One cannot drink water out of sea; as it dehydrates the body and cause serious medical complication. The fresh water reserves on land are limited and so are costly. A ship or cruise can only take much water it can hold from shore facilities. Not only it be uneconomical but also vary from port to port based on quality.
So the ship only takes non drinking water at dock; available at low cost. While it produces fresh water on board ship for drinking purposes; using fresh water generator or reverse osmosis plant. Basically it takes in the water from sea to produce drinkable fresh water; using a process known as desalination.
In simple words a fresh water generator or water maker ( a non professional word ) is the answer to the question; ” How we can make sea water drinkable “. It mainly consists of two large compartments within a cylindrical shell or body. The one compartment is called evaporator; where the heat source from steam or jacket water is used to produce steam.
In another compartment sea water is used to cool off the steam and thus called as condenser. Together they extract fresh water from salty sea water in the form of steam; separating latter in the form of condensed steam. Other than that it also contain ejector pumps; that creates vacuum and discharge brine out of the fresh water generator.
A fresh water generator consists of parts such as; heat exchanger, demister, ejector pump, distillate pump, salinometer, sea water pump, orifice and control panel. For both plate and shell type fresh water generator; everything remains the same other than heat exchanger. A plate type heat exchanger is used with plate type water generator; while a shell and tube heat exchanger being used with shell type fresh water generator.
1) Heat Exchanger
A heat exchanger allows for the heat to move from high temperature medium towards the low temperature medium; without actual contact in between. In the process a medium loose heat while the other gained it. A heat exchanger basically consists of a number of segregated elements; in form of plates or tubes with high thermal conductivity.
This keeps the medium’s separated from one another without sacrificing on efficient heat transfer. A fresh water generator consists of a pair of heat exchanger; with each having a specific purpose. The one which use jacket cooling water to heat the sea water feed to the generator is called evaporator.
The average temperature of jacket water is 70 to 80 degree Celsius. It loose some of its heat on bringing the sea water temperature; to 45 to 60 degree Celsius. While it takes a temperature of 100 degree for water to evaporate under normal condition; it just did evaporate at 50 to 60 degree under vacuum condition.
Demister is a thickened layer of mesh structure; fitted in between the evaporator and the condenser element. This is used to separate sea water from the steam vapour. A demister can be made of nickel, monel metals, copper, stainless steel and synthetic fibers; such as Polypropylene and PVC. Typically; demisters made of monel metal are used for fresh water generator.
What really happens is; when the water evaporates it carry over some fine little molecules of water along with the rising steam. When the source of such water is sea; it can considerably increase the salinity of output water. So to maintain salinity as low as 5 to 15 ppm; we use demisters which restrict the passage of mist and pass dry steam.
The restricted mist latch on the demister surface which then combined with other mist particles; it became heavy enough to fall off by the gravity. For efficient results it is placed as close as possible from the final steam outlet point. This allows for the larger heavier water particles to fall by itself; before ever reaching to the demister.
A demister is a light weight simple structure used to separate mist from dry stream. It is installed in system where the quality of steam required is very high. Some of the silent features of demister are its high separating efficiency; easy installation, corrosion resistance, long life and low pressure drop.
3) Ejector Pump
Both Brine and air ejector in combined are called as ejector pump in general. In maritime industry specially with marine engineers; the sea water pump supplying operating water to these ejector are many a times also considered in ejector pump. An ejector is similar to that of eductors with no moving parts.
It consists of parts such as; a nozzle, throat and a diffuser; and use under pressure water for its pumping action. The pump takes its operating water from sea water pump and discharge overboard; with the sucked it brine or air. Working on the principle of venturi effect; the sea water flow through a nozzle followed by a diffuser to create suction pressure.
When a constant velocity fluid flowing through a pipe encounters a sudden narrow path; its velocity decrease within a short time interval. Now to maintain its flow rate and overall energy; the lost energy in form of velocity is then converted to pressure energy. It is this pressure energy that is used to suck air and brine out of the fresh water generator.
The air ejector is connected to the upper most part of the generator shell’ while the brine ejector to the bottom most part. The air ejector is used to build up vacuum within the shell; of at least 90 percent in eight to twelve minutes. Brine ejector on other hand tends to continuesly pump out the sea water; that weren’t evaporated in the process.
4) Distillate Pump
A distillate pump is a normal centrifugal pump located at the lower most part of the fresh water generator. It takes suction from the steam condensate; and discharge to the drinking water tank on ship. The output pass through a salinometer which checks for the salt content in the output water.
The average salt content in the water is maintained at 10 ppm; but should never exceed more than 15 ppm. If the salt content exceeds the limit it is dumped to the bilges or send back to the generator shell with an alarm. These pumps are preferred over others; for their ability to pump high flow rate at variable dynamic head.
The pump is usually kept at much lower position to nullify the effects of vacuum condition; providing sufficient head for the pump to perform. The process also helps avoid the hassle to prime distillate pump; as it is self primed by the flowing distill water under the effect of gravity. Like most centrifugal pumps; a distillate pump also consist of parts such as shaft, impeller, bearing, casing and sealing.
A salinometer is the device installed on fresh water generator; capable of detecting even the slightest of salt content in the sampling water. It is connected to the distillate output just before the solenoid operated three way valve. The salinometer output is feed to the control panel; which then based on required salt limit will send fresh water for storage or back to generator.
A salinometer works on the simple principle that pure water do not conduct electricity; and its conductivity increase with increase in dissolved impurities and salts. The whole salinometer instrument consists of parts such as; temperature compensator, sensitive multi meter, relay and potentiometer.
