The invention relates to a heat exchanger, in particular for a supercritical refrigeration cycle as described in the preamble of patent claim 1.
Heat exchangers for supercritical refrigeration cycles require a pressure-resistant design of tubes and headers, since the refrigeration process takes place at high pressures, up to about 120 bar. Heat exchangers of this type have been disclosed by DE-A 199 06 289, DE-A 100 07 159 and WO 98/51983 A. These known heat exchangers are in some cases used as gas coolers in a supercritical refrigeration cycle operated with CO2 (R744); they are substantially characterized by a single-row design with two header tubes, i.e. one row of flat tubes which are formed as extruded multichamber tubes and have their ends secured in the header tubes and sealed, for example by brazing. The refrigerant flows through the gas cooler—as shown in DE-A 100 07 159—in serpentine form, i.e. in multiple flows, with the refrigerant being diverted in a plane perpendicular to the direction of flow of the air, i.e. over the height or width of the gas cooler.
EP-B 414 433 has disclosed a refrigerant condenser in which two single-row heat exchangers are arranged in series in the direction of air flow and are connected in series on the refrigerant side (known as duplex heat exchangers). In the known condenser, refrigerant and air are routed in cross-countercurrent to one another, i.e. the refrigerant enters the leeward heat exchanger (row of tubes) and leaves the condenser via the windward heat exchanger (row of tubes). Each row of tubes of a heat exchanger is divided into tube groups or tube segments, resulting in a decreasing cross section of flow for the condensing refrigerant. The rows of tubes comprise extruded flat tubes, between which corrugated fins are arranged. Each row of tubes, together with header tubes, forms a heat exchanger unit which is connected to the other heat exchanger unit by pieces of tube on the refrigerant side.
A similar multi-row heat exchanger, a liquefier for a refrigerant of a vehicle air-conditioning system, has been disclosed by EP-B 401 752. In this case too, refrigerant, i.e. a conventional refrigerant, such as R 134a, is routed in cross-countercurrent with ambient air; in general four rows of tubes are arranged in series on the air side. These are round tubes with flat fins, i.e. a mechanically joined heat exchanger block.
In motor vehicle air-conditioning systems, the condenser is arranged in the engine compartment of the motor vehicle upstream of the coolant/air cooler. The warmed air emerging from the condenser then flows through the coolant/air cooler. An arrangement of this type is also provided for gas coolers for CO2 air-conditioning systems of the type described in the introduction—i.e. the single-row design with a relatively large end face which is matched to the coolant/air cooler located downstream of it. This design and arrangement has various drawbacks: firstly, arranging a gas cooler upstream of the coolant cooler impedes the power of the coolant cooler, on the one hand on account of the additional pressure-side pressure drop caused by the gas cooler and on the other hand on account of the warming of air caused by the dissipation of heat from the gas cooler to the air flowing through. Secondly, the gas cooler arranged upstream of the coolant cooler, at certain driving operating points, only receives certain air quantities as a function of the driving speed or the fan power. The air-conditioning of the motor vehicle is therefore very much dependent on the driving state of the vehicle. Consequently, one problem on which the invention is based consists in providing a heat exchanger, in particular for a supercritical refrigeration cycle, which avoids the abovementioned drawbacks.
The article “Design Strategies for R744 Gas Coolers” von J. M. Yin, C. W. Bullard and P. S. Hrnjak (published in IIF-IIR Commission B1, B2, Purdue University USA-2000) compares and contrasts two configurations of gas coolers, namely what is known as the multi-pass heat exchanger, i.e. a single-row heat exchanger with medium flowing through it in multiple flows, and the multi-row countercurrent heat exchanger, in which three rows of tubes are provided connected in series on the refrigerant side. Since the refrigerant CO2 (R744) enters the gas cooler in the supercritical state, i.e. in a single phase, it has a relatively high temperature gradient, unlike conventional refrigerant (R134a), which condenses at a constant temperature. This temperature gradient can be effectively reduced in a three-row countercurrent heat exchanger, for which reason the authors prefer this solution. Similar conclusions are reached by the authors J. Peterson, A. Hafner, and G. Skaugen in their article “Development of compact heat exchangers for CO2 air-conditioning systems” (published in Int. J. Refrig. vol. 21, no. 3 pages 180-193, 1998). In this case too, the countercurrent heat exchanger (counterflow heat exchanger) with a reduced size of end face and increased depth in the direction of air flow is described as an advantageous gas cooler.
It is an object of the present invention to design a heat exchanger of the type described in the introduction which takes into account the conditions of a supercritical refrigeration cycle in terms of pressure and temperature gradient and has the highest possible efficiency (COP, i.e. coefficient of perfomance). Furthermore, the dimensions of this heat exchanger should be such that it can easily be accommodated in the engine compartment of a motor vehicle and can be supplied with sufficient cooling air.
