Fresh Water Generator Output Collapse in Tropical Waters
A shell-and-tube vacuum fresh water generator rated at 20 t/day is producing only 9 t/day. The ship is in tropical waters at 30 °C sea temperature. The seawater feed flow is set as per the manual, the ejector/vacuum is holding at 90 % vacuum, the brine density is being maintained at the specified value, and the heating medium is jacket water at 78 °C inlet and 70 °C outlet. The shell-and-tube bundle was last chemically cleaned eight months ago. The engineer has already increased the jacket water flow and opened the feed valve further, which raised output only marginally.
1. Diagnose the most probable cause and explain why the engineer's two adjustments were ineffective.
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The strongest single indicator is the combination of tropical sea temperature and reduced output with vacuum and brine density apparently normal. In a vacuum evaporator the achievable output depends on the temperature difference between the heating medium and the evaporating brine. With sea temperature at 30 °C, the condenser is warmer, so the vacuum that can be sustained and the condensing capacity are both reduced; the evaporation temperature rises and the temperature difference across the tube bundle falls. That alone can cut output substantially.
Scale is the second candidate. Eight months since cleaning, with the jacket water outlet only 8 °C below inlet, the heat transfer across the bundle is poor - a small approach suggests either low flow or fouled surfaces. Increasing jacket water flow would help only if flow were the limit; the marginal response suggests it is not. Opening the feed valve further is counterproductive: more feed means more brine to heat and a lower flashing fraction, and it can raise the brine level and carry-over risk.
So the diagnosis is a combination of reduced temperature difference from warm seawater plus progressive scaling of the tube bundle. The engineer's adjustments addressed neither the condenser-side limitation nor the fouling. The reasoning to show is: identify the driving temperature difference as the controlling variable, use the jacket water approach as evidence of fouling, and recognise that feed flow is not a free output control.
2. State the corrective actions you would take, in order, and explain how you would verify each one worked.
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First, confirm the instruments before touching the plant: compare the vacuum gauge with a second gauge or the ejector suction, and check the sea temperature and condenser outlet temperature to confirm the condenser is the limiting side. Verification is agreement between the two readings and a condenser outlet close to sea temperature plus a small rise.
Second, clean the tube bundle chemically with a suitable descalant, following the manufacturer's procedure, and inspect for pitting or erosion at the tube ends. Verification is the jacket water outlet temperature falling further below inlet at the same flow, and a measurable rise in output after restart.
Third, optimise the condenser side: confirm the ejector is clean and the seawater flow to the condenser is at design, and if the ship is fitted with a condenser that can use colder seawater, route it. Verification is recovery of vacuum toward the design figure and a lower evaporation temperature.
Fourth, re-set the feed flow and brine density to the manual values rather than leaving them opened up, and record output over a full watch. Verification is stable output at or near rated capacity with salinity within the potable limit. The answer must show sequencing - instrument confirmation, then fouling removal, then condenser optimisation, then feed re-set - and a specific check for each step.