Heat Exchanger

Abstract

The invention relates to a heat exchanger (5) comprising a block (2) which is provided with parallel ducts and can be penetrated in opposite directions in at least two passages (2a, 2b) by a medium that is to be cooled. A bypass duct (6) which be penetrated by the medium that is to be cooled is assigned o the first passage (2a).

Description

[0001] The invention concerns a heat exchanger according to the preamble of Claim 1.

[0002] Such heat exchangers, which have a heat exchanger core, known as a core for short, with flow channels arranged parallel to one another, are known, for example, as coolant/air radiators in motor vehicles. A medium to be cooled, for example, the coolant of a cooling circuit of an internal combustion engine of a motor vehicle, flows through the flow channels. The coolant is preferably cooled by air (ambient air), wherein secondary exchange surfaces in the form of fins can be provided. Various flow patterns are known for such a core, for example, downflow radiators or crossflow radiators with one or two flow filaments. In the latter case, the throughflow of the core is in the shape of a U. In this respect, two collection boxes are provided on the core, wherein the first has an inlet and an outlet chamber and the second is designed as a deflection box. The deflection of the flow thus takes place "in the width," that is, in the longitudinal direction of the deflection box. The division of the core into a first and a second passage, as a rule, is done 50:50, so that the flow rates in the tubes of the two core halves are the same. The flow direction of the cooling air is perpendicular to the flow direction of the medium to be cooled, so that the heat transfer occurs in crossflow. As a result of cooling, the temperature of the medium in the tubes of the first passage is higher than the temperature of the medium in the second passage. In comparison to a parallel flow radiator (wherein the entire core receives a throughflow in one direction), flow rates which are increased due to the deflection are produced in the flow channels, which lead to an improved heat transfer with small coolant throughputs. Different expansions of the tubes are produced due to the temperature differential between the tubes in the first and second passage; these lead to thermal stresses in the heat exchanger. In particular, with an increase in the coolant inlet temperature as a result of a higher motor load, there is an increased temperature differential between the first and second passage of the core, since with such a transient process a temporary lag appears, with the medium flowing through the two passages successively.

[0003] By the applicant's DE 197 22 099 B4, a heat exchanger became known, which has a collection box with an inserted separation wall and inlet and outlet connections. Thus, a U-shaped throughflow of the heat exchanger is made possible, which leads to the aforementioned temperature differences in the first and second flow passages.

[0004] A heat exchanger designed for internal combustion engines became known from the applicant's DE 32 12 891 C2; it consists of a fin/tube core, an upper and a lower box, and side parts which are designed as flow channels and receive a coolant throughflow. The medium to be cooled is withdrawn from the boxes and thus cools the side parts, which in this way obtain a lower component temperature. Thus, excessively high temperature differences between the cooling tubes and side parts and increased temperature stresses are avoided.

[0005] Proceeding from a heat exchanger which can receive a throughflow in the shape of a U, it is the objective of the present invention to avoid or to reduce thermal stresses produced by temperature differences in the heat exchanger, in particular in its flow channels.

[0006] This objective is attained by the features of Claim 1. Advantageous developments of the invention can be deduced from the subclaims.

[0007] According to the invention, provision is made first of all for a bypass to be associated with the first passage of the heat exchanger, i.e. the first U leg of the flow path; this means that a proportion of the medium to be cooled is diverted before entry into the first passage of the heat exchanger, conducted through the bypass, and again supplied, uncooled, to the main flow after the first passage or before the second passage. In this way there is the advantage that the temperature in the second passage is raised and thus the temperature difference is reduced. Therefore, the thermal stresses in the flow channels, for example, the tube and tube plate connections, are also reduced.

[0008] Advantageously, a first collection box with an inlet and outlet chamber and a second collection box in the form of a deflection box are associated with the core of the heat exchanger. In this case, the bypass channel extends between the inlet chamber and deflection box, wherein the local inlet of the bypass channel into the deflection box can be designed as variable, that is, dependent on the desired temperature increase in the second passage. Preferably, the inlet of the bypass into the deflection box can be at the level of a separation wall that separates the inlet and outlet chambers from one another. In this case, the heat exchanger preferably has horizontal flow channels and collection boxes arranged vertically. The deflection box has an inlet opening. The bypass channel discharges into the deflection box. The bypass channel and/or the deflection box carries only slightly cooled medium. The bypass channel is located in the deflection box. Slightly cooled medium arrives at the deflection box via an inlet opening. The closer the inlet opening of the bypass channel to the inlet to the second passage, the less mixing there will be with the cooled medium of the first passage, with a greater rise in the temperature in the second passage.

