How Does A Diesel Generator's Cooling System Work?
How does a diesel generator's cooling system work?
This chapter talks about the most important parts of diesel engine cooling systems and why each one is important for the engine to work well.
Cooling the engine mechanically
25–30% of all the heat that comes from the fuel and goes into the engine is taken up by the cooling system.
If this heat doesn't get rid of itself, the engine's internal temperature will rise quickly to a point where parts break and the engine stops working. All commercial diesel engines have a cooling system to collect this heat and move it to a medium that soaks up heat outside the engine.
Many modern engines have turbocharging systems that make sure there is enough air for the fuel to burn and make the power that is needed. The mechanism for turbocharging makes the combustion air hotter. Before the combustion air goes into the engine cylinders, it needs to be cooled to make sure there are enough pounds of air to burn the fuel (to maintain the air density). A heat exchanger that looks like a radiator is put in the pipe between the turbocharger compressor outlet and the engine air manifold. This is called an air intercooler or aftercooler. The job of this radiator is to remove heat from the combustion air. This heat exchanger can use either the jacket water system or the service water system to get its water (the ultimate heat sink).
When service water is used, there may be an extra heat exchanger between the service water system and the intercooler water system to clean and maintain the water in the intercooler water system so that it doesn't damage the air intercooler.
Cooling System Fundamentals
Most diesel engines have a cooling system that looks like a jacket and has a closed loop. As the coolant flows through the engine, it picks up heat from the cylinder liners, cylinder heads, and other parts.
The cooler the coolant is when it leaves the engine, the better the engine will work. On the other hand, coolant temperatures that are too high can cause structural damage by letting engine parts overheat. Lubricating oil can also be cooled using jacket water and a heat exchanger. Most diesel engines work best with a jacket water discharge temperature of about 180oF and a rise in temperature through the engine of between 8 and 15oF.
Most diesel engines cool down with water as their coolant. Still, water by itself can cause rust, mineral buildup, and freezing.
Antifreeze, like ethylene glycol or propylene glycol, needs to be added to engines that might be near or below freezing. The most common solution is to mix antifreeze and water, which works at temperatures as low as -40°F. Commercial antifreeze has chemicals in it that stop rust from happening. Adding antifreeze makes it harder for heat to move.
Most of the time, diesel engines used at nuclear reactors for emergency service are not exposed to freezing temperatures. Under these conditions, there is no need for antifreeze. Still, corrosion can be stopped by mixing chemicals that stop corrosion with water that has been stripped of its minerals.
Chemistry of Water: Water used to cool an engine should not have any chemicals that cause deposits or scale. Most of the time, de-mineralized water is used. The pH of the water should be somewhere between 8 and 9.5.
It is best to add a corrosion inhibitor like Nalco 2000 to keep scale from building up on the cylinder liners and cylinder heads. One sixteenth of an inch of scale is the same as adding one inch of steel to the engine to make it less likely to let heat through. A chemical analysis of the coolant is done every so often, and the right amount of corrosion inhibitor is added to keep the water's chemistry right.
How to Keep an Engine Cool
On some setups, the water in the intercooler and the water in the jacket are cooled by different parts of the radiator. Most of the time, the jacket water circuit is used to cool the lubricating oil in these situations.
With the help of an expansion tank (also called a "head" or "make-up tank"), which is installed above the engine to keep a head on the system, coolant is stored in the engine system itself. The engine drives the pump, which pulls air from the system and sends coolant to the engine. In most systems, the water leaves the engine through a valve that is controlled by a thermostat. If the water is too cold, a line lets it go around the heat exchanger. The water goes through the heat exchanger if it is too hot.
The thermostatic control valve (TCV) finds out how hot the coolant is and reacts to it.
As soon as the temperature of the engine coolant drops below the setpoint of the valve, coolant is sent through the jacket water heat exchanger. When the temperature of the coolant is higher than the set point, the valve sends the coolant through the heat exchanger. The excess heat is then sent to the raw or service water system. When a diesel engine starts, the flow of service water starts by itself.
Through the heat exchanger's exit, or bypass line, water goes back to the jacket water pump and, eventually, the engine. In many systems, the lubrication oil system is cooled by a heat exchanger in the jacket water system. For engines where it's important to keep the lube oil cooler than the jacket water, the oil heat is sent directly to the service/raw water system through the heat exchanger in the lube oil system.
When the coolant gets to the cylinder block, it flows through internal channels and/or pipes to the bottom of the cylinder liners. As the liquid goes up, it flows around the cylinder liners and into the cylinder heads. When the coolant leaves the cylinder heads, it goes into an outlet header and then to the thermostatic valve.
On engines with intercoolers or aftercoolers, some of the jacket water goes through the intercoolers to take in heat from the incoming air charge that isn't needed. On many engines with intercoolers or aftercoolers, this extra heat is sent to the service/raw water system by a separate heat exchanger. This is good because the water in the intercooler should be cooled to a temperature lower than the water in the jacket water system. Most ALCO engines use the jacket water system to cool the water in the intercooler.
Expansion Tank - Many engine use an expansion tank with a pressurized closure, or the expansion tank is mounted high enough to maintain the requisite head (net positive pressure head - NPSH) on the system. Most of the time, the expansion tank is placed just above the highest point of the jacket cooling water system, and vent lines are used to keep the system free of air. Some expansion tanks can be pumped up to keep a higher pressure, which helps raise the boiling point of the cooling fluid.
