11.2 Fresh Water Generators (Evaporators) & Heat Exchangers

11.2 Fresh Water Generators (Evaporators) & Heat Exchangers

Fresh water is essential aboard sea-going merchant vessels for engine cooling systems, steam boiler feed water, and domestic crew consumption (drinking, cooking, showers, and sanitation). Carrying thousands of tonnes of fresh water in structural tanks would drastically reduce cargo deadweight capacity. Merchant ships overcome this by producing their own high-purity fresh water using vacuum fresh water generators (evaporators) powered entirely by waste heat from the main propulsion engine. In parallel, marine heat exchangers manage thermal equilibrium across engine cooling, lubricating oil, and fuel systems.

---

1. Fresh Water Generator (Vacuum Evaporator)

The Thermodynamics of Low-Temperature Evaporation

At standard atmospheric pressure ($1.013\text{ bar}$ or $101.3\text{ kPa}$), water boils at $100^\circ\text{C}$. Lowering absolute pressure lowers the boiling point, allowing water to boil well below this temperature.

+-------------------------------------------------------------------------+
|                    PRESSURE VS. BOILING POINT OF WATER                  |
+------------------------------------+------------------------------------+
| Absolute Pressure inside Chamber   | Approximate Water Boiling Point    |
+------------------------------------+------------------------------------+
| 1.013 bar (Atmospheric Pressure)   | 100°C                              |
| 0.500 bar                          | 81°C                               |
| 0.200 bar                          | 60°C                               |
| 0.074 bar (about 93% vacuum)      | 40°C                               |
| 0.096 bar (about 91% vacuum)      | 45°C                               |
+------------------------------------+------------------------------------+

A vacuum evaporator reduces absolute pressure so that jacket-water heat can boil the feed. At $40^\circ\text{C}$, water's saturation pressure is about $0.074\text{ bar absolute}$; at $45^\circ\text{C}$, it is about $0.096\text{ bar absolute}$. Relative to atmospheric pressure of $1.013\text{ bar}$, these correspond to about $93\%$ and $91\%$ vacuum respectively. Percentage vacuum is $100(1-P_{abs}/P_{atm})$. Read the plant's actual pressure and temperature together.

This low boiling temperature provides two decisive advantages:

1. Zero Fuel Consumption: Seawater can be boiled using waste thermal energy extracted from main engine jacket cooling water ($80^\circ\text{C}\text{--}85^\circ\text{C}$ supply, leaving at $70^\circ\text{C}\text{--}75^\circ\text{C}$), requiring no auxiliary boiler steam while underway.
2. Reduced Scale Formation: Low-temperature operation helps limit scaling, but concentration of salts in the brine still matters. Maintain the specified feed-to-production ratio, brine extraction and approved antiscalant dose; low boiling temperature alone does not prevent deposits on the heating surfaces.

VACUUM FRESH WATER GENERATOR: SEPARATE FLUID PATHS

HEATING SIDE
ME jacket water ----> Evaporator heating passages ----> Jacket-water return
                         | Heat crosses metal plates
                         v
SEAWATER / BRINE SIDE
Feed seawater ------> Boiling chamber ----> Concentrated brine
                         |                        |
                         | Vapour rises           v
                         v                 Combined brine/air ejector
                    Demister pad             |             ^
                    |          |             v             | Air/gases
                    |          +-- Brine droplets     Vacuum chamber
                    |              fall to chamber
                    v
               Condenser vapour side ----> Distillate collection
                    ^                              |
                    | Heat crosses plates          v
COOLING SIDE                               Distillate pump
Seawater pump -> Condenser water side              |
                    |                              v
                    +-> Feed and ejector water  Salinometer/divert valve
                              |                    |              |
                              v                    v              v
                           Overboard          Accepted water   Reject route
                                             for intended use

