Fresh water generators (FWG) convert seawater (saltwater) to fresh water. FWGs are a common site on many marine vessels as it allows them to generate the fresh water they need whilst at sea. The process of generating fresh water is achieved via distillation.
There are three main methods employed for generating fresh water from seawater, these are boiling, evaporating and reverse osmosis (RO). Most marine vessels and industrial plants produce fresh water using either evaporators or RO plants. Large desalination plants may be connected to the steam circuit of a thermal power station, the steam is used to evaporate seawater.
The 3D model in the saVRee database is based upon a low pressure flash evaporator (flash distiller) that uses plate type heat exchangers, but FWGs that use shell and tube type heat exchangers are also available.
Fresh Water Generator Components
A FWG consists of the following main components:
- Hot and cold water connections.
- Fresh Water Pump
- Temperature Monitoring Devices
- Air-Purge Valve
- Safety Relief Valve (SRV)
Fresh Water Generator Components
All of the above components are explained in greater detail below.
Low pressure flash evaporators are connected to a seawater and hot water system. On marine vessels, seawater is sourced directly from the sea chest whilst hot water is sourced from the engine jacket water system (engine cooling water system).
The upper part of the FWG houses a condenser plate heat exchanger whilst the lower part houses an evaporator plate heat exchanger. The condenser allows seawater to pass through the heat exchanger in a closed system. The evaporator allows hot water to pass through the heat exchanger in a closed system. Both the condenser and evaporator are not fully sealed heat exchangers, the gaskets are modified to allow seawater to evaporate from the evaporator plates and fresh water to condense on the condenser plates. In short, plate heat exchangers usually have two fully closed systems, but in a FWG, one system per heat exchanger is closed and the other is open.
A demister is installed between the condenser and evaporator.
The condenser, evaporator and demister are housed within the FWG shell.
Fresh water from the FWG condenser is pumped to a storage tank by a fresh water pump (usually a small centrifugal pump).
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 temperature within the shell, seawater system and jacket water system is continuously monitored using thermometers (local indication) and PT 100 sensors (remote indication).
An air-purge 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.
A salinometer measures the salinity ('saltiness') of the generated fresh water. If the fresh water has too high a salinity, it is dumped/rejected (usually to the bilge). If the fresh water is within limits (typically <10 ppm), it is sent to a fresh water storage tank.
How Fresh Water Generators Work
Seawater is pumped from the sea chest to the FWG condenser. It passes through the condenser, then the evaporator, and finally through the ejector. A small amount of seawater is diverted directly from the condenser to the ejector, this maintains the vacuum within the shell. The seawater passes through the condenser first because it absorbs some heat prior to entering the evaporator, which increases the overall efficiency of the FWG.
Jacket water is pumped from the main engine to the FWG evaporator. The jacket water has a temperature of approximately 80°C (176°F). Because the shell is under vacuum, 80°C is sufficient to evaporate some of the seawater passing through the evaporator. It is important not to evaporate too much seawater as this will lead to salt forming on the plates.
Water evaporated from the evaporator forms a water mist which passes through the demister. The demister removes any carry-over salts; the water mist then reaches the condenser. Because the condenser plates are below the condensing temperature of the water mist, the water mist condenses upon the condenser plates. The condensed fresh water is then extracted using the fresh water pump.
Fresh water directly after the FWG is called distilled water and is used for washing and cleaning applications etc. Correcting the water PH value, then passing it through a mineralizer and bacterial treatment plant, yields drinking water (potable water).
Chemical dosing and UV filters are two of the most common bacterial treatment plants. For the health and welfare of drinking water consumers, it is essential bacteria levels are continually monitored and controlled.
Hardness is caused by magnesium and calcium ions in the water. Water hardness is a concern due to its tendency to form scale upon system surfaces e.g. heat exchanger surfaces, piping etc. Water softeners dose the water with sodium (salt) to reduce water hardness and reduce the likelihood of scale occurring.