US20090114380A1

US20090114380A1 - Spiral flat-tube heat exchanger

Info

Publication number: US20090114380A1
Application number: US12/302,239
Authority: US
Inventors: Michel Grabon
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2006-05-23
Filing date: 2006-05-23
Publication date: 2009-05-07
Also published as: CN101490494A; EP2029952A4; WO2007136379A1; EP2029952A1

Abstract

A spiral flat tube heat exchanger comprising a first flat tube and a second flat tube thermally coupled to the first flat tube, the first flat tube and the second flat tube forming substantially concentric spirals. A first fluid flows through the first flat tube in a first flow direction, and second fluid flows through the second flat tube in a second flow direction. The flat tubes preferably contain microchannels to increase heat transfer efficiency.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention generally relates to heat exchangers. More particularly, the present invention relates to a counterflow heat exchanger having flat tubes configured to form concentric spirals. The flat tubes may include interior microchannels.
2. Description of the Related Art
Counterflow heat exchangers for gases and liquids are known, and typically include an inlet and an outlet for each of two or more fluids. The fluids generally do not mix and follow separate flow paths within the heat exchanger. The most common types of counterflow heat exchangers include flat plate, tube in tube and shell and tube heat exchangers. Shell and tube heat exchangers generally have a series of small tubes within a larger pressure vessel or shell. Heat is transferred between a fluid flowing within the small tubes and another fluid flowing within the shell.
In a flat plate heat exchanger, heat is transferred between two fluids flowing in opposite directions along separate flow paths created between a series of parallel plates. The plates are typically made of stainless steel and may contain chevron-shaped channels to increase heat transfer efficiency. The plates may be sealed at their joints by using elastomer gaskets or by a brazing process. The large surface area of the plates provides for greater heat transfer efficiency when compared to a shell and tube design.
Flat plate heat exchangers are usually lighter and more compact than shell and tube heat exchangers. However, to achieve the higher heat transfer efficiency, the fluid flowing through a flat plate heat exchanger must be in contact with the plates for a sufficient amount of time, meaning that the plates have to be of a certain length. For chiller applications, (evaporation of refrigerant in one side of the wall and cooling of the water in the other side) flat plate heat exchangers must be installed vertically to facilitate refrigerant distribution in each adjacent plate. This configuration results in a relatively large height-to-footprint ratio. This height-to-footprint constraint limits the applications for flat plate heat exchangers. The stainless steel typically used in flat plate heat exchangers is also relatively heavy, and has a relatively high thermal resistance and a low heat transfer coefficient, which also limits its performance.
Microchannel tubes are also known, and have been commonly used in heat exchangers for automotive applications. However, this technology has been developed primarily for air-to-liquid heat exchangers.
Accordingly, there is a need for a heat exchanger that applies microchannel flat tube technology to a liquid to liquid or liquid to two-phase refrigerant heat exchanger while simultaneously overcoming the inherent limitations of shell and tube and flat plate heat exchangers by providing a compact, flexible and efficient heat exchanger design.

BRIEF SUMMARY OF THE INVENTION

In an exemplary embodiment of the present invention, there is a spiral flat tube heat exchanger comprising a first flat tube and a second flat tube thermally coupled to the first flat tube. The first flat tube forms a first spiral and the second flat tube forms a second spiral, the first and second spirals being substantially concentric. A first fluid flows through the first spiral in a first flow direction, and a second fluid flows through the second spiral in a second flow direction. The flat tubes preferably contain microchannels and are preferably made of aluminium.
In another exemplary embodiment of the present invention, a heat exchanger having a plurality of modules is provided. Each module comprises at least two collectors and a plurality of microchannel flat tubes. The microchannel flat tubes are positioned vertically between the collectors so that a longitudinal axis of each of the microchannel flat tubes is substantially collinear with the longitudinal axis of each of the other microchannel flat tubes. The plurality of modules wrap around each other in a spiral configuration so that the plurality of microchannel flat tubes are in thermal contact.
A method of making a spiral flat tube heat exchanger of the present invention is also provided. The method comprises providing a first module having a first inlet collector connected to a first outlet collector by a first flat tube and providing a second module having a second inlet collector connected to a second outlet collector by a second flat tube. The first module is then positioned adjacent to the second module with first inlet collector in proximity to second outlet collector and first outlet collector in proximity to second inlet collector. The first outlet collector and the second inlet collector are then rotated together in the same direction while holding the first inlet collector and second outlet collector in a fixed position so as to deform the first flat tube and second flat tube to form two substantially concentric spirals.
The present invention thus provides an extremely efficient counter flow heat exchanger that provides greater flexibility in terms of physical dimensions as well as control of desired heat transfer properties. These features will allow the present invention to find use in a wide variety of applications. These and other features of the present invention will be appreciated and understood from the following drawings, detailed description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of the spiral flat tube heat exchanger of the present invention;