This object is achieved by the features of patent claim 1. According to the invention, it is provided that the heat exchanger, which is preferably operated in countercurrent, has at least four rows of tubes, which are arranged in series in the direction of air flow. In this context, the term countercurrent is to be understood as meaning that the flow medium, preferably CO2, first of all enters the leeward row of tubes and emerges again from the windward row of tubes. The cooling air which enters the heat exchanger meets a flow medium which has already been (pre)cooled in at least three rows of tubes. In these four rows of tubes, through which the medium flows in succession, the temperature gradient with a temperature difference of approx. 100 degrees Celsius can be effectively eliminated with a sufficiently low pressure drop on the air side. Forming the heat exchanger in at least four rows allows the surface area of the end face to be reduced, so that the heat exchanger acquires compact dimensions verging on a cube, which has the advantage that the heat exchanger, in particular if it is used as a gas cooler of a CO2 air-conditioning system in the motor vehicle, can be accommodated at any desired point in the engine compartment of the vehicle. There is no need for it to be arranged upstream of the coolant cooler, which has the abovementioned drawbacks. The heat exchanger can be cooled by additional air passages and a special fan. This also makes it independent of the driving states of the motor vehicle, with the result that constant air-conditioning of the vehicle interior is ensured. Furthermore, it has been found that the efficiency (COP) of the heat exchanger according to the invention is scarcely any worse than a comparable prior art heat exchanger.
According to an advantageous configuration of the invention, at least five or optimally six rows of tubes are arranged in series, resulting in the advantage of a further boost to the power of the heat exchanger, without the air-side pressure drop and the weight rising excessively.
According to a further advantageous configuration of the invention, the tubes are formed as flat tubes, preferably as extruded multichamber tubes, and the fins are formed as corrugated fins, which together result in a brazed, pressure-resistant heat exchanger block with a high power.
In a further advantageous configuration of the invention, medium flows through all the tubes belonging to a row in parallel, and preferably medium flows through these rows of tubes in succession, in which case from tube row to tube row a so-called diversion over the depth takes place. The individual rows of tubes therefore alternately have medium flowing through them from the top downward and from the bottom upward, resulting in a long path for the flow medium in the tubes and effective cooling.
In an advantageous configuration of the invention, the individual rows of tubes have tube segments or tube groups through which medium can flow in succession—the flow medium is diverted “over the width” of a row of tubes, resulting in the advantage of a longer flow path and more extensive cooling of the flow medium.
In an advantageous refinement of the invention, just some or all of the rows of tubes can be divided into tube segments, so as to lengthen the flow path still further. The number of tubes in the tube segments corresponds to approximately half the number of tubes of a row of tubes, but may also deviate from this number, resulting in different tube segments. Therefore, for example in the case of horizontally arranged tubes, the flow velocity can be varied in the lower or upper region of the block, and therefore so too can the heat transfer.
In an advantageous configuration of the invention, each row of tubes has its own corrugated fins, i.e. the corrugated fins of adjacent rows of tubes are thermally decoupled or thermally isolated, resulting in maximum cooling of the flow medium.
However, in a further configuration of the invention, it may also be advantageous to provide one common, i.e. continuous corrugated fin for adjacent rows of tubes, for example two rows of tubes. This in particular has manufacturing technology benefits.
In a further advantageous configuration of the invention, one common, continuous corrugated fin is provided for all the rows of tubes, i.e. thermal coupling is implemented between the individual rows of tubes, resulting in a different temperature profile for the flow medium.
In an advantageous configuration of the invention, the tubes of adjacent rows of tubes are arranged aligned with one another, which is a precondition for example for continuous corrugated fins. This results in a lower pressure drop on the air side.
In a further advantageous configuration of the invention, however, the tubes may also be arranged offset with respect to one another, which although leading to a higher pressure drop on the air side does achieve a better heat exchanger power.
In a further advantageous configuration of the invention, the end face of the heat exchanger is square or at least approaches a square in terms of its height and width dimensions. An advantageous ratio of width to height is in the range from 0.8 to 1.2. This has the advantage that a fan behind or in front of the end face is sufficient to deliver the cooling air, since it sufficiently covers the end face.
In a further advantageous configuration of the invention, the end face has a surface area in the range from 4 to 16 dm2, resulting in a smaller end face compared to conventional heat exchangers combined, at the same time, with a greater depth, i.e. the heat exchanger has a compact shape approaching a cube and can therefore be arranged at any desired locations in the engine compartment. On the other hand, the power of the coolant cooler is no longer adversely affected by an upstream condenser or gas cooler.
In a further advantageous configuration of the invention, the abovementioned heat exchanger having the large number of refinements is used as a gas cooler in a supercritical refrigeration cycle of a motor vehicle air-conditioning system operated with CO2. This achieves all the advantages referred to above.
