Fig. 1 Exhaust Gas Recirculation (EGR) cooler. (Photo courtesy of Hoganas AB, Sweden)
Exhaust Gas Recirculation (EGR) coolers have been used extensively for many years on diesel truck engines, and are now also being used on gasoline engines. EGR coolers are specialized heat-exchanger assemblies that make extensive use of furnace-brazing (usually vacuum brazing) to create strong, leak-tight brazements that are capable of handling the very high temperatures involved in engine operations. EGR coolers used in diesel-engine applications help to reduce the formation of various nitrogen-oxides, such as N-O (nitrogen monoxide) or N2O (nitrogen dioxide), since such emissions are considered atmosphere pollutants, and are formed within a narrow temperature band in the combustion cycle. By recirculating some of the engine’s exhaust gases back to the engine through the EGR cooler, this cooled recirculated-gas gets mixed in with the incoming air entering the engine-cylinders and helps to reduce the combustion temperatures just enough so that less of these pollutants are formed.
Figure 1 shows what a modern diesel EGR-cooler looks like.
Notice the many brazed joints, not only of the many tubes to the header plate, but the many brazed joints around the rest of the cooler (brackets, external tubes, etc.). All these brazed joints are made at the same time within the brazing furnace, along with dozens, or perhaps hundreds, of other similar assemblies.
Fig. 2 and 3 show different methods whereby the header plate is coated with brazing filler metal (BFM) in either screen-printed paste form or sprayed-on powder/paste form.