In order to ensure the normal operation of the diesel engine, it is necessary to cool the heated components of the diesel engine, such as the cylinder head, cylinder liner, piston, etc., to reduce thermal stress, cool the lubricating oil of the bearing shell to ensure normal lubrication, and cool the scavenging air to reduce exhaust temperature. Therefore, the cooling system of the diesel engine is formed. Most large and medium-sized marine diesel engines use water cooling. According to the different purposes of the diesel engine (main engine or main generator set), the configuration of the internal and external cooling water systems of the diesel engine also varies, but the principle of the cooling water system is roughly the same.
There are two main types of cooling water systems for marine diesel engines: traditional seawater cooling systems and currently popular central cooling water (high and low temperature freshwater) systems.
The fresh water system is a closed cycle, and fresh water is used to cool the cylinder liner, cylinder head, and turbocharger of the engine. The heat absorbed by the freshwater in the circulation of the freshwater system is transferred to the seawater system through the freshwater cooler, and the freshwater circulation is controlled automatically or semi-automatically by the freshwater constant temperature valve. The seawater system is an open cycle system, where seawater is used to cool the pressurized air, lubricating oil circulating inside the machine, and fresh water. The seawater in the seawater system absorbs heat during operation and its temperature rises. According to the set temperature of the seawater constant temperature valve, it is discharged from the ship or returned to the seawater system to continue working. The seawater thermostatic valve adjusts the seawater volume during seawater circulation in an automatic or semi-automatic manner.
The high-temperature fresh water system in the central cooling water (high and low temperature fresh water) system is a closed cycle, and high-temperature fresh water is used to cool the cylinder liner, cylinder head, and turbocharger of the engine. The heat absorbed by the high-temperature fresh water in the circulation of the high-temperature fresh water system is transferred to the low-temperature fresh water system through the high-temperature fresh water cooler, and the high-temperature fresh water circulation is controlled by the high-temperature fresh water constant temperature valve in an automatic or semi-automatic manner. The low-temperature fresh water system is also a closed-loop system, where low-temperature fresh water is used to cool the pressurized air, lubricating oil circulating inside the machine, and high-temperature fresh water. The low-temperature fresh water system absorbs heat during operation and its temperature rises,
The heat is transferred to the seawater system on board through the central cooler, and the water volume during low-temperature freshwater circulation is adjusted automatically or semi-automatically by the low-temperature freshwater constant temperature valve.
Internal cooling water system
The cooling system of a diesel engine consists of a low-temperature (LT) circuit and a high-temperature (HT) circuit.
Low temperature cooling water system
The low-temperature cooling water system includes cooling of pressurized air, cooling of lubricating oil, and cooling of the generator (if the generator is water-cooled). The low-temperature cooling water system is designed to use fresh water as the cooling medium. Sea water can be used for selection.
In order to prevent the temperature of the pressurized air from being too high, the design value of the fresh water temperature for the low-temperature cooling water system entering the machine is up to 36 ℃. This temperature is suitable compared to the maximum seawater temperature of 32 ℃. As for the oil cooler, the inlet temperature of the low-temperature cooling water shall not be lower than 10 ℃.
System Layout
For example, the internal cooling water system of MAN B&W-ZJMD standard is represented in the basic system (as shown in the figure below). The system is arranged to connect to external systems in a holistic form.
The external cooling water system can be adjusted for high temperature (HT) and low temperature (LT), and is also equipped with high temperature (HT) fresh water pumps, low temperature (LT) fresh water pumps, and fresh water coolers. This indicates that these components can be shared for the propulsion of diesel engines and generator sets. The separation of high-temperature (HT) and low-temperature (LT) circuits means that the cooling medium for the low-temperature system can be either seawater or freshwater. Therefore, Basic System 2 can be matched with both traditional seawater and freshwater systems, as well as central cooling water systems. In order to match any external cooling water system, the internal system can be arranged in two cycles or one cycle, with or without a pump and thermostatic valve for the high-temperature system. Therefore, the cooling system of diesel engines can be fully or partially connected to external systems, or arranged as an independent system.
High temperature (HT) fresh water circulation pump
The high-temperature (HT) fresh water circulation pump is centrifugal, installed on the front cover of the diesel engine, and driven by the crankshaft through elastic gears.
thermostat valve
The thermostatic valve is a fully automatic three-way valve equipped with a constant temperature element set at a fixed temperature inside.
Pre-heating arrangement
As an option, diesel engines can be equipped with a pre heating device in the high-temperature circuit, including a temperature controller that controls the electric heating element and a safety valve.
The system is based on a thermosiphon cycle.
This system is designed as a single loop system, with only two flange interfaces connected to the external central low-temperature (LT) cooling water system. The diesel engine is equipped with a high-temperature water cooling water circuit that controls its own temperature. Used for cooling cylinder liners and cylinder heads. Therefore, only one fresh water cooler is needed on the cooling water side of the diesel engine, and the diesel engine can be integrated with the ship's cooling water system as an independent unit.