Exemplary embodiments of the invention are illustrated in the drawings and described in more detail in the text which follows. In the drawings:
a, 1b, 1c diagrammatically depict a heat exchanger according to the invention with four, five and six rows,
a, 2b show the heat exchanger according to the invention having four rows, with the last two or three rows divided into tube segments,
a, 3b show a heat exchanger according to the invention having four rows, with all the rows divided into tube segments of an identical or different arrangement,
a, 1b and 1c diagrammatically depict first exemplary embodiments of a heat exchanger according to the invention, which is designed and can be used as a gas cooler for a supercritical refrigeration cycle. In particular, this gas cooler can be used for a motor vehicle air-conditioning system operated with the refrigerant CO2 (R744).
a shows a four-row tube system for a gas cooler 1, through which the refrigerant CO2 flows and which is cooled by ambient air, the direction of flow of air being indicated by arrows L. The gas cooler 1 has four rows of tubes 1.1, 1.2, 1.3, 1.4 arranged in series in the direction of air flow L, which rows of tubes each have tubes running parallel to one another, indicated by arrows R. Each row of tubes 1.1 to 1.4 has the same number of tubes, through each of which medium flows in parallel. The individual rows of tubes are connected in series on the refrigerant side, i.e. they are connected to one another by refrigerant connections, represented by dotted arrows V. This connection V is referred to as a diversion of the refrigerant “over the depth”, with the depth direction being opposite to the air direction L. The refrigerant first of all enters the leeward row 1.1, illustrated by a dotted arrow E, and then, after it has flowed through the individual rows, is three times diverted over the depth and after it has flowed through the windward row 1.4 leaves the gas cooler via the outlet A, as illustrated by a dotted arrow A. This flow model for air and refrigerant is known as cross-countercurrent. The refrigerant CO2 enters the gas cooler, i.e. the row of tubes 1.1, approximately at a pressure of 125 bar and a temperature of about 130 degrees Celsius. The temperature of the air which enters the gas cooler 1 through the row of tubes 1.4 is approximately 45 degrees. Since the CO2 air-conditioning system is working in the supercritical range, the dissipation of heat takes place not through condensation at constant temperature—as is the case in the refrigeration cycle using R134a—but rather with a falling temperature, i.e. a temperature gradient from 130 degrees Celsius to approximately 50 degrees Celsius. This temperature difference of 80 degrees Celsius is gradually reduced during flow through the individual rows of tubes 1.1 to 1.4. The numbers mentioned are examples, and in some cases the temperature difference is even greater, i.e. approx. 100° Celsius. The gas cooler 1 has what is known as a finned end face, which is the surface area of the row of tubes 1.4 acted on by air and having the dimensions B×H (width×height). The definition applies to all the gas coolers according to the invention.
b shows another exemplary embodiment of the invention, namely a gas cooler 2 with five rows of tubes 2.1, 2.2, 2.3, 2.4, 2.5, which are arranged in series in the direction of air flow L and are also connected in series on the refrigerant side. Identical reference designations are used for parts which correspond to those shown in
c shows a further exemplary embodiment of the invention, namely a gas cooler 3 with six rows of tubes 3.1 to 3.6. It is once again based on the same flow model as in
a and
b shows a refinement of the embodiment illustrated in
a and
a shows a four-row gas cooler 6 with rows of tubes 6.1, 6.2, 6.3, 6.4, the individual rows of tubes each being divided into uneven tube segments 1a, 1b, 2a, 2b, 3a, 3b and 4a, 4b. The number of arrows symbolizes the number of tubes per tube segment, i.e. in this case there are tube segments having in each case two and three tubes, which constantly alternate with one another. Within a row of tubes, there is a diversion over the width from a three-tube segment to a two-tube segment, and from the latter there is a diversion over the depth to a three-tube segment, and so on, as shown in the drawing, which provides sufficient explanation. Therefore, the cross section of flow changes constantly from diversion to diversion, and therefore so does the flow velocity of the refrigerant, resulting in locally different heat transfer conditions within the gas cooler 6.
b shows a gas cooler 7 which represents a modification of the gas cooler 6, specifically with regard to the arrangement of the tube segments in each row of tubes. The only difference with respect to the gas cooler 6 is that the tube segments with two tubes are in each case located at the top and the tube segments having three tubes are in each case located at the bottom, resulting in a diversion V over the depth in each case from an upper two-tube segment 1b, 2b, 3b to a lower three-tube segment 2a, 3a, 4a. The power of the gas cooler can be further enhanced by dividing each row of tubes into two tube segments (cf.
Number | Date | Country | Kind |
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10 2004 001 786.7 | Jan 2004 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP04/13830 | 12/6/2004 | WO | 10/10/2006 |