FIG. 2 is a top view of a module of the heat exchanger of FIG. 1;

FIG. 3 is a front view of the module of FIG. 2;

FIG. 4 is a cross-sectional view of the heat exchanger module of FIG. 3 taken along the line IV-IV;

FIG. 5 is a cross-sectional view of a flat tube of the heat exchanger of FIGS. 1 through 4;

FIGS. 6 through 8 illustrate a method of making the heat exchanger shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and in particular to FIG. 1, an exemplary embodiment of a spiral flat-tube heat exchanger is illustrated.
Heat exchanger 100 includes a first tube 102 and a second tube 104. First tube 102 is thermally coupled to second tube 102, and the tubes are configured to form alternating spirals that are preferably substantially concentric. Substantially concentric means that a first spiral 106 formed by first tube 102 and a second spiral 108 formed by second tube 104 share a common central area 110. As used in this disclosure, the term spiral means an object wound around a fixed central area at a continuously increasing distance from the area. A first working fluid 112 flows through first tube 102 and a second working fluid 114 flows through second tube 104.
First tube 102 and second tube 104 are in thermal contact, so that heat may be transferred between the first working fluid 112 and the second working fluid 114. In a preferred embodiment, heat exchanger 100 is a counterflow heat exchanger, meaning that the flow direction of first working fluid 112 is opposite the flow direction s of second working fluid 114. Generally, counterflow heat exchangers transfer heat more efficiently than cross flow or parallel flow heat exchangers. In the exemplary embodiment shown in FIG. 1, first working fluid 112 enters heat exchanger 100 through first inlet collector 116, located in central area 110. First working fluid 102 then flows in a clockwise direction through first spiral 106, and exits heat exchanger 100 through a first outlet collector 118, disposed near a periphery 124 of the heat exchanger. In contrast, second working fluid 114 enters heat exchanger 100 near periphery 124 through a second inlet collector 120. Second working fluid 104 then flows in a counterclockwise direction through second spiral 108, and exits heat exchanger 100 through a second outlet collector 122 located in central area 110.
Alternatively, the spiral heat exchanger of the present invention may be a parallel flow heat exchanger, in which two working fluids each flow in the same direction, either clockwise or counterclockwise, within separate spirals. In this embodiment, the working fluids could enter at the periphery of the heat exchanger and exit at the center of the heat exchanger or enter at the center and exit at the periphery.
Heat is transferred between the walls of first tube 102 and the walls of second tube 104 by conduction. For example, first working fluid 112 may be a cooler fluid than second working fluid 114. In this case, heat would be transferred from first working fluid 112 to the walls of first tube 102, and then transferred by conduction to the walls of second tube 104 and finally transferred to second working fluid 114. Advantageously, the spiral configuration of heat exchanger 100 provides a large surface area for heat transfer, providing greater heat transfer efficiency while requiring less space than prior art heat exchangers. Also, the spiral configuration of heat exchanger 100 provides for nearly complete counterflow within the heat exchanger, thus increasing heat transfer efficiency.
Working fluids 112, 114 may be a liquid, a gas, or a two-phase medium. Preferably, one of the working fluids 112,114 is water, and the other fluid is a refrigerant, such as, but not limited to, R134a, R404A, R410A, CO₂, propane, ammonia, isobutene, and other commonly used refrigerants. Most preferably, working fluids 112,114 are water and ethylene glycol.
In the embodiment shown in FIG. 1, first inlet collector 116, first tube 102 and first outlet collector 118 form a first module 126, while second inlet collector 120, second tube 104 and second outlet collector 122 form a second module 128.
Referring now to FIGS. 2 through 5, details of first module 126 are shown. It should be understood that second module 128 will have a similar structure. For clarity, first module 126 is shown in an unwound state, before being assembled with second module 128 into the spiral configuration of heat exchanger 100.
In one exemplary embodiment, first tube 102 comprises a plurality of tubes. The embodiment shown in FIG. 3 has five tubes 102 arranged vertically between first inlet collector 116 and first outlet collector 118. Of course, any number of tubes 102 may be used, including a single large tube.
