises the boiling point and blankets condensation; vacuum first, always |
| Defective ejector, low vacuum — inspect and renew | Worn nozzle or weak motive pressure cannot hold vacuum or clear brine |
| Jacket water too cool — raise temperature | Slow-steaming or cold engine starves the evaporator of heat |
| Defective distillate pump — check pressure and motor load | Cavitation or wear leaves made water stranded in the condenser |
6. The Rival Method & Drinking-Water Rules
How it works
Natural osmosis pulls pure water toward the salty side. Forcing salt water against a semi-permeable barrier reverses it — light water molecules pass, large dissolved salt molecules cannot. The barrier is hollow fine fibres spirally wound into cartridges, one-way by construction.
What sets output
Net drive pressure across the membrane, pressure loss through it, feed-water temperature, and how salty the feed is. Membranes must survive high pressure, and big production needs big membrane area.
Drinking-water law
Low-temperature evaporation never sterilises. Never generate near coasts where micro-organisms load the feed — sterilise every drinking batch by chlorination, electro-katadyn or ultraviolet, dosed and monitored to current notices.
Warning: Never near shore, never unsterilised: coastal water carries the organisms vacuum boiling cannot kill. Offshore production plus sterilisation is not optional — it is the licence to drink.
Practise this in the question bank
- Boiler Water Treatment — 7 questions
Found a mistake, or something unclear?
Report it in a few seconds — it goes straight to the editors.