EVAPORATOR AND FRESH WATER GENERATOR: Most Frequently Asked Questions on Fresh Water Generator

   Most Frequently Asked Questions on Fresh Water Generator

(1)   Explain the Freshwater generator working principle.

Ans:-The basic principle of all low-pressure freshwater generators is that the boiling point of water can be reduced by reducing the pressure of the atmosphere surrounding it. By maintaining a low pressure, water can be boiled at low temperatures say 50 degrees Celsius. The source of heat for the freshwater generator could be waste heat rejected by main engine jacket cooling water.

(2) How to Minimize Scale Formation in Shell & Tube type Fresh water generator?

Ans;-Scale formation in the freshwater generator can be controlled and minimized by continuous chemical treatment. Polysulphate compounds (like sodium polysulphate) with anti-foam are preferred by marine engineers and are extensively used on ships. Their trade name is different, like:
 
Vaptreat (by MONITOR) Ameroyal (by DREW CHEMICALS)

These chemicals minimize calcium carbonate scale formation and the possibility of foaming. the compound is non-toxic, no-acidic, and can be used in freshwater generators producing water for drinking purposes. If would be continuously fed into the feed line using a metering pump or by gravity. The amount of chemical to be dosed depends on the capacity of freshwater produced. The important thing is that this chemical is effective only on low-pressure freshwater generators. The seawater temperature to be less than 90 degrees. Freshwater generator chemical treatment to be religiously carried out to maintain its performance.

(3) what is Reverse is Osmosis?

Ans:-Osmosis is a natural process by which molecules of solvent pass through a membrane from a less concentrated solution (example Fresh water) to a more concentrated solution ( example Sea water) until the same concentration is achieved. Now if a considerable pressure is applied to the concentrated solution then we are able to force the water molecules in the reverse direction & that is Reverse Osmosis

(4) what are the arrangements for making Fresh water fit for drinking purposes on board a ship?

Ans:-

• After the water has been produced by desalinization, the following process has to be applied to it to make it fit for drinking purposes.
• It has to pass through a Re-hardening filter.
• It has to pass through a sterilizer which can be either by passing ultraviolet rays through it or controlled Chlorination dosage or combination of both.

(5) what precautions are taken before water is de-salinated in a Reverse Osmosis process?

Ans:-

• Care has to be taken to save the Semi-Permeable membranes used for the Osmosis process as they are a costly item. Following are the precautions
• The plant should be started at least 20 Nautical miles from the shoreline & the depth of the water should be at least 50 Mts
• A sand filter incorporated in the Feedline after Feed pump
• A Cartridge Filter or an activated charcoal filter incorporated in the Feedline before the high-pressure pump.
• A relief valve incorporated in the high-pressure line after the pump.

(6) Why are distillate pumps placed at the lowest possible location in the freshwater generator plant?

Ans.

In order to get better suction head, the distillate pump is placed at the lowest possible location in the freshwater generator plant. This is because the freshwater generator shell is at a lower pressure. The distillate pump gets a maximum net positive suction head with the height of the liquid column in the suction line.

(7)  explain the principle of a Freshwater Generator on board a ship (Shell & Tube type)

Ans;-
Freshwater Generators are used onboard ships to produce fresh water from seawater. It utilizes the principle of boiling Sea water at low pressure & then condensing the vapour to produce fresh water.
It consists of two sets of shell and tube heat exchangers, one acting as evaporator or heater and other as a condenser. The low pressure/ vacuum is created inside the shell by a Brine& air ejector. This ejector is supplied with driving sea water by an ejector pump. A part of this sea water is diverted to the evaporator tubes. The evaporator is supplied on the outside with heated (70  80 Dg C) ME Jacket cooling water. The low pressure inside the shell allows the evaporation to take place at a lower temperature. The vapour produced rises up in the shell & passes up through the demister. The demister & a deflector mounted on top of the evaporator removes the water spray & droplets from the vapour, which fall back into the brine. Also, the unevaporated water is discharged as brine (by combined air /brine ejector).

The desalted vapour, which passes through the demister, will come in contact with the condenser, where it will be condensed by means of incoming cold seawater.
The distilled water is then taken out by an integral freshwater pump (distillate pump) and controlled by salinometer and solenoid valve. If the salt content of produced water is high, the solenoid valve diverts the freshwater to the shell side of the freshwater generator and issues an alarm signal. Thermometers are installed for control of seawater to the condenser and jacket cooling water to the evaporator. These thermometers permit control of both heating and cooling of these units. The salinometer or salinity indicator is connected to remote alarm so that very high salinity is immediately registered at the engine control room of the ship.

(8 ) explain the working of a Freshwater Generator on board a ship  (Plate type)

Ans:-
The working principle of plate type fresh water generator is the same as that of the submerged tube type. Only difference is the type of heat exchangers used. Here plate type heat exchangers are used for condenser and evaporator unit.
The low pressure/ vacuum is created inside the shell by a Brine& air ejector. This ejector is supplied with driving sea water by an ejector pump. A part of this sea water is diverted to the evaporator plates via an orifice which fixes the amount of feed water. The other side of the evaporator plates are supplied with heated (70  80 Dg C) ME Jacket cooling water. The low pressure inside the shell allows the evaporation to take place at a lower temperature. The vapor produced rises up in the shell & passes up through the demister. The demister mounted on top of the evaporator removes the water spray & droplets from the vapor, which fall back into the brine. Also, the unevaporated water is discharged as brine (by combined air /brine ejector).

The desalted vapor, which passes through the demister, will come in contact with the condenser, where it will be condensed by means of incoming cold seawater.
The distilled water is then taken out by an integral freshwater pump (distillate pump) and controlled by salinometer and solenoid valve. If the salt content of produced water is high, the solenoid valve diverts the freshwater to the shell side of the freshwater generator and issues an alarm signal.
Thermometers are installed for control of seawater to the condenser and jacket cooling water to the evaporator. These thermometers permit control of both heating and cooling of these units. The salinometer or salinity indicator is connected to remote alarm so that very high salinity is immediately registered at the engine control room of the ship.