Heat Pump Comprising a First Heating Circuit and a Second Heating Circuit

Abstract

Heat pump comprising a first and a second heating circuit, comprising: an evaporator; one or more than one expansion valve; a compressor; a first condenser arranged in the first heating circuit and a second condenser arranged in the second heating circuit, wherein the first and the second heating circuit share a common line; wherein, during normal operation, only one of the first heating circuit and the second heating circuit defines an active circuit, whereas the other one simultaneously defines an inactive circuit; and wherein the heat pump further comprises a distributor comprising at least two valves, and a controller that is configured to selectively redistribute a refrigerant charge from the first heating circuit to the second heating circuit or vice versa to thereby, for the active circuit, actively optimize the amount of refrigerant in the condenser and in the evaporator for the actual operating conditions.

Claims

1. A heat pump comprising a first heating circuit and a second heating circuit, said heat pump comprising: an evaporator; one or more than one expansion valve that is arranged upstream of the evaporator; a compressor that is arranged downstream of the evaporator; a first condenser arranged in the first heating circuit and a second condenser arranged in the second heating circuit, wherein the first heating circuit and the second heating circuit share a common line passing through the evaporator and the compressor; wherein, during normal operation, only one of the first heating circuit and the second heating circuit defines an active circuit that is involved in an active heating cycle, whereas the other one of the first heating circuit and the second heating circuit simultaneously defines an inactive circuit; and a distributor comprising at least two valves, wherein the distributor comprises a controller that is configured to selectively redistribute a refrigerant charge from the first heating circuit to the second heating circuit by controlling the at least two valves of the distributor in dependency of actual operating conditions, wherein the controller of the distributor is configured to selectively redistribute a refrigerant charge from the first heating circuit to the second heating circuit or vice versa by controlling the at least two valves of the distributor in dependency of actual operating conditions, to thereby, for the active circuit, actively optimize the amount of refrigerant in the condenser and in the evaporator for the actual operating conditions, and wherein excess refrigerant charge that is not needed for optimizing the refrigerant charge in the active circuit is temporarily stored in the inactive circuit.

2. The heat pump according to claim 1, wherein the controller is configured to redistribute the refrigerant charge from the first heating circuit to the second heating circuit or vice versa, in dependency of the actual operating conditions that are defined by a level of subcooling of the refrigerant in at least one of the first heating circuit and the second heating circuit.

3. The heat pump according to claim 2, wherein the controller is configured to determine at least one of: a level of subcooling in the first heating circuit by calculating a temperature difference between a condensing temperature at the first condenser and a temperature of the refrigerant leaving said first condenser; or a level of subcooling in the second heating circuit by calculating a temperature difference between a condensing temperature at the second condenser and a temperature of the refrigerant leaving said second condenser.

4. The heat pump according to claim 3, wherein the controller is configured to at least one of: redistribute refrigerant charge from the first heating circuit to the second heating circuit if the subcooling in the first heating circuit is above a pre-determined upper threshold temperature difference; redistribute refrigerant charge from the second heating circuit to the first heating circuit if the subcooling in the first heating circuit is below a pre-determined lower threshold temperature difference; redistribute refrigerant charge from the second heating circuit to the first heating circuit if the subcooling in the second heating circuit is above a pre-determined upper threshold temperature difference; or redistribute refrigerant charge from the first heating circuit to the second heating circuit if the subcooling in the second heating circuit is below a pre-determined lower threshold temperature difference.

5. The heat pump according to claim 4, wherein the temperature difference between the lower threshold temperature difference and the upper threshold temperature difference is in the range of 0.2 to 3 C.

6. The heat pump according to claim 4, wherein the lower threshold temperature difference is in the range of 0.5 to 1.2 C.

7. The heat pump according to claim 4, wherein the upper threshold temperature difference is in the range of 0.3 to 1.2 C.

8. The heat pump according to claim 1, further comprising: one or more than one further condenser arranged in a further heating circuit, wherein the first heating circuit, the second heating circuit and the one or more than one further heating circuit share the common line passing through the evaporator and the compressor, wherein the distributor is configured to redistribute a refrigerant charge from at least one of the first heating circuit, the second heating circuit and the one or more than one further heating circuit to at least one other of the first heating circuit, the second heating circuit and the one or more than one further heating circuit.

9. The heat pump according to claim 1, comprising a branch that is arranged downstream of the compressor and configured to branch the common line off into a first line associated with the first heating circuit and a second line associated with the second heating circuit.

