Filtration assembly and system

1. A filter comprising:

an upstream side defining a first aperture;

a downstream wall comprising filter media, a length of the filter extending between the upstream side and the downstream wall;

a first pair of opposing side walls comprising filter media and extending lengthwise between the front side and the back wall; and

a second pair of opposing side walls comprising filter media and extending lengthwise between the front side and the back wall,

wherein the front side, the downstream wall and the first and second pairs of opposing side walls together define an inner plenum in flowable communication with the first aperture,

wherein the filter has a longitudinal axis extending along the plenum between the upstream side and the downstream wall,

wherein the inner plenum has a cross-sectional area extending crosswise of the filter, the cross-sectional area decreasing from adjacent the upstream side toward to downstream wall to a contracted location disposed intermediate the upstream side and the downstream wall, the cross-sectional area of the inner plenum increasing from adjacent the contracted location toward the downstream wall,

wherein the opposing side walls of the first pair of opposing side walls extend inward toward one another at an angle relative to the longitudinal axis from the upstream side to the contracted location,

wherein the opposing side walls of the second pair of opposing side walls do not extend inward toward one another at an angle relative to the longitudinal axis from the upstream side to the contracted location.

2. The filter according to claim 1 , wherein the inner plenum has a generally hourglass shape in longitudinal section.

3. The filter according to claim 1 , wherein the first and second pairs of opposing side walls define a second aperture at the contacted location, the first aperture having a cross-sectional area greater than a cross-sectional area of the second aperture.

4. The filter according to claim 1 , further comprising a frame attached to the upstream side.

5. The filter according to claim 4 , wherein the frame at least partially surrounds the first aperture.

6. The filter according to claim 1 , further comprising a contracting retainer attached to the first pair of opposing side walls, the contracting retainer configured to retain the cross-sectional area of the inner plenum.

7. The filter according to claim 6 , wherein the contracting retainer is attached to the first pair of opposing side walls at the contracted location, the contracting retainer configured to retain the cross-sectional area of the inner plenum at the contracted location.

8. The filter according to claim 7 , wherein the contracting retainer surrounds an exterior of the filter.

9. The filter according to claim 8 , wherein the contracting retainer comprises a band.

10. The filter according to claim 9 , wherein the band is rigid.

11. The filter according to claim 9 , wherein the band is flexible.

12. The filter according to claim 7 , wherein the contracting retainer is disposed on the inner plenum.

13. The filter according to claim 12 , wherein the contracting retainer comprises at least one vane in the inner plenum.

14. The filter according to claim 13 , wherein the at least one vane extends through the first pair of opposing side walls.

15. The filter according to claim 14 , wherein the at least one vane comprises a plurality of vanes.

16. The filter according to claim 1 , wherein the cross-sectional area of the inner plenum tapers from the upstream side toward to the contracted location, wherein the cross-sectional area of the inner plenum tapers from the downstream wall to the contracted location.

17. The filter according to claim 1 , wherein the first pair of opposing side walls comprises top and bottom side walls, wherein the second pair of opposing side walls comprises left and right side walls.

18. The filter according to claim 1 , wherein the opposing side walls of the first pair of opposing side walls flare away from one another at an angle relative to the longitudinal axis from the contracted location to the downstream wall.

19. The filter according to claim 18 , wherein the opposing side walls of the second pair of opposing side walls do not flare away from one another at an angle relative to the longitudinal axis from the contracted location to the downstream wall.

20. The filter according to claim 1 , wherein the filter has a generally hourglass shape in cross section taken in only one cross-sectional plane.