Introduction

Fresh water is of dire need on a ship. It is used for various purposes such as drinking, for preparing food, in laundry services and in many other day to day activities. In the past, people used to carry tons and tons of fresh water in barrels or in specially made tanks. This was because there were no other sources of fresh water once the ship was in sea, except rain, which was quite unpredictable.

Today in spite of means on board to generate fresh water, ships carry tons and tons of fresh water in segregated fresh water tanks. But there are many other processes in which fresh water is used and as generally the fresh water tanks provided are not sufficient, the need arises to generate fresh water onboard. Fresh water is always in dearth also because it is very difficult to keep a check on the water usage by the crew members and also the usage cannot be avoidable as water is continuously required onboard ship.

Fresh water is generated on ship by a fresh water generator, also known as evaporator. The process by which fresh water is generated is known as distillation process. Sea water is converted into fresh water by evaporating and condensing the sea water. Evaporation of the sea water is either done by boiling or by flash process. This process enables the reduction of 32,000 parts per million in sea water to 1 to 2 parts in fresh water. Fresh water generators can be tube type or plate type.

Single stage fresh water generator

A fresh water generator consists of mainly two parts. A condenser and an evaporator. The boiling process is done either by a heating coil or by using the high temperature main engine jacket water. In some FWGs, heating is also done by superheated steam. Generally water is boiled at 100 degree Celsius, but in freshwater generator it is done at around 60-70 degree Celsius. For this purpose, eductors or air ejectors are use. Eductors reduces the chamber’s pressure to such a point that water starts boiling at a temperature of 60-70 degrees Celsius. A flow meter is used to regulate the flow of sea water into the generator. The freshwater generated in the condenser is later collected by a distillate pump. A salinometer is used to check the salinity in the fresh water generated.

Working of the FWG

The hot jacket water from the diesel engine is passed through the evaporator’s heated nest. The sea water enters the evaporator through the flow meter and due to the low pressure of the chamber; it boils and gets converted to steam. Generally the feed of the seawater is kept half the stated quantity to facilitate adequate boiling of sea water. The steam then passes through a steam separator, in which the water particles in the steam are separated and collected.

The steam then enters the condenser, where it cools down to form fresh water. It is then removed from the condenser with the help of a distillate pump. The remainder sea water particles or the brine which gets collected at the bottom is drawn out with the help of an ejector pump.

Flash process- Increasing the efficiency

A liquid which is already having sensible amount of heat when introduced in a pressurized chamber, immediately gets converted to steam, i.e. in a flash, without the boiling process. Thus the phenomena can be used to acquire higher level of efficiency by controlling the exact pressure and temperature of the water, which will lead to higher energy efficiency.

This can be done by using a double stage fresh water generator.

In the previous article we learnt as to how fresh water is generated on ship. Fresh water generators (evaporators) use heated water from the diesel engines to boil the sea water. Flash process can be used to increase the efficiency of the whole system. In the previous article we saw as to how water is generated in a single stage evaporator. Flash process can be used in a system having two stages of evaporation to obtain a better quality of fresh water. For this process 2-stage fresh water generators are used. A 2-stage FWG is similar to the single stage FWG, the only difference being that the whole single stage process is repeated twice in a 2-stage generator. Let’s see how a double stage evaporator works.

Construction

We know that a dual stage fresh water generator has the main chamber divided into two parts. Both the parts follow the same process. This to distillate the water to the finest quality by reducing the salt particles per million to the lowest number. There is also a pre-heater provided which pre-heats the salt water to facilitate the flash process. Rest of the arrangement is almost same as single stage FWG.

How does it work?

Sea water enters the evaporator chamber through a flow meter. This sea water is pre heated with the help of a pre heater to bring the temperature of the sea water as close to the jacket water temperature and also to enable the flash process from taking place. Pre heater uses steam for the process. Some of the sea water after entering the first stage evaporator flashes off due to the reduced pressure. The steam generated passes through the demister (steam water remover) where the water droplets are removed. The steam is then condensed into water droplets in the condenser.

The heated sea water now enters the second stage where it first passes through the evaporator. Due to the low pressure of the chamber, the hot water flashes off. The steam is again demisted and then condensed. The fresh water thus formed is collected with the distilled water from the first stage, with the help of a distiller pump.

The concentrated sea water, brine and the solid particles which remained at the end of the second process is collected with the help of a brine pump. Air ejector is used to maintain the air pressure inside the chamber and also to remove any access air in the chamber.

Maintenance

Due to high boiling temperature of sea water, scales are formed on the heating surface. The higher the boiling temperature required the more the formation of scales takes place. Other factors on which the formation of scale depends are flow rate and the density of brine formed.

Due to excessive scale deposits on heating surfaces, greater requirement of boiling temperature arises. Excessive scales will lead to low quality of fresh water and also reduction in the quantity of water produced. It will also prevent higher reduction of pressure inside the chamber from taking place.

Scale formation can be prevented by using a technique known as cold shocking. In this method the heating surface is alternatively heated and cooled, which reduces the formation of scales.

