Freshwater is an essential resource on board ships during extended voyages. It is required for human consumption, cooking, cleaning, and various other purposes. The traditional method of obtaining freshwater on a ship was by storing it in tanks before setting sail. However, this method had its limitations, as the supply was finite and could run out quickly. Also, the cargo-carrying capacity is reduced by storing large amounts of freshwater.
To overcome these challenges, engineers developed the freshwater generator for ships that use the process of desalination to produce freshwater from seawater while at sea. This technology is one of the best examples of the application of Gay Lussac’s Law on ships to build up their reserves of freshwater throughout their voyage. In this article, we will explore the functioning, benefits, and different types of freshwater generators on a ship.
- What is a freshwater generator?
- Types of Freshwater Generators
- How Freshwater Generator Works?
What is a freshwater generator?
A desalination plant onboard ships
A Freshwater generator is machinery equipment that is used on a ship for the production of freshwater from seawater for domestic and auxiliary use. The freshwater generator uses a variety of methods to convert seawater into potable water, including distillation, reverse osmosis, and evaporation.
An average freshwater consumption of a ship of about (20000-50000 DWT) is around 5-7 tons with Vacuum Sewage Plant or about 10 tons with a conventional freshwater flush system.
On average, a human on a ship consumes about 100-200 litres of water per day for domestic use. The remaining water is used for the operation of various auxiliary machinery
Need for Freshwater Generator
Humans operate the ships and freshwater is life
As humans are still operating ships and hence freshwater is a valuable and essential resource for any ship voyaging in the vast oceans. Without it, the crew and passengers would be unable to complete their voyage due to dehydration and other health issues. However, obtaining freshwater on a ship can be challenging and costly, especially during long journeys where there are no sources of freshwater nearby.
To address this issue, ships have turned to freshwater generators as a solution. These generators use advanced technologies that allow ships to produce clean and safe water from seawater or brackish water found in various parts of the world’s oceans. This article will explore how these generators work, their benefits over traditional methods of obtaining freshwater on board ships, and their importance in ensuring safe travels for everyone onboard.
The primary objective of a marine vessel is to convey goods, and owners strive to transport as much cargo as possible. Throughout time, the ship’s living quarters and crew sizes have decreased in order to maximize earnings. Constructing sizable freshwater reservoirs for personal and supplementary purposes is illogical. Consequently, freshwater is generated, and the restricted tank capacity is employed for storage.
The freshwater generator doubles as a colossal radiator for the vessel’s primary propulsion system. The machinery generates a substantial amount of heat while under load, which must be dissipated to ensure safe operation. As a result, the freshwater generator is employed as a conduit for eliminating the surplus heat from the system.
Types of Freshwater Generators
List of various freshwater generators
- Distillation / Evaporators.
- Shell & Tube Type
- Plate Type
- Reverse Osmosis.
Freshwater Generator Components
Let’s consider the Distillation/Evaporator type of freshwater generator on a ship. The following components make the freshwater generator operational.
Distillation or Evaporator Type Components
The evaporator is where the sea water is heated up using Main Engine Jacket Cooling Water which further evaporates giving off water vapours
The condenser is where the seawater helps to condense the water vapours generated from the evaporator.
- Freshwater Distillate Pump
The condensed vapours from the Condenser are pumped to the storage tank using this freshwater centrifugal pump.
- Air and Brine Ejector
The ejector or eductor is responsible for developing a vacuum inside the shell by removing the brine solution generated due to evaporation. Air from the system is also removed along with brine through the ejector with a venturi effect.
The salinometer is installed in the discharge line of the Freshwater Distillate pump. It is used to detect the salinity of the produced water. The typical alarm setting is > 20ppm+
The demister is present inside the shell separating the condenser and evaporator. This helps to avoid carry-over of the seawater from the evaporator and only allows the vapours to pass through to the condenser
- Ejector Pump
The ejector pump is usually a centrifugal pump supplying the seawater to the ejector.
The shell is referred to as the outer casing or housing inside which the vacuum is created and the above-mentioned components are arranged.
- Vacuum Breaker/Air vent valve
A vacuum breaker is installed on the shell which helps to break the vacuum inside the shell after stopping the freshwater generator.
