WO1998022770A1

WO1998022770A1 - Heat exchanger having a pleated mat of fibers

Info

Publication number: WO1998022770A1
Application number: PCT/US1997/021058
Authority: WO
Inventors: Lloyd E. Hackman; Gilbert C. Brunnhoeffer, Iii
Original assignee: Micrometal Fibers, Inc.
Priority date: 1996-11-19
Filing date: 1997-11-14
Publication date: 1998-05-28
Also published as: AU5261998A

Abstract

A heat exchanger (10) having a housing (12) into which a continuous mat (30) of semi-continuous fibers is pleated in layers and compressed upon one another. A fluid barrier (26) extends through the middle of the housing (12) and the fibers extend form one chamber (22) through the barrier (26) and into the other chamber (24).

Description

TITLE: HEAT EXCHANGER HAVING A PLEATED MAT OF FIBERS

Technical Field

The invention relates to heat exchangers generally, and specifically to a heat exchanger having fibrous materials making up the heat conductive path.

Background Art

A heat exchanger is a device which transfers heat energy from one fluid to another. Typically, the heat is conducted from the hotter fluid to the cooler fluid through an interposed fluid impervious wall which is exposed to both fluids.

Increasing the surface area of fluid contacting materials in the fluid flow paths of a heat exchanger increases its efficiency. Fins have been used for that purpose . Some in the prior art have found it advantageous to use a screen to conduct heat to or from a fluid rather than using fins. Poole, in U.S. Patent No. 2,112,743, shows a sinusoidally shaped screen having lower peaks clamped against a central panel. A second, similarly shaped screen has its upper peaks clamped against the central panel . Hot fluid flows through and across one screen and the heat is conducted through the central panel to the other screen. Cooler fluid flows across the other screen and removes heat from it . It is an object and feature of the present invention to provide a heat exchanger which permits a substantial increase in the surface area of the fluid contacting materials and also to permit the use of thermally conductive materials having a higher conductivity. It is also an object and feature of the present invention to permit the design of the surface area of the material which makes up the heat conductive path over a wide range of surface areas. With most prior art heat exchangers, the structure making up the heat conductive path has a specific shape and cooperation with other structures which precludes wide variations in the surface area of the heat conductive path material. This is due to the dependence of the surface area upon the shape and strength of the heat conductive path. Forming the barrier between the flow of hot fluid and the flow of cool fluid has also been a problem in conventional heat exchangers. Additionally, most heat exchangers consist of multiple fins or panels which are strategically constructed to have a particular spacing and/or shape. This complex design requires attention to detail, which increases the cost of construction.

It is also an object and feature of the present invention to provide a heat exchanger having a broad range of density and surface area parameters which can be selected by the designer while keeping the heat exchanger inexpensive to build. The present invention is a structure which combines high surface area, flexible design and low cost.

Brief Disclosure Of Invention The invention is a heat exchanger which has a housing containing two fluid flow paths. The heat exchanger comprises a continuous mat of semi- continuous fibers pleated in layers. Each layer is connected to an adjacent layer at a fold along adjoining layer ends. A fluid barrier is attached to the layers between their adjoining ends and interposed between the flow paths. The fibers extend through the barrier into the two flow paths . The invention contemplates a method of making a heat exchanger having two fluid flow paths. The method comprises a first step of pleating an elongated mat of semi-continuous fibers along lateral creases, forming multiple layers. Each layer is connected to an adjacent layer at a fold along adjoining layer ends. A second step comprises forming a fluid barrier between the adjoining layer ends and interposed between the flow paths . The fibers extend through the barrier into the two flow paths. The preferred method of forming the fluid barrier separating the fluid flow paths is to position spacers in gaps between layers and clamp the spacers against the layers between them.

Brief Description Of Drawings Fig. 1 is a view in perspective illustrating a heat exchanger embodying the present invention.

Fig. 2 is an end view in section along the line 2-2 of Fig. 1.

Fig. 3 is a side view in section along the line 3-3 of Fig. 1.

Fig. 4 is a side view illustrating a schematic of the preferred pleating of the mat of the present invention.

Fig. 5 is a side view illustrating the detail of the fibers. Fig. 6 is an end view illustrating a schematic of an alternative embodiment of the present invention.

