Ship cooling system
The term ship cooling system refers to systems on ships which, in addition to cooling the main engine and the auxiliary diesel, also serve to utilize waste heat and to cool other auxiliary systems such as air conditioning , food cooling systems , air compressors, excess steam condensers and fresh water generators .
Type of cooling
Direct cooling, throughflow cooling system
In the early days of marine diesel engines, the cooling water system was designed as a "once-through cooling system" around 1920. The engines were cooled directly with seawater that was pumped by a pump attached to the main engine .
Indirect cooling, circulation cooling system
Due to erosion, corrosion and sedimentation problems, a changeover to indirect cooling with fresh water took place later. This cooling was called "circulation cooling" because the fresh water circulated in a circulation circuit and the fresh water gave off the heat to the sea water in a cooler.
Central cooling system
In addition to the main and auxiliary diesel engines, other cooling points (auxiliary diesel, air conditioning, provision cooling system, air compressor, excess steam condenser and fresh water generator) were also supplied with sea cooling water. With the previous decentralized cooling systems, the cooling points (heat sources) distributed in the machine room are supplied with seawater, depending on where they are installed. Due to the large number of heat sources, many and sometimes very long seawater supply and return lines are required. The heat sources were flowed through in parallel or in series, depending on their size and temperature level.
When the main engines could be operated with inexpensive heavy oil from the 1950s onwards, steam was needed to preheat the heavy oil. This was followed by a step-by-step optimization of the cooling systems in order to use as little primary energy as possible for heating and preheating purposes (air conditioning, drinking water generation, preheating). The cooling water system was combined into a central unit with short lines carrying sea water. All systems and machines were now mainly cooled with fresh water.
Central cooling system, fresh water cooling with low and high temperature systems
The further energetic optimization led in the seventies to the division of the fresh cooling water systems into low and high temperature systems (LT, HT cooling water system). This enabled a more effective use of the cooling water heat in different temperature levels.
Shortly afterwards, some engine manufacturers also switched their charge air cooling from seawater to freshwater cooling. By dividing the charge air cooler into the NT and HT system, there was another rich source of heat for utilizing waste heat. In addition, this resulted in an improvement in the heavy oil operation of the engines; the charge air could be preheated at low outside temperatures and low partial loads.
Requirements for the cooling system
The requirements result from the heat sources (e.g. main diesel engine, auxiliary diesel), the environment and the engine room (structured vertically or horizontally).
Also important on today's ships is fully automated, economical operation, high reliability and ease of use. 40 - 50% of the energy supplied by marine diesel engines with the fuel is converted into mechanical energy; approx. 50 - 60% remains. About half of these losses are carried away by the cooling media and with the exhaust gas. This energy balance is graphically referred to as a Sankey diagram. The data of the heat sources are established relatively early, with the determination of the drive power and the main motor type, specific information is available from the main motor manufacturer. With the E-balance, selection of the auxiliary diesel generator sets and auxiliary machines, the other sources of waste heat are specified and can be roughly determined.
Sea water cooling systems
With the sea water cooling system, apart from the exhaust gas heat and the radiant heat, all heat losses occurring on board are to be dissipated. The type and supply of the seawater cooling system (pumps, scoop coolers , sea chest or outer skin cooling) is different. The design of the sea cooling water system has an impact on the space required in the engine room, the electrical energy requirement and influences the design of the outer skin in the engine room area (scoop cooler).
Sea cooling water pumps
The sea cooling water systems that are predominantly used today work with pumps that suck in the sea water, pump it through the heat exchanger and then transport it outboard. The drive of the sea cooling water pumps is usually done electrically, the necessary electrical power (5 - 15% electrical demand in sea operation) has to be taken into account in the electrical balance and has an impact on the auxiliary diesel and generator design. The sea cooling water pumps are designed for the maximum sea water temperatures that occur (according to the sailing area). A sea cooling water pump of the same size is connected in parallel as a reserve and a smaller (approx. 1/3 volume flow) harbor sea cooling water pump is often provided for port operations.
The sea cooling water pumps suck in sea water from sea chests and through filters. There are various sea chests (deep suction and high suction) available. The vacuum cleaners are normally in operation, the high vacuum cleaner is intended for driving in shallow water and rivers in order to avoid heavy pollution. In addition to sufficient ventilation pipes or slots, there are steam or compressed air lines for blowing out in the event of contamination or icing. For ships with an ice class, the sea chest volume is stipulated by the classifications depending on the installed main and auxiliary diesel output, the seawater outlet line is then returned to the sea chest as a bypass and steam connections are prescribed. In special cases, circulation cooling from the ballast tanks is prescribed. Filters with bypasses are arranged between the sea chest and the pump, which can be cleaned during operation.
The necessity of these facilities became clear in the ice winter of 1995/96, when several ships on the Elbe had to emergency anchor due to a failed main engine or caused collisions. The main or auxiliary diesel failure was caused by excessively high fresh cooling water temperatures due to ice gravel in the sea chest.
Alternatively, processes such as scoop, sea chest or outer skin cooling can be used that work without pumps.