10. The heat pump according to claim 9, comprising a combiner that is arranged upstream of the evaporator and configured to re-combine the first line of the first heating circuit and the second line of the second heating circuit into the common line.

11. The heat pump according to claim 9, wherein the distributor comprises a three-way valve that defines one of the branch and the combiner.

12. The heat pump according to claim 11, wherein the distributor comprises a further three-way valve that defines the other one of the branch and the combiner.

13. The heat pump according to claim 9, wherein the distributor comprises at least two shut-off valves, wherein: a first shut-off valve is arranged downstream of the second condenser and upstream of the combiner; and a second shut-off valve is arranged downstream of the first condenser and upstream of the combiner.

14. The heat pump according to claim 13, wherein: the first shut-off valve is defined by a second expansion valve of the one or more than one expansion valve, wherein this second expansion valve is arranged downstream of the second condenser and upstream of the combiner; and the second shut-off valve is defined by a first expansion valve of the one or more than one expansion valve, wherein this first expansion valve is arranged downstream of the first condenser and upstream of the combiner.

15. The heat pump according to claim 10, wherein the distributor comprises at least two shut-off valves, wherein: a third shut-off valve is arranged downstream of the branch and upstream of the first condenser; and a fourth shut-off valve is arranged downstream of the branch and upstream of the second condenser.

16. The heat pump according to claim 1, wherein the distributor is configured to selectively activate one of: the first heating circuit by opening the third shut-off valve and the second shut-off valve and closing the fourth shut-off valve and the first shut-off valve; and the second heating circuit by opening the fourth shut-off valve and the first shut-off valve and closing the third shut-off valve and the second shut-off valve.

17. The heat pump according to claim 1, wherein the distributor is configured to selectively redistribute the refrigerant charge: from the first heating circuit to the second heating circuit by opening the second shut-off valve and the fourth shut-off valve while the first shut-off valve and the third shut-off valve are closed; or from the second heating circuit to the first heating circuit by opening the first shut-off valve and the third shut-off valve while the second shut-off valve and the fourth shut-off valve are closed.

18. (canceled)

19. The heat pump according to claim 5, wherein the temperature difference between the lower threshold temperature difference and the upper threshold temperature difference is in the range of 0.5 to 2 C.

20. The heat pump according to claim 5, wherein the temperature difference between the lower threshold temperature difference and the upper threshold temperature difference is in the range of 1 to 1.5 C.

Description

[0025] In the following description preferred embodiments of the present invention are further elucidated with reference to the drawing, in which:

[0026] FIG. 1A is a schematic view of a prior art heat pump in a first mode, e.g. a heating mode;

[0027] FIG. 1B is a schematic view of a prior art heat pump in a second mode, e.g. a cooling mode;

[0028] FIG. 2A is a schematic view of a heat pump according to a first preferred embodiment of the invention, comprising two heating circuits, wherein a first heating circuit of the two heating circuits is active in a heating mode;

[0029] FIG. 2B is a schematic view of the heat pump according to the first preferred embodiment of the invention, wherein a second heating circuit of the two heating circuits is active in a heating mode;

[0030] FIG. 2C is a schematic view of the heat pump according to the first preferred embodiment of the invention, wherein the first heating circuit of the two heating circuits is active in a cooling mode;

[0031] FIG. 3A is a schematic view of a heat pump according to a second preferred embodiment of the invention, comprising two heating circuits, wherein a first heating circuit of the two heating circuits is active in a heating mode;

[0032] FIG. 3B is a schematic view of the heat pump according to the second preferred embodiment of the invention, wherein the refrigerant charge in the first heating circuit is increased by migrating refrigerant charge from the second heating circuit to the first heating circuit;

[0033] FIG. 3C is a schematic view of the heat pump according to the second preferred embodiment of the invention, wherein the refrigerant charge in the first heating circuit is reduced by migrating refrigerant charge from the first heating circuit to the second heating circuit;

[0034] FIG. 4 is a schematic view of a heat pump according to a third preferred embodiment of the invention, comprising three heating circuits; and

[0035] FIG. 5 is a schematic view of a heat pump according to a fourth preferred embodiment of the invention, wherein the first and the second heating circuit each comprise a dedicated expansion valve that also functions as the shutoff-valve in said respective heating circuit.

