EP2095053A1

EP2095053A1 - Charge minimized heat exchanger

Info

Publication number: EP2095053A1
Application number: EP06818951A
Authority: EP
Inventors: Hans-Joachim Huff
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2006-12-01
Filing date: 2006-12-01
Publication date: 2009-09-02
Also published as: WO2008064709A1

Abstract

Heat exchanger (2) comprising an inlet header (4), an outlet header (6) and a plurality of heat exchanger tubes (8) connecting the inlet header (4) and the outlet header (6) with each other, and further comprising a volume reducing insert (14) in at least one of the headers (4,6).

Description

CHARGE MINIMIZED HEAT EXCHANGER

The present invention relates to a heat exchanger comprising an inlet header, an outlet header and a plurality of heat exchanger tubes connecting the inlet header and the outlet header with each other.

Heat exchangers of such type are frequently used in refrigeration circuits like car and building air conditioning, household refrigerators, refrigeration displays, etc. Particularly heat exchangers which are used for cooling, possibly liquifying the compressed refrigerant are required to withstand substantial pressures of the refrigerant. Therefore, particularly the headers are designed, so as to withstand such high pressures with the effect that the headers have typically a relatively large volume. This is, generally, of no concern and the refrigerant charge of the system is correspondingly larger than the refrigerant charge which would be required to provide the refrigeration performance.

A problem, however, exists in some specific situations, for example if the amount of refrigerant charge is to be limited due to costs and weight reasons or due to the fact that the allowable charge of a burnable or environmentally hazardous refrigerant is limited by law. For example in case of flammable or burnable refrigerant like propane, the maximum amount of charge in the system is subject to legal requirements. For example in Europe, the maximum propane charge is 15O g per refrigeration circuit. Thus, measures are taken to reduce the amount of charge in the system to what is necessary to obtain the required refrigeration performance. In doing so, the cross-sectional areas of the conduits in the refrigeration circuit are reduced to a size, which, on the one hand reduces the volume thereof and on the other hand is still sufficient for the fluid dynamic be- haviour of the system so as to avoid restriction in the conduits.

For reducing the volume of the heat exchanger, conventional heat exchangers have been replaced by mini-channel heat exchangers which typically comprise

10 to 20 heat exchanger channels within a single heat exchanger tube. The mini channel heat exchanger may comprise any other number of heat exchanger channels within each heat exchanger tube. The heat exchanger tube is, due to manufacturing reasons etc., made of aluminium material and the individual channels are formed during the manufacturing process and are arranged within the aluminium material heat exchanger tube like bores extending the length thereof. The channels can have a various cross sectional shapes, like round, square, rectangular, triangular, elliptical, etc. The use of such mini channel heat exchanger has substantially reduced the refrigerant charge in the refrigeration circuit. Nevertheless there is the need to further reduce the amount of charge of refrigerant in the refrigeration circuit.

Accordingly, it is an object of the present invention to provide a heat exchanger which allows for a further reduction of the refrigerant in the refrigeration circuit.

In accordance with the present invention, this object is solved by a heat ex- changer comprising an inlet header, an outlet header and a plurality of heat exchanger tubes connecting the inlet header and the outlet header with each other, and further comprising a volume reducing insert in at least one of the headers.

As such, the present invention is not going to make any further attempt to re- duce the volume as encased by the heat exchanger which would result in substantially increasing the wall thickness of the heat exchanger walls, the weight and the manufacturing costs thereof. Instead, it uses conventional heat exchangers and places a low cost volume reducing insert into at least one of the headers of the heat exchanger. Such insert can be sized and shaped as appropri- ate and can also be located within the header in any desired position. A substantial reduction of the refrigerant charge of the system can thus be obtained with minimum expenses.

