Heat exchanger

The invention relates to a heat exchanger, in particular a heat exchanger for a heating or air-conditioning system for motor vehicles in which a gaseous refrigerant is cooled by heat exchange contact with ambient air.

EP 0 845 648 A2 discloses a flat-tube heat exchanger, in particular a condenser of the serpentine type, with a flat-tube block consisting of one or more flat tubes which issue with preferably twisted end portions on the opposite or on the same tube block side into respective connection-space components, that is to say header tubes, so that, should the header tubes be arranged on the same tube block side, two header tubes running adjacently and parallel to one another are provided. In this case, a plurality of serpentine-shaped flat tubes may be provided, in which adjacent flat tubes are arranged with their inlet-side or their outlet-side tube portions adjacent to one another in the longitudinal direction of the header tubes, the serpentines comprising a plurality of 180° bends. A corresponding arrangement prevents heat transmission losses, but still leaves much to be desired.

EP 0 414 433 discloses a duplex heat exchanger which allows a coolant throughflow in cross countercurrent, in that two flat heat exchangers arranged one behind the other, designated hereafter as blocks, in each case with two header tubes which are connected to one another via a multiplicity of flat tubes, are provided. The two blocks are connected to one another by means of flanges and O-ring seals, for which purpose they have to be constructed, tensioned and soldered separately and, after soldering, connected to one another. The supply of the coolant to the block through which the flow passes first takes place in an upper region, the outlet at the bottom, and, in the second block through which the flow subsequently passes, inlet can take place both at the bottom and at the top, and outlet takes place correspondingly at the top or at the bottom. A duplex heat exchanger of this type which consists of two blocks entails a multiplicity of individual parts and a relatively high outlay in production terms, so that production is costly. Furthermore, a heat exchanger of this type still leaves much to be desired with regard to thermal properties.

Moreover, DE 100 43 439 A1 discloses a radiator for a supercritical steam compression refrigerating circuit, in which a coolant outlet is provided in a higher position than a coolant inlet, with respect to a vertical direction, such that coolant flows from an underside of the radiator to a top side, as a result of which an improvement in the cooling efficiency of the coolant is promised. Even a radiator of this type, however, still leaves much to be desired in terms of coolant efficiency.

The object of the invention is to improve a heat exchanger of the type initially mentioned.

The main idea of the invention is to make the surfaces of the subblocks dependent on the size of installation-space-related zones having different air temperatures and to cause coolant to flow first through the subblock within an installation-space-related zone having a higher air temperature, the subblock being arranged preferably within the zone having the highest air temperature.

In an advantageous embodiment of the heat exchanger, the height of the subblock through which coolant flows first is at least as great as the height of the zone having an increased air temperature.

In a further advantageous embodiment of the heat exchanger, the number of tubes arranged in the horizontal direction in a subblock is dependent on the installation-space-related air temperature zone within which the corresponding subblock is arranged.

In a particularly advantageous embodiment of the invention, the number of tubes of a subblock within a zone having a higher temperature is larger than the number of tubes of a subblock which is arranged within a zone having a lower temperature, the ratio of the number of tubes of the subblock within the zone having a higher temperature to the number of tubes of the subblock within the zone having a lower temperature being selectable in the range of 1:1 to 3:1.

In a particularly advantageous embodiment of the invention, at least two subblocks are arranged one behind the other and at least two subblocks are arranged one above the other, the coolant flowing through the subblocks in succession, and the order of throughflow being predeterminable, as desired, by means of structural measures.

Preferably, the coolant flows through at least two of the subblocks in countercurrent to the airstream.

In a particularly advantageous embodiment, the heat exchanger is subdivided into four subblocks through which the flow passes in succession, the subblocks through which the flow passes first being arranged below the subblocks through which the flow subsequently passes, the first and the second subblock and also the third and the fourth subblock being arranged in each case at the same height. Such a heat exchanger is suitable, in particular, for an installation space in which, as a consequence of installation space, there is in a lower region of the installation space a zone having a higher air temperature than in an upper region.

In an alternative version of the heat exchanger, the subblocks through which the flow passes first are arranged above the subblocks through which the flow subsequently passes, the first and the second subblock and also the third and the fourth subblock being arranged in each case at the same height. This alternative version of the heat exchanger is suitable, in particular, for an installation space in which, as a consequence of installation space, there is in an upper region of the installation space a zone having a higher air temperature than in a lower region.

