Air to Air Recouperator

This invention relates to the use of air to air heat recouperators to obtain thermally efficient ventilation of buildings and dwellings, and in particular, to a rotary wheel heat exchanger for room ventilators.

Heat exchangers are used in ventilation systems installed in residential, commercial and industrial buildings to extract and remove heat or moisture from one air stream and transfer the heat or moisture to a second air stream. In particular, rotary wheel heat exchangers are known wherein a wheel rotates in a housing through countervailing streams of exhaust and fresh air, in the winter extracting heat and moisture from the exhaust stream and transferring it to the fresh air stream. In the summer rotary wheel heat exchangers extract heat and moisture from the fresh air stream and transfer it to the exhaust stream, preserving building air conditioning while providing desired ventilation. Fans or blowers typically are used to create pressures necessary for the countervailing streams of exhaust and fresh air to pass through the rotary wheel heat exchanger. Various media have been developed for use in rotary wheel heat exchangers to enhance heat and moisture transfer, for example, Marron et al, U.S. Pat. No. 4,093,435. Typical of rotary wheel heat exchangers are the devices shown by Hajicek, U.S. Pat. No. 4,497,361, Honmann, U.S. Pat. No. 4,596,284, and, those used by Mitani, U.S. Pat. No. 4,426,853 and Coellner, U.S. Pat. No. 4,594,860 in air conditioning systems.

It has been found in the prior art that to achieve thermally efficient ventilation of rooms and buildings, rotary wheel heat exchangers require installation in rather large, fixed, or non-portable heat recouperators, such as that disclosed by Berner, U.S. Pat. No. 4,727,931. The need exists, therefore, for smaller, portable heat recouperators which can still achieve thermally efficient ventilation. Further, the need remains for improved heat exchanger media for rotary wheel heat exchangers to increase the efficiency of heat transfer between the countervailing air streams.

Typically heat recouperators in the prior art employ heat exchangers having a plurality of parallel passages running in the direction of flow, as in Marron et al, U.S. Pat. No. 4,093,435 and Coellner, U.S. Pat. No. 4,594,860. Such passages must be sufficiently small to maximize the total surface area for heat transfer, yet sufficiently large relative to their length to minimize resistance to gas flow. These constraints have made the materials used critical to the effectiveness of such rotary wheel heat exchangers. Thus, for example, Marron et al, U.S. Pat. No. 4,093,435, disclose the use of corrugated paper of a specified composition, density, and thickness in a plurality of layers in a rotary wheel heat exchanger. Further combination with metal foil in a multi-layered material is disclosed. Coellner, U.S. Pat. No. 4,594,860 discloses the use of sheets of polymer film alternating with layers of corrugated or extruded polymer film or tubes, each layer having specified thermal conductivity and specific heat characteristics.

The need exists, therefore, for a compact, rotary wheel heat exchanger for heat recouperators which may be used without the necessity of building modification or connecting duct work as required, for example, with the devices of Tengesdal, U.S. Pat. No. 4,688,626 and Zenkner, U.S. Pat. No. 4,491,171. In addition to ordinary ventilation requirements of residential, commercial, and industrial buildings, the increasing importance of ventilation in residences due to the hazardous build-up of radon, formaldehydes, carbon dioxide and other pollutants presents a further need for inexpensive portable, compact, efficient heat recouperators which are capable of window-mounting. A continuing need exists for the improved design of rotary wheel heat exchangers, including improved, efficient heat exchanger media which avoid the exacting material and design restrictions found in the prior art.

The present invention meets these needs by providing a compact rotary wheel heat recouperator which may be designed to fit into room windows of a residence or satisfy the needs of commercial or large industrial buildings. The present invention is low cost in both construction and operation. Moreover, a new low cost, easily manufactured, heat exchanger medium is disclosed which has an average heat transfer effectiveness in excess of 90% regardless of temperature difference between inside and outside air.

