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SCAC, CAC, AATAC, What Does It All Mean?
The new emission regulations have led to major changes in the way diesel engines are cooled. Older engines had a single jacket water circuit (using a mixture of ethylene glycol and water) which was responsible for removing heat from the engine to the radiator. Simple enough, right?
Fast forward to today! Engines are now equipped with separate cooling circuits, aftercoolers, charge-air-cooled circuits and even oil cooling circuits. This article describes some of these systems and the radiators used to cool them.
Let’s start with the most basic, single jacket water circuit. Here, coolant is circulated through internal passages in the engine block to remove and carry away the heat generated by the combustion cycle. It is a single circuit because all cooling water is circulated through a single engine-mount water pump.
SCAC stands for “Separate Circuit After-Cooled”. This technology circulates the jacket water in the normal way, but a separate water circuit is added to cool the engine’s intake air after it is pressurized by the turbocharger. By circulating cold water through the aftercooler, the engine is able to run more efficiently and produce more horsepower. This system uses two engine-mounted water pumps, each pumped to a separate radiator core for circuit cooling.
ATAAC stands for Air-to-Air-After Cooled, also known as CAC (Charge-Air-Cooled). This system uses forced air (instead of water) to cool the turbocharged air before it enters the engine’s combustion chamber. Similar to the SCAC design, the goal is to lower engine intake temperatures to improve emissions and output power efficiency.
So, how do these systems affect external heat exchangers (radiators)?
Engines equipped with combined jacket water and aftercooler circuits are fairly straightforward. There are two water circuits, each piped separately to a radiator with a split core or with a dual core. When using a remote radiator (not engine-mounted), engine water pump and aftercooler pump head limitations have to be considered. In the worst case scenario, shell and tube heat exchangers are used to circulate the engine circuits and auxiliary pumps are used to circulate the coolant to the remote radiator. This is referred to as a dual-loop remote radiator cooling package.
Engines equipped with charge-air-cooled circuits are a bit more complicated. A charge-air circuit is extremely sensitive to even small pressure drops in its piping system. This requires a minimum gap between the engine and the charge-air cooler. Although set-mounted radiators are most common, remote radiator packages are sometimes required. In these instances, a liquid-to-air, charge-air cooler airbox assembly is installed by the engine. This assembly is then piped to a remote radiator, with auxiliary pumps providing the necessary circulation of coolant.
Engines equipped with SCAC designs use a stacked core radiator (a single radiator assembly with two stacked cores) or a split-core radiator (a single radiator assembly with side-by-side cores). Each core is connected to an associated circulation pump (JW pump for jacket water core and AC pump for aftercooler core). In remote radiator applications, pump head limitations may require the use of auxiliary pumps and shell and tube heat exchangers.
Here are some considerations to keep in mind when selecting a generator set for a particular application:
- If the radiator is engine-mounted, check the temperature rise across the engine (ie: radiator operating temperature = ambient air plus the ambient temperature rise when air flows over the engine.
- Calculate the airflow requirements and maximum external static resistance limits of the radiator. Communicate this information to the radiator duct fabricator and louver vendor so that radiator airflow is not excessively restricted.
- For remote radiator applications, the mechanical engineer must calculate the pressure drops associated with the piping from the engine to the radiator. These values should be compared to the engine’s water pump data sheet.
- When it comes to cooling systems, different engine manufacturers often apply different technologies for the same engine rating. A manufacturer using the SCAC design for a given rating may compete with another manufacturer using the AATAC design. If your project is competitively bid, your design should take worst-case scenarios into account.
- Consult with the various engine manufacturers or independent radiator manufacturers so that you are aware of any cooling system limitations that are important to your design.
How much exposure have you gotten with this new cooling system? Are you lacking information on this topic? Please share your comments or questions.
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