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Learning Objective:Learning Objective: Know the basic designKnow the basic design

features and functions of various marinefeatures and functions of various marine

auxiliary machineryauxiliary machinery

Machinery: Fresh Water GeneratorMachinery: Fresh Water Generator

Specific Objectives:Specific Objectives:Recognize the various names and locations of

auxiliary machinery found on board

Describe the basic operation of the machinery

Identify the main parts of the machinery

Sketch and label the main parts

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Specific Learning Objectives:Specific Learning Objectives:

Explain the uses of fresh and distilledExplain the uses of fresh and distilled water onboard.water onboard.

Classify the evaporator types.Classify the evaporator types.

Describe their principles of operation.Describe their principles of operation.

Explain what is safe drinking water.Explain what is safe drinking water.

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DISTILLATION

Sea water has a total dissolved solids (TDS) content of 32000mg/litre.

Potable water for crew requirements contains

less than 500 mg/litre of suspended solids.

Good quality boiler feed will contain less than

2.5 mg/litre. Potable water taken on in port is thus not suitable for

use in water tube boilers unless futher treated by distillation.

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It is a statutory requirement that ocean-going (foreign) ships

should carry distillation plant for emergency use even if

sufficient potable water for normal requirement is carried.

Thus it is a common practice to take only a minimal supply of

potable water and make-up the rest of the requirement is madeup by distillation of sea water

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EVAPORATORS

By bringing the sea waterto its boiling point

(temperature) using heat from some convenient

source (steam/jacket water) to boil it and draw

off the vapour leaving the dissolved solids

behind in the liquid and distilled water is

obtained by condensing the vapour.

Cont-

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The fresh water produced can be used to provide make-

up feed for boilers, or fordomestic purpose. In the latter

case it is referred to as potable water, and the vapour

must be passed through a suitable distilling plant and

also be sterilized before use

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TYPES OF EVAPORATORS

BOILING TYPE

FLASH TYPES

Both can be ofBoth can be ofsingle or multiplesingle or multiple

effect plant typeseffect plant types

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BOILING EVAPOPATORS

Boils the water at the saturation temperature

corresponding to the pressure in the evaporator,

the water

is maintained continuously at its saturation

temperature (here latentheat is added)

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SUBMERGED TUBE

HIGH VACUUM TYPE

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Shell in two parts

Both are fabricated from mild steel, with its internalsurfaces protected against corrosion by a bondedrubber coating.

The lower evaporating section contains a verticaltube stack which consists of plain aluminum brasstubes expanded into tube plates at both ends

The upper vapour shell contains the vapourcondenser consisting ofaluminum brass hairpin

tubes expanded into a single tube plate and placedhorizontally above a water catchments' tray

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To reduce carry-over to a minimum, thevapour entering the distilling section has to

pass through a mesh type demister. This consists of layers of knitted monel

metal wire. Alternatively polypropylene

mesh. (Vapour temperature below 75oC) The vapour pass freely through this but any

water particles impinge onto the wire mesh,where they accumulate and ultimately

coalesce into water droplets large enough tobreak free, dropping down against thevapour flow, to fall back into the brine

Cont-

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The evaporator feed, after passing through astrainer,

flow indicator, and flow controller, enters at thebottom of the evaporator. It then passes up through the vertical heating

tubes where its temperature is raised by steam orhot water (jacket cooling water) passing over theoutside of the tubes.

Sufficient heat is provided for the water to boilunder

the vacuum conditions existing in the shell, thisresulting

vapour rising to pass through the demister

Cont-

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Vapour leaving the demister then enters thevapour condenser, where its latent heat isremoved by cooling water circulating through the

tubes of the distiller. The resulting droplets of condensate are

collected in the catchments tray, from where itflows via a salinometer probe to the distillate

pump. This probe transmits a signal to the electrical

salinometer which measures the density of thedistillate.

Cont-

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With an acceptable reading (say set at 5ppm), thedistillate pump discharges it through a flowcontroller and a non-return valve to the storagetank.

If the density is unacceptable, the salinometerprovides a signal which stops the pump, thusallows the unacceptable distillate to pass over the

double loop to re-enter the evaporator feed line forre-distillation or pass to bilge.

As an alternative arrangement, the salinometermay be used to operate a series of diverter valves

which achieve a similar objective.

Cont-

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The brine density is controlled by fitting flowcontroller in the feed and distillate lines, thesebeing set to admit 2.75 times as much feed

water as the amount of distillate produced, theexcess being pumped out by the water operatedejector.

Both provides a continuous blow down of brine

so as to maintain the density low enough toprevent scale forming, and also removes air andother non-condensable gases released duringthe evaporation process, from the upper part ofthe vapour shell.

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M/E JACKET

COOLING WATER

or STEAM LINE

AIR EJECTOR

BRINE EJECTOR

FEED INLET

SALINITY DETECTOR

DISTILLATE

PUMP

EVAPORATOR

VAPOUR

CONDENSER

SEA WATERCOOLING WATER

SUPPLY

DEMISTER

CATCHMENT TRAY

FLOW METER

COMBINED EVAPORATOR AND DISTILLER

SIGHT GLASS

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BRINE EJECTOR

DIFFUSERNOZZLE

BRINE PLUSOPERATING

WATER

OUTLET

BRINE INLET

OPERATING WATER

SUPPLY