The heat exchanger can be a mini channel heat exchanger which comprises mini channel tubes which are each inserted through an opening of the header and extent into the header and are brazed thereto. As mentioned, the mini channel tubes are typically made of aluminium or aluminium alloy material. The brazing together of the mini channel tubes and the header is accomplished by way of applying a paste of braze material onto the interface between the mini channel tube and the header and placing the complete arrangement within an oven, where the arrangement is heated to a temperature above the melting point of the braze material. The braze material will flow and will fill the gap between the opening of the header and the outer circumference of the mini channel tube. In order to avoid wicking of the braze material into the small channels of the mini channel tubes, it is necessary to have the ends of the mini channel tube extending for a certain distance into the header. If the distance by which the mini channel tubes extend into the header is to small, liquid braze material will flow into the channels of the mini channel tubes and block at least some of them. The width of the mini channel tubes and the distance they extend into the interior of the header are two decisive dimensions which govern the shape and dimension of the header. In other words even if the skilled person would change to a cross section of the header other than circular which is preferred for pressure reasons, the minimum volume of the header would still be governed by the distance of the tubes extending into the header and the width of the individual tubes so that a reduc- tion of the volume of the header cannot easily be accomplished. The combination of the insert with the mini channel heat exchanger allows to substantially reduce the charge of the refrigerant within the refrigeration circuit. On the other hand, it substantial increases the refrigeration performance for a preset maximum refrigerant charge.

The header may have a tubular shape and may in particular have a circular or elliptical cross section. The header can be made from a thin sheet metal, for example a thin-walled metal tube, etc. The circular or elliptical cross section allows for a very even distribution of the pressure forces in the header.

It can be advantageous to position the insert in the outlet header, particularly where the heat exchanger is used as the condenser of the refrigeration system. Here the insert reduces the volume of the pressurized, liquid refrigerant and is particularly effective for reducing the refrigerant charge.

It is possible to locate the insert in the inlet header and the outlet header so as to form an expansion device. This can be done, for example in the outlet header by using/positioning the insert so as to reduce the cross-sectional area for the flow of the refrigerant as compared to the total cross section of flow area of the heat exchanger tubes. Similarly, the insert can be arranged in the inlet header so as to reduce the cross-sectional area in the inlet header as compared to that of the inlet conduit leading into the inlet header. Particularly within the outlet header the insert can be arranged so as to provide a partial expansion device which further reduces the amount of charge in the header by reducing the density of the refri- gerant in the remaining open volume of the header downstream of the insert. Such partial expansion does not necessarily have an adverse effect on the system performance, since the header does not significantly contribute to the heat transfer particularly with mini channel heat exchangers.

The tubular header can have caps or closing elements at its respective ends and it can be preferred to attach the insert to such closing elements. On the one hand this allows for a easy manufacture, since the closing elements are generally readily accessible for attachment of the insert thereto. On the other hand, this allows for welding a tubular insert readily sealingly into the header. A tubular insert can be preferred particularly for weight reasons. The insert can have a tubular shape of circular, elliptical or other cross-section, independent from its attachments to the header.

The insert can be attached within the header so as to be spaced or separated from the inner walls thereof in order to leave a refrigerant passage between the header and the insert. In case of a tubular header and attachment of the insert to the closing elements thereof, the passage can be uninterrupted as seen in a sectional view which will allow un-interrupted flow of the refrigerant through such passage.

The insert can comprise a closed outer surface which is impermeable to the refrigerant. Consequently, the refrigerant cannot penetrate into the insert but will flow there around. It is also possible to have an insert which comprises an open cell material which comprises internal flow passages. Such a material can be used as a partial expansion device as mentioned above in the inlet and/or the outlet headers. It is also possible to use such open cell or sponge-like material in the inlet of the heat exchanger to equally distribute the inflowing refrigerant to the individual heat exchanger tubes. In such case, it might be preferred to connect the open cell material to the inlets to the heat exchanger tubes and/or the outlet of the inlet conduit leading into the inlet header. The volume of the insert occupies preferably a fraction of between approximately 15 to 90%, approximately 25 to 80%, approximately 40 to 80% and possibly approximately 50 to 80% of the volume of the header. For cases without in- tended pressure reduction in the header, the insert should be sized such that the value of the ratio of refrigerant mass flow rate to free cross sectional area is similar to the corresponding value of this ratio in the channels. For cases with intended pressure reduction in the header, the insert should be sized such that the value of this ratio is smaller that the value of the ratio in the channels.

The invention further relates to a refrigeration circuit comprising a heat exchanger in accordance with the present invention. Such refrigeration conduit can be used in a building or vehicle air-conditioning system, a household refrigerator, a supermarket refrigerator, etc.

The present invention will now be described, by way of example, with reference to the following Figures, in which:

Figure 1 is a schematic lateral view of a heat exchanger in accordance with the present invention;

Figure 2 is a sectional view of the heat exchanger of Figure 2 taken at 2-2 of Figure 1 ; and

Figure 3 is a schematic lateral view of an alternative embodiment of a heat ex- changer in accordance with the present invention.