The heat recouperator features a random matrix media in a rotary wheel heat exchanger. As the heat exchanger rotates, it transfers sensible and latent heat energy between two streams of air through which it passes. The heat exchanger is located in a housing which is baffled to permit the two oppositely directed streams of air to pass through with a minimum of intermixing of the streams. Heat transfer efficiency achieved with random matrix media in the heat recouperator is at least 90%, regardless of the temperature differential between the oppositely directed air streams.

Against the backdrop of prior art heat exchangers, typified by media having a plurality of ordered parallel passages, the media of the present invention is comprised of a plurality of interrelated small diameter, heat-retentive fibrous material, which, relative to the prior art, appear random, thus the term "random matrix media." Random matrix media, however, may encompass more ordered patterns or matrices of small diameter heat-retentive fibrous material, resembling, for example, shredded wheat biscuits or similar cross-hatched patterns.

The interrelation or interconnection of such fibrous material, whether by mechanical or chemical means, results in a mat of material of sufficient porosity to permit the flow of air, yet of sufficient density to induce turbulence into the air streams and provide surface area for heat transfer. Such mats, further, may be cut to desired shapes for use in heat exchangers of various shapes. One fibrous material suitable for use is 60 denier polyester needle-punched felt having 90-94% porosity and approximately 6-6.5 pounds/ft..sup.3 density. However, Kevlar.RTM., numerous polyester or nylon strands, fibers, staples, yarns or wires may be used, alone or in combination, to form a random matrix media, depending on the application. Once size and flow are determined, material selection exists in a broad range of filament diameters, overall porosity, density, mat thickness, and material thermal characteristics.

In operation, the heat exchanger may be rotated by various means, such as by belts, gears or, as shown, a motor-driven wheel contacting the outer periphery of the heat exchanger container. The random matrix media is retained in the container by screens, stretched over the faces of the container, which have openings of sufficient size to permit substantially free flow of air. Radial spokes, separately or in addition to screens, may also be used extending from the hub of the container through and supporting the random matrix media. Seals are located between the heat exchanger and baffles, angles and brackets in the housing to prevent mixing of the separate streams of air.

Air streams may be provided to the heat recouperator from existing ducts or from fans located in the housing. When fans are used to introduce the air streams, inlet and outlet vents are provided in the housing and are oriented to inhibit recirculation of air from the separate streams. If desired, filters may be added to inlet or outlet air vents. However, the random matrix media itself performs some filtering functions, for example, of pollen, which although driven to the surface of the random matrix media at the inlet, generally does not penetrate the random matrix media and may be blown outward as the heat exchanger rotates through the countervailing exhaust air. Similarly moisture attracted to or condensed in the random matrix media at an inlet is reintroduced in the countervailing exhaust stream.

Because of the heat transfer efficiency of the random matrix media, and related material characteristics, the deliberate inducement of turbulence, and the large surface area for heat transfer, random matrix media lend themselves to minimizing heat exchanger thickness, and permit development of a low cost, compact, portable window-mountable heat recouperator ventilating unit for residential use. Nonetheless, for the same reasons, the present invention may also be applied to meet the largest commercial and industrial applications for rotary wheel heat exchangers.

FIG. 1 is an exploded perspective view of the heat recouperator of the present invention.

FIG. 2 is a perspective view of the heat recouperator.

FIG. 3 is a rear elevational view of the heat recouperator of FIG. 2 with the rear housing cover removed.

FIG. 4 is a side elevational view of the heat recouperator of FIG. 3 taken at line 4-4.

FIG. 5 is a side elevational view of an alternative embodiment of the heat recouperator.

FIG. 6 is a perspective view of an alternative application of the heat recouperator.

FIG. 7 is a perspective view of an alternative system application of the heat recouperator.

