Heat exchanger and method for producing matrix for recuperative heat exchanger

Abstract

According to one exemplary embodiment, a matrix (9) for a heat exchanger (1) is provided, the matrix (9) being made of a heat absorbing material and having a plurality of fluid flow passages (10) extending through the matrix (9) in a direction of fluid flow, the flow passages (10) including flow passage sections (11), each of which is continuous in the direction of fluid flow and at least partially staggered with respect to one another.

Description

[Technical Field]

[0001] The present invention relates to a heat exchanger. The present invention also relates to a method for manufacturing a matrix of a heat exchanger. [Background technology]

[0002] Approximately 30% of the energy delivered to a building is dissipated in the ventilation and leakage air streams. In buildings built to very high insulation standards, the proportion of airborne energy losses can be very high. Ventilation energy losses cause costs at the consumer level and impacts on primary energy demand and greenhouse gas emissions.

[0003] Energy consumption depends on the ventilation flow rate and the amount of air conditioning required to achieve thermal comfort. Additional energy is required to drive the mechanical ventilation system, to cool the air by refrigeration or evaporation, and to maintain adequate humidity.

[0004] Heat losses caused by ventilation can be reduced by heat exchangers placed in the intake and exhaust streams of the ventilation system. Heat is transferred from the exhaust stream to the intake stream in the heat exchangers. There are several types of heat exchangers that can be used for air-to-air heat exchange, including plate heat exchangers, run-around coil heat exchangers, and regenerative heat exchangers.

[0005] The most efficient types of heat exchangers for heating, ventilation, and air conditioning (HVAC) applications are regenerative heat exchangers, such as rotary heat exchangers and fixed matrix counterflow heat exchangers. In regenerative heat exchangers, heat from a warm fluid is intermittently stored in a heat storage medium and then transferred to a cold fluid. The heat storage medium is typically a matrix made of a heat-absorbing material.

[0006] A rotary heat exchanger includes a rotating wheel with a matrix of heat-absorbing material. The matrix has small fluid passages extending axially through the wheel. The wheel slowly rotates within the intake and exhaust streams of the system. As the wheel rotates, heat is extracted from the exhaust stream during one half revolution and transferred to the intake stream during the other half revolution. Thermal energy is transferred from the exhaust stream to the matrix material and from the matrix material to the intake air stream, increasing the temperature of the intake air stream by an amount proportional to the temperature difference between the fluid streams, the surface area available for heat transfer, and the heat transfer coefficient. Heat exchange is most efficient when the intake and exhaust streams flow in opposite directions. A fixed matrix counterflow heat exchanger includes at least one fixed matrix through which the intake and exhaust streams alternate. Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to increase the efficiency of a heat exchanger, particularly a regenerative or recuperative heat exchanger, by increasing the heat transfer within the matrix of the heat exchanger. [Means for solving the problem]

[0008] According to one aspect of the present disclosure, there is provided a recuperative heat exchanger featuring a matrix made of a heat absorbing material and having a plurality of fluid flow passages extending therethrough in a direction of fluid flow, the passages including passage sections, each passage section being continuous in the direction of fluid flow and at least partially staggered relative to one another.

[0009] According to another aspect of the present disclosure, there is provided a method for producing such a matrix using additive manufacturing techniques.

[0010] The invention is defined by the accompanying independent claims.

[0011] Some embodiments may include one or more features from the following itemized list. - consecutive channel sections of a channel are staggered in the transverse direction of said channel; - Successive channel sections are staggered relative to one another in two directions, which are transverse to one another and which are both transverse to the flow direction of the channel. - the flow channel comprises two or more, preferably three or more consecutive flow channel sections; - Successive channel sections are staggered such that the edge of the outlet opening of a channel section intersects with the inlet opening of the subsequent channel section. The edge of the outlet opening of a flow section intersects with the center of the inlet opening of the subsequent flow section. Every other channel section in the fluid flow direction is located at the same transverse position. - consecutive channel sections are staggered such that the outlet opening of a channel section is in direct fluid flow communication with the inlet opening of at least two subsequent channel sections; The cross-sectional shape of the flow channel is rectangular, triangular, polygonal or sinusoidal. - The matrix is ​​structured as a rotating wheel. The heat exchanger is a fixed matrix regenerative heat exchanger, which comprises at least one such fixed matrix and guide members for guiding alternate hot and cold fluid flows through the matrix. The heat exchanger is a rotary heat exchanger, which comprises a casing in which the matrix is ​​configured as a rotating wheel mounted on the casing, and a drive unit for rotating the rotating wheel about an axis of rotation.