[0009] The division of the core into a first passage and a second passage can be 1:1, but can also deviate from that. With an equal division, essentially the same flow rates are produced in the two passages. The flow rate in the bypass channel, on the other hand, is higher and can be established by dimensioning its cross section or flow resistance to the desired value. The higher the flow rate in the bypass channel, the more rapidly will the temperature front of the hot medium reach the deflection box or the inlet to the second passage. Thus, sudden temperature increases of the medium to be cooled and the related increased temperature differences between the first and the second passages can be compensated, since the temperature fronts in the first and in the second passages run opposite one another.

[0010] The bypass channel can be advantageously designed as a separate bypass line to the heat exchanger or can be integrated into the heat exchanger. The latter can, for example, be effected by integration of the bypass channel into a side part of the heat exchanger. The side part is thereby designed as a flow channel, that is, hollow, and is fluidically connected with the inlet box and the deflection box.

[0011] According to a preferred embodiment of the invention, the heat exchanger is designed as a coolant/air radiator in the coolant circulation of an internal combustion engine for a motor vehicle. The radiator core thus consists, as a rule, of tubes and fins which receive a coolant throughflow; they are impinged upon by the ambient air. The fin/tube core can be made mechanically or constructed as a soldered core. The collection boxes can be made of plastic or metal, in particular, aluminum, for example in all-aluminum radiators.

[0012] Advantageously, the bypass line has a diameter in the range of 7 to 16 mm. The proportion of the throughput through the bypass, relative to the total throughput through the radiator, is thus between 10 and 25%.

[0013] Embodiments of the invention are shown in the drawing and are described in more detail below. The figures show the following:

[0014] FIG. 1, a coolant radiator with U-shaped flow deflection according to the state of the art;

[0015] FIG. 2, a coolant radiator, according to the invention, with bypass line;

[0016] FIG. 3, a temperature/time diagram;

[0017] FIG. 4, a schematic representation of temperature fronts with a radiator according to the state of the art;

[0018] FIG. 5, a schematic representation of the temperature fronts with a radiator according to the invention;

[0019] FIG. 6, a schematic representation of an introduction tube to introduce the coolant from the bypass line; and

[0020] FIG. 7, another view of the schematic representation of an introduction tube to introduce the coolant from the bypass line.

[0021] FIG. 1 shows a heat exchanger 1, designed as a coolant/air radiator, according to the state of the art. The coolant radiator 1, designated below as radiator for short, is located in a not-shown coolant circulation for an internal combustion engine of a motor vehicle. The radiator 1 has a radiator core 2, designated below as core 2 for short, which has not-shown horizontal tubes (flow channels) and also not-shown fins, which are located on the outside of the tubes. Tubes and fins are preferably soldered to form a core, that is, core 2. However, other modes of construction, for example, mechanically constructed round or oval tube systems, can also be considered. The tube ends of the not-shown tubes individually discharge into collection boxes 3, 4, wherein the first collection box 3 is subdivided by a separation wall 3a into an inlet chamber 3b and an outlet chamber 3c, whereas the second collection box 4 does not have a separation wall, but rather is designed as a deflection box. As a result of the separation wall 3a, coolant that enters the inlet chamber 3b via an inlet connection 3d first flows in a first passage 2a (first tube group), at the tip in the drawing, in the direction of arrow A through the core 2. The coolant is subsequently deflected in the deflection box 4 and then flows back through a second passage 2b (second tube group), which is lowermost in the drawing, in the direction of arrow B, enters the outlet chamber 3c, and leaves the radiator 1 via an outlet connection 3e. The two passages or tube groups 2a, 2b are separated at the level of the separation wall 3a by a broken line m. The coolant which flows through the tubes is cooled by ambient air, which flows through core 2 perpendicular to the drawing plane.

[0022] On the one hand, an increased flow rate of the coolant, as a result of the deflection, and thus an improved heat exchange, are advantageous in this arrangement. On the other hand, with certain installation conditions, the arrangement of coolant inlet and outlet connections on the same side or on the same collection box can be advantageous.