Also a routine maintenance of the generator should be carried out by shutting down the plant and removing the scale manually or by chemical treatments. The internal walls of the chamber or the shell should also be cleaned when the overall cleaning is done.

Air ejectors and eductors should also be checked for holes or leakages, which can prevent the formation of desired vacuum.

The distiller, feed and brine pumps should also be properly maintained to prevent any interruption in the flow of fresh and sea water. The processes and the phenomena used in both plate and tube type FWGs are the same.

A constant check should be kept on the flow meter to prevent excessive or very less flow.

The salinometer alarm should be precisely set and given a constant watch. This is to prevent the degrading of the quality of fresh water produced.

Procedure For Starting F.W. Generator On Ships

Introduction

A ship normally sails on long voyages and certainly drinking water is one of the most vital elements required on board. If you are thinking that this should not be a problem since the oceans contains unlimited water, read this article on the effects of drinking sea water. Especially imagine the condition in areas such as the dead sea, hence a provision for converting salt water to fresh water exists on ships and the machine is known as the fresh water generator.

We have learnt about the machine and its working in our previous articles but it is a different thing to know how and when to start these water generators on a ship. This article will tell you exactly the same and first we will see if it is OK to start these water generators anywhere.

Well I must tell you that a fresh water generator can only be started when the ship is 12 nautical miles away from the port and after sea speed is given to the main engine from the bridge .And when all the temperature and pressure parameters are normal and main engine is on sea speed and not in congested waters, or not maneuvering.

Starting of Fresh Water Generator

Normally a Junior Engineer would be required to start the water generator once the above conditions have been achieved. Many JEs are not quite familiar or comfortable with this simple procedure, so just outlining the main points here.

1.Check the jacket cooling water temperature outlet from the main engine - it should be constant

2.Open the Ejector pump suction valve and discharge valve to OBD (over board)

3.Close the vacuum breaker valve

4.Open cooling sea water valve inlet to condenser and out let valve

5.Check salinometer (salinity indicator) and distillate pump

6.Start the ejector pump and maintain the pressure more than 5kg/cm²

7.Check for the vacuum inside the shell as slowly rising

8.When the vacuum gauge reading reaches about 17mm of mercury, open the feed water to heating tubes

9.Check through the sight glass and maintain the water level inside the shell

10.Open the jacket water inlet valve fully - out let valve to be throttled

11.The jacket cooling water circulating through the spaces outside the heating tubes causes vacuum to drop, shell temperature to rise and feed water level to fall

12.When all the above is stabilized to normal, steam starts rising through the demister units and get condensed in the condenser

13.Fresh water gets collected in the catcher plate which can be seen through the gauge glass

14.When the gauge glass level is more than 3/4 th, put the salinity indicator to 'ON' position

15.Drain the water to the bilges or send it to the feed system again till the salinity level comes to set value (5-10 ppm)

16.When the salinity level comes to set value, open the discharge valve of distillate pump to the fresh water tank through the flow meter and start the pump

17.The evaporation rate can be increased by throttling the jacket cooling water outlet

18.Check for the tank vacuum to stabilize after some time, and ensure that the shell temperature should not rise more than 40-45^o c

19.Observe all gauge readings of :

· Inlet and outlet temperature of jcw

· Condenser sea water inlet and out let temperature

· Feed water level inside the shell

20.Distillate level in gauge glass should be monitored and always maintained at half gauge glass level when the distillate pump is running

AQUA Blue The AQUA Blue freshwater generator

The AQUA Blue freshwater generator The AQUA Blue freshwater generator has an optimized process that uses just half the seawater of other freshwater generators. This minimizes pipework and allows the installation of smaller seawater pumps, which reduces both installation costs and energy consumption. The process is based on unique 3-in-1 plate technology, which enables desalination in a single plate pack with one type of titanium plate. In AQUA Blue, this pioneering technology is combined with a wide range of enhancements for easier installation and simpler operation. Since the plate pack contains the process vacuum, AQUA Blue has no outer shell and is smaller than other freshwater generators. Its construction is robust, with duplex piping and titanium plates that resist corrosion and erosion. Little maintenance is required, and the plate pack slides open for easy access to the interior without an additional service area

Application

AQUA Blue uses vacuum distillation to convert seawater into high-quality fresh water for domestic and process utilization. By providing a constant supply of low-salinity water and continuously controlling the water quality, it eliminates the need for bunker water. AQUA Blue is designed for start-and-forget operation in periodically unmanned engine rooms and other automated operations. It is suitable for installation on ships and rigs, as well as in remote onshore locations. Jacket water, hot water, steam injection or a Hot Water Loop can all be used as heating media.

