Table of Contents
Why we need a Fresh Water Generator on board?
Modern-day vessels are designed to run at their lowest cost. rather than carrying tons of quantity of freshwater vessel owners prefer to carry cargo instead. so it’s common practice the freshwater tanks are limited to a few hundred tons depends on the consumption of the particular vessel.
For domestic use, for boilers, for cooling systems, if the vessel is a cattle carrier for animals obviously we have to use a considerable amount of freshwater per day. because of this reason vessel, can’t wait until reaching a port to take freshwater but to produce it onboard. so it is more practicable to produce freshwater on board by using a freshwater generator. another advantage is this way vessels can produce quality freshwater compare to the shore supplies.
Distillation
Seawater has a total dissolved solids (TDS) content of 32000 mg/liter.
Potable water for crew requirements contains less than 500 mg/liter of suspended solids. Good quality boiler feed will contain less than 2.5 mg/liter. Potable water taken on in port is thus not suitable for use in water tube boilers unless further treated by distillation. Clean water free from impurities is essential for engines and boilers.
- Impurities in Shore water
- Chlorine & suspended solids
- Unsuitable for consumption
- unsuitable for boiler and engine cooling as it contains dissolved solids of about 500 mg/ltr
- Impurities in Seawater
- Salts and other particles
- Unsuitable for consumption
- Total dissolved solids (TDS) about 32000 mg/ltr
Types of distillation Processes
- boiling type
- lash type
- vapour compression
- reverse osmosis
Produces water vapor at low-temperature Seawater boils under vacuum condition Vapour condenses into water droplets
Boiling Type Fresh Water Generator
- Evaporator
- Condenser
- Eductor
- Demister
- Salinometer
- SW Pump
- Condensate Pump
Operation of Fresh Water Generator
- Condenser
- Demister Action
- Materials of FWG components Shell-Mild steel
- Demister- Knitted Monel wire gauge
- Evaporator d Condenser-Al-brass tubes brass tube plate
- AS pressure decreases, boiling temp decreases
- As pressure increases, boiling temp increases
Shell coated with epoxy to prevent any corrosion.
Single Flash Type Evaporator
It consists of one identical chamber made of fabricated mild steel with protective internal coating, demister screens of knitted Monel metal wire, and vapor condensers made up of aluminum brass U tubes expanded into rolled naval brass tubes plates.
This can complete in two stages, First, the feed to the vapor Condenser heat by the Outgoing vapor. It gives up its latent heat and in the Second stage by steam or M/E jacket cooling water.
Then the heated feed release into the flash chamber. In the chamber, the pressure is maintained low enough to ensure that the corresponding saturation temperature is below the incoming hot water by having an orifice in the suction line to the chamber.
The water cannot remain in the super-saturated state. So some of its mass flashes off into steam, leaving its dissolved solids behind in the water remaining in the flash chamber call brine.
A brine pump extracts low-density unevaporated water and discharges the bulk overboard. Some, however, may return to the suction side of the supply line to maintain the feed inlet temperature at about 30°C irrespective of how low the seawater temperature may be.
The released steam then passes to the vapor condenser, from where the resulting distillate via demister is pumped to the storage tanks via a potable filter, and sterilizer if necessary.
The density of the distillate measure by a salinometer. If it reaches a high value, the distillate pump stops and the unacceptable water passes over a loop to the brine pump suction from where it discharged along with the outgoing brine.
A water-operated ejector uses to remove air and other non-condensable gases from the vapor chamber and so maintain the necessary vacuum conditions.
Automatic Control is necessary to ensure satisfactory operation of the plant. The reason is, it is sensitive both to changes in sea temperature and to the water level in the chamber.
The special Starting up arrangements will require as until flash off commences, the second Stage heater has to supply all the necessary heat or restricting the flow of feed in.
Operation of the two-stage flash evaporator or Multi-stage flash evaporator or double effect flash evaporator is almost the same as single-stage flash evaporator plant.
Unevaporated water from the first chamber will flow through an orifice which maintains pressure difference- into the second chamber where more water evaporates since the pressure is lower than in the first chamber.
