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.

EGR header-plate coated with BFM-paste via a screen-printing technique. (Photo courtesy of Hoganas AB, Sweden).

Fig. 2 EGR header-plate coated with BFM-paste via a screen-printing technique. (Photo courtesy of Hoganas AB, Sweden).

Screen-printing is a method whereby a closely controlled amount of paste can be applied to the metal surface through a fine-mesh screen. Both the size of the powder particles (powder mesh-size) and the size of the openings in the screen (the screen mesh-size) must be carefully selected and controlled to optimize any screen-printing application in brazing. Additionally, the paste-binder system must be formulated to properly allow the paste to go through the screen, and to dry and remain adherent to the header-plate during subsequent assembly operations so that the dried-paste does not flake off prior to actual brazing operations.

In a similar manner, spray guns can be used to either spray braze-powder along with a sprayable binder system so that a uniform, thin coating of BFM powder will be deposited on the header plate, or a specialized spray gun can be used that sprays a thin paste (much like a paint-spray gun).

EGR header-plate coated with BFM-paste via a screen-printing technique. (Photo courtesy of Hoganas AB, Sweden).

Fig. 3 EGR-cooler header-plate coated with BFM powder from a spray-gun. (Photo courtesy of Hoganas AB, Sweden)

It is very important that just the right amount of paste/powder be applied to the header-plate, so that the BFM, when it melts, will form a complete, tight brazed joint between the header-plate and the end of each tube, without having excess BFM paste run into the tubes (and then perhaps plug the tubes) or causing erosion of the tubes during the brazing process. The ratio of BFM powder-to-binder used is also very important so that, when screen-printing, the resulting applied BFM-coating is not too viscous or too thin (runny), or if spray-coating, the applied spray coating builds up evenly to a controlled thickness.

Fig. 4 shows another type of diesel-engine EGR cooler that was vacuum-brazed using a nickel-based BFM.

Here is a look inside another type of diesel-engine EGR cooler showing corrugated fins brazed to inside flat surfaces (photo courtesy of Navistar).

Fig. 4 Here is a look inside another type of diesel-engine EGR cooler showing corrugated fins brazed to inside flat surfaces (photo courtesy of Navistar).

Notice that each of the corrugated fins in Fig. 4 is brazed firmly to the internal flat wall surfaces, but the quantity of BFM used is very small, i.e., just enough to bond it strongly to the base metal, without forming large fillets. Large visible fillets could be very detrimental to the operation because fillets are castings, and the larger they are, the more the risk of casting imperfections and cracking due to the large amount of thermal expansion and contraction of those fins each time the diesel engine is turned on and off.

Therefore, to keep the amount of BFM to the proper minimum amount needed, the nickel-based BFM can be applied using very thin amorphous-foil sheets between the corrugations and the flat stainless-steel surfaces, or the BFM may be applied as a very thin sprayed-on layer to the stainless flat surfaces. The corrugated fins are then laid on top of this BFM-foil or sprayed-on BFM, then additional layers of stainless sheet and corrugated fins added until the desired heat-exchanger size/thickness is achieved.

Vacuum brazing of EGR coolers is highly effective and allows many hundreds of such coolers to be produced under very controlled conditions every day.

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