The invention relates to a heat exchanger, in particular an evaporator, for a vehicle heating, ventilating and/or air-conditioning system (HVAC) .

In a widely-used evaporator design, a plurality of flat tubes are provided which are arranged staggered in one direction, with fin stacks being positioned in the clearances between the flat tubes. Air circulates through the fins of the fin stacks in an air inflow direction parallel to the flat sides of the flat tubes and perpendicularly to the direction of stagger, whilst coolant circulates in coolant canals which are constructed in the flat tubes. The coolant canals in the flat tubes are normally aligned perpendicularly to the fins so that the coolant moves perpendicularly to the airflow.

If a heat exchanger of this type is used as an evaporator, thermal energy is therefore transferred from the circulating air via the fins to the flat tubes and to the coolant flowing therein, whereby the air is cooled.

When used in a vehicle, it is desirable for the capacity of the heat exchanger to be as high as possible with small, often predetermined, installation dimensions.

The object of the invention is to provide a heat exchanger of this type. This objet is achieved with a heat exchanger which can be, in particular, an evaporator for a vehicle heating, ventilating and/or air-conditioning system. The heat exchanger has a flat tube arrangement with a plurality of flat tubes whereof the flat sides extend parallel to an air inflow direction of the heat exchanger and which are staggered perpendicularly to the air inflow direction, clearances being formed between the flat tubes in the direction of stagger. The flat tubes have canals through which coolant is circulated and which are each delimited by canal walls, the canals being arranged in succession in the air inflow direction. The flat tube arrangement has, in the air inflow direction, a circulation width which is determined by a first canal wall, which is closest to the air inlet side, and a second canal wall, which is closest to the air outlet side, of the outermost coolant circulation canals with reference to the air inflow direction. At least one fin stack is moreover provided, which has a plurality of fins arranged one above another and is arranged in a clearance between two flat tubes of the flat tube arrangement, it being possible for air to circulate through the fin stack in the air inflow direction from the air inlet side to the air outlet side. The fin stack is longer than the circulation width of the flat tube arrangement with reference to the air inflow direction and begins upstream of the first canal wall on the air inlet side and ends downstream of the second canal wall on the air outlet side.

In this case, the circulation width of the flat tube arrangement forms the region of the heat exchanger through which coolant circulates, as seen in the air inflow direction. This region at the same time also constitutes the region of the heat exchanger in which thermal energy can be exchanged between the coolant and the fins . According to the invention, the fin stack therefore projects beyond the circulation width of the flat tube arrangement both on the air inlet side and the air outlet side. The area of the fins is therefore increased with respect to the region of the flat tubes which is responsible for the heat exchange. It has been surprisingly shown that it is thus possible to effect a considerable increase in capacity in comparison with a conventional heat exchanger, in which, with the same fin length, the circulation width of the flat tube arrangement is selected to be precisely the same length as the fins . The flat tubes can be constructed continuously in the air inflow direction, although it is also possible to arrange two or more separate flat tubes in succession in the inflow direction, which can adjoin one another directly or be spaced from one another by a gap. In this case, the gap between the individual flat tubes is taken into account when determining the circulation width. As seen along one of the flat tubes, the circulation width always begins with the first canal through which coolant circulates on the air inlet side and ends with the last canal through which coolant circulates on the outlet side.

As with the known design, an upper and a lower coolant chamber can be provided to distribute and to collect coolant, which coolant chambers are each in fluid communication with the coolant circulation canals of the flat tubes and are arranged perpendicularly to the flat tubes. The coolant chambers are generally connected to supply and discharge lines by way of which the heat exchanger is interconnected in a fluid circuit. The coolant chambers are preferably subdivided internally in order to control the flow of coolant through the individual flat tubes. It is possible here to let coolant flow in the opposite direction through the flat tubes and to arrange both the supply and discharge line for example in the upper coolant chamber. According to a preferred embodiment of the invention, the length of the fin stack corresponds substantially to the dimension of the upper and/or lower coolant chamber as seen in the air inflow direction. It is thus possible to make use of the entire maximum width of the heat exchanger.

The air inflow direction and the flow direction of the coolant in the flat tubes are preferably perpendicular to one another as is customary.

In conventional manner, the fins can be soldered, in particular brazed, to the flat tubes in order to ensure both the stability of the heat exchanger itself as well as a good heat transfer between the fins and the flat tubes .

At least some of the fins can be provided with air slots to enable an exchange of air between the air canals formed by the fins arranged above one another. In the air slots, the material of the fins can be flared out of the plane of the respective fin in the direction of the fin located above or below.

However, it has been shown that the capacity of the heat exchanger can be increased if the fins in the regions projecting beyond the first and beyond the second canal wall do not have air slots.

