Drinking water supply on ships
The drinking water supply on ships consists of the drinking water storage in special tanks, the drinking water production, the drinking water treatment and the drinking water supply system. They are ship auxiliary systems that work fully automatically and are monitored by crew members such as ship engineers .
Review and introduction
There are many reports of the time of sailing, after which the drinking water on board became scarce. "Putrid" or stinking drinking water was also reported from this period. With steam shipping, the ships became bigger, tanks were used instead of drinking water barrels. With advancing boiler technology, fresh water ( synonym for fresh water in seafaring) replaced the initially used sea water in the boiler and finally only distillate was used as boiler feed water. The on-board production of the distillate from seawater was obvious and initiated the next step towards drinking water production. Both passenger ship lines and especially the whaling shipping companies were significantly involved here, because most of the water was used on their ships.
Since ships like a small town need a lot of completely different technical systems, high demands are made on the ship engineers, which results in an extremely broad training. Since they have to understand these systems in order to operate them and to repair them without outside help, it has never been difficult for them to link these systems. This resulted in energetic advantages that led to high overall efficiency levels and were often associated with other advantages. This example applies to a large extent to the subsequent drinking water systems.
Blaum reported to experts in 1937 about seawater evaporators for the production of drinking water . So far, this technology has only been used to generate feed water for the steam systems. The main advantage was space and weight savings, it also increased the comfort of the crew and passengers. The heating took place with steam, on motor ships with exhaust gas boilers without additional expenditure of primary energy.
In 1959 there was a report on evaporators in negative pressure operation, which are heated with the waste heat from the cooling water , which had already been proposed in 1937. With the previous methods for producing drinking water on board ships, the sea water was heated to at least 100 degrees Celsius for evaporation. This guaranteed bacteriologically perfect drinking water. With the production of drinking water at negative pressure , the evaporation temperatures fell to 40–70 degrees Celsius, usually below the threshold at which bacteria are safely killed. A reprocessing system was therefore prescribed.
Fresh water supply on today's ships
Fresh water (fresh water) is divided into drinking water and service water. A separate system with storage tanks, pipes, pressure tanks and pumps is required for each type of water. Drinking water is used to prepare food, drink, wash and wash dishes. The flushing water of the water closets (WC) is service water. Not only drinking water of inferior quality and drinking water purified after use, but also sea water is used as service water. Fresh water is required in the technical area as boiler feed water , for refilling the fresh cooling water and in the on-board laundry.
On most of today's new ships, only a common fresh water system for drinking and service water is installed if the sea water evaporator is sufficiently dimensioned. This saves additional pipelines, storage tanks and equipment. In order to roughly determine the fresh water required for the design of the storage tanks, pressure vessels and pumps, the need for technical purposes and the drinking water requirement are determined by the shipyard. Depending on the type of ship (merchant ship, ferry, cruise ship), around 200 to 500 liters are assumed for drinking water per person and day. A more precise determination, especially for ships with passenger facilities, is carried out using empirical values from the shipping companies and shipyards.
Drinking water and hot water
The cold and hot water supply are combined in this system. On cargo ships, the drinking water is pumped from the fresh water storage tank into the drinking water pressure tank using the drinking water pump. An overpressure is maintained in the pressure tank with the help of an air cushion, which is 4 to 7 bar, depending on the size of the ship and the extent of the drinking water system. The cold water taps are supplied from this cold water pressure tank. If the pressure falls below a pressure set on the pressure switch (pressostat), the drinking water pump is switched on by the pressure switch and switched off again when an upper pressure is reached. With this simple two-point control, a reliable water supply is possible as long as the storage tank contains enough drinking water. With the same technology and groundwater as a storage tank, many domestic waterworks in rural areas still work in residential buildings that are not yet connected to the central water supply.
Large passenger ships, such as cruise and ferry ships with high superstructures, have extensive drinking water systems, which usually no longer contain pressure tanks. Here the drinking water pressure pump is constantly in operation; in the simplest case, the volume flows, which depend on the time of day and operation, are returned to the storage tank via a pressure-controlled bypass line. However, parallel connections of pumps with the same or different volume flows are often chosen. Another technical possibility for adapting the volume flows to the consumption are pole-changing or speed-adjustable electric drive motors for the pumps. Circulation pumps are also used on passenger ships to avoid stagnant drinking water in the pipes.