Principal Components of a Vacuum Evaporator

1. Combined Ejector Pump: A dedicated high-capacity centrifugal seawater pump taking suction from the sea chest. It performs two duties:
   • Pumps cooling seawater through the upper condenser section.
   • Delivers high-velocity seawater through a venturi nozzle in the air ejector and brine eductor, generating a vacuum by entraining air and gases, and continuously pumping out concentrated brine.
2. Evaporator Section: Located in the lower chamber; consists of a bundle of titanium plates or aluminum-brass tubes. Main engine jacket cooling water ($80^\circ\text{C}$) circulates through one side, while feed seawater enters the bottom shell and boils at $40^\circ\text{C}\text{--}45^\circ\text{C}$.
3. Demister Pad (Moisture Separator): A thick mat of knitted stainless steel wire or Monel mesh mounted between the boiling chamber and condenser. Rising steam bubbles entrain tiny droplets of salty seawater (brine mist). As steam passes through the tortuous path of the demister, the heavier liquid droplets strike the wire mesh, coalesce, and drop back into the boiling chamber. Only dry, salt-free vapor passes through.
4. Condenser Section: Located in the upper chamber; cooled by seawater. The pure rising vapor strikes the cold condenser plates/tubes, condensing into fresh water distillate that collects in a distillate collection tray.
5. Distillate Extraction Pump: A small centrifugal pump that extracts pure water from the collection tray against the deep vacuum and discharges it to shipboard potable water tanks.
6. Feed Water Chemical Dosing Unit: Continuously injects anti-scale conditioning chemical (such as polyphosphate compounds) into the feed seawater line to further inhibit crystalline salt deposition.

Salinometer and Automatic Dump Valve

Fresh water produced for modern high-pressure marine boilers and engine cooling must contain virtually no dissolved mineral salts. Water salinity is monitored continuously by an electrical salinometer connected to a conductivity probe located in the distillate pump discharge pipe.

• Principle: Pure water is a poor electrical conductor; dissolved salt ions ($Na^+$, $Cl^-$) dramatically increase electrical conductivity. The salinometer measures conductivity and calibrates it into parts per million ($1\text{ ppm} = 1\text{ mg/L}$) of sodium chloride ($NaCl$).
• Alarm Setting: Use the approved salinity limit for the plant and intended water use. The salinometer has an adjustable alarm; 10 ppm is an example set point, rather than a universal statutory boiler-feed limit. Boiler-feed acceptance also follows the boiler water-treatment specification.
• 3-Way Solenoid Dump Valve: If salinity exceeds the selected limit, the salinometer gives an alarm and operates the divert valve. Off-spec water goes to the installed reject route, such as the bilge or evaporator return, instead of storage. Investigate foaming, excess feed, demister displacement or condenser leakage before accepting production again.

EXAMPLE: ALARM SET TO 10 ppm

Distillate   +-------------+  Salinity <= 10 ppm   +--------------------------+
Extraction ==| Salinometer |======================>| Fresh Water Storage Tank |
Pump         +------+------+                       +--------------------------+
                    | Salinity > 10 ppm
                    v (Alarm Triggered)
             +-----------------------+
             | 3-Way Solenoid Valve  |=====================> [Overboard / Bilge]
             +-----------------------+

Fresh Water Post-Treatment (Potable Water System)

Water produced by low-temperature distillation must be disinfected before drinking-water use. Its low salt content does not establish microbiological safety. The water also needs stabilisation because very low mineral content can make it corrosive and unpalatable. Follow the ship's approved potable-water treatment and monitoring arrangements, which can include:

1. Re-hardening / Mineralizer Filter: Calcium-carbonate media and other approved mineral treatment increase hardness and alkalinity, improve taste and reduce corrosivity. Check pH and treatment performance against the potable-water specification.
2. Ultraviolet (UV) Disinfection: An approved UV unit, commonly using 254 nm radiation, inactivates microorganisms as water passes the lamp. Lamp condition, sleeve cleanliness and correct flow matter; UV provides no disinfectant residual in the downstream pipework.
3. Chlorination Unit: Controlled sodium-hypochlorite dosing provides disinfection and a residual that limits bacterial regrowth. A far-tap residual of 0.2–0.5 mg/L is a common operating example; dose, contact time and monitoring follow the approved water-safety plan. Keep tanks and distribution pipes hygienic and investigate abnormal results.

