The buildup of deposits in EGR coolers causes significant degradation in heat transfer performance, often on the order of 20-30%. Deposits also increase pressure drop across coolers and thus may degrade engine efficiency under some operating conditions.
It is unlikely that EGR cooler deposits can be prevented from forming when soot and HC are present. The presence of cooled surfaces will cause thermophoretic soot deposition and condensation of HC and acids. While this can be affected by engine calibration, it probably cannot be eliminated as long as cooled EGR is required for emission control. It is generally felt that “dry fluffy” soot is less likely to cause major fouling than “heavy wet” soot. An oxidation catalyst in the EGR line can remove HC and has been shown to reduce fouling in some applications. The combination of an oxidation catalyst and a wall-flow filter largely eliminates fouling.
Various EGR cooler designs affect details of deposit formation. It is generally thought that fin type coolers, having large surface area, are less impacted by fouling than tube-in-shell type coolers. However, both types are subject to significant fouling and the choice of type is often made more by package considerations than by deposit performance. High gas velocities tend to reduce deposition although they lead to large pressure drop.
Cooler deposits tend to stabilize after long operating time, often 50-200 hours. The mechanisms leading to this stabilization are not clearly understood. There may be deposit removal mechanisms, or it may be that the rate of deposition decreases as deposits build (or both).