[0023] FIG. 2 shows a heat exchanger 5 according to the invention that is also designed as a coolant/air radiator for a motor vehicle and corresponds to the known radiator 1 according to the state of the art; the reference numbers of radiator 1 from FIG. 1 are therefore adopted for the corresponding parts of radiator 5. In contrast to the known radiator 1, radiator 5 has a bypass line 6 which bypasses the first passage 2a of core 2 without the coolant being cooled. The entering coolant flow is labeled by arrow V.sub.E; the outleting coolant flow, by arrow V.sub.A. Bypass line 6 therefore branches off before or in the inlet chamber 3b and is connected with the deflection box 4 via an inlet opening 7. Bypass line 6 can be designed as a separate line, for example, a tubular line, or, not shown in the drawing, integrated in radiator 5. This can be achieved, for example, with a radiator with side parts, wherein one side part, which fits snugly against the core half of the first passage, is hollow and designed as a flow channel, and receives a coolant throughflow from the inlet chamber to the deflection box.

[0024] The inlet opening 7 is preferably located in an area b, which deviates by approximately 15% of the width of core 2 on either side of line m. The inlet opening or inlet point 7 is understood to mean the location where the bypass flow (the coolant flow through the bypass channel 6) meets the coolant flow in the deflection box 4 and the two flows mix.

[0025] According to a preferred embodiment of the invention, the diameter of the bypass line for a radiator is in the range of 7-16 mm, thus establishing the proportion of the bypass flow relative to the total throughput through radiator 5 between 10% and 25%.

[0026] In the drawing, that is, in a preferred embodiment, the inlet opening 7 in the deflection box 4 is located above line m that separates the first passage 2a, which is uppermost in the drawing, from the second passage 2b, which is lowermost in the drawing. Since the first passage 2a and the second passage 2b have the same number of tubes (not shown) with the same flow cross sections, the upper and the lower core halves 2a, 2b are the same. However, design of the flow cross sections of the passages 2a, 2b in a ratio different from 50:50, for example, at 40:60, also lies within the scope of the invention.

[0027] By means of the bypass flow, that is, the proportion of the coolant flowing through the bypass line 6 and the reentry of the practically uncooled coolant in the middle area of the deflection box 4, hot or relatively uncooled coolant is supplied to the second passage 2b, so that the temperature of the coolant in the second passage 2b rises. This effect of the bypass flow according to the invention is explained in more detail below.

[0028] FIG. 3 is a diagram in which the inlet temperature TE of the coolant, that is, the coolant flow V.sub.E, is plotted over time t. The depicted temperature curve is based on the following two operating states in the vehicle: in the first operating state (short-circuit operation) the (not shown) thermostat of the coolant circulation is closed; the engine is running in the partial load range. The coolant radiator cools the coolant almost to the ambient temperature (T1). The volume flow in the radiator is equal to zero or very low in this operating state. In the second operating state, the engine runs under load; more heat is therefore removed from it, that is, the thermostat is opened. The volume flow rises, and coolant with a temperature T2, which is increased in comparison to T1, flows into the radiator.

[0029] In the diagram, T1 is the low coolant inlet temperature, whereas T2 represents the increased coolant inlet temperature, which as mentioned above can arise with an increased motor load. The continuous lines, which represent the time dependence of the temperature TE on the time t, show the delay with which a temperature increase of T1 to T2 at the radiator inlet is propagated up to the deflection box. While the coolant inlet temperature increases to T2 in a time period (t2-t1), a time period (t4-t2) also elapses until the temperature T2 has arrived at the deflection box, that is, at the inlet to the second passage.

[0030] FIG. 4 shows a schematic representation of a radiator according to the state of the art, that is, according to FIG. 1, which shows the temperature fronts in the first (upper) passage with a temperature of T1, and an increased temperature of T2, at a time t=t3 (see FIG. 3). With a temperature jump, in particular at the inlet, the upper core 2a of the radiator has essentially the shaded area A in which the coolant in the tubes has reached the inlet temperature T2. In a transition area with T2>T>T1, cold coolant and hot coolant are mixed with one another. Cold coolant with the temperature T=T1 is found in an area T=T1. From this it can be clearly seen that the temperature difference (T2-T1) is in full effect, that is, the tubes of the upper passage for the most part already have an elevated temperature T2, whereas the tubes of the lower passage still have a lower temperature of T1. The aforementioned thermal stresses result from this.

