Question: What Is A Fresh Water Generator? What Is Its Purpose Onboard?
Answer: A Fresh Water Generator Is Equipment Used Onboard For Production Of Fresh Water From Sea Water. It Also Serves The Purpose Of Cooling The Hot Jacket Cooling Water At Around 85 Deg Celsius To 65 Deg Celsius. Generally The FWG Is Stopped, When Engine Attains Maneuvering Rpm, Because Further Cooling Of Jacket Water Is Not Required.
A Fresh Water Generator Uses The Heat Of Jacket Cooling Water Of The Main Diesel Engine To Evaporate The Sea Water Under High Vacuum, Through The Bilge Of The Separator, The Brine Is Separated From The Vapor, And Then The Separated Vapor Enter Into The Condenser Where They Are Condensed By The Cooling Sea Water In The Condensable Tube. By The Means Of The Salinometer, The Condensed Is Sent To The Fresh Water Tank When It’s Salt Content Is Less Than The Settled Value, Otherwise, It Is Sent Back To The Heat Exchanger To Evaporate Again.
In Addition, As The Fresh Water Generator Is Fitted With A Steam Ejector At Option, It Can Also Produce The Fresh Water Based On The Steam Of The Auxiliary Boiler When The Ship Berths At Dock And The Main Diesel Engine Stops Working.
Q. Explain The Working Principal Of FWG?
Ans: The Major Factor That Makes FWG Effective Is Evaporation And Condensation Of Water At Lesser Temperature, This Reduction In Saturation Temp. For Vaporisation And Condensation Is Due To Reduction In Saturation Pressure By Generating Vacuum Within The Chamber. It Is Generated By The Help Of An Air Ejector, Which Extracts The Air From The Chambers. An Air Ejector Works On Bernoulli's Principle And Continuity Equation. Heated Jacket Water At Around 85 Deg Celsius Enters The Evaporator And Transfers Its Heat To Cold Sea Water Supplied By The Ejector Pump. The Sea Water Flashes Off At Lesser Temperature And Pressure Due To Reduction In Its Saturation Pressure In Presence Of Vacuum. This Flash-off Water Vapour Passes Through Demister Plate Which Restrict The Passage Of Wet Vapour. This Water Vapour Now Reaches Into Condenser Chamber, Here Also The Sea Water Supplied By Ejector Pump Is Used To Condense The Water Vapour, Due To Reduction In Saturation Point, The Reduction In Condensation Temperature Also Take Place And Condensation Takes Place .The Condensed Water Passes Through A Salinometer Which Measures The Ppm Of Salt Present In Water. If It Is More Than 1-2 Ppm It Is Passed Back To Evaporator Chamber. Otherwise It Passed Towards Distillate Pump Via The Chemical Dosing Tank, Where Vaptreat Is Added At Regular Intervals To Remove Impurities. Distillate Pump, Pumps It Out Of The Fresh Water Generator And Is Passed To The Fresh Water Storage Tanks.
What Is Purpose Of Fitting FWG On The Ship?
Purpose Of FWG On Board
1. Production Of Fresh Water From Sea Water For Human Consumption With A Quality Demand Equal To World Health Organization (WHO).
2. Produce Fresh Water In Ample Flow For The Daily Demand.
3. To Make The Ship Independent Of Water Supply From Source Of Water Of Unknown Quality.
4. To Produce Fresh Water At Lowest Costs, As The FWG Unit Utilizes The Waste Heat From The Engine Jacket Cooling Water.
Explain General Working Principle Of A FWG?
A Controlled Amount Of Sea Feed Water Is Led To The Inside Of The Heat Exchanger Tubes, Where It Is Heated By The Surrounding Engine Jacket Cooling Water (Or From The Other Heat Sources). The Sea Feed Water Evaporates Under Vacuum To Reduce The Evaporation Temperature, Thus Avoiding Scale Formation Inside The Tubes.
The Generated Vapour Pass To The Separation Compartment Where The Sea Water Drops Are Separated As Brine.
The Dry Fresh Water Vapours Raise To The Condenser, In Which The Tubes Are Cooled Inside By Water, There They Are Condensed Outside The Tubes And By Gravity Is Falling To The Collecting Shell Of The Condenser And Flows To The Suction Pipe Of The Fresh Water Pump.
Briefly Explain The Function Of All The Parts/equipments Found In FWG System?
Ejector Pump:
It Is A Normal Centrifugal Pump Which Supplies The Necessary Service Water To The Ejector And The Preheated Sea Feed Water To The Heat Exchanger.
Sea Feed Water System:
By Means Of The Correct Ejector Pump Pressure And An Orifice, The Correct And Controlled Amount Of Sea Feed Water Is Supplied To The Plant.The Spring Loaded Diaphragm Valve Secures That The Plant Is Not Flooded With Sea Water When It Is Out Of Service.
Heat Exchanger:
It Exchanges The Latent Heat In Engine Jacket Cooling Water (Or From Other Heat Sources) To The Sea Feed Water.
Separator: A Deflector Plate Secures That No Sea Water Drops Pass On To The Condenser, As The Excess Of Boiling Feed Water From The Heat Exchanger Tubes Is Hold Back Drained Off. The Brine Accumulated In The Bilge Of The Separator Where It Is Led To The Brine Ejector.
Brine Ejector:
It Takes Out All Excess Of Sea Feed Water (Brine) And Secures That The Separator Is Not Flooded.
Air Ejector:
It Evacuates The Plant And Takes Out Continuously The Air Gases As To Maintain A Low Evaporation Temperature.
Condenser:
It Exchanges The Latent Heat In The Produced Fresh Vapours To The Cooling Water So That The Vapours Are Condensed And Accumulated In The Bottom Of The Condenser ‘s Shell.
Fresh Water Pump/Distillate Pump
It Is A Normal Centrifugal Pump Which Draws Out The Produced Fresh Water And Pumps It Into The Storage Tanks Of The Ship.
Salinometer System And Flow Meter: By Means Of An Ejector, A Salinometer, And Solenoid Valve, Produced Fresh Water With Too High Sea Salt Content Will Be Led Back To The Feed Water System.The Flow Meter Indicates The Accumulated Fresh Water Produced.
Q. What Are The Safeties In A FWG?
Ans: Safeties In A FWG Are:
1. Vacuum Breaker For Releasing The Vacuum At The Time Of Shutting Down.
2. Relief Valve For Releasing The Excess Pressure.
3. High Salinity Alarm: It Is Fitted To The Salinometer As It Measures Higher Salt Content In The Water Produced, It Sounds The Alarm.
4. Temperature Guage.
Q. Where Does The Ejector Pump Takes Suction From, What If Ejector Pump Fails And We Have To Run FWG?
Ans: Ejector Pump Has A Separate Sea Water Suction I.e. A Separate Sea Chest. In Case The Ejector Pump Fails And We Need To Run The FWG, There Is A Separate Line From Fire And General Service Pump As The Discharge Pressure Of This Pump Is Around 3-4 Bar And Ejector Pump Discharges At Pressure Not Less Than 4 Bar. Main Sea Water Cannot Be Used In This Case Because Msw Pump Has Discharge Pressure Around 1-2 Bar.