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How does the cooling system of a diesel generator work?
This chapter examines the key components of diesel engine cooling systems and why each is critical to the engine's reliable operation.
Engine Cooling through Mechanical Means
The engine cooling system absorbs between 25 and 30% of the total heat input from the fuel to the engine.
If this heat is not dispersed, the internal temperature of the engine will quickly rise to a level that may cause component failure and engine failure. A cooling system is used in all commercial diesel engines to trap this heat and transfer it to a medium that absorbs heat outside the engine.
Several modern engines are outfitted with turbocharging systems to provide enough air for fuel combustion to provide the required level of power. The turbocharging system raises the temperature of the combustion air. Cooling the combustion air before it enters the engine cylinders ensures that enough pounds of air are available for fuel combustion (to maintain the air density). An air intercooler or aftercooler, a radiator-like heat exchanger installed in the pipes between the turbocharger compressor outlet and the engine air manifold, is used to do this. This radiator is in charge of extracting surplus heat from the combustion air. This heat exchanger can use water from either the jacket or service water systems (the ultimate heat sink).
When service water is used, an extra heat exchanger may be installed between the service water system and the intercooler water system to clean and maintain the water in the intercooler water system so that it does not deteriorate the air intercooler.
The Basics of a Cooling System
The majority of diesel engines use a jacket-style closed-loop cooling system. Coolant, which circulates throughout the engine, absorbs heat from the cylinder liners, cylinder heads, and other engine components.
The higher the temperature of the coolant exiting the engine, the more efficiently it will operate. Very high coolant temperatures, on the other hand, can cause structural damage by causing engine components to overheat. Jacket water can also be utilized to cool the lubricating oil using a heat exchanger. A jacket water discharge temperature of approximately 180oF with a temperature rise through the engine of 8 to 15oF is appropriate for the majority of diesel engines.
Water is the most commonly utilized engine coolant in diesel engines. Water, on the other hand, can produce corrosion, mineral deposits, and freezing.
Engines that may be exposed to temperatures near or below freezing must be treated with an antifreeze, such as ethylene glycol or propylene glycol. The most common treatment is a mixture of antifreeze and water, which works at temperatures as low as -40°F. Commercial antifreeze comprises corrosion-inhibiting compounds. Heat transfer is hampered by the presence of antifreeze.
Diesel engines used for emergency service at nuclear reactors are not typically subjected to freezing temperatures. Antifreeze is unnecessary in these instances. To prevent corrosion, corrosion inhibitor chemicals can be mixed with demineralized water.
Water Chemistry - Engine coolant water should be free of deposits and scale-forming substances. De-mineralized water is commonly used. The water should be somewhat alkaline, with a pH of 8 to 9.5.
A corrosion inhibitor, such as Nalco 2000, is recommended to prevent scale from accumulating on cylinder liners and cylinder heads. A sixteenth of an inch of scale is equivalent to adding one inch of steel to the engine's heat transmission resistance. A chemical analysis of the coolant is performed on a regular basis, and the proper amount of corrosion inhibitor is supplied to keep the water's chemistry stable.
Engine Cooling Methods
Separate radiator sections may be used to cool the intercooler water and the jacket water in some configurations. The lubricating oil is generally cooled by the jacket water circuit in such cases.
Coolant is stored in the engine system with the help of an expansion tank (head or make-up tank) mounted above the engine to keep a head on the system. The engine-powered pump draws suction from the system and provides coolant to the engine. In most systems, water exits the engine through a thermostatically regulated valve. If the water is excessively cold, a bypass line allows it to bypass the heat exchanger. The heat exchanger is used if the water is too hot.
The thermostatic control valve (TCV) senses and responds to coolant temperature.
As the temperature of the engine coolant goes below the setpoint of the valve, coolant is routed over the jacket water heat exchanger. When the coolant temperature exceeds the set point, the valve directs the coolant via the heat exchanger, where the excess heat is transferred to the raw or service water system. As a diesel engine starts, the flow of service water begins automatically.
Water returns to the jacket water pump and, eventually, the engine via the heat exchanger's exit, or bypass line. A heat exchanger in the jacket water system cools the lubrication oil system in various systems. When it is intended to keep the lube oil cooler than the jacket water in engines, the oil heat is supplied directly to the service/raw water system via the lube oil system heat exchanger.
After reaching the cylinder block, coolant is routed by internal channels and/or piping to the bottom end of the cylinder liners. When it ascends, the fluid flows around the cylinder liners and into the cylinder heads. The coolant that exits the cylinder heads is routed through an outlet header to the thermostatic valve.
A portion of the jacket water in certain engines equipped with intercoolers or aftercoolers passes through the intercoolers to absorb excess heat from the incoming air charge. A separate heat exchanger delivers this excess heat to the service/raw water system on many engines with intercoolers or aftercoolers. This is excellent since it is preferable to keep the intercooler water colder than the jacket water system. ALCO engines typically use the jacket water system to cool intercooler water.
Expansion Tank - Many engines employ an expansion tank with a pressured closure, or the expansion tank is situated high enough on the system to maintain the required head (net positive pressure head - NPSH). The expansion tank is often located somewhat higher than the highest point of the jacket cooling water system, and vent pipes are used to continuously purge the system of air. Some expansion tanks can be pressed to maintain a higher pressure, which helps to raise the boiling point of the cooling fluid.