[0012] Significant benefits can be achieved with the present invention.

[0013] The matrix according to the present invention comprises several successive channel sections in the direction of fluid flow. The successive channel sections are staggered relative to one another across the channel (typically both horizontally and vertically). In this way, air flowing through the matrix encounters a new initiation and development of the boundary layer that covers the entire periphery of each channel section. In the inlet region of the channel section, the thermal conductivity is significantly higher than in the fully developed region. The length of the staggered channel sections is selected so that the flow does not reach fully developed state, resulting in increased heat transfer.

[0014] The increased efficiency allows for the size of heat exchangers to be reduced, thereby facilitating their use in new applications. Although staggering flow sections may slightly increase the pressure drop of gases flowing through the heat exchanger, the amount of energy required to cover that pressure drop is significantly less than the energy savings achieved.

[0015] In conventional heat exchangers, air flows in continuous channels within the matrix, and the periphery of the matrix does not adequately participate in heat recovery and transfer in any application. In the present invention, the air flow within the matrix is ​​offset transversely to the channels at regular intervals, making the entire wetted periphery of the matrix available for heat transfer.

[0016] Additionally, the staggered channel sections create discontinuities that prevent or at least reduce heat transfer in the direction of fluid flow within the matrix, further increasing the thermal efficiency of the heat exchanger.

[0017] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0018] [Figure 1a] 1 shows a schematic perspective view of an exemplary rotary heat exchanger, partially sectioned; [Figure 1b] 1b shows an enlarged view of the rotary heat exchanger matrix of FIG. 1a. [Figure 2a] 1 shows a schematic perspective view of a conventional matrix with straight channels. [Figure 2b] 2b shows an enlarged top elevation view of the matrix of FIG. 2a. [Figure 3a] FIG. 1a shows a perspective view of a matrix that can be used in a heat exchanger according to one embodiment of the present invention. [Figure 3b] 3b shows an enlarged top elevation view of the matrix of FIG. 3a. [Figure 4a] 1 shows a schematic perspective view of a matrix according to another embodiment of the invention having continuous channel compartments. [Figure 4b] 4b shows an enlarged top elevation view of the matrix of FIG. 4a. [Figure 5] 1 shows an elevated perspective view of a matrix according to one embodiment. [Figure 6] FIG. 6 shows an isometric detail view of one channel of the matrix of FIG. 5. [Figure 7] 1 shows a diagram illustrating measured and theoretically predicted results for the relationship between air flow rate and efficiency achieved by a conventional heat exchanger and a heat exchanger according to an embodiment of the present invention. [Figure 8] The diagram in Figure 7 shows the relevant values ​​as dashed lines. [Figure 9] 1 shows test results and a diagram depicting the relationship between air flow rate and the corresponding pressure drop across a matrix of test heat exchangers. DETAILED DESCRIPTION OF THE INVENTION

[0019] FIG. 1 illustrates a rotary heat exchanger 1 that can be placed in the supply air and exhaust air streams of a ventilation system or industrial process to recover thermal energy. The rotary heat exchanger 1 includes a casing 2 and a rotating wheel 3 mounted on the casing 2. The rotating wheel 3 is configured to rotate about an axis of rotation 4. As the rotating wheel 3 rotates, it transfers heat and / or moisture from the exhaust air to the supply air or from the supply air to the exhaust air. However, it should be noted that the principles disclosed herein with respect to the application of a rotating wheel are applicable to counterflow or crossflow matrices having separate flow paths for different streams for recuperation.