The total projection of the fin stack beyond the circulation width of the flat tube arrangement can be for example approximately one and a half times the width of the fin stack in the direction of stagger of the flat tubes. These dimensions have proven advantageous for increasing the capacity of the heat exchanger, although it is of course also possible to use other dimensions. In particular, in the air inflow direction, the length of the fin stack can be approximately 32 mm and the circulation width of the flat tube arrangement can be approximately 26 mm. The dimension of 32 mm corresponds to the currently popular dimension for evaporators in vehicle HVAC systems, which means that the heat exchanger according to the invention can be readily installed in place of a conventional heat exchanger.

On the air outlet side and/or on the air inlet side, it is possible to provide extensions, which extend in the air inflow direction and do not have coolant circulating through them, on the flat tubes, which extensions extend as an elongation of the circulation region of the flat tubes and are located between the fin stacks. These extensions do not form part of the circulation width of the flat tube arrangement. So that the overall dimensions of the heat exchanger are not increased, the extensions advantageously end in each case at the ends of the fin stacks. The length of the extensions together with the length of the circulation width of the flat tube arrangement can correspond substantially to the overall length of the fin stacks.

On the one hand, the extensions can have the function of specifically discharging condensation water which is moved along the fins. To this end, the extensions are constructed for example on the air outlet side as vertically extending water discharge surfaces.

On the other hand, the extensions can also be used to facilitate the assembly of the heat exchanger during the soldering of the fin stacks by forming a fixed bearing surface for the flat tubes.

In one possible embodiment, the flat tubes are constructed as extruded profiles and the extensions are part of the extruded profile, which means that they are constructed in one piece with the canals of the flat tubes during the extrusion process. The extensions can end for example in a T shape and be approximately as wide as the flat tubes as seen in the direction of stagger so that the gap between adjacent fin stacks is substantially closed in order to reliably discharge condensation water.

According to another embodiment, the extensions can be formed by a separate spacer component which has webs extending between two fin stacks as well as two transverse webs connecting the webs. The transverse webs here extend in particular parallel to the coolant chambers and can be arranged directly below or above the coolant chambers. The region covered by the lower transverse web can be constructed without fins in order to achieve better water drainage at the bottom of the heat exchanger and to reduce splash water.

The spacer component can also be used as an aid to assembling the heat exchanger in that the weight of the heat exchanger bears principally on the spacer component during soldering, which reduces the forces acting on the fins.

It should be noted that the flat tube arrangement does not have to comprise all the flat tubes of the heat exchanger. The flat tube arrangement described can also relate to only a sub-region of the heat exchanger in the direction of stagger. In theory, it is likewise unnecessary for all the fins of a fin stack to be longer than the circulation width; there could also be fins of a shorter design. Fin stacks are preferably provided at the ends of the heat exchanger, these are then only delimited by a flat tube on one side in each case . Although the heat exchanger is principally described as an evaporator within the frame of this application, it can however equally be used, for example, for heating the circulating air, i.e. for emitting heat from the coolant instead of absorbing heat.

It is particularly possible to use R744 as a coolant. The invention is described in more detail below with the aid of a plurality of examples and with reference to the attached drawings. The drawings show:

Figure 1 a schematic perspective illustration of an evaporator according to the invention;

Figure 2 a schematic cross-sectional view of a section of an evaporator according to the invention, according to a first embodiment;

Figure 3 a schematic cross-sectional view of a section of an evaporator according to the invention, according to a second embodiment; - Figure 4 a schematic cross-sectional view of a section of an evaporator according to the invention, according to a third embodiment; and

Figure 5 a schematic perspective section of the evaporator of Figure 4.

Figure 1 shows a heat exchanger 10 which can be used in particular as an evaporator in an HVAC system of a car. A plurality of flat tubes 12, whereof the flat sides 14 extend parallel to an air inflow direction and which are staggered parallel to one another in a direction of stagger S perpendicularly to an air inflow direction L, form a flat tube arrangement 16. The flat tube arrangement 16 can extend over the entire heat exchanger 10, although it can also constitute only a portion of the heat exchanger 10.

All flat tubes 12 of the flat tube arrangement 16 have coolant circulation canals 18 (see for example Figures 2 to 4) . The canals 18 are connected at an upper end 20 and a lower end 22 of the flat tube 12 to an upper coolant chamber 24 and a lower coolant chamber 26 respectively. The coolant chambers 24, 26 collect and distribute the coolant to the individual flat tubes 12. Internal subdivisions in the coolant chambers 24, 26 ensure that the flow of coolant through the individual flat tubes 12 (not illustrated) is controlled so that the coolant can also circulate through the flat tubes 12 in the opposite direction. In this example, the upper coolant chamber 24 is connected to a coolant inlet 28 and to a coolant outlet 30 which enable a connection to a fluid circuit of an HVAC system which is not illustrated in more detail.