Scoop cooling
With the scoop cooling system, the sea cooling water runs through inlet and outlet pipes with a very large cross-section. A low-resistance cooler is located between the outlet and inlet pipes, through which the water flows when the ship is moving. For a good flow, the inlet and outlet nozzles are shaped in such a way that overpressure is created at the inlet nozzle and negative pressure at the outlet nozzle. The advantage of the scoop cooler lies in the lower electrical energy requirement in sea operation, which is bought with the disadvantage of the increased resistance of the ship and the large space and weight requirements of the scoop cooler.
Sea chest cooler
Sea chest coolers are almost unknown on large merchant ships, although here too they offer considerable advantages for decentralized systems that are far away. The application has so far been limited to inland and coastal motor vessels that cover the entire heat exchange of the cooling water system via the sea chest cooler. Similar to the scoop cooler, no sea cooling water pump is required. The heat-exchanging coolers, previously mainly tube coolers, hang directly in the seawater and, similar to the scoop cooler, are flowed through as the ship moves. To reduce drag, the coolers are installed within the ship's lines in boxes provided for this purpose, which are covered with perforated or slotted sheets according to the ship's contour. It is important that the top wall is inclined to support the flow from below to above. The flow of the sea water through the cooler is from bottom to top and is caused by the difference in density between the cold and the sea water that heats up during heat transfer.
Outer skin cooling and combinations
A method often used on fishing ships consists of pipelines laid on the outer skin. These are protected against mechanical damage by a grid. With the pocket cooling system, pipes are laid in pockets that are inserted into the outer skin. The fresh water flows through these pipes, which is cooled by the seawater flowing outside. This corresponds to the principle of the box cooler, however, no compact tube bundle is used, but flat-laid pipelines.
For particularly demanding purposes, outer skin coolers are used, which are located in a special double floor or in a double outer skin. In the intermediate space, meander-shaped structures are welded on the outer skin and / or the inner skin, which serve to guide the flow but also to enlarge the surface area according to the finned tube cooler principle. This type of outer skin cooler is mainly used for small and medium-sized watercraft that also operate in extremely shallow water, in ice areas and in heavily polluted or algae-rich sailing areas. In addition to eliminating the seawater pump, this system does not have the disadvantage of costly repairs by divers or docking in the event of a serious malfunction.
All of these systems and processes require little or no electrical energy for the sea cooling water system. The energy is provided by the main engine, which due to the increased resistance requires slightly more power at the same speed. However, this additional power is very small, the disadvantage is the poor accessibility of these cooling systems in the event of a fault. Repairs require divers or costly docking.
Low-temperature cooling water system (NT)
This system consists of the pipelines, fittings, control valve, LT fresh water pumps, the LT central cooler and cooled auxiliary machines. Due to the large number of these auxiliary machines, this system has taken over the main task of the previous decentralized sea cooling water system.
The LT system is regulated to a constant temperature at the outlet of the central cooler. This regulation takes place by activating a bypass valve or three-way valve, which connects the hot pressure side with the cold suction side. Depending on the temperature measured by the temperature sensor behind the three-way valve, the bypass is opened to a greater or lesser extent and the temperature is thereby set. On the pressure side, the fresh cooling water flows through the heat exchangers and auxiliary machines, which are connected in parallel or in series depending on the temperature difference and the amount of heat to be dissipated.
The high tank or equalizing tank for the LT fresh water circuit, which fulfills various tasks, is arranged on the suction side.
- expansion
- Loss of evaporation
- Venting
If fresh cooling water is lost, it acts as a buffer and compensates for the loss. A float with a contact switch gives an alarm if the water level is too low. An overflow pipe leads to the fresh cooling water storage tank (usually in the double floor), from which the high tank can be filled using the filling pump. A water level device or a sight glass is installed to monitor the cooling water. At the height of the water level a drain device is arranged so that, for. B. floating oil can be drained into the sludge tank. A heating coil is used to heat up when the main engine is at a standstill and the temperature has dropped too far.
High temperature cooling water system (HT)
The HT system is equipped like the NT system, it supplies the cold spots at a higher temperature level. These are usually the cylinder cooling water system and the second stage of the charge air cooler, if this is designed in two or more stages. In the HT system, the inlet temperature, the outlet temperature or both are regulated depending on the philosophy of the engine manufacturer. Depending on the engine manufacturer, the temperature is between 70 and 90 ° C and in the past there was a trend towards increasing temperatures. This trend is expected to continue in the future (hot cooling), high temperatures reduce the material loads in the engine and enable better waste heat utilization in ship operation. The waste heat from this system is used to generate fresh water, often also for bunker and settling tank heating, domestic water heating and heating of living spaces.
literature
- Henschke (ed.): Shipbuilding manual . Volume 4. VEB Verlag Technik, Berlin 1958
- K.-H. Hochhaus, JD Mehrkens: Economical design of cooling systems . In: Schiff und Hafen , 1987, issue 8
- G. Großmann: Development of calculation methods for optimizing box coolers for seagoing vessels . FDS report 169, Hamburg 1986
- W. Milde: Design of central coolers without seawater pumps . In: Yearbook of the Shipbuilding Society , 1989
- Centrifugal pumps Lexicon, Klein, Schanzlin & Becker . Frankfurt 1974
- Basics for the planning of centrifugal pump systems , publisher SIHI Group, 1978 Ludwigshafen
- H. Mann: Marine pipelines . VEB Verlag Technik, Berlin 1973
- G. Großmann: Pumps and Pipelines . In: Handbuch der Werften , Volume XVI, 1982, Hansa Verlag