First inlet collector 116 is in fluid communication with first outlet collector 118 by way of flat tubes 102. An inlet 128 is in fluid communication with first inlet collector 116, and an outlet 130 is in fluid communication with first outlet collector 118. In operation, first working fluid 112 enters module 126 through inlet 128. After passing through inlet 128 into first inlet collector 116, first working fluid 112 travels through the plurality of flat tubes 102 into first outlet collector 118. Finally, the fluid flows out of first outlet collector 118 through outlet 130. In an application where refrigerant distribution has an impact on overall heat transfer behavior (refrigerant boiling), specific inserts may be applied inside inlet collector 116 to prevent refrigerant maldistribution among the plurality of flat tubes 102 operating in parallel.
First tube 102 may be of any cross-sectional shape that will allow heat to be transferred between adjacent tubes. First tubes 102 are preferably flat tubes, that is tubes having substantially flat walls to aid in heat transfer. Preferably, the cross-sectional shape of first tube 102 is obround, that is a shape having two substantially semicircular ends connected by two substantially parallel lines. An obround cross section of first tube 102 is illustrated in FIG. 5. First tube 102 has a top semi-circular section 132, a bottom semi-circular section 134 and substantially parallel flat walls 136. First tube 102 has a height, h and a width w, with the height preferably being greater than the width. Flat walls 136 provide the primary heat transfer surfaces between first tube 102 and the adjacent second tube 104 when first module 126 and second module 128 are wrapped around central area 110 to form heat exchanger 100. First tubes 102 may be manufactured using an extrusion process, a welding process, or any other process capable of producing a suitable cross-sectional shape. The internal shape of tubes 102 may be specifically adapted to accommodate various fluid and operation types to maximize the heat transfer characteristics of tubes 102. For example, tubes 102 may be of a distinct shape when used for one phase cooling, refrigerant boiling, and condensation, respectively.
The quality of thermal contact between flat walls 136 of adjacent tubes is a major factor in the efficiency of heat transfer in heat exchanger 100. Flat walls 136 of adjacent tubes should be as close together as possible. However, dry contact may not provide sufficient thermal contact, since any air remaining between adjacent flat walls will act as an insulator, thus decreasing the heat transfer efficiency. To overcome this problem, the preferred embodiment of the present invention uses metal tubes that are brazed together. The brazing process eliminates air gaps, resulting in low thermal resistance between adjacent tubes. Advantageously, the brazing process is also relatively easy to control. Alternatively, the quality of thermal contact between adjacent tubes may be increased by using a high conductivity grease, a glue, or by means of a mechanical bonding process.
First tubes 102 are oriented vertically between first outlet collector 116 and first inlet collector 118, so that a longitudinal axis y of each of the first tubes 102 is substantially collinear with the longitudinal axis of the other first tubes 102.
Tubes 102 are preferably microchannel flat tubes. Microchannel flat tubes are flat tubes containing small interior channels. The interior microchannels typically have channel widths between 10 and 1,000 μm. These channels increase both the rate of heat transfer and heat transfer efficiency by creating turbulent flow. The use of microchannel flat tubes allows for optimization of the heat transfer coefficient and of the pressure drop within the tubes. Tubes 102 have interior microchannels 140, as illustrated in FIG. 5. Microchannels 140 may be of any size and shape
Advantageously, the present invention allows microchannel flat tubes to be used in a liquid to liquid or liquid to two-phase medium heat exchanger. Previously, microchannel tubes had been used principally for air to liquid heat exchangers. In existing microchannel heat exchangers, a working fluid flows through horizontally oriented microchannel flat tubes and transfers heat to air flowing across the tubes in a cross flow configuration, meaning that the air flows perpendicular to the flow direction of the working fluid. In the prior art, the microchannel tubes were typically connected to a plurality of fins installed horizontally between the flat tubes. In contrast, the present invention uses flat microchannel tubes 102 arranged vertically between two collectors, without fins. Heat is transferred from a first working fluid to a second working fluid in a counterflow configuration. The present invention thus applies microchannel technology in a new way to produce a liquid to liquid heat exchanger having exceptional heat transfer efficiency.