[0020] The rotary heat exchanger 1 further includes a drive unit 6 that rotates the rotating wheel 3 about the rotation axis 4. The drive unit includes a drive motor 5 and a belt drive system. The drive motor may be an electric motor, and may typically be a constant speed motor or a variable speed motor. The belt drive system includes a pulley 7 connected to the output of the drive motor 5. The belt drive system further includes a drive belt 8 that is wrapped around the rotating wheel 3 and the pulley 7. The drive motor 5 rotates the pulley 6, and the rotational motion is transmitted to the rotating wheel 3 by the drive belt 8.

[0021] The rotating wheel 3 comprises a matrix 9 of a heat-absorbing material (e.g., aluminum, plastic, or synthetic fiber). As can be seen in FIG. 2, the matrix 9 has a plurality of flow channels 10 for a fluid (e.g., air or other gas). The flow channels 10 extend through the matrix 9 in the direction of fluid flow. The flow channels 10 are parallel to one another. The flow channels 10 extend parallel to the rotation axis 4 of the rotating wheel 3. The cross-sectional shape of the flow channels 10 may be square, rectangular, triangular, polygonal, or sinusoidal. The height and / or width of the flow channels 10 are typically 1 to 3 mm. The depth of the rotating wheel 3 is 150 to 250 mm, typically 200 mm.

[0022] In use, the rotating wheel 3 rotates within the supply air stream and the exhaust air stream (e.g., the air / gas stream of a heat recovery system). As the rotating wheel 3 rotates, heat is picked up from the exhaust air stream during one half rotation and added to the intake air stream during the other half rotation. In this way, waste thermal energy and / or moisture from the exhaust air stream is transferred to the matrix 9 of the rotating wheel 3 and then transferred from the matrix 9 to the supply air stream, or vice versa. The supply air stream and the exhaust air stream flow through the matrix 9 in opposite directions. Figure 1 shows an exemplary rotary heat exchanger 1 fitted into a building to recover heat from air being exhausted from the building. Thus, outside air 21 enters the rotating wheel 3 from the environment side and is heated by the warm matrix 9. The warmed outside air exits the building side as supply air 22. Sufficiently warm return air 23 enters the matrix 9 from the building side and heats it. This air transfers heat into the matrix by passing through it and exits the environment side as exhaust air 24.

[0023] Exemplary structures of the matrix 9 and channels 10 are shown in more detail in Figures 3a and 3b. Channels 10 include channel sections 11, each continuous in the direction of fluid flow and staggered or at least partially staggered relative to one another. In comparison, the conventional matrix 9 shown in Figures 2a and 2b shows straight, non-staggered channels 10. Figure 3a shows a layered matrix with ten channel sections 11.1-11.10 that are overlapping and sequentially translated with one another. Successive channel sections 11.1...11.10 are staggered (i.e., offset) relative to the preceding channel section 11.1...11.10 in the transverse direction of the channel 10. The transverse direction of a channel 10 is perpendicular to the direction of fluid flow within that channel 10. The transverse direction of a channel 10 is perpendicular to the rotation axis 4 of the rotating wheel 3 (the embodiment of Figure 1). The exemplary channels shown have a hexagonal cross section.

[0024] Typically, consecutive channel sections 11 are staggered relative to one another in two directions that are transverse to one another and that are both transverse to the flow direction of the channel 10. The two transverse directions are typically perpendicular to one another or within 20° of being perpendicular to one another.

[0025] The channel 10 includes at least three consecutive channel sections 11. The number of consecutive channel sections 11 depends on the overall length of the channel 10, the length of the developing flow in the channel 10, and the tolerable pressure drop of the flow across the matrix 9, which in turn depend on the geometry of the channel 10, the fluid flow rate, and the thermodynamic properties of the fluid. The length of the channel sections 11 is selected so that the fluid flow in the channel sections 11 does not reach a fully developed state. Each consecutive channel section 11 has the same cross-sectional shape and / or size.