Features and benefits

• Half the seawater flow. AQUA Blue requires only half the seawater needed by other freshwater generators, which means smaller seawater pumps can be used. • Lower energy consumption and emissions. The reduction in seawater pumping needs has a corresponding effect on the consumption of electrical energy. Less fuel has to be burned, which reduces both operating costs and CO2 emissions. AQUA Blue also makes use of an energy-efficient IE3 motor. • 3-in-1 plate technology. AQUA Blue incorporates the evaporation, separation and condensation processes into a single type of titanium plate. Desalination is handled within a single plate pack that also contains the process vacuum. No outer shell is necessary. • Simple, compact installation. With no outer shell and no additional service area, AQUA Blue has a minimal weight and footprint. Assembly can be handled on site and installation is simplified by the ability to use smaller seawater pumps and pipes. Since AQUA Blue is not sensitive to roll and pitch, it can also be installed in any direction. • Easy operation and maintenance. AQUA Blue is operator-friendly and offers start-and-forget operation, which saves time for the crew. Optimized flow distribution across the plates prevents dry spots and keeps scaling to a minimum. Maintenance intervals are long and the plate pack slides open for easy access to the interior. • Proven technology. AQUA Blue has been thoroughly tested in full scale, both in Alfa Laval’s thermal laboratory and aboard vessels at sea. • Low-salinity water. Regulation of the seawater flow optimizes the performance of AQUA Blue, ensuring high-quality fresh water with a maximum salinity of 2 ppm. The fresh water has a lower content of dissolved solids than water supplied by other freshwater generators and can be used directly by the steam boilers. • Continuous quality control. AQUA Blue’s salinometer-equipped control system continuously monitors the quality of the outgoing fresh water.

Basic equipment

The AQUA Blue freshwater generator consists of a single plate pack containing a flexible number of titanium plates. These plates are suspended within a frame, which comprises a carrying bar, frame plate and pressure plate. Evaporation, separation and condensation all occur within the same plate pack. Among the plate pack features are gaskets with rubber flaps that indicate correct plate assembly. Connected to the plate pack is a combined system for condenser cooling water, feed water and ejector water. The seawater flow is regulated to ensure optimized production and fresh water with the correct quality. The freshwater system consists of a freshwater pump and a freshwater control sensor that ensures a stable outgoing flow of highquality fresh water. Connecting flanges can be delivered according to DIN, JIS or ANSI standards.

Additional equipment

• Combined cooling and ejector water pump with electric motor • Control panel with motor starters and salinometer • Anti-scale chemical dosing unit for feed water treatment Optional equipment • Steam heating system with direct steam injection • Equipment for steam boosting with a Hot Water Loop system • Build-on control panel • Build-on anti-scale chemical dosing unit • Distance indicators that ensure proper plate alignment and correct tightening • Cleaning-in-Place (CIP) unit • Freshwater pH adjustment equipment • Freshwater disinfection equipment

Operating principle

AQUA Blue’s flow of feed water is taken from the flow of seawater coolant. Feed water enters the lower (evaporator) section of the plate pack, in which the plates are warmed by the heating medium. Here the water is evaporated at around 40-60°C in a vacuum of 85-95%, which is maintained by the brine/air ejector. The vapour produced rises between the plates into the middle (separator) section of the plate pack, where any droplets of entrained seawater are removed. Gravity causes these droplets to fall back into the brine sump at the bottom of the freshwater generator. Only clean freshwater vapour reaches the top (condenser) section of the plate pack, which is cooled by a flow of seawater. Here the vapour is condensed into fresh water, which is pumped out of the freshwater generator by the freshwater pump

Installation

AQUA Blue is easily installed on ships and rigs, as well as in remote onshore locations. Since there is no need for an extra service area, the installation is highly compact. The heating medium is either hot water, such as jacket water from the engine, or a closed circuit heated by steam. An ejector pump supplies seawater coolant for the condenser, feed water for evaporation and drive water for the combined brine/air ejector. This pump is separately installed and connected to its own seawater intake. The fresh water produced is pumped into the storage tank by a built-in freshwater pump. A control panel, which incorporates motor starters and a salinometer, supplies electrical power to the ejector pump and freshwater pump, as well as control voltage to the salinometer and dump valve .

Operations •

AQUA Blue is designed for intuitive operation and easy maintenance. If Alfa Laval recommendations are followed, only a few service inspections are needed. • Connections for Cleaning-in-Place (CIP) are available. • Standard spare part kits are available. • The installation manual provides detailed information regarding correct installation in electronic or printed format: - System description - Installation instructions - Technical data and drawings • The operation manual provides detailed information regarding operation and maintenance in electronic or printed format: - System description - Operating instructions - Chemical dosing of anti-scaling chemicals - Troubleshooting - Maintenance of major components - Spare parts drawings - Technical data and drawings • Commissioning and technical services are available from all Alfa Laval offices, including installation assistance and advice on operation and maintenance. • Training in all aspects of freshwater generation can be provided by Alfa Laval. • Spares and maintenance can be provided by Alfa Laval Service Centres. Details are available from local Alfa Laval offices.

Capacity range

The AQUA Blue series covers a capacity range from 10 to 60 m3 /24 h, depending on the heating medium and cooling water temperatures. An AQUA Blue freshwater generator can be dimensioned to suit any jacket water temperature from 55 to 95°C at cooling water temperatures from -2 to 36°C.