Referring to FIG. 1, a heat recouperator 10 consisting of a rotary wheel heat exchanger 12, and a housing 14 with baffles 16, 18 and peripheral baffle 20, provides for two oppositely directed streams of air 22, 24 to pass through heat exchanger 12. Flexible seals 19 and 21, preferably of a Teflon.RTM.-based material, attach to peripheral baffle 20, to prevent streams of air 22 and 24 from circumventing heat exchanger 12.

In the preferred embodiment of FIGS. 1-4, motor driven fans 26 and 28 are located at alternate inlets 27 and 29, respectively, and are mounted on fan mounting plates 30 and 32 which are supported, in part, by mounting angles 34 and 36, and connected to a source of electricity (not shown). In an alternative embodiment, FIG. 5 shows fans 26 and 28 mounted on the same side of heat exchanger 12 at inlet 27 and outlet 29', respectively. Regardless of the location of fans 26 and 28, inlet and outlet vents 27 and 29', and 27' and 29 are oriented to inhibit recirculation of streams of air 22 and 24.

All components of heat recouperator 10 are commercially available and made of materials known and used in the art, unless otherwise specified. Housing 14, various baffles 16, 18 and 20, fan mounting plates 30, 32, and mounting angles 34, 36 are preferably made of light weight materials such as plastics, aluminum or mild steel, and are connected by conventional means such as bolts and nuts, welding, sealing or the like. Conventional seals or sealant material (not shown) may also be further used to seal the various elements where connected to prevent intermixing of streams of air 22, 24.

As seen in FIGS. 1-4, heat exchanger 12 is rotatably mounted on an axle assembly 38 such as is known in the art, typically comprising bearings 38a. Axle assembly 38 is supported by mounting angles 34 and 36. Seals 34a and 36a, such as Teflon.RTM.-based tapes, cover flanges of mounting angles 34 and 36, respectively, and abut screens 44 covering the faces of heat exchanger 12. Seals 34a and 36a typically are designed to contact screens 44 initially and wear to a level which maintains a desired seal between air streams 22 and 24', and 22' and 24. Mounting angle holders 52 and 54 are attached to housing 14 by conventional means and support mounting angles 34 and 36. Seals 52a and 54a, such as Teflon.RTM.-based tapes, are placed on surfaces of mounting angle holders 52 and 54 adjacent to the container 42. The surfaces of mounting angle holders 52 and 54 are made or machined to match as closely as possible the outer circumference of container 42. Designed to initially contact container 42, seals 52a and 54a wear to a level which is designed to maintain the desired seal between air streams 22 and 24', 22' and 24, 22 and 22', and 24 and 24'.

Heat exchanger 12 contains random matrix media 40 consisting of a plurality of interrelated small diameter, heat-retentive, fibrous material. Such materials may be interrelated by mechanical means, such as needle punching, or thermal or chemical bonding. Whether entirely random or maintaining some semblance of a pattern, such as a shredded wheat biscuit or cross-hatched fabric, the fibrous material, so interrelated, forms a mat of material which is easy to work with, handle and cut to shape. The random matrix media may be made from one or more of many commercially available filaments, fibers, staples, wires or yarn materials, natural (such as metal wire) or man-made (such as polyester and nylon). Filament diameters from substantially about 25 microns to substantially about 150 microns may be used. Below substantially about 25 microns, the small size of the filaments creates excessive resistance to air flow, and above about 150 microns inefficient heat transfer results due to decreased surface area of the larger filaments. Single strand filaments from substantially about 25 microns to substantially about 80 microns in diameter are preferred, for example a 60 denier polyester needle-punched felt having filament diameters of about 75 to 80 microns.