Figure 1 shows a heat exchanger 2 in accordance with the present invention. The heat exchanger 2 comprises an inlet header 4, an outlet header 6 and a plurality of heat exchanger tubes 8 connecting the inlet header 4 and the outlet header 6 with each other. An inlet conduit 10 and an outlet conduit 12 are provided for connecting the heat exchanger 2 with the remaining components of a refrigeration circuit (not shown) which typically comprises serially connected with each other in flow direction a compressor, a condenser, an expansion device and an evaporator which is finally connected to the compressor again. The condenser and the evaporator are both heat exchangers. One or both thereof can be a heat exchanger in accordance with the present invention.

Particularly, the heat exchanger 2 is a mini channel heat exchanger which is typically used in combination with a fan or blower to increase the heat exchange by forcing air over the individual heat exchanger tubes 8. The heat exchanger tubes 8 are typically made of extruded aluminium material and comprise a plurality, typically between 10 and 20 bore like conduits each having a diameter of approximately 1 mm. As can be seen by comparing the Figures 1 and 2, the heat exchanger tubes 8 have a width as seen in Figure 2 which is larger than the height thereof as seen in Figure 1. A typical heat exchanger tube can have a width of approximately 18 mm and a hight of approximately 3 mm. The invention is not limited to a mini channel tube heat exchanger with such dimensions.

As can be seen in Figure 2, the inlet header 4 and the outlet header 6 are both of cylindrical shape. A header can be made from thin-walled tubes made of alumini- um material or any other material to which the heat exchanger tubes 8 can be brazed. The individual heat exchanger tubes project through respective openings in the headers 4, 6 into the interiors thereof and are brazed thereto. Similarly, the inlet conduit 10 and the outlet conduit 12 can be soldered or brazed to the head- ers 4, 6. As shown in Figure 1 , both conduits 10, 12 are mounted centrally to the headers 4 and 6. Practically, the conduits 10 and 12 are frequently mounted to the headers 4, 6 so that the inlet conduit 10 will be close to the upper part of the inlet header 4, while the outlet conduit 12 will be close to the bottom of the outlet header 6.

As can be seen in Figure 2, the diameter of the headers 4, 6 is dependent on the width of the heat exchanger tubes 8. The circular cross section is preferred for the headers 4, 6 to sustain the pressure in the system. As can be further seen in the Figures, headers 4 and 6 encompass a substantial volume and the refrigerant in such volume will typically not contribute to the systems performance. If used as a condenser in the refrigeration circuit, the inlet header 4 typically includes gaseous refrigerant, while the outlet headers 6 includes liquid refrigerant. Accordingly, the density of the refrigerant in the outlet header 6 is substantially higher than in the inlet header 4 and accordingly more mass of refrigerant is in the outlet header 6 as compared to the inlet header 4.

An insert 14 is provided within one of the headers 4, 6 and, as shown in the Figures, in the outlet header 6. Generally, the insert 14 can be of any shape. Generally, a shape corresponding more or less to the shape of the space as confined by the inner walls of the header 4, 6 but somewhat smaller than that is preferred. The insert 14 can be shaped such as to resemble the shape of the the free internal volume formed by the inner walls of the header 6 and the outer walls of the heat exchanger tubes 8 placed inside of the header 6. The insert 14 can be placed in the space as defined by the header 6 so as to allow passage of the re- frigerant there around and towards the outlet conduit 12 (or towards the heat exchanger tubes 8 if placed in the inlet header 4). In order to provide flow passages free of disturbances, it might be preferred to attach the insert 14 to cover elements 16, 18, 20, 22 which close the respective ends of the headers 4, 6. The insert 14 can be a single insert as shown in the Figures, but likewise a plurality of inserts 14 can be present. It is possible to use one or a plurality of tube-like inserts for heat exchanger purposes. Thus it is possible to direct gaseous refrigerant leaving the evaporator through such insert tubes in order to further cool down the liquid refrigerant leaving the outlet header 6 and at the same time superheating the gaseous refrigerant.

The insert can be made of any suitable material, for example any aluminium material which allows easy brazing thereof to the remainder of the header 4, 6. Al- ternatively, any other material which is impermeable to the refrigerant and which is sufficiently corrosion resistant can be used. It is also possible to use a potting material which can be filled into the header in liquid form and cured therein, for example in the lateral areas of the headers 4, 6 as seen in the sectional view of Figure 2 so as to retain the flow path between the heat exchanger tubes and the conduits 10, 12. Such potting material, might, however, affect equal pressure distribution within the headers 4, 6.