Fig. 7 is a close-up view illustrating the preferred semi -continuous fiber mat.

Fig. 8 is a close-up view illustrating an alternative semi-continuous fiber mat.

Figs. 9, 10, 11 and 12 are side views in section illustrating alternative embodiments of the present invention.

In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word connected or terms similar thereto are often used. They are not limited to direct connection but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art .

Detailed Description

Fig. 1 shows the preferred heat exchanger 10. The heat exchanger 10 includes a housing 12, made of a rigid, preferably insulating material for containing the heat in the interior thereof. The housing 12 has inlets 14 and 16 and outlets 18 and 20 which lead into and out from two distinct flow paths contained within two longitudinal chambers 22 and 24 shown in Figs. 2 and 3. A fluid barrier, preferably including a plurality of spacers 26, divides the housing 12 into the chambers 22 and 24. With reference to Figs. 2 and 3, the inlet 14 permits fluid to enter the chamber 22 in the interior of the housing 12, and exit through the outlet 18 on the opposite end. The inlet 16 allows fluid to enter the second chamber 24 of the housing 12 and exit through the outlet 20. The housing 12 contains an elongated mat 30 which is made up of semi -continuous fibers. The mat 30 is of the type commonly referred to as a fine fiber mat or tow of fibers. The mat 30 is pleated into multiple layers, and each layer is contiguous with the neighboring layer, connecting at folds formed laterally across the mat 30. The mat 30 is arranged in the housing 12 with a portion of each layer on opposite sides of the fluid barrier, with approximately one-half of the exposed mat in the chamber 22 and one-half in the chamber 24.

Fig. 4 shows schematically the heat exchanger 10 of Fig. 1 to illustrate the pleating of the preferred mat. In the present invention, the term "pleating" is defined as folding the continuous mat at regular intervals along the length of the mat. The folds are formed along lateral lines which are generally perpendicular to the axis of the elongated mat. The line 40 shows where the center of mass of the continuous mat 30 is in the preferred embodiment, and shows the pleated layers formed by the mat 30. Referring again to Fig. 3, one spacer 26 is preferably interposed between each pair of layers, and the spacers 26 and layers of the mat 30 are compressed together by a clamp, preferably a screw 32 extending longitudinally through the housing 12. The amount of compression is predetermined to give the desired density to the mat 30 between the spacers 26 and the rest of the mat 30. When the screw 32 clamps the spacers 26 together against the mat 30, the density of the mat 30 in the localized region between each pair of spacers becomes extremely high. This high density mat portion between the spacers 26 prevents, or at least restricts, the flow of fluid through the region between the spacers. If it is desired, a metal solder can be added to the sides of each spacer 26 to infiltrate the mats on both sides under heat and pressure. The solder can be of any conventional solder metal, such as lead or copper-based alloys.

The combination of the spacers 26 and the clamped mat 30 forms the fluid barrier. The fluid barrier is preferably fluid impervious, but if the fluids used with the heat exchanger 10 can intermix with little harm, the fluid barrier may be made merely resistant to the flow of fluid from one chamber to the other.

The fibers of the mat 30 form a path for conducting heat between the chambers, and therefore the fibers must be longitudinally heat conductive. The preferred fibers used to form the mat 30 are made of copper or aluminum, and most preferably nickel aluminide, since nickel aluminide becomes more conductive as temperature increases. The fibers can also be made of platinum to act as a catalyst if it is desired to chemically change fluid flowing through one or both of the chambers 22 and 24. An alternative fiber material is high modulus, high thermoconductivity graphite. The longitudinal heat conduction of the graphite is significantly higher than the radial conduction. A coating on these graphite fibers, such as nickel, provides a bonding agent to fuse the fibers to each other to form the fluid barrier, if desired, and also permits heat to be conducted radially inwardly into each graphite fiber. The fibers of the mat 30 are semi-continuous.

The term "semi -continuous" means the fibers are long enough to extend from one chamber 22 into the other chamber 24, to provide a continuous, heat conductive path. Some fibers may be so long that they extend from one end of the mat 30 to the opposite end, weaving through every layer of the mat 30. Furthermore, some fibers may extend through multiple layers, but not all of the layers. Additionally, some fibers may be much shorter, extending only from one chamber to the opposite, and may be contained within a single layer of the mat 30. All of these conditions meet the definition of semi-continuous . The term "semi-continuous" does not exclude entirely the existence of some fibers which are contained completely within one chamber. The term "semi- continuous" means that the majority of fibers are at least long enough to extend from one chamber to the other.