[0026] 4a and 4b illustrate the principle of staggering channel sections 11 according to one embodiment of the present invention. The hexagonal cross-section shown in FIGS. 3a and 3b has been replaced with a square cross-section. These figures disclose a channel 10 including three channel sections 11.1, 11.2, and 11.3 that are consecutive in the fluid flow direction. The first channel section 11.1 is at least partially staggered (i.e., stepped) relative to the second channel section 11.2 in the transverse direction of the channel 10. Similarly, the second channel section 11.2 is staggered relative to the third channel section 11.3 in the transverse direction of the channel 10. The third channel section 11.3 is at the same transverse position as the first channel section 11.1. If an additional channel section 11 is disposed in the channel 10 after the third channel section 11.3 or before the first channel section 11.1, the additional channel section will be at the same transverse position as the second channel section 11.2, etc. Therefore, every other channel section 11 in the fluid flow direction is located at the same transverse position.

[0027] Consecutive flow path sections 11 are staggered such that the leading edge 12 of the outlet opening 14 of a flow path section 11 intersects with the inlet opening 13 of the subsequent flow path section 11, and vice versa. Typically, the leading edge 12 of the outlet opening 14 of a flow path section 11 intersects with the center of the inlet opening 13 of the subsequent flow path section 11, and / or vice versa. Consecutive flow path sections 11 are staggered such that the outlet opening 14 of a flow path section 11 is in direct fluid flow communication with the inlet openings 13 of at least two subsequent flow path sections 11. Typically, the outlet opening 14 of a flow path section 11 is in direct fluid flow communication with the inlet openings 13 of three or four subsequent flow path sections 11.

[0028] The matrix 9 is manufactured using additive manufacturing techniques (e.g., 3D printing).

[0029] 5 and 6 show a stack of the staggered matrix designs shown in FIGS. 4a and 4b. The matrix includes multiple flow channels 10A, 10B, and 10C in an adjacent or stacked configuration. In the illustrated example, the matrix includes three such flow channels 10A, 10B, and 10C. The purpose of each flow channel may be different. For example, every other flow channel 10A, 10C is intended to receive ambient air 21, and every other flow channel 10B is intended to expel exhaust air 24. That is, air flows in opposite directions between adjacent flow channels, allowing heat exchange between the flows. Alternatively, the direction of air flow may be repeatedly reversed to store heat and extract heat absorbed by the matrix.

[0030] Each flow channel 10A, 10B, 10C has several (specifically, four) channel sections 11.1, 11.2, 11.3, 11.4 arranged consecutively in the fluid flow direction. Similar to the embodiment of Figures 4a and 4b, consecutive channel sections 11.1, 11.2, 11.3, 11.4 are staggered (i.e., offset transversely to the air flow direction). Consecutive channel sections 11.1, 11.2, 11.3, 11.4 also differ in shape and size. In the example of Figure 4, every other channel section 11.1, 11.3 is relatively low, and every other channel section 11.2, 11.4 is relatively high. Unlike the embodiment of Figures 4a and 4b, channel sections 11.1, 11.2, 11.3, 11.4 are constructed as open contours rather than closed perimeter contours. The flow sections may be made from strips of material such as aluminium and folded into square corrugations, with alternating straight or curved parts 15, 16 joined in succession over 180 degrees in alternating directions, the folding axis extending in the flow direction. The flow sections 11.1, 11.2, 11.3, 11.4 are attached to and between partitions 17, which may be in the form of simple plates (e.g. aluminium plates).

[0031] The embodiment shown in Figures 5 and 6 can be modified by changing the shape of the channel sections 11.1, 11.2, 11.3, 11.4, for example, to a sinusoidal, triangular, or sawtooth shape. Additionally or alternatively, the difference in shape or size between consecutive channel sections 11.1, 11.2, 11.3, 11.4 can vary. Additionally or alternatively, the number of consecutive channel sections 11.1, 11.2, 11.3, 11.4 can vary. Additionally or alternatively, the number of adjacent channels 10A, 10B, 10C can vary.

[0032] The matrices proposed herein and illustrated in Figures 1-5 may be made of a heat-absorbing material (e.g., aluminum, plastic, or synthetic fibers), which is highly advantageous in rotary or cross-flow heat exchangers. Alternatively, the matrix material may be a thermally conductive material, which is advantageous in return-flow heat exchanger applications.

[0033] According to one embodiment, the proposed matrix may be applied to a heat exchanger with reciprocal airflow. Thus, a regenerative heat exchanger may comprise a fixed matrix and guide members for guiding alternate hot and cold fluid flows through the matrix.