This greatly reduces installation costs. This is also beneficial for certain power generation devices to be updated, as not only does the diesel engine power increase, but the distance between diesel engines also increases.
Low temperature circulation loop
The circulating components or temperature regulating valves are installed in the internal system. The air cooler and oil cooler are set in parallel. To ensure that these two coolers have the lowest cooling water inlet temperature.
The cooling of the high-temperature circuit is achieved by adjusting the amount of water flowing out of the low-temperature circuit from the oil cooler. Therefore, the amount of cooling water flowing through the cooling system is always adjusted according to the load of the diesel engine.
High temperature cycle circuit
The centrifugal high-temperature (HT) fresh water circulation pump high-temperature water through the cooling water main pipe into the cooling water chamber between the cylinder liner and the frame of each cylinder, and then flows from the hole at the top of the frame through the cooling water jacket to the cylinder head, cooling the cylinder head and valve seat.
The high-temperature water flowing out of the cylinder head passes through the outlet manifold to the thermostatic valve. Depending on the load of the diesel engine, a small amount or a large amount of water is directed to the external system or recycled.
Selection
Diesel engines can be equipped with the following components as needed:
-Thermostatic valve, installed at the outlet of the low-temperature system;
-Equipped with a low-temperature fresh water pump;
-High temperature system preheating device;
Other Choice
-External preheating pipeline interface;
-Generator cooling water pipeline;
If the generator is water-cooled, the cooling pipeline can be integrated with the generator set.
The internal cooling water system is designed with separate low-temperature and high-temperature circuits, which can be integrated into the external cooling water system as a whole. The external system can be a traditional seawater cooling system or a central cooling water system. Using this system, the propulsion engine and diesel generator can use a shared pump and heat exchanger. However, we recommend that the high-temperature circuit of the diesel engine for power generation should have a separate temperature regulator.
Low temperature circulation loop
As a standard, the low-temperature circuit of the system uses fresh water, the pipeline uses steel pipes, and the oil cooler uses stainless steel plates. However, as an option, seawater can be used for the low-temperature circuit, and the materials used in the system need to be adjusted and treated accordingly.
High temperature cycle circuit
High temperature water enters the cooling water chamber between each cylinder liner and the frame through the cooling water main pipe of the external high-temperature system, and then flows out from the top of the frame through the cooling water jacket to the cylinder head to cool the cylinder head and valve seat.
The high-temperature water flowing out of the cylinder head is discharged to the external cooling water system through the outlet manifold.
Selection
Diesel engines can be equipped with the following components as needed:
-The low-temperature system is cooled by seawater;
Including the use of titanium plates for oil coolers, aluminum brass pipes or galvanized pipes for low-temperature water pipes, and bronze covers for air coolers.
-Thermostatic valve, installed at the outlet of the low-temperature system;
-Thermostatic valve, installed at the outlet of the high-temperature system;
-Low temperature water system machine with pump;
-High temperature water system machine with pump;
-High temperature system preheating device;
-External preheating pipeline;
-Generator cooling water pipeline;
If the generator is water-cooled, the cooling pipeline can be integrated with the diesel generator set.
COOLING WATER SYSTEM
The diesel engine is designed to be cooled only by fresh water. Therefore, the cooling water system must be arranged as a central or enclosed cooling water system.
All recommended cooling forms will be described below.
Diesel engines are almost designed to be ductless, meaning fresh water flows through the internal chambers of the front-end box and cylinder unit. The front-end box includes all major pipeline interfaces. The pipeline for water to enter the gearbox oil cooler in the rear box must be installed by the shipyard. The diesel engine is equipped with a built-in fresh water pump for both high and low temperature cooling water systems. To facilitate the start-up of the backup pump, a check valve is installed. The constant temperature valve element that controls the high-temperature and low-temperature water systems is also a comprehensive component of the front-end box. The diesel engine is equipped with two-stage air coolers. The first level is set in the high-temperature cooling water system. The temperature of the pressurized air after the turbocharger
At the highest value, when heat is lost to the high-temperature water system, a high degree of heat recovery may occur.
The second stage air cooler is installed in the low-temperature water cooling system.
Further cool the pressurized air before it enters the combustion chamber.
For special applications, such as sailing in Arctic waters with low air temperatures and direct intake from the deck, a regulating system can be used to control the flow of water into the second stage air cooler to increase the pressurized air temperature at low loads.
water quality
Fresh water used as a cooling medium should be kept as clean as possible. The pH value should be between 6.5 and 8 at 20 ℃. The total hardness of water must be Max.10 ° dH (German hardness). If the hardness increases, it must be diluted with some softened water.
The content of chloride ions, chlorides, silicates, and sulfates must be as low as possible and must not exceed the following values:
Chloride ion: 10PPM Chloride: 50PPM
Silicates: 150PPM Sulfate: 100PPM
In order to reduce the risk of corrosion inside diesel engines, fresh water must be treated with additives. The fresh water-cooling system must be treated before conducting navigation tests.