Fig. 3 shows an alternative embodiment of a heat exchanger 2. Like references are used throughout the figures and refer to corresponding elements, so that a detailed description of Fig. 3 can be omitted as far as they had already been described in Fig. 1 and 2. The main difference in the embodiment of Fig. 3 as compared to that of the previous embodiment relates to the shape of the insert 14. Particularly, insert 14 is shaped so as to provide an increased volume inside the outlet header 6 closer to the outlet conduit 12. The volume may increase gradu- ally or in a stepped manner. This accommodates the increased mass flow rate closer to the outlet conduit 12. The volume increase is preferably so as to provide a constant resistance within each of the heat exchanger channels 8. Alternatively, a predetermined resistance distribution for each individual heat exchanger channel 8 can be designed by selecting a specific shape of the insert 14. Vice versa it is also possible to accordingly shape an insert 14 which is placed within the inlet header 4 so as to provide for a desired mass flow rate therein.

Fig. 3 further shows the inlet conduit 10 placed in the upper area of the inlet header 4 while the outlet conduit 12 is placed next to the bottom of the outlet header 6. It is also possible in the embodiment of Fig. 1 to have a specifically shaped insert 14 in order to provide for a desired mass flow rate therein in the case of having the inlet and outlet conduits 10, 12 placed more or less centrally.

Claims

1. Heat exchanger (2) comprising an inlet header (4), an outlet header (6) and a plurality of heat exchanger tubes (8) connecting the inlet header (4) and the outlet header (6) with each other, and further comprising a volume reducing insert (14) in at least one of the headers (4, 6).

2. Heat exchanger (2) according to claim 1 , wherein the heat exchanger tubes (8) are mini channel tubes which are inserted through an opening of the header (4, 6), extend into the header and are brazed thereto.

3. Heat exchanger (2) according to claim 1 or 2, wherein the header (4, 6) with the insert (14) has a tubular shape.

4. Heat exchanger (2) according to claim 3, wherein the header (4, 6) with the insert (14) has a circular or elliptical cross section.

5. Heat exchanger (2) according to any of claims 1 to 4, wherein the header (4, 6) with the insert (14) is made from a thin sheet material.

6. Heat exchanger (2) according to any of claims 1 to 5, wherein the insert (14) is positioned in the outlet header (6).

7. Heat exchanger (2) according to claim 6, wherein the insert (14) is placed in the outlet header (6) so as to reduce the cross sectional area in the outlet header (6) as compared to the total cross sectional area of the heat exchanger tubes (8) so as to act as an expansion device in use.

8. Heat exchanger (2) according to any of claims 3 to 7, further comprising closing elements (16, 18, 20, 22) closing the tubular header (4, 6) at its ends and wherein the insert (14) attached at least to one of such closing elements (16, 18, 20, 22).

9. Heat exchanger (2) according to any of claims 3 to 8, wherein the insert (14) has a tubular shape.

10. Heat exchanger (2) according to any of claims 3 to 9, wherein the insert (14) is attached within the tubular header (4, 6) and separated from the inner walls thereof, so as to leave a refrigerant passage between the tubular header (4, 6) and the insert (14).

1 1. Heat exchanger (2) according to any of claims 1 to 10, wherein the insert (14) comprises a closed outer surface which is impermeable to the refrigerant.

12. Heat exchanger (2) according to any of claims 1 to 1 1 , wherein the insert (14) comprises an open cell material which comprises internal flow passages and is connected to inlets of the heat exchanger tubes (8).

13. Heat exchanger (2) according to any of claims 1 to 12, wherein the volume of the insert (14) occupies a fraction of between approximately 15 to 90%, approximately 25 to 80%, 40 to 80%, and possibly 50 to 80% of the volume of the header (4, 6).

14. Heat exchanger (2) according to any of claims 1 to 13, wherein the insert (14) is sized so that in use the value of the ratio of the refrigerant mass flow rate to free cross sectional area is approximately the same as the corresponding value of this ratio in the heat exchanger tubes (8).

15. Heat exchanger (2) according to any of claims 1 to 13, wherein the insert (14) is sized so that in use the value of the ratio of the refrigerant mass flow rate to free cross sectional area is smaller than the corresponding value of this ratio in the heat exchanger tubes (8).

16. Refrigeration circuit comprising a heat exchanger (2) according to any of claims 1 to15.