Storing large quantities of fresh water on board ship is costly and takes up valuable space. An alternative solution, in the form of a process and technology for sea water desalination - fresh water generator.
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.
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)
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 relief valve gradually opens as the inlet pressure increases above the set-point. A relief valve opens only as necessary to relieve the over-pressure condition. Relief valves are typically used for liquid systems.
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.
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.
Fresh water generator troubleshooting
Low fresh water production:
- sea water pressure low because of Ships draft, choked filters, fault in pump etc.;
- level of brine is too high;
- faulty Ejector nozzle/nozzle choked;
- incorrect feed;
- scale formation in evaporator or condenser;
- shell temperature is too high;
- condenser cooling water flow is reduced;
- condenser cooling water temp. too high;
- incorrect assembly of plates;
- improper vacuum due to the leakage in plants like from pressure gauge, vent, distillate ump seal etc.
Vacuum is not maintaining:
- air leaks into the evaporator shell in large quantities and the air ejector cannot cope;
- the cooling water flow through the condenser is reduced or the cooling water temperature is high. This causes saturation temperature and hence saturation pressure within the condenser to rise;
- malfunctioning of the air ejector;
- the flow rate of the heating medium increased and excess water vapour produced. Since this excess vapours cannot be condensed, shell pressure increases or vacuum falls.
Increase in Salinity of Freshwater:
- brine level inside the shell too high;
- leaking condenser tubes or plates;
- operation of evaporator near shore with contaminated feed water;
- shell temperature and pressure are too low;
- increased solubility of CO2 generated from the salt water due to reduced seawater temperature. This dissolved CO2 makes water acidic and conductivity of water increases. Hence salinometer shows increased salinity, which is a measure of conductivity and not the presence of salt.