Filter units, filtration methods, and filtration systems

A filter unit comprising:

• a housing having an upstream wall disposed at an upstream side of the housing, a downstream wall disposed at a downstream side of the housing, and a cavity therebetween, the upstream wall having a first entrance opening therein and first and second upstream wall portions separated by the first entrance opening, the downstream wall having first and second exit openings therein and a downstream wall portion between the first and second exit openings, the first entrance opening being disposed in the upstream wall so as to be aligned with the downstream wall portion, the first and second exit openings being disposed in the downstream wall so as to be aligned with, respectively, the first and second upstream wall portions;
• a heat exchanger disposed within the cavity, the heat exchanger having fluid flow conduits comprising at least first and second upstream conduits within the first entrance opening, a first downstream conduit within the first exit opening, and a second downstream conduit within the second exit opening;
• tube fins attached directly to the first downstream conduit and tube fins attached directly to the second downstream conduit, wherein the heat exchanger does not have any fins attached directly to the first and second upstream conduits;
• first and second entrance fins adjacent the first entrance opening;
• first and second baffles adjacent, respectively, the first and second exit openings;
• wherein the first entrance opening, the first and second exit openings, the first and second entrance fins, and the first and second baffles are configured and arranged to define first and second S-shaped air flow paths through the housing between the first entrance opening and the first and second exit openings;
• and
• wherein air entering the first entrance opening flows between the first and second upstream conduits and is then separated into first and second flow portions, the first flow portion flows around a downstream side of the first upstream conduit, then between the first entrance fin and the first baffle, then around an upstream side of the first downstream conduit and through the tube fins attached thereto, then exits the housing through the first exit opening, and the second flow portion flows around a downstream side of the second upstream conduit, then between the second entrance fin and the second baffle, then around an upstream side of the second downstream conduit and through the tube fins attached thereto, then exits the housing through the second exit opening.