Radiator is as an important part in cooling system of diesel engine, its cooling capacity determines the working temperature of the cooling system to a great extent. Cummins diesel engine water radiator is usually closed DC forced circulation copper tube belt type. The core structure of radiator is tube belt type, and the core tube (i.e. cooling water pipe) is pancake, so as to reduce air resistance and increase heat dissipation area. The heat dissipation belt is wavy, on which there are many small windows arranged according to rules, which increases air turbulence and heat dissipation efficiency.
In the circulating water of the water cooling system of high-speed diesel engine, there is often a small amount of air and steam. The main reasons for these air and steam sources include:
1.The limitation of structural design, the existence of "dead zone" in the diesel engine water jacket and some parts, and the accumulation of some air.
2.During the full load process, some air is always absorbed into the system in the coolant. When the diesel engine is working under high load, the cooling water will boil in the high temperature zone and generate steam bubbles. When the steam bubbles increase and form into a piece, they will form air bags.
3.When the water pump of the water pipe joint is not well sealed, the air penetrates into the inside of the cooling system to form air bubbles, when the diesel engine burns, the combustion pressure is very high, the cylinder gasket is slightly poorly sealed, or the cylinder head bolt is slightly loose, so the high-pressure gas can easily flow into the cooling system from the cylinder and form the air bag in the cooling system. If the cooling liquid level is too low, there will be a lot of air in the cooling system space. If these gases can not be discharged, they will cause air resistance when the coolant circulates.
The gas resistance is very harmful to Cummins diesel engine:
1.When the water pump is working, the gas and coolant circulate together to reduce the flow of water, which causing the internal temperature of the diesel engine to rise, and will cause the diesel engine overheat, so that causes local boiling in the water jacket of diesel engine and the local thermal stress increases sharply.
2.The residual air in the coolant will cause uneven local cooling, and the burning exhaust gas dissolved in the coolant is easy to form acid, which will cause rust and corrosion of parts and shorten the service life of parts.
3.The penetrated gas will cause cavitation of cylinder liner and water pump and accelerate the damage.
In order to understand the cavitation of water pump, the low water chamber of radiator, and the removal of air and water vapor in cooling system etc. Cummins diesel engine adopts the high post type forced degassing device expansion water tank. The main functions of the expansion water tank are as follows:
1.In the cooling circuit, the expansion space of the coolant (i.e. as the expansion chamber) is provided to separate the coolant and air, drive off the gas in the water circuit and eliminate the air resistance of the coolant.
2.Contain the coolant overflowing from the radiator and return it to the cooling system to prevent the decrease of coolant in the cooling system. This is more important for the cooling system filled with antifreeze and antirust agent, because if there is no expansion tank in the cooling system of general diesel engine, after the water is heated and expanded, the water vapor will be discharged through the steam valve of radiator. As time goes on, the coolant decreases, while the expansion tank can reduce the loss of coolant. The expansion water tank can also be used to store the cooling water lost due to the after boiling of the diesel engine after flameout. After a long period of hot running or high-speed and heavy load running, the diesel engine will immediately stop running or run at idle speed, which will cause after boil phenomenon.
This is because at this time, the coolant stops or greatly slows down the circulation speed, so that the heat of the coolant can not be emitted, resulting in after boiling. In a word, the expansion tank can avoid the loss of coolant.
Arrangement of expansion water tank in the cooling system: the expansion water tank and radiator are connected in parallel in the water circuit and higher than the radiator. The higher its position is, the more favorable it is to restrain the formation of air bag, but the higher the pressure resistance requirement of the system is. Generally, the bottom of expansion water tank is at least 600mm higher than the main outlet pipe.
The lowest pressure of the cooling system occurs at the inlet of the water pump impeller, so the expansion water tank is connected with the inlet of the water pump through the balance pipe to prevent the pressure drop there.
Since the position of the expansion tank is higher than any point in the diesel engine water circuit, the vacuum degree that may be generated at the water suction of the water pump can be partially or completely balanced by using its water level and pressure. This is conducive to reducing the cavitation on the impeller and the shell of the water pump and prolonging the service life of the water pump. In the place where the water supply chamber of the radiator is prone to air stagnation, the degasser pipe is used to connect with the expansion water tank, which is conducive to leading the air to the expansion water tank and separating the air and water. Generally, one end of the water outlet pipe of the cylinder head is also connected with a degassing pipe, which is the highest point of the water jacket of the cylinder head. After the start of the large cycle of the cooling system, the water mixture flows into the expansion water tank through the degasser, and the gas (steam) water mixture is separated after entering the expansion water tank, and the gas (steam) body is eliminated to maintain the system pressure in a certain range. The coolant without gas (steam) body flows through the return pipe from the bottom of the water tank and returns to the water inlet of the water pump to participate in the circulation again.
During the operation of Cummins generator, the coolant heated by the diesel engine has expansion space and seldom drips out from the overflow pipe, so as to avoid the loss of coolant. The expansion water tank provides a certain amount of coolant reserve, which can ensure that there is no need to add water for a long time, and it is also convenient to add coolant. In this closed cooling system, the coolant flows back and forth between the radiator and the expansion water tank. Although the coolant level in the expansion water tank sometimes rises or falls, the radiator is always full of coolant.
At the same time, in the forced degassing cycle, it is very important to match the diameter of the degasser with that of the same water pipe. If the diameter of the degasser is too large, the flow of the degasser cycle will increase, the return flow will increase, the degassing time of the gas (or steam) water mixture in the expansion water tank will decrease, and some bubbles will be discharged into the expansion space and brought back to the diesel engine water jacket in the future, so the degassing is not clean. If the diameter of degasser pipe is too small, the flow of degasser cycle will be reduced, and some gas or steam will not enter degasser cycle, but will be taken away by large cycle or small cycle, resulting in unclean degasser. Therefore, the degasser pipe and return pipe of diesel engine are selected through many tests, and can not be replaced by other caliber pipes at will.
In the cooling system with expansion water tank, the radiator cap is basically permanently installed and not disassembled. The cooling system can only be disassembled after thorough cleaning during overhaul. The radiator cap should not be removed when only checking the coolant level or adding coolant. The liquid level can be checked on the expansion water tank (generally, it is marked on the expansion water tank to indicate the normal, hot and cold coolant level position), and the coolant can be added from the expansion water tank.
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