A weir fitted to maintain a water seal between the two stages or orifice in the suction line to the second chamber act to maintain pressure difference.
Distillate formed in the two vapor condensers collects in the catchment trays, from where it flows to the distillate pump via salinometer probe.
As the usual operation of salinometer, detecting the ppm or salt contains in water. The material used was the same as a single flash evaporator plant
Shell and Col Evaporators
It consists of one identical chamber made of fabricated mild steel with protective internal coating, Demister screens of knitted Monel metal wire, and vapor condensers made up of aluminum brass U tubes expanded into rolled naval brass tubes plates
The incoming seawater feed is first heated to some temperature below its boiling point in the tubular heat exchanger. Tow stages to follow:
In the first, formed by the vapor condenser, the feed heats by the outgoing vapor which gives up its latent heat and in the second stage by steam or M/E jacket cooling water.
The heated feed then releases into the flash chamber where the pressure is maintained low enough to ensure that the corresponding saturation temperature is below that of the incoming hot water by having an orifice in the suction line to the chamber.
The water cannot remain in the super-saturated state, so some of its mass flashes off into steam, leaving its dissolved solids behind in the water remaining in the flash chamber call brine.
A brine pump extracts low-density unevaporated water and discharges the bulk overboard. some, however, may return to the suction side of the supply line to maintain the feed inlet temperature at about 300C irrespective of how low the seawater temperature may be.
The released steam then passes to the vapor condenser, resulting in distillate via demister pumped to the storage tanks via a potable filter and sterilizer if necessary.
The density of the distillate measures by a salinometer. If it reaches a high value the distillate pump stops. And the unacceptable water passes over a loop to the brine pump suction from where it discharged along with the outgoing brine.
A water-operated ejector uses to remove air and other non-condensable gases from the vapor chamber and so maintain the necessary vacuum conditions.
Automatic control is necessary to ensure satisfactory operation of the plant, as it is sensitive both to changes in sea temperature and to the water level in the chamber.
Special starting up arrangements require as until flash off commences, the second stage heater has to supply all the necessary heat or restricting the flow of feed in.
Two-Stage Flash Type Evaporators
Operation of the two-stage flash evaporator or Multi-stage flash evaporator or double effect flash evaporator is almost the same as single-stage flash evaporator plant.
Unevaporated water from the first chamber flow through an orifice which maintains pressure difference – into the second chamber where more water evaporates since the pressure is lower than in the first chamber.
A weir fitted to maintain a water seal between the two stages or orifice in the suction line to the second chamber use to maintain pressure differently.
Distillate formed in the two vapor condensers collects in the catchment trays, from where it flows to the distillate pump via salinometer probe.
As the usual operation of salinometer, detecting the ppm of salt contains in water.
A brine pump extracts low-density unevaporated water and discharges the bulk overboard.
Some, however, may return to the suction side of the supply pump through the regulating or auto-valve to maintain the feed inlet temperature about 30C irrespective of how low the seawater temperature may be.
non-condensable gases extract by the air ejector which maintains the high vacuum condition in the chambers. The material used was the same as a single flash evaporator plant
A typical multistage flash system is based upon preheating of a pressurized seawater stream. Or more typically a recycled brine stream to which the feed seawater is added. The steam heats in the heat input section brine heater.
From here the recycle stream passes into the first stage of a series of flash chambers. Here the pressure release, permitting a portion of the brine stream flash to form salt-free vapor which condensed to give the freshwater.
In condensing the vapor gives off its latent heat to the recycled brine steam. From the first stage, the flashing brine stream passes to the second stage which keeps at a slightly lower pressure; more vapor flashes off.
In the same way, the flashing brine stream passes to the next stage and so on through the plant with a portion of the vapor flashing off at each stage. A heat balance shows that the heat supplied in the brine heater should reject or not. This happens in the last two stages of the plant which cools by a seawater stream which subsequently passes to waste.
Related Article-What is Reverse osmosis water treatment plant?
