2nd Episode -
Fresh Water Generator at Sea
By Tat YEUNG
This is Tat Yeung again, a seagoing marine engineer.
A ship very often sails at sea for weeks before berthing a port for supply. Food can be stocked easily in the freezer. How about fresh water? Health experts have found that human can survive much longer without food than without water. Note that there is ample supply of sea water but sea water is harmful to human.
Simple solution is to store a lot of fresh water. However, fresh water is demanded in various aspects on board – drinking, washing, machinery cooling, raising steam in steam boiler… A commercial trading ship will rather spare the space to carry revenue earning cargo than non-revenue earning fresh water.
Another solution is to generate fresh water from the sea, an endless supply of sea water. Someone may call it desalination.
Modern desalination uses reverse osmosis method. This method produces fresh water of high quality. The filter is said to be so fine that even bacteria can be screened off. However, this technique may prove to be too complicated and expensive that it is not virtually viable on board a commercially orientated cargo ship as compared to a passenger ship offering luxurious cruises.
Reverse osmosis, on the other hand, is widely used in shore-based desalination plants.
The popular method is to let sea water boils. Fresh water evaporates from the boiling water, leaving behind water concentrated in salt, commonly known as brine water. The water vapour is subsequently cooled to condense to liquid state and collected as fresh water. Simple isn’t it?
How to boil the sea water? Heat it to boil, of course. Well, it demands fuel to heat and hence considerable cost. There is a cheaper way.
Water boils when its vapour pressure equals the ambient pressure. At sea level water boils at 100oC. At high attitude water may boil at a much lower temperature due to a low atmospheric pressure. How about creating a low pressure atmosphere, i.e. vacuum, inside a compartment partly filled with water, will the water boil without heating or very minimal heating? The answer is yes. That is the principle of fresh water generation on board.
Below is a schematic diagram of a fresh water generator (FWG) from the internet (Google master).
Sea water is first taken from the sea chest (sea chest is the common name of the opening at the underwater ship hull for taking in sea water) by a pump. Part of the sea water is pumped through an ejector pump. The ejector pump is virtually a venturi nozzle where water is forced under the pump pressure to go through a narrow path (venturi) thus moving fast to create a low pressure zone, vacuum. The low pressure zone is connected to the FWG internal compartment to extract the air inside the compartment thus creating a vacuum environment.
The medium in the above diagram is compressed air but can be pressurised water.
Part of the sea water from the pump goes through an orifice plate (serves to limit/control the amount of sea water going into the FWG, if too much incoming sea water, the vacuum may be broken) into the generating shell.
The diagram shows two lines from/to “jacket cooling water” and the evaporator before the generating shell. The two lines denote warm jacket cooling water to warm/heat the incoming sea water. This is a design to recover some of the waste heat from the jacket cooling water otherwise the heat will be dumped.
Let me explain what jacket cooling water is. Any reader who drives or owns a car, other than a total electric car or an air cooled Volkswagen Beetle, may be aware that the car’s engine is cooled by water circulating inside the engine casing called jacket which houses the combustion chambers (cylinders). The jacket has hollow passages for water to circulate around the hot cylinders. The cooling water extracts heat from the engine to prevent the engine from overheating. This is the jacket cooling water. The water carries the waste heat to the radiator where the hot water is subsequently cooled by air blowing through the radiator by virtue of the car’s running through air or by a fan. Do you notice that the car’s jacket cooling water is added with chemical which is indicated by a colour? The colour helps to distinguish this water as having chemicals added.
The jacket cooling water shown in the diagram comes from the main propulsion engine cooling system. This water is also added with chemical to prevent rusting and to prevent corrosion (alkalinity). Such chemical additive may be harmful to human when leakage occurs leading to toxic contamination of the fresh water generated.
In the above diagram, jacket cooling water is used to heat the sea water to enhance evaporation. I do not quite agree with the use of jacket cooling water to heat up unless the fresh water product is not for human consumption, drinking or washing (laundry/ shower etc). During my time on board, we never used jacket cooling water to heat. Steam was the alternative. In fact, most of the time the sea water temperature was warm enough that heating was not necessary. Of course, it was different at Alaska or North Sea.
Back to the fresh water generation, sea water boils in the generating shell under vacuum at low temperature. Some water evaporates as vapour leaving the sea water more concentrated in salt, the brine water. The brine water is led away and discharged overboard while new sea water comes in to replenish and evaporate. It is a continuous process.
The water vapour rises and passes a demister. The demister blocks solid particles such as salt and dirt, and brine water droplets.
The water vapour is then condensed to liquid fresh water and led away to service. If the water vapour has not been duly condensed and led away, the vacuum will be broken then the production chain fails.
Samples of the fresh water generated are taken continuously and tested by a salinometer to monitor the quality. An alarm will go off if the salt content is, usually, over 10 ppm. Besides giving an alarm, the monitoring device will also automatically recirculate the “not so fresh” water back to avoid contamination.
Otherwise, when the quality is good, the fresh water is led to various storage tanks to service. In particular, potable (drinking and cooking) tank is a separate tank from others in order to ensure protection to crewmembers on board.
This method to generate fresh water is simple, compact, economical and easy to operate.
However, the quality is not high. Water from the potable tank is usually sterilised by an ultra-violet light steriliser before consumption by the crewmembers but the effect is minimal.
The fresh water generated is virtually distilled water. It contains little or none minerals. This may have adverse effect on human health in the long term.
Unlike the reverse osmosis method which claims that almost all contaminants/pollutants would have been filtered out, the low temperature evaporation method is prone to contamination, say bacteria, germs, micro-organism. These can be effectively killed at temperature higher than 60oC. Unfortunately, the evaporation takes place below this temperature.
As a matter of practice, when the ship is in the port/harbour, close to the coast or in a sea area known of contamination, the fresh water generated is directed away from human consumption i.e. directed to storage tanks for washing, steam boiler feed water etc.