The present invention is distinguished from the prior art in that deliberate turbulence, rather than directed flow through parallel passages is encouraged by and adds to the effectiveness of the random matrix media. While turbulence in the random matrix media is desirable, resistance to air flow should not be excessive. The mat of material which forms the random matrix media should have a porosity (i.e., percentage of open space in total volume) of between substantially about 83% and substantially about 96%. Below substantially about 83%, resistance to air flow becomes too great, and above substantially about 96% heat transfer becomes ineffective due to the free flow of air. Preferably the mat thickness should be less than 6" to prevent excessive resistance to air flow. Porosity is preferable from substantially about 90% to substantially about 94%, as for example, with 60 denier polyester needle-punched felt, having a porosity of about 92.5%. Representative of random matrix materials which may be used in heat exchanger 12, 60 denier polyester needle-punch felt has a specific gravity of approximately 1.38, thermal conductivity of approximately 0.16 watts/m.degree. K. and specific heat of approximately 1,340 j/Kg.degree. K.

With reference to FIGS. 1-4, in heat exchanger 12, the random matrix media 40 is retained in container 42. Container 42 encloses random matrix media 40 around its periphery, and supports and retains the random matrix media 40 with screens 44 stretched tightly over the faces of container 42. Alternatively, radial spokes 46, shown in phantom on FIG. 1, may be used in lieu of or in addition to screens 44 to support and retain random matrix media 40.

In operation, heat exchanger 12 is rotated by contact between wheel 48, driven by motor 50, and the outer circumference of container 42 as shown in FIGS. 1, 3 and 4. Motor 50 is connected to a source of electricity (not shown). Rotation of heat exchanger 12 is preferably between about 10 revolutions per minute (rpm) and about 50 rpm. Below about 10 rpm, overall efficiency of the heat recouperator 10 declines. Above about 50 rpm, cross-over or mixing between air streams 22 and 24 occurs as heat exchanger 12 rotates, reducing the amount of ventilation provided.

The random matrix media 40 may be used in heat exchangers 12 of various sizes for various applications. One embodiment, shown in FIG. 2, is a window-mounted heat recouperator 12 for ventilation of rooms. For example, a 20 inch.times.20 inch.times.8.5 inch housing may contain a 17 inch diameter by 1.6 inch thick heat exchanger which may be rotated at 35 rpm-45 rpm with appropriate fans to supply from 80 to 150 cubic feet per minute, (cfm) of air with a thermal efficiency of generally 90% over a wide range of temperature differences. Shown in FIG. 2 embodied in a compact portable window-mounted heat recouperator 10, the random matrix media 40 of the present invention may be used in heat recouperators of many sizes for ventilating applications ranging from approximately 20 cfm for rooms to in excess of 30,000 cfm for large commercial and industrial applications, shown typically in FIG. 6. In other applications, heat recouperators using random matrix media 40 may be placed in forced-air systems and connected to one or more ducts which carry counter-flow streams of air or gas, shown typically in FIG. 7.

In any application, filter screens (not shown) may be added to filter inside or outside air at inlets or outlets 27, 27', 29, or 29'. The random matrix media 40 itself functions as a filter for some particulates. For example, pollen driven to the surface of the heat exchanger 12 at the inlet of a first stream does not substantially penetrate the surface of the random matrix media 40 and may be removed with the exhaust of the second stream. Similarly, moisture condensed at the inlet of a first stream is carried away from the surface of the random matrix media 40 by the exhaust air of the second stream. Thus, humidity and air quality are maintained by the random matrix media 40.

Precise selection of material, composition, filament size, porosity and width of the random matrix media 40 as well as the rate of rotation of heat exchanger 12 and selection of size of fans 26, 28 may vary with each application. However, once the size and flow required for a particular application are fixed, the fans and other components may be sized, and the random matrix media 40 may be selected from appropriate materials within the range of characteristics, particularly filament size and porosity, noted above. Chart 1 below lists typical parameters for the present invention in representative applications.

While certain representative embodiments and details have been shown and described for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the apparatus disclosed herein may be made without departing from the scope of the invention which is defined in the appended claims. It is further apparent to those skilled in the art that applications using the present invention with gases other than air may be made without departing from the scope of the invention defined in the appended claims.