When a forced induction vehicle generates boost it is compressing air to produce the pressure, hence the compressor side. When air goes through compression phases, this charged air amplifies temperature through a process called “heat of compression”. This process negatively impacts air density, and reduces the power potential of a vehicle. An intercooler combats this issue by reducing air density fluctuation through thermal management. Even though the term “intercooler” is incorrect, being its technically a charge air cooler (CAC), we will use it for the remainder of this article for the sake of simplicity and popularity.
An intercooler recaptures air density by reversing the heat produced by the turbocharger or supercharger. The air density recuperated by an air-to-air intercooler relies heavily on the ambient air temperature. The efficiency of a turbocharger also plays a vital role in how much heat is generated at the outlet of the compressor; if the turbocharger is operating outside of its efficiency range, it can significantly increase the outlet temperatures seen at the turbo. The more competent an intercoolers’ thermal retarding abilities are, the higher the detonation threshold becomes.
Fundamentally there are two different types of intercoolers, air-to-water and air-to-air. Each design has beneficial attributes, and they also have shortcomings. Mike Reagan of Vortech said, “On a street car, it basically comes down to a balance of cost, packaging and performance. An air-air intercooler is typically less costly to produce, however it can be simpler to install. Also an air-air type usually exhibits higher pressure drop than air-water, depending on the application plumbing. A properly engineered air-to-water system can achieve a lower pressure drop when compared to an air-to-air. This is due to the fact that air-to-water cores can be placed ‘in-the-run’, and is not burdened by multiple bends and lengths of tubing. He also noted “In most cases water is more effective than air at removing heat”. Since air-to-air is the more commonly used style, we’ll be discussing that in this article.
There are two main types of air-to-air intercoolers; one being a tube and fin design, while the other is a bar and plate design. Reagan says “ Tube and fin charge air coolers are usually lighter in weight, which presents an advantage for OE’s when deciding which type to use in a vehicle. In high-volume, there may be a cost advantage argument for the tube- and fin as well. Regarding performance, the fin and passage configuration will have more of an impact on the overall effectiveness of the intercooler than will the type.”
The plain square fin type will allow charged air to travel faster through the intercooler, minimizing pressure drop. An offset fin will allow the charged air to pass through slower, however this delayed transition will produce more of a cooling effect at the cost of a higher pressure drop. Tube and fin intercoolers have oval tubes that are extruded from thin wall material, and at higher boost levels they are susceptible to damage from the tubes ballooning.
Bar and plate type intercoolers are inherently heavier by design, however this is advantageous for an effective charge air cooler. The core acts as a heat sink, the heavier design has the ability to soak more heat up. Bar and plate designs can also take considerably more damage before failing, when in comparison to tube and fin types. This is durability is beneficial for road racing or even for daily driving. Sealing is also superior for bar and plate designs because the brazing sheets and bars run the entire length of the core, which provides 100 percent sealing between the charged and ambient side.
Since bar and plate intercoolers utilize bars to harness internal fins, the charged air passage height can be manipulated for taller fins or even fin density. This dramatically increases the design capabilities for an engineer, allowing them to tailor intercoolers to meet the need of specific applications. A bar and plate design can house more rows per a given area, with more area per passage when in comparison with a tube and fin. This allows the bar and plate to push more CFM (airflow) through the core and makes a more effective cooling unit. The thickness of an intercooler is a key factor in its ability to cool the charged airflow. The area of the core determines the cooling properties of the ambient airflow. When the charged air intersects with the ambient air, this is where the heat exchanging process happens.
Fin density and offset plays a crucial role in how an intercoolers’ operational effectiveness is measured. A dense fin design will facilitate higher cooling rates, but will adversely affect the pressure drop by increasing it. This pressure drop is how much boost is lost during the heat exchanging process in the intercooler. With a plain fin design, fin density and offset allow the airflow to travel much faster and has minimal pressure drop. However this does not provide efficient heat transferring abilities, like a louvered or an offset fin design.
Reagan said “Fin density and type will have a significant effect on performance. Achieving the best balance of thermal performance and pressure drop is the challenge. Both hot and cold-side passage size, fin type and density play a role in overall performance. Balance is key.” So whatever air-air intercooler you decide to choose for your application, you can see how design implementations affect certain aspects of performance.