The term "fiber" refers to long, thin pieces of the material making up the mat 30. The term "fiber" also includes the short pieces of material which are fused together to form a continuous mat or a bonded block of fibers. Although it is preferred that the continuous mat of semi-continuous fibers of the preferred embodiment comprise multiple long, thin, single-strand fibers intertwined along the length of the continuous mat, it is also within the scope of the invention for the mat to comprise chopped or milled fibers sintered together to form the mat. For example, a mass of short fibers may be sintered together with a heat conductive bond to form a mat which is later pleated. Additionally, short fibers may be sintered to form a block of fibers which are bonded together and to a fluid barrier. These chopped or milled fibers can be formed into a mat or block having a porosity which is varied by controlling the processing of the fibers. Examples of the semi -continuous fibers making up the preferred continuous mat are illustrated in Fig. 7 and the alternative chopped or milled fibers are shown in Fig. 8.

The invention operates in the following manner, with reference to the preferred embodiment shown in Figs. 1, 2 and 3. Hot fluid, preferably air or some other gas, flows into the housing 12 through inlet 14. As the gas flows across and between the fibers in the chamber 22, heat is transferred from the fluid to the fibers. Heat energy flows along the length of the fibers, through the fluid barrier 26, and into the portion of the fibers in the opposite chamber 24. The heat flows in this direction due to the well-known principle of heat flowing from a point of higher temperature to a point of lower temperature. After it has flowed the length of the chamber 22, the hot gas passes out of the outlet 18 at a lower temperature than when it entered.

At the same time the hot gas flows through the chamber 22, the cooler gas flows in the inlet 16 into the chamber 24. As the cooler gas flows across and between the fibers in the chamber 24, the gas molecules impinge upon the outer surface of the fibers, absorbing heat and removing it through the outlet 20. Heat is therefore transferred from the hot gas to the cool gas without any mixture of the gases. The terms "hot" and "cool" do not refer to any particular range of temperatures, but merely indicate a temperature difference between the gases. This relative difference creates a temperature gradient which causes a removal of heat from near one end of the fibers and an addition of heat to near the opposite end of the fibers .

In the preferred embodiment, the majority of the fibers extend continuously longitudinally from the chamber 22 to the chamber 24, and therefore heat is conducted rapidly from the portion of the fibers in the chamber 22 toward the portion in the chamber 24. There is no transition from a fiber to an intermediate plate or wall, and back to a fiber. This continuous fiber flow path permits a significant amount of the heat energy which enters the chamber 22 to be removed from the heated gas and transferred into the cool gas flowing through the chamber 24. This heat transfer occurs without any significant amount of the actual fluid in the chamber 22 communicating with the fluid in the chamber 24, due to the fluid barrier.

The preferred fluid barrier shown in Fig. 3 is advantageous in many respects, but many possible equivalent fluid barriers will be understood by one of ordinary skill in the art. Fig. 5 shows one such equivalent, consisting of a fused layer 50 made up of fibers which are fused together forming the fluid barrier. The fused layer 50 can be formed in a number of ways, including merely clamping the mat 52 and applying heat to the clamped portion. An electrical current can be passed through the portion which is to be fused, causing heating and fusing of the fibers. Additionally, the fused layer 50 can be formed by applying a sealant between each layer of the mat 52 and heating this material until it cures. In addition to variations in the fluid barrier, variations can be made in the manner of forming the heat conductive path on opposite sides of the fluid barrier. Fig. 9 shows a pair of pleated mats 120 and 122 within a housing 124 which are each pleated similarly to the mat 30 of the preferred embodiment. The mats 120 and 122 can be made of chopped or milled fibers sintered into a mat, or can be merely intertwined fibers as in the preferred embodiment . The mats 120 and 122 are bonded to the fluid barrier 125 by a heat conductive bond, such as soldering or a heat conductive adhesive.