The flat tubes 12 are arranged at a consistent interval in the direction of stagger S. Provided in the thus- defined clearances 32 is a respective fin stack 34 having a plurality of fins 40 arranged above one another in zigzag form as seen in plan view of an air inlet side 36 or an air outlet side 38 of the heat exchanger 10. The fins 40 are normally part of a folded metal strip, the individual folds adjoining one another in a zigzag shape or at a right angle. The fins 40 could therefore also naturally run parallel. Air ducts extending continuously from the air inlet side 36 to the air outlet side 38 are formed between the fins 40 in each case. The inflowing air therefore flows in the air inflow direction L through the fin stacks 34 and along the fins 40 from the air inlet side 36 to the air outlet side 38. During this, heat is absorbed from the fins 40 or emitted to them (depending on whether the heat exchanger is operating as an evaporator or as a heat pump) . The thermal energy is transmitted to the flat tube 12, and therefore to the coolant circulating therein, via a soldered connection. In particular, R477 can be used as a coolant.

Figure 2 shows a first embodiment of the heat exchanger 10. Since the individual embodiments only differ from one another in terms of details, the reference sign 10 is used for all embodiments. Likewise, already- established reference signs are maintained throughout for components which are not different or are only different in terms of details.

In all the embodiments, as seen in the air inflow direction L, the flat tube arrangement 16 has a circulation width b which is determined by the interval between a first canal wall 42 and a second canal wall 44 of the flat tubes 12 in the air inflow direction L. The first canal wall 44 in this case describes that canal wall of a coolant circulation canal 18 which is closest to the air inlet side 36 of the heat exchanger 10, whilst the second canal wall 44 denotes that canal wall of a coolant circulation canal 18 which is closest to the air outlet side 38.

As shown in Figures 2 to 4, it is possible to provide a plurality of flat tubes 12 in succession in the air inflow direction L in the flat tube arrangement 16, with a small gap 46 between the individual flat tubes 12 in the air inflow direction L. In this case, the circulation width b specifies the interval between the canal wall 42 of the flat tube 12 on the left in the figures, which is closest to the air inlet side, and the canal wall 44 of the flat tube 12 on the right in the figures, which is closest to the air outlet side, inclusive of the width of the gap 46. The individual fin stacks 34 can be folded in known manner from a single metal strip, each side of a fold forming a separate fin 40. In order to achieve an exchange of air between the individual air ducts between adjacent fins 40, air slots 48 in which the material of the fins 40 is flared upwards and/or downwards are provided in the fins 40 The flat tubes 12 and the fins 40 are arranged perpendicularly to one another so that the coolant flow and the air flow are likewise perpendicular to one another . The overall length b _L of the fin stack 34 in the air inflow direction L is greater than the circulation width b of the flat tube arrangement 16.

The fin stacks 34 project beyond the circulation width b of the flat tube arrangement 16, i.e. beyond the first and second canal wall 42, 44 which delimit the coolant circulation canals 18 of the flat tubes 12, both on the air inlet side 36 and the air outlet side 38. In this example, the projection I is distributed evenly on the air inlet side 36 and the air outlet side 38 so that the fin stacks 34 protrude over the first and second canal wall 42, 44 in each case by the length of 1/2. However, it would also be possible to select different lengths here. The projection 1/2 can be, in particular, 3 mm on each side with a circulation width b of 26 mm and a fin stack length b _L of 32 mm.

In this example, the length b _L of the fin stacks 34 also corresponds to the width of the coolant chambers 24, 26 in the air inflow direction L and therefore the total width of the heat exchanger 10. It is possible to dispense with the air slots 48 in the region of the projection I (see Figures 3 and 4) . It is also possible to dispense with the air slots 48 in the region of the gap 46 between two adjacent, successive flat tubes 12.

The total projection I is for example approximately one and a half times the width b _P of the fin stack 34 in the direction of stagger S.

It has been shown that, with this arrangement, it is possible to achieve an increase in capacity of approximately 10% over that of a conventional heat exchanger in which the circulation width b of the flat tubes 12 is equal to the length b _L of the fin stack 34 (and with the same fin stack width bp) .

Figures 3 and 5 show two further embodiments, in which the above-described features of the heat exchanger 10 are present in identical manner.

In the only essential difference from the first embodiment, extensions 60 are provided on the flat tubes 12 in the second embodiment shown in Figure 3, which extensions extend as an elongation of the flat tubes 12 of the first embodiment in the air inflow direction L and do not have coolant flowing through them. These extensions 60 do not form part of the circulation width b of the flat tube arrangement 16.

On the one hand, the extensions 60 serve for discharging condensation water from the fin stacks 34. On the other hand, they can be used as an aid to assembly when soldering the fin stacks 34 to the flat tubes 12.

