Freshwater Generation on ships

Ever wondered how the seafarer's get the freshwater onboard the ships? It intrigued me when I was about to join this career. Many of us have watched the movies where protagonist searches for drinking water and finds it empty. Well all my questions are now answered, after I studied about the Freshwater generation plant on board the vessels. 

Freshwater Generator on ships works on a very basic principle. When we evaporate Sea water, it produces vapors, which can be cooled to produce distilled water. When added with required salts and minerals, it is fit for consumption. A typical freshwater must therefore contain an evaporator, a condenser, a mineralizer and a unit to check the consumption fitness of the produced water. 

Before we go in depth study of the freshwater generator, lets be aware with the basic line diagram which will facilitate our learning. 

Let's understand the whole working principle of the FWG system:

• Firstly the sea water is taken from the sea chest, and pumped through an ejector pump (explained later). The sea water in this state is at ambient sea water temperature which can be used as a cooling medium, thus, to increase the overall efficiency this sea water goes into the condenser unit of the Freshwater Generator. This causes the seawater to be in a preheated situation and also the water vapor coming from evaporator to cool down into distillate form. 

• The hot sea water coming out from the condenser unit, is sent into the evaporator unit, and is also sent to the Air/Brine Ejector (Ejector pump) as the main working fluid. While sending it into the evaporator unit we pass it through the orifice plate which, fixes the feed rate of the sea water going into the FWG.

• Now the preheated sea water is in the Evaporator unit. We have to evaporate the sea water in order to produce the distillate, thus we need a heating medium. To increase overall efficiency the Main Engine Jacket Cooling water (JCW) system provides for the heating medium of the evaporator unit. 

• Here one thing is very important to understand. The JCW system has freshwater at 80 degree Centigrade. To boil the sea water, we need either to elevate the temperature or decrease the boiling point. To decrease the boiling point we remove air from the unit using the Air/Brine Ejector. You can see a line coming from the top connected to the Ejector pump. 

• Thus vacuum is created inside the Freshwater generator, which causes the sea water to boil at a reduced temperature of 80 degree centigrade. Now the vapors, go up through the Demister, and the left salty mixture (Brine) is settled in the bottom of the Evaporator unit. Demister ensures that no salts by any means go into the condenser unit from the evaporator unit to ensure so, it restricts the salt and only allows the vapor to go from evaporator to condenser. 

• Now the vapor in the condenser exchanges heat with the sea water (as explained above), and the distilled water is formed in between the plates of the heat exchanger. The distillate is then taken out using a distillate pump which pumps out the distilled water prepared inside the generator. ***

• After this additional salts and minerals are added into the distillate to make it suitable for consumption in a Mineralizer. Following which the obtained freshwater is sent to the Freshwater Tank and is ready for consumption. 

*** SALINOMETER 

Salinometer is a device which keeps a check on the salinity levels of the distillate produced. The reading in the Salinometer is kept below 10 ppm. Usually the Salinometer is placed before mineralizer unit and after the distillate pump. As shown in the daigram:

Salinometer

Some Important Topics: 

1. What is an Ejector Pump?
2. Why do we have a safety relief valve and a air purge valve at the top?
3. Do we have any Pressure or Temperature sensors on FWG? If so, why?

Let's answer them one by one. 

Ans 1) Ejector pump basically works on the principle of Venturi effect. A high velocity fluid moving through a small cross sectional region, creates a vacuum which can be used to create a suction from a pipe attached in the same region. High velocity of working fluid creates an area of low pressure near the nozzle which creates a suction in the inlet region. 

A simple Ejector Pump 

Ans 2) The production of vapors and vacuum inside the FWG creates a region of variable pressure, which exposes a risk of rupture and crumbling under the pressure. A safety valve is necessary to remove excess pressure from inside of the FWG, similarly an air purging valve is use to fill the FWG with the air from atmosphere. 

Ans 3) The FWG is also equipped with Pressure and Temperature sensing units, which continuously monitors the abnormal growth of pressure and temperature in the FWG unit. 

HOW TO BATTLE SCALING IN FWG....?

One of the major problem in the FWG is formation of scales, which can minimize the heat transfer thus decreasing the overall efficiency of the system, and also result in less water production. Major scales forming compounds in FWG are: 

• CaCO3
• Mg(OH)2
• CaSO4

To minimize the scale formation it is advised to run the FWG on its rated condition. Chemical treatment and regular inspection and maintenance reduces chances of Fouling and scaling inside the tubes in FWG. 

I've tried to keep it short and cover all the topics in the blog. Please ask if I need to add anything....

Thank you for reading, Have a great day..!