Flat tubes 102 are preferably made out of aluminum. Aluminum provides several advantages over traditional steel or copper tubes that have been used in prior-art heat exchangers. Aluminum has a lower thermal resistance, is lighter and more easily deformable than steel or copper, and demonstrates good resistance to corrosion. Alternatively, flat tubes 102 could be made of other metals or a plastic material.
Referring now to FIGS. 6 through 8, a method of making the spiral heat exchanger 100 as shown in FIG. 1 is illustrated. First module 126 is positioned adjacent to second module 128 with first inlet collector 116 in proximity to second outlet collector 122, and first outlet collector 118 in proximity to second inlet collector 120. To compensate for the relative path length difference between the two spirals, the length of second module 128 may be greater than the length of first module 126. First inlet collector 116 and second outlet collector 122 are held in a fixed position, while first outlet collector 118 and second inlet collector 120 are rotated together in the same direction so as to deform the first tube 102 and second tube 104 and form two substantially concentric spirals, 106 and 108. To accomplish the rotation of first tube 102 and second tube 104, a rolling mechanism 138 may be used to apply force to the tubes. The arrows shown in FIGS. 6 through 8 illustrate the direction of movement of the rolling mechanism 138 and the collectors 118, 120 as heat exchanger 100 is being formed. First tube 102 and second tube 104 may be deformed by rolling, using rolling mechanism 138. Concentric spirals 106, 108 will share a common central area 110, with first inlet collector 116 and second outlet collector 122 being disposed in central region 110, and first outlet collector 118 and second inlet collector 120 being disposed near periphery 124 of the spiral heat exchanger 100. The spiral heat exchanger 100 resulting from this method is shown in FIG. 1.
Although the embodiment shown in FIG. 1 uses two modules, a spiral heat exchanger having any number of modules is contemplated by the present disclosure. For example, the spiral heat exchanger may have three modules, comprising two refrigerant circuits and one water circuit, possibly for use in a two circuit chiller. Another possibility includes a spiral heat exchanger having either three or four modules, with two water circuits and one or two refrigeration circuits, which would allow for a heat recovery option in a double bundle condenser.
In one preferred embodiment, first tubes 102 of heat exchanger 100 have different structures than adjacent second tubes 104, based on the working fluid being carried through the tube. For example, if first tubes 102 were carrying water and second tubes 104 were carrying refrigerant in boiling or condensation, the size and shape of the tube, as well as the size, shape, and number of microchannels within the tubes could be varied to compensate for the different heat transfer coefficients and pressure drops on the water side and on the refrigerant side.
Advantageously, the present invention allows for a flexible design of a heat exchanger, both in terms of physical dimensions and desired performance. By selecting the number of tubes, corresponding to the overall height of the heat exchanger, and by selecting the tube length between collectors, corresponding to the overall diameter, critical properties such as fluid capacity and pressure drop within the device can be carefully controlled. The spiral configuration of the inventive heat exchanger allows for a very compact design with flexibility in the selection of the height to footprint ratio. This flexibility makes the inventive heat exchanger suitable for a greater number of applications when compared to previous heat exchanger designs.
It should be understood by one of ordinary skill in the art that the exemplary embodiments of the heat exchangers described herein can be utilized with various heat transfer systems that utilize heat exchangers. The particular components of the heat transfer systems can be chosen by one of ordinary skill in the art to increase the thermal efficiency of the system.
The terms “first,” “second,” and the like may be used herein to modify various elements. These modifiers do not imply spatial, sequential, or hierarchical order to the modified elements unless specifically stated.
While the present invention has been described with reference to on or more exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the scope thereof. It is intended that the present disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated, but that the disclosure will include all embodiments falling within the scope of the appended claims.