[0034] The staggered layers of flow channels 10 greatly enhance the efficiency of the matrix 9 in transferring heat between two fluid streams associated with a heat exchanger containing such a matrix. Figure 7 shows a plot of airflow rate (liters per second) on the horizontal axis versus heat exchanger efficiency on the vertical axis. The figure shows four data sets. The first data set is theoretical values ​​calculated for a conventional test matrix having a cylindrical shell 102 mm in diameter and 100 mm deep, enclosing straight (i.e., non-staggered) hexagonal flow channels, each with an edge length of 0.75 mm and a diagonal length of 4 mm. The theoretical values ​​are plotted on the lower line extending between 0.8 on the efficiency scale and approximately 3 l / s on the airflow scale, and approximately 0.45 on the efficiency scale and approximately 11 l / s on the airflow scale. The measured data points for the test matrix are shown as the second data set, represented by circles, and appear to closely match the theoretical model, represented by triangles.

[0035] The third data set is theoretical values ​​calculated for a novel test matrix with a cylindrical shell, 102 mm in diameter and 100 mm deep, enclosing hexagonal channels, each with an edge length of 0.75 mm and a diagonal length of 4 mm. The test matrix has 10 layers of channel sections, each 10 mm long. Each successive layer translates by half the diameter of the channel opening (i.e., the largest dimension across the opening). In this example, the translation relative to the previous layer is 2 mm. The theoretical values ​​are plotted on the lower line extending between 0.92 on the efficiency scale and approximately 3 l / s on the airflow scale, and approximately 0.72 on the efficiency scale and approximately 11 l / s on the airflow scale. The measured data points for the test matrix are shown in the fourth data set, represented by solid squares, and appear to closely match the theoretical model, represented by hollow squares.

[0036] As can be seen from these measured data, the staggered flow design significantly outperforms the conventional straight flow design in terms of efficiency. When the Nusselt number is calculated for the test matrix, these data show that the Nusselt number for the staggered design is three times that of the conventional straight flow design.

[0037] It should be noted that while this novel proposal improves the efficiency of heat recovery, it also increases the pressure drop across the heat exchanger. However, it has been found that the energy savings realized by the improved heat recovery outweigh the energy losses due to the increased flow resistance. To illustrate this trade-off, consider a comparative example of a heat exchanger having a 100 mm deep matrix, on the one hand, configured as a conventional straight channel (as shown in Figures 2a and 2b) and, on the other hand, configured as the proposed staggered channel (as shown in Figures 3a and 3b). This comparative test was carried out on a ventilation device including a fan generating an air flow and on the associated heat exchanger, which recovers the heat contained in the volume of air flowing through it.

[0038] In this comparative test, the power used by the fan is calculated using the following formula: P=(q dp) / eta where q is the air flow rate (m 3 / s), dp is the pressure drop across the heat exchanger (Pa), and eta is the combined efficiency of the electric motor and fan (which is assumed to be 0.5). During the test, the target indoor temperature was set to 21°C when the outdoor temperature was 4°C.

[0039] The ventilation heating energy, i.e., the energy required to heat or cool the outside air to the desired supply air temperature, is calculated as follows: F=q m ·c P dT However, q m is the air mass flow rate (kg / s), and c P is the heat capacity of the air (i.e., 1 J g -1 ·K -1 ) and dT is the temperature difference between the supply air and the outside air (K).

[0040] If the air flow rate is 7.8 l / s and the average dT between the indoor and outdoor temperatures is 17 K, the power to heat or cool the supply air is: F=0.0078[m 3 / s] 1.2 [kg / m 3 ]·1[J / (g·K)]·17[K]=159[W]

[0041] As can be seen from Figure 8, the heat recovery efficiency of the staggered design for the selected airflow is 78% while that of the conventional design is 60%. That is, the energy recovery achieved by the staggered configuration is: 0.78·159[W]=124[W] In contrast, the conventional configuration is 0.6·159[W]=95[W] As a result, the energy recovery increases by 129W-95W=29W.

[0042] Meanwhile, as shown in FIG. 9, the pressure drop caused by the staggered heat recovery configuration is 63 Pa, and the resulting fan energy consumption is: (0.0078[m 3 / s]·63[Pa]) / 0.5[W]=1.0[W] However, the air flow rate selected is 7.8 l / s.