There are two basic types of chemical additives:
Chromate base
Nitrite based or similar
Chromate based additives are often considered the most effective, but we should advise those who use this additive that its toxicity is extremely strong, and if a freshwater generator is installed in the device, the use of this additive is not allowed.
For information on the additives we recommend, please refer to "Cooling Water Inhibitors", which can provide the required information.
The new diesel engine has been cleaned and treated with nitric acid. If the freshwater inhibitor is still properly retained, the piping system will hardly need to be cleaned in the future. But if cleaning is needed, we are willing to help recommend a fat removal method,
Acid pickling rust removal method and inhibitor.
Silicates: 150PPM Sulfate: 100PPM
In order to reduce the risk of corrosion inside diesel engines, fresh water must be treated with additives. The fresh water cooling system must be treated before conducting navigation tests.
There are two basic types of chemical additives:
Chromate base
Nitrite based or similar
Chromate based additives are often considered the most effective, but we should advise those who use this additive that its toxicity is extremely strong, and if a freshwater generator is installed in the device, the use of this additive is not allowed.
For information on the additives we recommend, please refer to "Cooling Water Inhibitors", which can provide the required information.
The new diesel engine has been cleaned and treated with nitric acid. If the freshwater inhibitor is still properly retained, the piping system will hardly need to be cleaned in the future. But if cleaning is needed, we are willing to help recommend a fat removal method,
Acid pickling rust removal method and inhibitor.
Treatment of freshwater systems
Corrosion prevention of freshwater cooling system
The fresh water system of diesel engines must be carefully treated, maintained, and managed to avoid rusting or scaling (scaling can reduce heat transfer efficiency). MAN B&W-ZJMD recommends following the following procedure for processing.
--Clean the cooling water system
--Add deionized water or distilled water (such as water produced by a freshwater generator) and add preservatives.
--Conduct a systematic inspection of the cooling water system and its condition.
If the above program is guaranteed and the system has correctly released air, the operational failures caused by cooling water can be minimized.
Cleaning of cooling water system
Before anti-corrosion treatment, it is necessary to remove the scale, rust, and sludge present in the system to improve heat transfer efficiency and ensure uniform protection of the metal surface after the use of anti-corrosion agents.
Cleaning should include degreasing, removing oil sludge, and acid washing to remove rust and scale.
You can purchase pre formulated cleaning agents specifically designed for cleaning cooling water systems from professional cleaning agent manufacturers. These companies provide assistance and guidance for cleaning all major components. The attached table lists some professional manufacturers. It should be pointed out that the guidance provided by these manufacturers must be strictly followed. It is particularly important to rinse the entire system after cleaning. Defatting treatment can use water emulsions of cleaning agents and slightly alkaline cleaning agents; It is obvious that pre prepared cleaning agents with fire hazards cannot be used. When pickling and descaling, it is particularly recommended to use products such as amino sulfuric acid, citric acid, and tartaric acid, as they are usually solid and easily soluble in water without producing harmful gases. Cleaning agents should not be directly added to the cooling system, but should be dissolved in water before being added.
Usually, cleaning can be carried out without disassembling any components of the diesel engine. It should be pointed out that circulating water inside the machine can achieve better cleaning results.
Due to cleaning, leakage may occur at the joint surface of the parts due to poor assembly or local damage to the gasket. Therefore, inspection should be carried out during the cleaning process. Immediately check the acid content in the system oil after cleaning and 24 hours after cleaning.
Cooling water, preservatives
After cleaning, add cooling water and anti-corrosion additives directly to prevent rust on the cleaned surface.
Raw Water
The scale on the surface of the cylinder liner and the inner cavity of the cylinder head will weaken the heat transfer, causing the metal to produce high temperatures that it cannot withstand. Therefore, it is recommended to use deionized water or distilled water (such as water produced by a freshwater generator) as cooling water. However, due to its lack of hardness and considerable corrosiveness, preservatives are always added.
If there is no deionized water or distilled water, normal drinking water can be used in special circumstances, but the total hardness of the water cannot exceed 9 ° dH (German hardness value). The content of chlorides, chlorides, sulfates, and silicates should also be checked and should not exceed the following values:
Chloride 50ppm (50mg/L)
Chlorine 10ppm (10mg/L)
Sulfate 100ppm (100mg/L)
Silicates 150ppm (150mg/L)
Water should not contain sulfides and ammonia. Rainwater cannot be used as it may be severely contaminated.
It should be noted that softening water does not reduce the content of sulfates and chlorides in it.
preservative
To prevent corrosion of the cooling water system of marine diesel engines, various preservatives can be provided.
Usually, only nitrite and borate based preservatives are recommended for use.
Table 6-1 lists some of the products sold by major companies in the market, along with the necessary dosages and blending procedures. It is recommended to strictly comply with these regulations. It is not recommended to use rust proof oil to treat cooling water. Because this treatment poses a risk of oil adhering to the heat transfer surface.