A standpipe is a tank that is set up vertically and is at the same height as the engine. It holds engine coolant and has a space for air to make up for the coolant's expansion when it gets hot.
Standpipes are usually vented to the air, making a cooling system that is not under pressure. The water level in the standpipe must be high enough to reach the required NPSH, or the tank must be pressurized.
Jacket Water Pump: The engine drives the single-stage centrifugal jacket water pump, which is powered by the crankshaft of the engine through a series of gears.
As seen, water enters the suction input of the pump. The engine gear train drives the pump drive gear, which in turn rotates the pump shaft and impeller. The speed of the coolant is increased by centrifugal force when the impeller turns. As the coolant enters the pump casing, its velocity reduces and its pressure rises proportionally. Coolant spills from the pump casing into the jacket water header to the lower end of the cylinder liners at a higher pressure.
The coolant for the engine comes up through the bottom of the thermostatic control valve. When the coolant temperature is low, as shown on the right side of the diagram, the sliding valve poppet stays in the up position and the coolant goes around the heat exchanger.
As the coolant temperature rises, the wax pellets inside the temperature control elements expand. This pushes the element tube and valve poppet down. So, the flow through the bypass is limited or throttled, as shown on the left side of the diagram, and coolant is sent to the heat exchanger.
In use, the valve changes its position over a temperature range of about 10 to 150 degrees Fahrenheit to keep the temperature of the coolant pretty stable.
Jacket Water Heat Exchanger - Jacket water heat exchangers are usually made of a shell and tubes. On the shell side, engine coolant usually flows over the tubes, while service water flows through the tubes.
Jacket Water Keepwarm Systems
When a motor is turned off for a while, the temperature inside the engine drops by a lot. Rapidly starting and quickly loading a cold engine, which is typical of nuclear application diesels in emergency situations, puts the engine under a lot of stress and wears it out faster until it gets to its normal operating temperature.
The jacket water keepwarm system is shown on the same plan as the standard jacket water cooling system. This part keeps the temperature of the engine coolant at or close to the normal operating temperature. This doesn't mean that all the parts are at their normal temperature.
Because diesel engines use the heat from compression to start, keeping the engine warm makes it start much faster and makes it less likely that the engine won't start because the intake air temperature is too low.
Keepwarm Pump: The keepwarm pump is a single-stage centrifugal pump that is powered by electricity. It is similar to the engine-driven pump in that it keeps heated coolant moving through the engine even when the engine is turned off.
Keepwarm Heater: The jacket waterkeep warm heater is an immersion-style electric warmer, just like the lubricant oil keepwarm heater.
It is put in a separate standpipe or heating tank. It is controlled by a thermostat to keep the engine at the right temperature.
How the System Works: When the engine is in the "standby" mode, the "keepwarm" system turns on. The keepwarm pump creates a vacuum in the system and sends water into the engine's jacket water inlet. When the engine is running, check valves may be put in the keepwarm system to stop flow in the wrong direction. The heated coolant flows through the engine, warming the cylinders, cylinder heads, and other parts that are cooled by water.
System for Cooling Water
The intercooler water system gives water to the intercooler or aftercooler, which is installed on the engine's combustion air intake pipes. It is a heat exchanger like a radiator that cools the combustion air after the turbocharger compressor and before the air manifold/plenum of the engine.
Cooling makes the air more dense, which lets more oxygen burn more fuel and make more power. Moreover, the combustion air cools the piston crowns.
The water used for intercooling must typically be quite close to the temperature of the surrounding air. For this reason, it is usually better to use service water instead of jacket water, which has a much higher temperature (160 to 180oF).
A typical intercooler and aftercooler water system diagram
Because these parts are the same as those used in the jacket water system, we won't talk about them any more.
In some intercooler water systems, a thermostat may be used to keep the intercooler water from getting too cold, especially in cold weather or when the engine isn't doing much work. This keeps moisture from condensing in the combustion air as much as possible. In some systems, the jacket water system and the intercooler water system are linked so that the intercooler can be heated when it needs to be.
If the combustion air coming into the engine is too cold, it may take longer for the engine to start, it may not work as well when the load is low, and the cylinder liner may not be as well lubricated. To mitigate this impact, several manufacturers thermostatically restrict the flow of cooling water to the intercooler and/or supply warm jacket water as necessary.
The thermostatic valve in the circuit keeps the water in the intercooler from getting too cold, which keeps the air going into the engine from getting too cold as well. When the air is too cold, it can cause condensation in the engine and "white" smoke to come out of the tailpipe.
More things that make it cool
Most of the time, the diesel generator is kept in a building with few openings.
There are several sources of heat in the EDG room, such as the engine and generator. For the best performance, the switchgear, control panels, monitoring equipment, fuel day tank, air compressor(s), and air storage tank(s) in this area must be kept at a cold temperature.
The EDG room can't get any hotter than 122 °F (50 °C). So, it is necessary to bring in enough cool air (ambient air) to get rid of the heat and keep the room's temperature below the highest level that is allowed. Even though room temperature doesn't have much of an effect on the engine itself, very high EDG room temperatures can have an effect on the generator and other parts. If the air for the engine's combustion comes from the room, hot air coming into the engine can make it less powerful.