---

2. Marine Heat Exchangers

Heat exchangers transfer thermal energy between two fluids of differing temperatures without allowing them to mix. In a ship's machinery space, they function as coolers (lubricating oil cooler, jacket water cooler, scavenge air cooler) and heaters (fuel oil heaters, feed water heaters). Modern ships primarily use two structural designs: Plate Type and Shell-and-Tube Type.

PLATE HEAT EXCHANGER (PHE)                 SHELL-AND-TUBE HEAT EXCHANGER

  Fixed Frame       Tightening Bolts        Tube Bundle Inside Cylindrical Shell
      |                    |                +--------------------------------+
      v                    v                |===Seawater In======+           |
   +----+ +--+ +--+ +--+ +----+             |  |--------------|  |           |
   |    | |  | |  | |  | |    |             |  |   Baffles    |  | Zinc      |
   |    | |  | |  | |  | |    |             |  |--+        +--|  | Anodes    |
   |    | |  | |  | |  | |    |             |  |  |        |  |  | In Cover  |
   |    | |  | |  | |  | |    |             |  |--------------|  |           |
   +----+ +--+ +--+ +--+ +----+             |===Seawater Out=====+           |
      ^                    ^                +--------------------------------+
      |                    |                   ^                  ^
  Fixed Plate        Movable Plate             |                  |
   (Titanium Plates & Gaskets)             Hot Oil In         Cool Oil Out

1. Plate-Type Heat Exchangers (PHE)

Plate heat exchangers consist of a pack of thin, corrugated metal plates held between a stationary frame plate and a movable pressure plate by high-tensile steel tightening bolts.

• Plate Metallurgy: Made of pressed titanium for seawater services (impervious to saltwater corrosion and impingement attack) or AISI 316 stainless steel for fresh water and lubricating oil duties. Corrugations (chevron or herringbone patterns) create severe fluid turbulence even at low flow velocities, yielding heat transfer coefficients 3 to 5 times higher than tubular designs.
• Elastomeric Gaskets: Molded Nitrile Butadiene Rubber (NBR) or EPDM gaskets are cemented or clipped into perimeter grooves on each plate. The gaskets route warm and cold fluids alternately through odd and even plate channels in a counter-flow arrangement.
• Double-Gasket Safety Barrier: A double-gasket seal with atmospheric tell-tale leakage grooves is provided around nozzle ports. If an elastomer gasket fails, fluid weeps externally down the plate pack to the bilge rather than cross-contaminating the opposing fluid system.
• Operational Advantage: Extremely compact (requires only one-third the space and weight of a shell-and-tube unit of equivalent heat duty); easily opened for chemical cleaning or mechanical brushing; capacity can be expanded simply by adding more plates.
• Operational Cautions: Elastomer gaskets age in service. Use the approved gasket material within its specified temperature, pressure and fluid-compatibility limits. During reassembly, tightening bolts must be tightened evenly in cross-pattern to a precise manufacturer-specified dimension (known as Dimension 'A'). Over-tightening crushes and permanently deforms the corrugated plates.

2. Shell-and-Tube Heat Exchangers

Shell-and-tube heat exchangers consist of an outer cylindrical steel shell housing a longitudinal bundle of hundreds of small-diameter tubes held in place by transverse tube sheets.

• Flow Mechanics: One fluid circulates through the interior of the tubes (typically corrosive seawater in modern practice), while the second fluid circulates through the shell space around the tubes (lubricating oil, jacket water, or steam). Baffle plates inside the shell force fluid to flow across the tubes in a serpentine pattern, maximizing turbulence and heat exchange.
• Thermal Expansion: To prevent tube buckling caused by thermal expansion, one tube sheet is fixed securely to the shell, while the opposite end tube sheet is 'floating' (sealed with resilient O-rings) or the tubes are bent into a hairpin 'U-tube' configuration.
• Sacrificial Zinc Anodes: Seawater water boxes (end covers) are made of cast iron or mild steel, while tubes are made of copper-nickel ($70/30\text{ or }90/10\text{ Cu-Ni}$) or aluminum-brass. The electrical coupling of these dissimilar metals in electrolytic seawater induces aggressive galvanic corrosion. To protect the expensive tube sheets and tubes, replaceable zinc sacrificial anodes are threaded into the water box covers. The zinc has a more negative electrochemical potential and corrodes preferentially.