Here we see a liquid-to-air core (left) and air-to-air units (right). A liquid cooler work
"A key advantage of bar and plate is its extraordinary variety of fin designs on both sides of the cooler. Further, the height of the passages can be changed by using taller bars and fins, which dramatically expands the possibilities for the intercooler designer. A 3.5-inch thick core gets a 3.5-inch row of fins, not the 2.87 inches found in tube-and-fin designs.
"Also, a tube-and-fin design has a lot of room between rows where a bar-and-plate unit can get more rows into a given area with more area per passage which means more cfm and a more efficient unit. As far as big boost goes, the ability of a bar-and-plate core to handle high boost is determined by the thickness of the braze sheets, fins, side bars and top plates."
The intercooler is a crossroads of sorts. Charge air and ambient air intersect inside the unit and heat exchange occurs. The cooling properties of intercoolers are decided by its thickness (for charge air flow) and area (for cooling ambient airflow).
The thickness (and the type of fins used) dictates how quickly the charge air will move across the unit while the surface area affects how quickly ambient air will flow across the cooler. The curveball here is fin density. The number of fins per inch will dramatically impact the cooling characteristics of the unit.
The popular plain square fin works its magic on the charge side.
"Fin combination is very crucial when designing an effective core with a very low pressure drop," says Wang. "A dense fin design offers superb heat transfer, however, this design also creates high pressure drop. Pressure drop is the amount of boost lost by the system as it pumps the charge air through the intercooler. The type of fin-plain, offset, louvered or bump-each has its own unique heat transfer characteristics.
"The key when designing any intercooler is that a median is met where the effectiveness is attained and a low-pressure drop is achieved. For example, a plain fin design offers an exceptional low pressure drop but does not provide great heat transfer compared to a louvered fin or an offset fin. The air moves quickly over/through the plain fin (low pressure drop), but is so fast it may not be in the core long enough to maximize heat transfer. Consequently, air moving through a louvered or offset fin takes longer to move through the core (more pressure drop), but it also has more time to transfer heat."
Area vs. Thickness
Wang continues, "The rule of thumb here is obvious-bigger is better. More surface area means more cooling power. However, a larger surface area is favored over a thicker core. A larger surface area provides more cooling face for the ambient air where a thicker core might be less effective. As the core gets thicker, the ambient air traveling through the core is heated by transfer and the farther through the core it travels the hotter it gets. By the time the ambient is at the back side of the unit, it's lost a good percentage of its cooling capacity. Beyond that, thick cores cause an ambient pressure drop, which means less air will make its way to the radiator causing overheating issues.
"When it comes to area, achieving the maximum height is more beneficial than widening the unit. The main reason for this is pressure drop is greater when the charge air has to travel through longer passage compared to a shorter passage. It should be noted that pressure drop through a longer-passage core can be compensated for, to a certain extent, via fin design."
Here we see an ambient side-louvered fin.
Spearco produces both tube-and-fin and bar-and-plate intercoolers. With bar and plate being so superior, one has to wonder why. It's all about cost. The tube-and-fin core is much cheaper to produce and is often used in OE applications, which inherently run lower boost levels.
Spearco put a new twist on tube-and-fin cores with its extruded tube design that incorporated the fins as part of the extrusion. In this design, the overall weight was about equal to a bar-and-plate core. tube-and-fin units are cheaper to manufacture.