In the example of Figure 3, the extensions 60 are of a T-shaped design, their dimension in the direction of stagger S corresponding approximately to the dimension of the flat tubes 12 in this direction so that they at least substantially close the gaps between the individual fin stacks 34. The T-shaped end forms water discharge surfaces 62 along which condensation water can flow off downwards.

In this example, extensions 60 are provided both on the air outlet side 38 and on the air inlet side 36, said extensions extending respectively to the ends of the fin stacks here. However, it would also be conceivable for the extensions 60 to be shorter in design or to provide extensions 60 only on the air outlet side 38. The flat tubes 12 here are each constructed as an extruded profile and the extensions 60 form a single- piece portion of this extruded profile; they are therefore extruded together with the canals 18 during manufacture .

Figures 4 and 5 show a further embodiment, in which the extensions 160 are not constructed in one piece with the flat tubes 12 but are constructed as a separate spacer component 164. The spacer component 164 here consists of a plurality of vertically extending webs 166, which each cover the clearance between two fin stacks 34, and two transversely extending webs 168, which connect all the vertically extending webs 166 to one another. The vertically extending webs 166 can be constructed in a V shape, with the V opening towards the air outlet side 34 (see Figure 4) . The webs 166 form water discharge surfaces 162.

In this example, only one spacer component 164 is provided, which is arranged on the air outlet side 38.

The transverse webs 168 here are located directly below the upper coolant chamber 24 and directly above the lower coolant chamber 26 (Figure 5 shows only the upper part of the heat exchanger 10) .

The spacer component 164 can be manufactured from aluminium sheet by means of a forming process.

The spacer component 164 is mounted on the outside of the heat exchanger 10, it being possible for the heat exchanger 10 to bear against the spacer component 164 during the soldering procedure.

In this example, both the extensions 60 and the extensions 160 extend substantially over the entire length of the projection 1/2 of the fin stacks 34 beyond the circulation width b of the flat tube arrangement 16.

Example 1 A heat exchanger 10 described above produces for example a cooling capacity of approximately 11 kW as an evaporator .

As a result of the fin stack 34 projecting beyond the circulation width b of the flat tube arrangement 16, with a circulation width b of 26 mm and a fin stack length b _L of 32 mm, the heat transfer area can be increased by approximately 23%. If an internal heat transfer coefficient is 2500 W/m ² K and an external heat transfer coefficient _a is 250 W/m ² K in the regions which are in direct contact with the flat tube 12 and 120 W/m ² K in regions which are not in direct contact with the flat tube 12, with an air passage of 500 kg/h in each case, the result is a mean value _a of 220 W/m ² K. For a standardized 26 mm wide heat exchanger core, the ratio between the external and internal heat transfer area is A _a / A _± = 4. For a 32 mm core, this ratio is 4.92. This results in a heat transfer coefficient k=l/ ( A _a /Ai/oii+l/oii) of 157 W/m ² K, i.e. approximately 12% less than for a 26 mm core. For a specific heat exchanger capacity of k A _a , with 193 W/K there is an increase in capacity of approximately 8.5% over that of the standard heat exchanger core which is 26 mm wide.

This applies for an air inlet temperature of 25°C and an air outlet temperature of 3°C.

To cool inflowing air from 25°C to 3°C, it is possible to increase the evaporating temperature accordingly from -3°C to -1.8°C when compared to a conventional heat exchanger. The efficiency of the heat exchanger 10 is therefore increased from 78.5% to 82%. For this reason, icing of the fins 40 is also lower. With an air temperature of 3°C and an evaporating temperature of -1.8°C, the surface temperature at the fins 40 is still slightly above 0°C, which means that icing does not occur.

Example 2 :

Operated as an evaporator, the heat exchanger 10 described above has a cooling capacity of approximately 11 kW with an air inlet temperature of 40°C, a relative humidity of the air of 40%, a coolant pressure at a coolant inlet valve (not illustrated) of 12 MPa, a coolant pressure at a coolant discharge 30 of the heat exchanger 10 of 3.5 MPa, a coolant temperature at the inlet valve of the heat exchanger 10 of 30°C and a circulating air quantity of 600 kg/h. This applies for the above-mentioned lengths of the fin stacks 34 of 32 mm and the circulation width b of the flat tube arrangement 16 of 26 mm.

Example 3 :

If the heat exchanger 10 described above is operated as an evaporator, with an air temperature at the air outlet side 38 of 2°C, an air temperature at the air inlet side 36 of 25°C, a relative humidity of the circulating air of 80% and an air throughflow of 600 kg/h, then it is possible to achieve an efficiency of the heat exchanger 10 of approximately 81%.

In particular, at this operating point, icing of the fins 40 does not occur for at least 15 minutes, in particular for at least 30 minutes, since a temperature of slightly above 0°C is established at the fins 40.