[0043] As further shown in FIG. 9, the pressure drop caused by the conventional thermal matrix configuration is 11 Pa, which results in the fan energy consumption: (0.0078[m 3 / s]·11[Pa]) / 0.5[W]=0.2[W] However, the air flow rate selected is 7.8 l / s.

[0044] As a result, the fan's energy consumption increases by 0.8W.

[0045] These results are summarized in the table below.

[0046] [Table 1]

[0047] In conclusion, the impact of the incremental heating energy savings (i.e., 64W - 35W = 29W) is much greater than the incremental power consumption (i.e., 1.0W - 0.2W = 0.8W). In the above example, the power savings is 29W - 0.8W = 28.2W.

[0048] It should be noted that the above example represents a non-optimized test setup. Greater savings could be achieved by using a thinner matrix wall than that designed for the test fixture. Furthermore, efficiency could be increased even further by increasing the length of the matrix. However, even with a non-optimized solution, energy savings are evident.

[0049] Although the above examples are given for recovering heat from exhaust air, the reverse is also possible: a staggered heat exchanger design can be used for cooling to save energy in hot climates. [Explanation of symbols]

[0050] 1 heat exchanger 2 Casing 3 Spinning Wheels 4 rotation axes 5 motors 6 Drive Unit 7 Pulley 8. Belt 9. Matrix 10 Flow path 11 Channel division 12 Leading Edge 13 Entrance opening 14 Exit opening 15 parts 16 parts 17 Dividers 21 Outside Air 22 Air supply 23 Return air 24 Exhaust

Claims

1. 1. A recuperative heat exchanger (1) comprising a matrix (9), the matrix (9) being made of a heat absorbing material and having a plurality of fluid flow passages (10) extending therethrough in a fluid flow direction, the flow passages (10) comprising flow passage sections (11) which are continuous in the flow direction and at least partially staggered with respect to one another.

2. 2. The recuperative heat exchanger (1) according to claim 1, wherein the successive flow passage sections (11) of the flow passage (10) are staggered in the transverse direction of the flow passage (10).

3. The successive flow passage sections (11) are staggered relative to one another in two directions, the two directions being: - transverse to each other, and - in the flow direction or transverse to the flow path (10) 3. The recuperative heat exchanger (1) according to claim 1 or 2,

4. The recuperative heat exchanger (1) according to any one of claims 1 to 3, wherein the flow path (10) comprises two or more, preferably three or more, consecutive flow path sections (11).

5. The recuperative heat exchanger (1) according to any one of claims 1 to 4, wherein the successive flow passage sections (11) are arranged in a staggered manner such that an edge (12) of the outlet opening of a flow passage section (11) intersects with an inlet opening (13) of a subsequent flow passage section (11).

6. 5. The recuperator (1) according to claim 4, wherein an edge (12) of the outlet opening of the flow section (11) intersects with a center of the inlet opening (13) of the subsequent flow section (11).

7. The recuperator (1) according to any one of claims 1 to 6, wherein every other flow section (11) in the fluid flow direction is arranged at the same transverse position.

8. 8. The recuperative heat exchanger (1) according to any one of claims 1 to 7, wherein the successive flow passage sections (11) are staggered such that the outlet opening of a flow passage section (11) is in direct fluid flow communication with the inlet openings (13) of at least two subsequent flow passage sections (11).

9. The recuperator (1) according to any one of the preceding claims, wherein the cross-sectional shape of the flow passages (10) is rectangular, triangular, polygonal or sinusoidal.

10. The recuperator (1) according to any one of the preceding claims, wherein the matrix (9) is configured as a rotating wheel (3).

11. The recuperative heat exchanger (1) according to any one of claims 1 to 10, wherein the heat exchanger (1) comprises a plurality of adjacent flow paths (10A, 10B, 10C), each flow path constituting such a matrix (9).

12. A method for manufacturing a matrix (9) of a recuperative heat exchanger (1) according to any one of the preceding claims, characterized in that said matrix (9) is manufactured by means of an additive manufacturing technique.

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