The both electrodes of salinometer is dipped in the sampling water; conducting electricity between them. The circuit is connected as such the conducting current pass through the actuating relay. Now as the current increase with increase in salinity; the actuating relay actuates at desired value.
This close the alarm circuit raising alarm with a negative feedback; back to the fresh water generator control panel.
6) Sea Water Pump
Since the both ejectors works on the principle of venturi effect; you need something to provide them with the operating water. This is where the sea water pump came into play; it not only pass through the ejector creating suction pressure but also provide feed water for the fresh water generator to produce steam.
A sea water pump is a centrifugal pump that takes; suction from the sea water through sea chest followed by the strainer. Working on this simple principle of centrifugal forces and change in pressure upon change in flow area. The impeller rotates throwing fluids on the walls of the casing; which then converts major part of its kinetic energy to pressure head.
The output to the generator is controlled by the pump pressure along with correct orifice plate along the line. A spring loaded diaphragm plate is specially fitted into the generator feed inlet; to avoid flooding of shell when the generator is on stand by. On other hand the ejectors receive operating water as discharged by the sea water pump.
7) Control Panel
A control panel is what that makes it easier and possible to control and operate any machinery including fresh water generator. It is what that automate the starting and stop procedures; letting us just to monitor and see if its all well. It just makes it easier for us to start, stop, test alarms, test salinometer and set desired ppm level.
In simple terms a control panel is a set of dials, toggles, buttons, and bulbs that helps; operate and monitor the working of associated system even with remote position. Such automation along with remote position control is vital for operation; during the periodic unmanned engine room periods.
Working Principle Of Fresh Water Generator
A fresh water generator works on two simple principle of distillation and effect of pressure on boiling point. The sea water is a liquid mixture of pure water and lots of salt. Now when a liquid mixture is boiled; the one having lower boiling point boils and escape the mixture. So when we boil sea water and condense the vapor; what we get is the fresh water.
But the whole process has one flaw; that its not always economical to burn fuel to boil sea water at 100 degree Celsius. So to better utilize the waste heat improving overall efficiency of the plant; it is required to boil water at temperature much lower than its actual boiling point. So air ejectors are used to suck out air creating vacuum condition; which then help to complete the process with lower operating cost.
The sea water pump takes suction from one of the sea chest to discharge as; the operating water of ejector pump and feed to the generator. The feed water pass through the heat exchange where it gain heat from hot jacket water. At the same time ejector pumps suck out air to generate vacuum.
The sea water start to evaporates under the influence of the vacuum condition; and can be seen by the sudden drop in vacuum from 90-95% to 85 percent. The dry vapour then pass through a dimmister to move on to the second heat exchanger; to loose on heat and convert back to the liquid state.
This condensed water is then collected and pumped to the tanks using a distillate pump. The pump output is passed on through a salinometer; which checks and monitor salt content and take action to store or recirculate water accordingly.
Manual Starting Procedures For Fresh Water Generator
It is not that easy to start a fresh water generator manually with just a press of a button as with automatic system. It takes proper steps to be followed in correct sequence; take in consideration of the environmental factor such as port or at sea.
It is a common practice not to use fresh water generator while at port; as it generally contains waste products and harmful chemicals. A fresh water generator is usually started at 20 nautical miles away from the nearest shore. This not only provide protection against sewage and industrial waste; but also ensure we get high temperature jacket water.
Under maneuvering speed the rpm of the ships engine is low; and thus not provide enough heat to efficiently operate the fresh water generator.
12 Key Steps To remember
Check and ensure that the jacket water temperature is constant and is not below 70 degree Celsius.
Open the vent valve of the generator shell while closing the drain valve in the bottom if opened.
Line up the sea water pump line by opening its suction and discharge valve and the overboard discharge valve.
Start the sea water pump and slowly close the vent valve on the generator shell.
Maintain the sea water pump pressure at 3 to 5 bars with ejector pump pressure at 4 to 5 bar.
Wait for another 10 to 15 minutes till you get a 90% vacuum condition (About 18 mm of mercury) in the generator shell.
Once achieved; slowly open the valve to the jacket water inlet to evaporator. Complete this process by giving one turn to the outlet valve; followed by another turn on inlet valve.
Watch through the sight glass to confirm evaporation of water; followed by a sudden drop in vacuum pressure, rise in temperature and fall in feed water level.
As the system stabilize itself; open the cooling sea water vales to the condenser.
Start the distillate pump and salinometer to check quality of feed water.
Cross check the output water with salinometer reading; to ensure it is not salty and fit for consumption.
Open necessary valves to start discharging produced water to the fresh water tanks.
Water Maker ( Fresh Water Generator ) For Your Yacht Or Sailboat
While few yacht and recreational boats do come with company fitted mater maker; most others just don’t. Its more kind of a luxury than necessity with the boats; rather than on ships where its more of a necessity. A boat is a vessel capable of floating on water with a weight less than 500 tonnes; while a ship is a large complex vessels capable of sailing across the oceans with weight more than 5oo tonnes.
SSI offers a full range of alternative spares compatible with Alfa Laval Nirex fresh water evaporators.
We stock titanium plates and gaskets compatible with Nirex evaporators :
All parts as with all SSI parts are guaranteed to fit and perform exactly the same as parts supplied by Nirex themselves.
We stock and make all the various feed pump impellers and associated parts along with the distillate pumps. Ejectors and nozzles are also available.