The temperature of the coolant in the various subblocks differs as a function of the zones having different temperature. Thus, in an embodiment of the heat exchanger which is arranged in an installation space in which there is in a lower region of the installation space a zone having a higher air temperature than in an upper region, the temperature of the coolant is higher in the lower subblocks than in the upper subblocks, the temperature of one or of both rear subblocks being higher than the temperature of the corresponding front subblock. In an alternative embodiment of the heat exchanger which is arranged in an installation space in which there is in an upper region of the installation space a zone having a higher air temperature than in a lower region, the temperature of the coolant is higher in the upper subblocks than in the lower subblocks, the temperature of one or of both rear subblocks being higher than the temperature of the corresponding front subblock.

In all the instances mentioned, for example, R 134a and carbon dioxide may be used as coolant. In particular, carbon dioxide in a supercritical state, that is to say when there is a pure gas flow in the heat exchanger, is suitable for a heat exchanger according to the invention.

Preferably, a throughflow of at least two of the four subblocks by coolant takes place in cross countercurrent to the air. More effective heat transmission occurs as a result of cross countercurrent operation.

In particular, a diagonal deflection is provided between the second subblock and the third subblock, so that cross countercurrent operation takes place in all the subblocks.

Preferably, the diagonal deflection is formed by means of a one-part transition flange which is connected to two header tubes, to be precise to the header tube assigned to the second subblock and to the header tube assigned to the third subblock.

Preferably, in the region of the diagonal deflection, a tube, in particular a flat tube, is provided, through which coolant does not flow or flows to only a minimal extent, with the result that a decoupling of heat transmission takes place.

Preferably, the tubes which connect the header tubes and in the region of which heat transfer takes place are formed by flat tubes, the flat tubes being twisted through 90° upstream and downstream of a 180° bending point in the vicinity of the header tubes and on that side of the heat exchanger which is located opposite the header tubes.

In a further embodiment, the subblocks are closed off on both sides by means of header tubes, in which case at least two subregions may also be closed off on at least one side by means of a common header tube.

Preferably, the air flowing through the heat exchanger comes into contact with two or more regions of different temperature, the maximum air temperature difference between air inlet and air outlet being smaller than one and a half times the temperature difference between coolant inlet and coolant outlet, the coolant used being carbon dioxide in the supercritical state. In this case, temperatures of around 150° C. prevail at the coolant inlet and of around 50° C. at the outlet.

Preferably, the tubes arranged essentially in the horizontal direction are thermally separated from one another, for example by means of an air gap.

Preferably, the individual subblocks, too, are thermally separated from one another.

Preferably, the header tubes, too, are decoupled essentially thermally. There is thermal contact only at the diagonal deflection and, depending on design, also at the connecting flanges.

Preferably, the cooling ribs arranged between the tubes are likewise decoupled thermally. This is achieved, for example, by each subblock having its own cooling ribs.

The invention is explained in detail hereafter by means of an exemplary embodiment, with reference to the drawings, in which:

FIG. 1 shows a front view of a flat-tube heat exchanger according to the exemplary embodiment;

FIG. 2 shows a section through the flat-tube heat exchanger of FIG. 1 along the line II-II in FIG. 1 ;

FIGS. 3 to 6 show a transition flange in various views;

FIGS. 7 to 9 show a connection piece in various views;

FIGS. 10A to 10B show the cross sections of the flat tube heat exchanger along the line XA-XA and the line XB-XB in FIG. 1 , respectively; and

FIG. 11 shows a partial perspective view of the flat tube heat exchanger showing the transition flange.

FIGS. 1 and 2 show a flat-tube heat exchanger for a heating or air-conditioning system of a motor vehicle, which serves as a radiator 1 and is part of a coolant circuit, not illustrated, and which serves for cooling a coolant, in particular CO 2 , with the aid of the air flowing through the radiator 1 . FIG. 2 illustrates the airstream symbolically by an arrow pointing to the radiator 1 from the left. The CO 2 is normally in a supercritical state as a pure gas flow, temperatures of around 150° C. prevailing at the inlet 2 into the radiator 1 . A cooling of the coolant takes place in the radiator 1 , so that temperatures of around 50° C. prevail at the outlet 3 .