The hot water system on cargo ships is heated with a steam-heated, rarely thermal oil-heated heat exchanger. The desired temperature (approx. 60 to 80 degrees Celsius) is set on a thermostat, which influences the heating. The pressure is maintained via the cold water system. With this on-board hot water heating, the waste heat from the exhaust gases is used; in addition, electrical heating is installed in order to ensure the hot water supply even in the port when the main engine is not running or in the shipyard. So that hot water flows immediately from the hot water taps, the hot water consumers are connected to a ring line, in which there is constant circulation. A second pump is required as a circulation pump.
The fresh water required today is mostly generated on board. A distinction is made between two basic methods, the evaporation and subsequent condensation of seawater or microfiltration ( reverse osmosis ).
Fresh water production through evaporation
With this method, the sea water is evaporated and the steam is then condensed again. So that the heating can also take place with the waste heat from the engine cooling water, evaporation now takes place at negative pressure. The evaporation temperatures are around 40 - 50 ° C. The engine cooling water from the high temperature system (75–95 ° C) is usually used for heating. Steam is rarely used as a heating medium on cargo ships today, but is the rule on cruise ships. The condensate or distillate obtained in this way is processed depending on the intended use. Disinfection systems and filters are installed for the treatment of drinking water.
Single-stage immersion tube evaporators, each with a tube heat exchanger for heating and condensation, were common on cargo ships. On new buildings, plates are almost exclusively installed as heat exchangers instead of pipes. Depending on the size and type of ship, services of 10 to 40 tons of water are installed per day. This supplies the entire ship. The evaporator is switched off in coastal areas, when sailing through the territories and in ports.
Flash evaporator
The expansion evaporator is heated outside the evaporator in a separate heat exchanger. After heating, the seawater is expanded in an expansion valve to the lower pressure prevailing in the evaporator and evaporated. The separation of heating and evaporation is the main difference to the immersion tube evaporator. For passenger ships, several expansion evaporators are connected in series for better utilization of waste heat, these systems are then very large and have a total output of up to 700 tons of fresh water per day.
Drinking water treatment
The drinking water storage tanks are mainly located in double-bottom tanks on cargo ships, which are lined with a coating and provided with suction, filling, air and sounding pipes. The pipe system consists of steel, galvanized steel or stainless steel. With the on-board fresh water generation, a distillate, i.e. very soft water, is available. Very hard but also very soft water can be used for human consumption without harm (see water hardness ). However, soft water has a bland, dull taste. However, distilled water should not be drunk in large quantities or on a regular basis. Calcium bicarbonate is therefore often added to drinking water via hardness filters in order to achieve a pleasant, refreshing taste.
Activated carbon filters and disinfection devices are also provided for treatment as drinking water . The following three chemical and three physical processes are and have been used on ships for disinfection:
- Chlorination
- Ozonation
- Silver ion treatment
- UV radiation
- Filtration
- Heating
Fresh water production through reverse osmosis
Since drinking water consumption is very high on passenger ships, high-performance reverse osmosis systems are often used. Osmosis is a natural transport phenomenon that takes place as follows: In the case of two salt solutions of different concentrations, which are separated by a semi-permeable membrane (microfilter), the solvent is transported from the low to the highly concentrated side. The liquid level rises on the high concentration side. This transport takes place until a state of equilibrium is reached. The difference in height between the two liquid levels in the equilibrium state corresponds to the difference between the "osmotic pressures" of the two solutions.
If the solution with the higher concentration is subjected to an external pressure that is greater than the osmotic pressure, the process is reversed and the solvent flows through the membrane from the high to the low concentration side. Now we speak of reverse osmosis.
In water treatment systems, a pump exerts pressure on the salty seawater in order to force the water molecules through the semi-permeable membrane. A pure water permeate is produced . Most of the dissolved salts, organic components, bacteria and suspended matter are unable to pass through the membrane due to their size. The remaining water enriched with these undesirable substances is called concentrate and is thrown overboard. The permeate is usually post-treated before it is used as drinking water.
literature
- I. Jung: Use of heat from exhaust gases and cooling water in diesel systems. In: Yearbook of the Shipbuilding Society . 1959.
- K. Illies: Handbook for marine engineers and marine machinists. Friedr. Vieweg & Sohn, Braunschweig 1958.
- W. Janda: Evaluation criteria for the selection of reverse osmosis systems. In: Hansa . No. 23/24, 1987.
- T. Hesse, K.-H. Hochhaus, JD Mehrkens: Drinking water systems on ships. (= Handbook of the shipyards. Volume 26). Hansa Verlag, Hamburg 2002.