[0031] FIG. 5 shows the propagation of the temperature fronts at a time t=t3 (see FIG. 3) with a radiator according to the invention having a bypass line 6 and inlet point 7 of the bypass flow in the deflection box 4 corresponding to the embodiment according to FIG. 2. The inlet point 7 is advantageously located in the area +/-15% of the radiator width from the position of the separation wall (line m). By means of the bypass flow and its entry in the vicinity of line m, coolant at the elevated temperature T2 is conducted directly to the inlet of the second passage 2b. In this way, a temperature distribution or a temperature front is formed that is depicted by a shaded area 8 in an exemplary manner (in an idealized manner). The area with the elevated coolant inlet temperature T2 in the first passage is also shaded and provided with the reference number 9. The shaded areas 8, 9 form areas A.sub.u and A.sub.o which correspond to the coolant volumes with the temperature T2. The corresponding shaded area of temperature T2 is designated by A in FIG. 4. In comparing FIGS. 4 and 5, the relationship A=A.sub.o+A.sub.u holds. The diagram shows that the temperature fronts of area 8 (A.sub.o) in the first passage and area 9 (A.sub.u) in the second passage run contrary one another, that is, toward one another. In this way, the increased temperature differences known from the state of the art are reduced, and consequently the stresses resulting therefrom are also reduced.

[0032] Based on the flow resistance and the cross section of the bypass line, the delay between the rise in temperature in box 3a and in box 4 at location 7 can be varied and adjusted.

[0033] FIGS. 6 and 7 show an embodiment of an introduction tube 21. The introduction tube 21 is connected to the bypass channel 6 by means of a tube flange 20 and is used to introduce the coolant from the bypass channel 6 into the deflection box 4. The introduction tube 21 thereby projects at least in part into the deflection box 4. Furthermore, the introduction tube 21 is at least offset at right angles and/or has at least one opening 22 to introduce the coolant from bypass channel 6.

Claims

1. A heat exchanger comprising a core having flow channels arranged parallel to one another which can receive a throughflow of a medium to be cooled in at least a first passage and a second passage in opposite directions, and a bypass channel for the supply of the medium, which can receive a throughflow of the medium to be cooled, wherein the bypass channel is associated with the first passage.

2. The heat exchanger according to claim 1, wherein a first collection box is associated with the core; the collection box having an inlet chamber for the first passage and an outlet chamber for the second passage.

3. The heat exchanger according to claim 1, wherein a second collection box is associated with the core as a deflection box for the first and second passages.

4. Heat exchanger according to claim 1, wherein the bypass channel is branched off before entry into core.

5. heat exchanger according to claim 4, wherein the bypass channel is branched off from the inlet chamber.

6. The heat exchanger according to claim 3, wherein the bypass channel discharges into the deflection box.

7. The heat exchanger according to claim 2, wherein the inlet chamber and the outlet chamber are separated from one another by a separation wall.

8. The heat exchanger according to claim 1, wherein the passages have flow channels having cross sections which behave in a 1:1 ratio for the first and the second passages.

9. The heat exchanger according to claim 7, wherein the bypass channel discharges into an inlet opening of the deflection box.

10. The heat exchanger according to claim 9, wherein the inlet opening is located in an area (b) of a line (m) marking the position of the separation wall and that the area (b) deviates to either side of the line (m) by approximately 15% of the width of the core.

11. The heat exchanger according to claim 1, wherein the bypass channel is a separate bypass line.

12. The heat exchanger according to claim 1, wherein the bypass channel is integrated into the heat exchanger.

13. The heat exchanger according to claim 12, wherein the heat exchanger has at least one side part, arranged parallel to flow channels in the first passage, which is designed as a flow channel and can receive a throughflow as the bypass channel.

14. A coolant/air radiator for the coolant circulation of an internal combustion engine for a motor vehicle comprising the heat exchanger according to claim 1.

15. The heat exchanger according to claim 10, wherein the bypass line has a diameter of 7 to 16 mm.

16. The radiator according to claim 14, wherein the proportion of the throughput through the bypass channel can be established at 10 to 25% of the throughput through the radiator.

17. The radiator according to claim 16, wherein the flow rate of the coolant in the bypass channel is higher than the flow rate in flow channels of the first passage.

18. The radiator of claim 17, wherein the flow channels are designed as tubes, and the cored is designed as a tube/fin core.

19. The heat exchanger according to claim 3, wherein the bypass channel projects at least in part into the deflection box by means of an introduction tube.