Instead of being formed into a mat which is later pleated into layers, the fibers can be randomly stacked in a pair of chambers 126 and 128 forming mats 130 and 132, as shown in Fig. 10. The mats 130 and 132 are bonded to the fluid barrier 134 with a heat conductive bond. The fibers making up the mats 130 and 132 are not bonded to each other fiber, but are merely intertwined together. On the contrary, the fibers making up the mats 140 and 142 shown in Fig. 11, are small chopped or milled fibers which are sintered together to form a porous mass bonded to and separated by a fluid barrier 144. The sintering connects the fibers of the mats 140 and 142 to each other and to the fluid barrier 144 with a heat conducting bond.

The fibers which have been chopped and milled and sintered into a mat can also be arranged similarly to the preferred embodiment. Fig. 12 shows a mat 150 pleated into layers within a housing 152. The housing 152 is divided into chambers 154 and 156. A fluid barrier is formed by spacers 158 clamping the mat 150 between the spacers 158 with a clamp, as in the preferred embodiment. Each layer of the preferred mat 30 is generally planar, and each layer is generally parallel with every other layer. In the preferred embodiment, the flow of fluid is along a fluid flow path perpendicular to the planes containing the layers of the continuous mat 30. As the fluid travels through the chamber 22 or 24, it flows along a path directed generally perpendicularly to the planes of the layers . An alternative to the preferred layer orientation is shown schematically in Fig. 6 in which the planes of the layers are oriented generally parallel to the fluid flow path axis. Fig. 6 is an end view of a housing 99 showing an inlet 100 and outlet 102. In both the preferred embodiment and this alternative embodiment, the fluid flow path is always transverse, and preferably perpendicular to a majority of fibers. The primary difference between the effect of the layer orientation in the preferred embodiment and the alternative embodiment is that in the preferred embodiment, it is less critical how much the layers are compressed against one another, since any small gaps between the layers will have little effect on fluid flow through the layers. However, in the alternative embodiment shown in Fig. 6, the compression of the layers is more critical, since sizeable gaps may be formed between each layer, permitting the fluid to flow through the gaps without having significant contact with fibers.

By making the heat exchanger in accordance with the present invention, a very efficient, inexpensive, and variable heat exchanger is constructed. The preferred heat exchanger is inexpensive since it uses a continuous mat product which can presently be made inexpensively. This mat is then pleated forming layers which are merely compressed together, rather than arranged in particular shapes to be held in place or cut, positioned relative to an existing fluid barrier and adhered to that same barrier.

The heat exchanger is efficient since there is a substantial amount of surface area exposed, which is directly related to efficiency. Additionally, the fibers extend continuously from the hot side to the cold side through the fluid barrier, providing a continuous heat conduction path between the hot side and the cool side. Even when the sintered mat is used, the heat conductive bonds provide a network of interconnected continuous paths. T h e variability of the invention arises from the variability in the dimensions and shapes of the fibers. As the fibers are made to have smaller thickness, their surface area to mass ratio increases and they attain a different cross- sectional shape. The smaller thicknesses are more "D" shaped and the larger thicknesses are more kidney shaped. The smaller thickness fibers function better in the heat exchanger so long as fluid flow is not significantly impeded. Decreasing the thickness of the fibers increases the density for a given fiber mass, which can inhibit the flow of fluid, thereby making the heat exchanger less efficient . Therefore, the variability advantage with the present invention is that a balance between surface area to mass ratio and density can be reached according to the parameters required for each application. And this balance can be reached relatively free of other design constraints such as mat shape and structural rigidity of the mat . Since the preferred mat is merely compacted into the housing, it is of little concern what the rigidity and shape are of the final product. There is very little consideration given to whether the continuous mat can support a force directed onto it without deforming into a shape that inhibits fluid flow. Furthermore, because of the configuration the flow of fluid need not be in the preferred direction, and therefore the flow could be reversed with little or no loss of efficiency.

The thickness of fibers in each application is determined by the requirement of the application. In one experimental heat exchanger, about two-thirds of the fibers ranged from 70 to 98 microns in diameter. A range from 20 microns to 200 microns is also useful. However, the dimensions of the fibers can be varied significantly, from less than 1 micron to more than a thousand microns. The size depends primarily on the needs of the system with which the heat exchanger is to be used.

