SCAC, CAC, AATAC, What Does It All average?

New emission regulations have led to enormous changes in the way diesel engines are cooled. Older engines had a single jacket water circuit (using a combination of ethylene glycol and water) that was responsible for removing heat from the engine and transporting it to a radiator. Simple enough, right?

Fast forward to today! Engines are now equipped with separate cooling circuits, aftercoolers, charge-air-cooled circuits and already oil cooling circuits. This article with describe some of these systems, and the radiators used to cool them.

Let’s begin with the most basic, the single jacket water circuit. Here, the cooling fluid is circulated by internal passages in the engine block to extract and carry away some of the heat produced by the combustion cycle. It is a single circuit because all cooling water is circulated by a single engine-mounted water pump.

SCAC stands for “separate circuit after-cooled”. This technology circulates jacket water in the normal manner, but a separate water circuit is additional to cool the engine’s intake air after it is pressurized by the turbocharger. By circulating cooling water by the aftercooler, the engine is able to function more efficiently and produce more horsepower. This system uses two engine-mounted water pumps, and each circuit is pumped to an individual radiator chief for cooling.

ATAAC stands for air-to-air-aftercooled, 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 enhance emissions and output strength efficiency.

So, how do these systems affect external heat exchangers (radiators)?

Engines equipped with a combined jacket water and aftercooler circuit are fairly straightforward. There are two water circuits, each piped separately to a radiator with a divided chief, or a with a dual chief. When using a far away radiator (not engine-mounted), the engine’s water pump and aftercooler pump head limitations have to be considered. In the worst scenario, shell & tube heat exchangers are used to circulate the engine circuits, and auxiliary pumps are used to circulate the cooling fluid to the far away radiator. This is referred to as a dual-loop far away radiator cooling package.

Engines equipped with charge-air-cooled circuits are a bit more complicated. The charge-air circuit is very prone to already the smallest pressure drop in its piping system. This requires that minimal distance exist between the engine and the charge-air cooler. While set-mounted radiators are most shared, far away 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 far away radiator, with auxiliary pumps providing the necessary circulation of coolant.

Engines equipped with SCAC designs use a stacked chief radiator (a single radiator assembly with two stacked cores) or divided-chief radiator (a single radiator assembly with side-by-side cores). Each chief is connected to the corresponding circulation pump (JW pump for jacket water chief, and AC pump for the aftercooler chief). In far away radiator applications, pump head limitations may require the use of auxiliary pumps and shell & tube heat exchangers.

Here are some thoughts to keep in mind when selecting a generator set for a particular application:

  • if the radiator will be engine-mounted, be sure to explain temperature rise across the engine (i.e.: radiator operating temperature = ambient air plus ambient temperature rise observed as the air flows over the engine.
  • calculate airflow requirements and radiator maximum external static resistance limitations. Communicate this information to the radiator duct fabricator and louver vendor to ensure that the radiator airflow is not excessively restricted.
  • for far away radiator applications, a mechanical engineer should calculate pressure drops associated with the piping runs from the engine to the radiator. These values should be compared against the engine’s water pump data sheets.
  • when it comes to cooling systems, different engine manufacturers often apply significantly different technologies for the same engine rating. A manufacturer that uses an SCAC design for a given rating may compete against another manufacturer using an AATAC design. If your project will bid competitively, your design should take into account the worst case scenario.
  • consult with various engine manufacturers or with an independent radiator manufacturer so you can be aware of any cooling system limitations that are basic for your design.

How much exposure have you had with these newer cooling systems? Have you encountered a shortage of information on this topic? Please proportion your comments or questions.

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