In order to allow an optimum utilization of the air flowing through the radiator 1 , the radiator 1 is subdivided into 2×2 subblocks which are designated hereafter as T 1 , T 2 , T 3 and T 4 . In this case, in the installed state, the subblocks T 1 and T 2 are arranged within a zone 4 having a higher air temperature and below the subblocks T 3 and T 4 . The height h of the two subblocks T 1 , T 2 which are arranged within the zone 4 having the higher air temperature is greater than the height H of the zone 4 having an increased air temperature, the value of the air temperature in the zone 4 being higher than the air temperature in the remaining regions of the installation space of the radiator 1 . A header tube S 1 , S 2 , S 3 , S 4 is connected to each subblock, in each case two header tubes S 1 , S 2 and S 3 , S 4 being arranged at the corresponding height of the subblocks T 1 , T 2 and T 3 , T 4 . Between the header tubes S 1 , S 2 and S 3 , S 4 are arranged a plurality of flat tubes 5 , through which the coolant can pass from one header tube S 1 or S 3 to the adjacent header tube S 2 or S 4 , for which purpose the flat tubes 5 have a U-shaped run. They are twisted in each case through 90° in a known way in the vicinity of the respective header tube S 1 , S 2 , S 3 , S 4 , as seen in FIGS. 10A , 10 B, and 11 . Between the flat tubes 5 are arranged ribs (see FIG. 11 ) which assist the heat exchange, and these ribs may be divided in two, that is to say the subblocks T 1 , T 2 and T 3 , T 4 arranged one behind the other have in each case their own ribs. It is also possible, however, to decouple the ribs of the subblocks thermally by means of slots.

So that the coolant can flow through the radiator 1 in cross countercurrent to the air, a diagonal flow deflection 6 from subblock T 2 to subblock T 3 is provided, as is indicated in FIG. 2 by an arrow depicted into the radiator 1 . For this purpose, a transition flange 7 , as is illustrated in FIGS. 3 to 6 , 10 B, and 11 , is provided between the two header tubes S 2 and S 3 , the zone of the flat tube 5 ′ lying at the boundary of the two subblocks T 2 , T 3 being utilized, in that the partitions of the two header tubes S 2 and S 3 are mounted so as to be offset by the amount of one transverse division. The middle flat tube 5 ′ is thus “short-circuited” and has scarcely any flow passing through it, at the most as a result of a slight pressure difference which occurs between the two header tubes S 2 and S 3 on account of the slight throttling effect in the transition flange 7 . In this case, the flat tube 5 ′ through which no flow or only a minimal flow passes has the secondary effect that thermal decoupling is achieved between the subblocks T 1 , T 3 and T 2 , T 4 . The transition flange 7 is conventionally produced, together with the two partitions, as one component and is also soldered during the soldering of the radiator 1 .

The header tubes S 1 and S 2 or S 3 and S 4 are connected to one another in each case at the inlet 2 or at the outlet 3 via a connection piece 9 , as is illustrated in FIGS. 7 to 9 and 10 A, so that coolant can also pass directly into the header tube S 2 or can flow directly out of the header tube S 3 .

For thermal decoupling, the collection of the coolant takes place, after the latter has flowed through the subblocks T 1 and T 2 or T 3 and T 4 , in header tubes S 1 , S 3 and S 2 , S 4 designed separately. The thermal coupling of the subblocks T 1 and T 2 or T 3 and T 4 via the one-part ribs may be reduced by the slotting of the rib or by any other suitable measure.

According to the exemplary embodiment illustrated, there is a division of the subblocks T 1 , T 2 in relation to the subblocks T 3 , T 4 of 50:50, but the division should preferably be made decreasingly, that is to say, for example, 60:40 or 70:30, since, as in the condenser, the outlet density is higher and consequently the volume of flow lower than at the inlet. Moreover, the gas radiator likewise serves in a subcritical state as a condenser.

List of Reference Symbols

1 Radiator

2 Inlet

3 Outlet

4 Zone having a higher temperature

5 , 5 ′ Flat tube

6 Diagonal reversal

7 Transition flange

9 Connection piece

S, S 2 , S 3 , S 4 Header tube

T 1 , T 2 , T 3 , T 4 Subblock

H Height of the zone having a higher temperature

h Subblock height