[0036] The prior art reversible heat pump 1 shown in FIGS. 1A and 1B comprises an evaporator 2, a compressor 3, a condenser 4, and an expansion valve 5. A controller 6 may control the setting of a four-way reversing valve 7 to allow the flow direction of a refrigerant inside the heating circuit to be reversed between a heating mode (FIG. 1A) and a cooling mode (FIG. 1B), as well as other components, such as the compressor 3.

[0037] In the heating mode of FIG. 1A, the evaporator 2 is capable of extracting heat from air (such as ambient air or air of an indoor ventilation system) or from the earth. The temperature of the refrigerant may be increased by the compressor 3 compressing the refrigerant, after which this heat may be extracted from the refrigerant in the condenser 4, where it may be used for e.g. indoor heating.

[0038] In the cooling mode of FIG. 1B, the flow direction of the refrigerant is reversed relative to the flow direction of FIG. 1A. The heat exchanging element that functioned as the evaporator 2 in FIG. 1A now acts as the condenser 4, and the heat exchanging element that functioned as the condenser 4 in FIG. 1A now acts as the evaporator 2.

[0039] A first preferred embodiment of the invention is shown in FIGS. 2A-2C, wherein the heat pump 1 comprises a first heating circuit C.sub.1 and a second heating circuit C.sub.2. The heat pump 1 comprises an evaporator 2, an expansion valve 5 that is arranged upstream of the evaporator 2, and a compressor 3 is arranged downstream of the evaporator 2. A first condenser 41 is arranged in the first heating circuit C.sub.1 and a second condenser 42 is arranged in the second heating circuit C.sub.2, wherein the first heating circuit C.sub.1 and the second heating circuit C.sub.2 share a common line 21 passing through the evaporator 2 and the compressor 3. In this first preferred embodiment, the common line 21 also passes through the expansion valve 5, but this is not essential, as will be elucidated with reference to the fourth preferred embodiment shown in FIG. 5. The heat pump 1 further comprises a distributor 12 comprising at least two valves V, wherein the distributor is configured to redistribute a refrigerant charge from the first heating circuit C.sub.1 to the second heating circuit C.sub.2 or vice versa by controlling the at least two valves V. In the first preferred embodiment, the two valves V comprise two three-way valves T.sub.1 and T.sub.2. It is remarked that the distributor 12 comprises the controller 6 and the valves V. For clarity, only the control lines between the controller 6 and the valves V that belong to the distributor 12 are shown with dashed lines. In addition, the controller 6 may comprise further (not shown) control lines to other components, such as the compressor 3 and the optional four-way reversing valve 7.

[0040] The four-way reversing valve 7 is optional and only necessary to make the heat pump 1 reversible, also allowing a cooling mode as shown in FIG. 2C. FIG. 2C may be considered a defrost mode, wherein the evaporator 2 is defrosted.

[0041] The first condenser 41 is arranged in the first heating circuit C.sub.1, which in FIG. 2A-2C is connected to a space heating circuit 8 that is configured to provide a continuous heated water flow for indoor heating when it is active. The active state of the first heating circuit C.sub.1 is represented by the thick lines in FIGS. 2A and 2C.

[0042] The second condenser 42 is arranged in the second heating circuit C.sub.2, which in FIGS. 2A-2C is connected to a water tank 11 of a domestic water heating circuit. The second condenser 42 may be used to heat up the water in the water tank 11. The active state of the second heating circuit C.sub.2 is represented by the thick lines in FIG. 2B.

[0043] Heat pump 1 comprises a branch 13 that is arranged downstream of the compressor 3 and configured to branch the common line 21 off into a first line L.sub.1 associated with the first heating circuit C.sub.1 and a second line L.sub.2 associated with the second heating circuit C.sub.2.

[0044] Heat pump 1 further comprises a combiner 14 that is arranged upstream of the evaporator 2 and configured to re-combine the line L.sub.1 of the first heating circuit C.sub.1 and the line L.sub.2 of the second heating circuit C.sub.2 into the common line 21. For this first preferred embodiment, as well as for the second preferred embodiment (FIGS. 3A-3C) and the third preferred embodiment (FIG. 4), the combiner 14 is also arranged upstream of the expansion valve 5. This is however not essential, as becomes apparent from the fourth preferred embodiment that is shown in FIG. 5.