One measure of size is aspect ratio: the length divided by the effective diameter. Fibers are considered semi-continuous if their aspect ratio is above about 1000. They are "chopped" if their aspect ratio is between 200 and 1000, and "milled" if their aspect ratio is less than 200.

While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications may be adopted without departing from the spirit of the invention or scope of the following claims.

Claims

1. A heat exchanger having a housing containing first and second fluid flow paths, the heat exchanger comprising:

(a) a continuous mat of semi -continuous fibers pleated in layers, each layer connected to an adjacent layer at a fold along adjoining layer ends ; and

(b) a fluid barrier attached to the layers between their adjoining ends and interposed between the flow paths, said fibers extending through the barrier into the flow paths .

2. A heat exchanger in accordance with claim 1, wherein the fluid barrier is transverse to a majority of the fibers in each layer.

3. A heat exchanger in accordance with claim 1, wherein each layer is substantially planar and substantially parallel to every other layer.

4. A heat exchanger in accordance with claim 3 , wherein the layers are transverse to a fluid flow path.

5. A heat exchanger in accordance with claim 3, wherein the layers are substantially parallel to a fluid flow path.

6. A heat exchanger in accordance with claim 1, further comprising multiple spacers, multiple pairs of layers and multiple gaps, one gap between each pair of layers, wherein a spacer is interposed in each gap, and the layers and spacers are compressed against one another, forming said fluid barrier.

7. A heat exchanger in accordance with claim 1, wherein the fluid barrier comprises fibers fused together along a plane extending through the layers.

8. A method of making a heat exchanger having first and second fluid flow paths, the method comprising: (a) pleating an elongated mat of semi- continuous fibers along lateral creases, forming multiple layers, each layer connected to an adjacent layer at a fold along adjoining layer ends; and (b) forming a fluid barrier between the adjoining layer ends and interposed between the flow paths, the fibers extending through the barrier into the flow paths .

9. A method in accordance with claim 8, wherein the step of forming a fluid barrier further comprises positioning one of multiple spacers in each of multiple gaps between each of multiple pairs of layers, and subsequently clamping the layers and the spacers together.

10. A method in accordance with claim 8, wherein the step of forming a fluid barrier further comprises fusing the fibers together along a plane extending through the layers

11. A heat exchanger having a housing containing first and second fluid flow paths, the heat exchanger comprising:

(a) at least one mat having irregularly intertwined, unwoven fibers in at least one of the flow paths; and

(b) a fluid barrier attached to the mat and interposed between the flow paths .

12. A heat exchanger in accordance with claim 11, wherein the fibers of the mat are attached together with a heat conductive bond.

13. A heat exchanger in accordance with claim 12, further comprising first and second mats, wherein the mats are attached to opposing surfaces of the fluid barrier with a heat conductive bond.

14. A heat exchanger in accordance with claim 12, wherein the mat is pleated in layers, each layer connected to an adjacent layer at a fold along adjoining layer ends, and wherein the fluid barrier is attached to the layers between their adjoining ends, said mat extending through the barrier into the flow paths.

15. A heat exchanger in accordance with claim 12, further comprising first and second mats pleated in layers, each layer connected to an adjacent layer at a fold along adjoining layer ends, the first mat in the first flow path, the second mat in the second flow path, and the first and second mats attached to opposing surfaces of the fluid barrier with a heat conductive bond.

16. A heat exchanger in accordance with claim 11, further comprising at least one continuous mat of semi -continuous fibers.

17. A heat exchanger in accordance with claim 16, further comprising first and second mats, wherein the mats are attached to opposing surfaces of the fluid barrier with a heat conductive bond.

18. A heat exchanger in accordance with claim 16, wherein the mat is a continuous mat of semi- continuous fibers pleated in layers, each layer connected to an adjacent layer at a fold along adjoining layer ends, and wherein the fluid barrier is attached to the layers between their adjoining ends, said fibers extending through the barrier into the flow paths .

19. A heat exchanger in accordance with claim 16, further comprising first and second continuous mats of semi-continuous fibers pleated in layers, each layer connected to an adjacent layer at a fold along adjoining layer ends, the first mat in the first flow path, the second mat in the second flow path, and the first and second mats attached to opposing surfaces of the fluid barrier with a heat conductive bond.