Capillary Fin Media

This invention relates generally to evaporative air coolers having an integral indirect evaporative cooling stage, and more particularly to wetted surface air-to-air heat exchangers and materials for forming the same.

The principles upon which indirect evaporative coolers utilizing a wetted surface air-to-air heat exchanger operate are well known. A stream of water, preferably in sheet form, trickels down through a first channel. A current of air is blown in the same channel typically in the opposite direction of the flow of water to partially evaporate the water. As the air evaporates the water, the temperature of the unevaporated water which remains in the channel decreases. The temperature decrease of the unevaporated water in turn cools the material which defines the channel through which the water is moving. This channel is an element of the heat exchanger media, and typically consists of some sort of tube or other channel-defining structure.

A second stream of air is blown or drawn across the opposite surface of the channel-defining structure. This second stream of air is kept separate from the air and water in the first channel. The channel-defining heat exchanger media which has been cooled as described above in turn cools this second stream of air. This cooled air can then be circulated directly into a space to be cooled or circulated through a direct evaporative cooler for further cooling before circulation into the space to be cooled.

Many attempts have been made to create improvements in evaporative coolers. Particularly relevant recent improvements are disclosed in U.S. patent application Ser. No. 444,805, filed Nov. 26, 1982. In that application, an indirect cooling unit is disclosed which comprises a plurality of pairs of opposed plates, each pair of plates forming a channel to receive the water in the first current of air referred to previously. The plates are disclosed to be embossed to form a plurality of vertically and horizontally disposed ridges which serve as spacers to maintain the plates in the desired spaced relation with respect to each other. The plates are said to be advantageously made of plastics and the ridges formed thereon by continuous extrusion, vacuum-forming, or other conventional methods. The plates are said to be treated such that one face of each plate has a wettable surface for distributing the water uniformly across the plate which will enhance the evaporation and, therefore, improve the indirect evaporative cooling effect. The recited advantages of forming the plates of plastics are low cost, light weight, cleanability, ease of replacement, and so forth.

U.S. Pat. No. 4,263,967 discloses a heat exchanger comprising a plurality of parallel plates composed of plastics. The plates include a regular pattern of conical projections extending from one side of each sheet which acts to space each sheet from the adjacent sheet. Edges of each sheet are folded to form return flanges which engage edges of an adjacent sheet to form a channel for containing the flow of air and other fluids. The transfer of heat through the sheets is generally limited by the poor heat conduction characteristics of most plastics. The effective heat transfer area of each media plate in the prior art is generally limited to the product of the exterior dimensions of each plate. The present invention is intended to achieve a substantial increase in heat transfer effectiveness without any corresponding increase in heat exchanger size. The present invention is also intended to permit the construction of smaller heat exchangers having the same heat transfer effectiveness as much larger conventional units.

In accordance with the present invention, the plates used to form the media comprise a sheet of water-impervious material which includes a plurality of slot-like depressions dimensioned thin enough to exhibit capillarity, and thick enough to enable the slot to exhibit good heat transfer fin characteristics when filled with chilled water. The capillary fins including the slot-like depressions are preferably oriented to be generally perpendicular to the flow of the water so as to laterally distribute the water within the first channels by capillary action. The depth dimension of the capillary fins is desirably such as to maximize the heat transfer through the sheet by providing an enlarged area for heat transfer without increasing the overall dimensions of the sheet. The capillary fins have the additional advantage of increasing the rigidity of the sheets, thus making the media more form stable, and permitting a reduction in thickness of the sheets forming the media, thereby enhancing the heat conduction through the sheets. Experimental indirect cooling units employing the present invention have achieved the same heat transfer effectiveness as prior structures while using one-third less material.

These and other features of this invention and their inherent advantages will become apparent to those skilled in the art from the following description of preferred embodiments and the accompanying drawings illustrating the best mode of carrying out the invention as presently conceived wherein:

FIG. 1 is a side elevation view, partially broken away, of a wetted surface air-to-air heat exchanger constructed in accordance with the present invention;

FIG. 2 is a top plan view, partially broken away, of the heat exchanger shown in FIG. 1;

FIG. 3 is a sectional view taken along lines 3--3 of FIG. 1;

FIG. 4 is a front elevation view of a media sheet of the present invention;

FIG. 5 is an enlarged view, broken away, of the media plate assembly used in FIG. 1; and

FIG. 6 is a top plan view of a media plate as viewed from line 6--6 of FIG. 5, partially broken away.

A wetted surface air-to-air heat exchanger 10 constructed in accordance with the present invention comprises generally a lower collar 12, end plates 14, and an upper collar 16. Water is caused to trickle downward over water distribution skirts 18 into and through a first set of channels in the media 20, while a first flow of air is caused to flow upward through the same channels to partially evaporate the water, thereby cooling the media. A second flow of air is caused to flow through the media 20 in a direction substantially normal to the plane of FIG. 1 to be cooled by contact with the media.

The lower collar 12 as shown in FIG. 1 comprises a generally rectangular annular element having a substantially horizontal inwardly protruding lip 22 which terminates to define bottom opening 24 of the exchanger 10. An upwardly and outwardly inclined portion 26 extends between lip portion 22 and a substantially vertically projecting rim portion 28 which envelopes the lower extremities of end plates 14 and media 20.