Reverse Osmosis Type Components
- Feed Water Pumps
The feed water pump is usually a centrifugal pump supplying the seawater for the reverse osmosis plant.
Prefilters are installed after the feed pump in order to filter out the large suspended solids from the feed water.
- Chemical Treatment Dosing
In order to reduce the formation of scales chemical dosing is required.
- Cartridge Filters
Before dosing to the semipermeable membrane, suspended solids in seawater are additionally filtered by cartridge filters.
- High-Pressure Feed Pumps
High-Pressure feed pumps are required to develop high pressure which assists in the process of Reverse Osmosis
- Semi Permeable Membranes
The semi-permeable membranes are the most important component of Reverse Osmosis type freshwater generator as the complete process of reverse osmosis is carried out due to these membranes. It is an array of multiple membranes through which the high-pressure water flows causing the separation of freshwater and brine.
How Freshwater Generator Works?
Working Principle of Freshwater Generator
Onboard ships, the majority of freshwater generators are either of the distillation or evaporation variety. However, for cruise ships that require large quantities of water production, reverse osmosis plants are commonly used.
Working of Distillation/Evaporator type of Freshwater Generator
In Distillation/Evaporation type of the freshwater, generator involves boiling and condensing the vapours of seawater.
The Distillation/Evaporator type of freshwater generator is a simple application of Gay Lussac’s Law which states that Pressure is directly proportional to the temperature of the gas.
Boiling of a Liquid: A liquid boils at a temperature at which its vapour pressure is equal to the pressure of the gas above it. Applying Gay Lussac’s Law if the pressure acting on the liquid is reduced the boiling point of the liquid is also reduced.
Low-pressure flash evaporators/shell and tube type FWG make use of the seawater from the sea-chest through the ejector pump which also helps in developing a vacuum in the shell. Main Engine Jacket water is the primary source of heating for this seawater and also acts as a cooler for Main Engine to withdraw the heat generated during its operation.
The ejector pump supplies seawater to condensor inlet, the outlet of condensor is connected to an ejector and a small tapping is drawn which is supplied as feed water which flashes in the evaporator. This flashing of the feed water in return cools the Main Engine jacket water acting as a radiator for the Main Engine.
Flashing of Liquid: As the seawater flashes inside the shell there are chances of boiling water getting carried over along with the vapours. A Demister is present between the evaporator and condenser that traps the boiling water from getting carried over with vapours and only vapours are allowed to pass through the demister.
Condensation of Vapours: The vapours are then further cooled in the condenser and Fresh water from the FWG condenser is pumped to a storage tank by a freshwater pump (usually a small centrifugal pump). Net Positive Suction Head (NPSH) is required for the centrifugal pump to take suction from and discharge to the required tank. Will be discussing the NPSH in detail later on in other topics.
Vacuum Production: An ejector/eductor is used to create and maintain a vacuum within the shell; it also removes brine (water with high salinity) from the lower part of the shell. The ejector is nothing but a carefully designed combination of converging and diverging nozzles. The Venturi effect occurs when the sea water passes through the ejector which in return helps to remove the excess air and brine solution from the shell, thus producing a vacuum.
The temperature within the shell, seawater system, and jacket water system is continuously monitored using thermometers (local indication) and PT 100 sensors (remote indication), etc. An air-purge/vacuum breaker valve is installed at the top of the shell. The air purge should be open when the FWG is not in service and closed when the FWG is in service.
As a precaution against over-pressurization, a safety relief valve (SRV) is installed on the top side of the shell.
Freshwater Testing: A salinometer measures the salinity (‘saltiness’) of the generated fresh water. If the freshwater has too high a salinity, it is dumped/rejected (usually recirculated back to the shell). If the freshwater is within limits (typically <10 ppm), it is sent to a freshwater storage tank.
Working of Reverse Osmosis type Freshwater Generator
What is Reverse Osmosis?
The process of movement of a solvent through a semipermeable membrane from the solution to the pure solvent by applying excess pressure on the solution side is called reverse osmosis.
The working principle of reverse osmosis, which is a widely utilized technique for water purification, has a long history and is highly favoured. Initially, it was primarily employed for desalinating seawater in the 1950s, but the process was initially sluggish and confined to specific laboratories. Nevertheless, through extensive research and technological advancements, particularly in polymers and membrane production, significant progress has been made.