A standpipe is a vertical tank that is installed at the same level as the engine. It holds engine coolant and offers an air space to compensate for thermal expansion of the coolant.
Standpipes are often vented to the atmosphere, resulting in a non-pressurized cooling system. The standpipe water level must be adequate to achieve the required NPSH, or the tank must be pressurized.
The engine-driven jacket water pump is a single-stage centrifugal pump that is powered by the engine's crankshaft via a series of gears.
Water enters the pump's suction input, as seen. The engine gear train rotates the pump shaft and impeller, which is driven by the pump drive gear. The rotation of the impeller causes the coolant's velocity to increase due to centrifugal force. The velocity of the coolant decreases as it enters the pump casing, but the pressure increases proportionately. With a higher pressure, coolant leaks from the pump casing into the jacket water header and flows to the lower end of the cylinder liners.
The thermostatic control valve receives engine coolant from the bottom. When the coolant temperature is low, as shown on the right side of the diagram, the sliding valve poppet remains in the upward position, allowing coolant to flow around the heat exchanger.
The wax pellets within the temperature control elements expand as the coolant temperature rises, pushing the element tube and sliding the valve poppet downward. As a result, bypass flow is controlled or throttled, as seen on the left side of the diagram, and coolant is channeled to the heat exchanger.
In operation, the valve modulates throughout a temperature range of around 10 to 150 degrees Fahrenheit to keep the coolant temperature roughly constant.
Jacket Water Heat Exchanger - Jacket water heat exchangers are typically shell and tube types. Engine coolant often runs over the tubes on the shell side, while service water goes through the tubes.
Jacket Water Warming Systems
When a motor is turned off for an extended period of time, the internal temperature of the engine drops dramatically. The rapid start and fast loading of a cold engine, characteristic of nuclear application diesels in emergency conditions, creates high stress and increased wear until the engine reaches its usual operating temperature.
The jacket water keepwarm system is shown on the same schematic as the regular jacket water cooling system. This system keeps the temperature of the engine coolant at or near the standard operating temperature. This does not imply that all components have attained their usual operating temperatures.
Because diesel engines rely on the heat of compression for ignition, keeping the engine warm reduces start time and reduces the probability of a failure to start due to low intake air temperature.
Keepwarm Pump - A keepwarm pump is an electrically powered, single-stage centrifugal pump similar to the engine-driven pump that keeps heated coolant flowing through the engine even after the engine is turned off.
Keepwarm Heater - The jacket waterkeepwarm heater, like the lubricating oil keepwarm heater, is an immersion-style electric warmer.
It is put in a standpipe or separate heating tank. It is thermostatically controlled to keep the engine at the desired temperature.
System operation - The keepwarm system is initiated while the engine is in standby mode. The keepwarm pump creates a vacuum in the system and releases water into the jacket water inlet of the engine. Check valves can be installed in the keepwarm system when the engine is running to prevent reverse flow. The heated coolant circulates through the engine, warming the cylinders, cylinder heads, and other water-cooled engine components.
System of Water Intercooler
The intercooler water system supplies water to the intercooler or aftercooler mounted on the engine's combustion air intake pipes. It is a heat exchanger that, like a radiator, cools the combustion air after the turbocharger compressor and before the engine's air manifold/plenum.
Cooling increases the density of the air, allowing more oxygen to burn more fuel for a higher power output. Also, the combustion air keeps the piston crowns cold.
Intercooling water must normally be near to the temperature of the surrounding air. As a result, it is usually preferred to use service water rather than jacket water, which has a much higher temperature, for this function (160 to 180oF).
Diagram of a typical intercooler and aftercooler water system
Because these components are so similar to those used in the jacket water system, they will not be discussed further.
A thermostat may be used in some intercooler water systems to prevent the intercooler water from becoming extremely cold, especially in cold weather or while the engine is not under load, in order to avoid condensation of moisture in the combustion air. In some systems, the jacket water system and the intercooler water system are linked to help heat the intercooler as needed.
Engine start time, light load performance, and cylinder liner lubrication may be impacted if the intake combustion air is excessively cold. To counteract this effect, numerous manufacturers thermostatically limit the flow of cooling water to the intercooler and/or supply warm jacket water as needed.
The thermostatic valve in the circuit keeps the intercooler water (and thus the air entering the engine) from getting too cold. When the air is too cold, condensation forms in the engine and "white" smoke forms in the exhaust.
Extra Cooling Conditions
The diesel producing unit is often housed in a structure with limited openings.
There are various heat sources in the EDG room, including the engine and generator. For optimal operation, parts of the equipment in this area, such as the switchgear, control panels, monitoring equipment, fuel day tank, air compressor(s), and air storage tank(s), must be kept cold.
The EDG room's standard temperature limit is 122 °F (50 °C). As a result, sufficient cold air (ambient air) must be introduced to remove the heat and keep the room temperature below the maximum permitted range. While room temperature has no effect on the engine, exceptionally high EDG room temperatures can have an effect on the generator and other components. If the engine's combustion air is pulled from the room, extremely hot inlet air can impair the engine's ability to produce power.
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