The upper collar 16 shown in detail in FIGS. 1-3 includes a downwardly extending rim portion 30 which envelopes the end plates 14 and media 20 in a manner similar to rim portion 28 of lower collar 12. The upper collar 16 extends upwardly from the rim portion 30 to an inwardly extending step 32 which rests upon the top edge of media 20. From step 32, the upper collar extends upwardly to a perimeter-defining ridge 34 which defines the perimeter of top opening 36 of exchanger 10.

The upper collar integrally includes water distribution means 38 comprising at least one upwardly opening channel 40 including tooth-like depressions 42 on the edge of the channel 40 which direct water 44 from the channel 40 outward on either side thereof and downward to media 20. Lower outer portions of the channels 40 include water distribution skirts 18 which can be integrally formed with the channel 40 or secured to the channel 40 by a securing means 46 such as an adhesive or weld. The water distribution skirts 18 can include on a lower edge thereof slits or serrations 48 for receiving upper edges 50 of media 20.

Each end plate 14 includes a top flange 52 and a bottom flange 54 which are held within the upper collar 16 and lower collar 12, respectively. Between the top flange 52 and bottom flange 54, the end plates 14 bow outwardly from the exchanger 10 to form a substantially planar vertical outer surface 56 which can be interrupted by one or more vertically extending depressions (not shown) for strengthening and rigidifying the end plate 14. Side flanges 60 are included which define the vertical edges of the exchanger 10.

The media 20 is shown in detail in FIGS. 4-6 to comprise a plurality of plates 62 of water-impervious material which are arranged in opposed pairs as shown in FIG. 5. Each plate 62 includes an upper flange 64 adapted to be secured to a similar upper flange of an adjacent plate 62 as shown at the top of FIG. 5. The space between non-contacting adjacent flanges 64 comprises an opening 66 through which water trickles downwardly after leaving the water distribution means 38. Each plate 62 also includes side flanges 68 adapted to be joined to a similar side flange 68 of an adjacent plate 62 to form a first set of vertically extending channels 78 through which the water proceeds downwardly and through which a first stream of air can proceed upwardly. Each sheet preferably includes one or more vertically extending depressions 70 for rigidifying the plate 62, and can also include periodic horizontally extending depressions 72 to assure the proper spacing between adjacent plates 62.

Each of the plates also includes a plurality of horizontally extending capillary fins 74 having slot-like depressions which are dimensioned to exhibit capillarity. That is, the generally horizontal slots within the capillary fins 74 have a sufficiently small vertical interior dimension A that water traveling downward through channels 70 is drawn into and caused to substantially fill the slot by capillary action. As shown in FIG. 5, the depth of the capillary fins 74 are chosen to be substantially identical to the horizontally extending reinforcing depressions 72. Further, the capillary fins 74 are spaced from each other by a distance D which, in the preferred embodiment, is approximately equal to the separation distance C between the major surfaces 76 forming channel 70. Thus, each plate 62 can be seen to include major surfaces 76 defining the boundaries of first channels 78, which major surfaces 76 are periodically interrupted by a series of horizontally extending capillary fins 74.

In the preferred embodiment, the major surfaces 76 as well as the interior of the capillary fins 74 are treated so as to exhibit enhanced wettability. This can be easily achieved by coating one side of each plate 62 with a chopped polyester fiber flocking 58 as disclosed in the previously referred-to application. The flocking 58 can most advantageously be applied to the plates 62 while they are in planar form prior to the formation of upper flanges 64, side flanges 68, depressions 70 and 72, and capillary fins 74. After the flocking or other wettabilitv-enhancing material 58 is applied, the various flanges, depressions, fins, and channels can be formed by subjecting the planar sheet to a conventional thermoforming step.

The plates 62, once formed, can be assembled in pairs 80 with the wettability-enhanced, major surfaces 76 facing each other. The contacting side flanges 68 can be secured to each other to form the vertically extending channels 78. The pairs 80 of plates 62 can then be assembled together with adjacent pairs of plates and the pairs secured to each other along contacting upper flanges 64 and lower flanges 82.

As shown in FIG. 5, the space 84 between adjacent pairs of plates is divided by the capillary fins 74 into a plurality of parallel, vertically spaced, rectangular openings or conduits 86 through which the second stream of air is caused to flow for cooling the same. Each rectangular opening 86 of dimensions C by D is shown to be substantially completely surrounded by water which is maintained near the wet bulb temperature of the air moving in the vertical channel 78 by virtue of the evaporation occurring in the vertically extending channels 78.

It will be appreciated by those skilled in the art that the use of capillary channels to achieve an even distribution of a film of water on the surface of the media can be advantageous in direct contact evaporative coolers as well as in the indirect coolers described herein. The capillary channels can generally be arranged substantially as shown in FIG. 4 so as to effect distribution of the water over the entire surface of the sheet. Although the invention has been described in detail with reference to the illustrated preferred embodiment, variations and modifications exist within the scope and spirit of the invention as described and as defined in the following claims.