Claims

1. A heat exchanger comprising:

a first flat tube; and

a second flat tube thermally coupled to the first flat tube,

wherein the first flat tube forms a first spiral and the second flat tube forms a second spiral, the first spiral and the second spiral being substantially concentric, and

wherein a first fluid flows through the first spiral in a first flow direction and a second fluid flows through the second spiral in a second flow direction.

2. The heat exchanger of claim 1, wherein the first flat tube and the second flat tube comprise flat tubes having microchannels.

3. The heat exchanger of claim 1, wherein the first flat tube and the second flat tube each have an obround cross section.

4. The heat exchanger of claim 1, wherein the first fluid is water and the second fluid is a refrigerant.

5. The heat exchanger of claim 1, wherein the first flow direction is opposite the second flow direction.

6. The heat exchanger of claim 1, wherein the first flow direction is substantially parallel with the second flow direction.

7. The heat exchanger of claim 1, further comprising a plurality of first collectors in fluid communication with the first flat tube and a plurality of second collectors in fluid communication with the second flat tube.

8. The heat exchanger of claim 7, wherein at least one of the plurality of first collectors and at least one of the plurality of second collectors is disposed substantially near a central area of the first and second spirals.

9. The heat exchanger of claim 8, wherein at least one of the plurality of first collectors and at least one of the plurality of second collectors is disposed on a periphery of the heat exchanger.

10. The heat exchanger of claim 1, wherein the first flat tube contacts the second flat tube along substantially an entire length of the first flat tube.

11. The heat exchanger of claim 1, wherein the first flat tube and second flat tube comprise a material selected from the group consisting of aluminum, copper, plastic, and any combination thereof.

12. The heat exchanger of claim 1, wherein the first flat tube and the second flat tube are joined by a brazing process.

13. The heat exchanger of claim 1, wherein the first flat tube and the second flat tube brought into thermal contact using a process selected from the group consisting of gluing, applying a conductive grease, mechanical bonding, and any combination thereof.

14. The heat exchanger of claim 1, wherein the first flat tube comprises a plurality of flat tubes.

15. The heat exchanger of claim 13, wherein the second flat tube comprises a plurality of flat tubes.

16. A heat exchanger having a plurality of modules, each module comprising:

at least two collectors; and

a plurality of microchannel flat tubes positioned vertically between the at least two collectors and each having a longitudinal axis, wherein the longitudinal axis of each of the plurality of microchannel flat tubes is substantially collinear with the longitudinal axis of each of the other microchannel flat tubes.

17. The heat exchanger of claim 15, wherein the plurality of microchannel flat tubes are in thermal contact with each other.

18. The heat exchanger of claim 15, wherein the plurality of modules wrap around each other in a spiral configuration.

19. A method of making a spiral heat exchanger, the method comprising:

providing a first module having a first inlet collector connected to a first outlet collector by a first flat tube;

providing a second module having a second inlet collector connected to a second outlet collector by a second flat tube;

positioning the first module adjacent to the second module with the first inlet collector in proximity to the second outlet collector and the first outlet collector in proximity to the second inlet collector; and

rotating the first outlet collector and the second inlet collector together in the same direction while holding the first inlet collector and the second outlet collector in a fixed position so as to deform the first flat tube and second flat tube to form two substantially concentric spirals.

20. The method of claim 18, further comprising the step of brazing the entire heat exchanger.

21. (canceled)