The working principle of reverse osmosis is now widely employed worldwide to treat water for various purposes such as industrial, residential, commercial, and scientific applications. Additionally, the shipping industry has also adopted this technique to meet their freshwater requirements.
Feed water and Filtration: The water that usually requires desalination is typically referred to as feed water. In order to supply the system with feed water, a feed pump is utilized. The feed water then proceeds through a strainer. The same pump is also responsible for backwashing the Sand filters, which will be discussed in the subsequent steps below. In a shipboard setting, the feed water is sourced from the sea. However, it is important to note that the reverse osmosis plant can only function beyond 4 nautical miles from the shore to prevent contamination from coastal waters.
The purpose of this procedure is to eliminate larger suspended solids that may be found in the untreated water. Sand filtration is conducted using multiple filter vessels (usually 2 or more, depending on the quality of the untreated water) that contain various sizes of grit stored in layers along with coarse sand. There is intentionally some empty space above the grit media to accommodate bed expansion during the backwashing of the filter. The filter is designed to produce enough filtered water to ensure the proper functioning of the reverse osmosis plant.
The desired value for the total design flow rate of parallel filters is established, and it is crucial not to surpass this flow. Exceeding the filtration rate necessary for optimal performance could result in a significantly diminished quality of filtrate, as per the working principle of reverse osmosis.
Chemical Treatment & Dosing: Antiscalants, which are a group of chemicals, are specifically designed to prevent the formation and deposition of crystallized mineral salts that create scale. The majority of antiscalants are synthetic organic polymers that are privately owned (such as polyacrylic acids, carboxylic acids, poly maleic acids, organo-phosphates, polyphosphates, phosphonates, anionic polymers, etc.). These polymers have a molecular weight ranging from 2,000 to 10,000 Dalton. To hinder the precipitation of these salts, a solution containing a unique antiscalant compound is introduced into the feed water. This compound acts as an inhibitor for scaling, thereby enabling more efficient functioning.
Cartridge Filters: To ensure optimal performance of the reverse osmosis membrane elements, the feed water is first treated with antiscalant and then passed through a 10-micron absolute-rated cartridge filter. This filter effectively removes any large suspended solids and fine particulate matter from the feed water, guaranteeing that the RO membrane elements receive clean water. Additionally, the filter serves as a safeguard for the high-pressure pump in case of a sand filter lateral failure.
High-Pressure Pumps and Reverse Osmosis: Feeding the membranes is the subsequent action, which is facilitated by high-pressure feed pumps. In the reverse osmosis process, pressure is applied to the concentrated fluid to counteract osmotic pressure. These pumps generate the necessary high pressure based on the rating of the membranes. The high-pressure pump provides the force required to propel water through the membrane, while simultaneously preventing salt from passing through it.
The desalination process involves passing pressurized, filtered water through the RO membrane elements. In each stream, the feed water is divided into two flows: permeate (high-quality, potable water) and reject (concentrated feed water). The reject from the first vessel then enters the second vessel where it is once again divided into permeate and reject flows. This same process occurs in all the remaining streams. It’s important to note that a stream is formed by several vessels joined together, with three or more vessels enclosed within a container known as a stream.
The reject from the final vessel in each of the three streams is combined and directed through the reject control valve before being discharged as waste. The purpose of the reject control valve is to maintain the desired pressure within the membrane stack, thereby controlling both the rate and quality of the permeate flow.
Freshwater is a vital resource for any ship, serving various purposes such as drinking, cooking, and sanitation. However, when sailing across vast oceans for extended periods of time, ships face a challenging dilemma – how to sustain an adequate supply of freshwater without relying on external sources. This predicament has led to the development and implementation of innovative technologies like freshwater generators on ships.
Throughout my experience in the maritime industry, I have encountered both Flash type and Reverse Osmosis type freshwater generators. Unfortunately, upon joining the ship, I discovered that the RO plant was not functioning properly due to a lack of spare parts. However, I took the opportunity to familiarize myself with the system. While freshwater generators are relatively simple machinery, they require regular maintenance and care. It is crucial to promptly identify and address any decrease in efficiency to avoid a shortage of freshwater. Trust me, implementing water rationing onboard is a dire situation that can result in extremely poor living conditions, especially in remote areas.