[0045] In the first preferred embodiment shown in FIGS. 2A-2C, the distributor 12 comprises a three-way valve T.sub.1 that defines the combiner 14, and a further three-way valve T.sub.2 that defines the branch 13. By selectively setting the two three-way valves T.sub.1 and T.sub.2, the controller 6 may activate the first heating circuit C.sub.1 or the second heating circuit C.sub.2, but also control the distributor 12 to redistribute the refrigerant charge from the first heating circuit C.sub.1 to the second heating circuit C.sub.2 or vice versa. In order to cause this redistribution of refrigerant charge, the first heating circuit C.sub.1 and the second heating circuit C.sub.2 are temporarily connected to each other. How this works, will be discussed in more detail when discussing the next, even more preferred, second embodiment.

[0046] A second and even more preferred embodiment of the invention is shown in FIGS. 3A-3C. This second preferred embodiment is closely related to the first embodiment, except that the distributor now comprises at least two shut-off valves S.sub.1, S.sub.2.

[0047] Also for this second preferred embodiment, the heat pump 1 comprises a first heating circuit C.sub.1 and a second heating circuit C.sub.2. The heat pump 1 comprises an evaporator 2, an expansion valve 5 that is arranged upstream of the evaporator 2, and a compressor 3 is arranged downstream of the evaporator 2. A first condenser 41 is arranged in the first heating circuit C.sub.1 and a second condenser 42 is arranged in the second heating circuit C.sub.2, wherein the first heating circuit C.sub.1 and the second heating circuit C.sub.2 share a common line 21 passing through the evaporator 2 and the compressor 3. In this embodiment, the common line 21 also passes through the expansion valve 5. The heat pump 1 further comprises a distributor 12 comprising at least two valves V, wherein the distributor 12 is configured to redistribute a refrigerant charge from the first heating circuit C.sub.1 to the second heating circuit C.sub.2 or vice versa by controlling the at least two valves V.

[0048] Instead of the two three-way valves T.sub.1, T.sub.2 of the first preferred embodiment, the at least two valves V of the distributor 12 now comprises at least two shut-off valves S.sub.1, S.sub.2, wherein a first shut-off valve S.sub.1 is arranged downstream of the second condenser 42 and upstream of the combiner 14, and a second shut-off valve S.sub.2 is arranged downstream of the first condenser 41 and upstream of the combiner 14. The two shut-off valves S.sub.1 and S.sub.2 are a preferred advantageous alternative for a three-way valve at the combiner 14. The first and second shutoff-valves S.sub.1, S.sub.2 may be solenoid operated valves, that allow for easy control by the controller 6.

[0049] The distributor 12 shown in FIGS. 3A-C comprises at least two further shut-off valves S.sub.3, S.sub.4, wherein a third shut-off valve S.sub.3 is arranged downstream of the branch 13 and upstream of the first condenser 41, and a fourth shut-off valve S.sub.4 is arranged downstream of the branch 13 and upstream of the second condenser 42. The two shut-off valves S.sub.3, S.sub.4 are a preferred advantageous alternative for the three-way valve T.sub.2 at the branch 13 of the first embodiment. The third and fourth shutoff-valves S.sub.3, S.sub.4 are preferably solenoid operated valves, that allow for easy control by the controller 6.

[0050] The distributor 12 is configured to selectively activate one of the first heating circuit C.sub.1 (as shown in FIG. 3A) and the second heating circuit C.sub.2. The first heating circuit C.sub.1 is activated by opening the third shut-off valve S.sub.3 and the second shut-off valve S.sub.2 and closing the fourth shut-off valve S.sub.4 and the first shut-off valve S.sub.1. The second heating circuit C.sub.2 is activated by opening the fourth shut-off valve S.sub.4 and the first shut-off valve S.sub.1 and closing the third shut-off valve S.sub.3 and the second shut-off valve S.sub.2.

[0051] The arrangement of the first heating circuit C.sub.1 and the second heating circuit C.sub.2 with the distributor 12 comprising at least two valves V allows the distributor 12 to selectively redistribute the refrigerant charge from the first heating circuit C.sub.1 to the second heating circuit C.sub.2 or vice versa. For example, refrigerant may be distributed from the first heating circuit C.sub.1 to the second heating circuit C.sub.2 by opening the second shut-off valve S.sub.2 and the fourth shut-off valve S.sub.4 while the first shut-off valve S.sub.1 and the third shut-off valve S.sub.3 are closed (FIG. 3C). Alternatively, refrigerant may be distributed from the second heating circuit C.sub.2 to the first heating circuit C.sub.1 by opening the first shut-off valve S.sub.1 and the third shut-off valve S.sub.3 while the second shut-off valve S.sub.2 and the fourth shut-off valve S.sub.4 are closed (FIG. 3B). By redistributing refrigerant charge between the first heating circuit C.sub.1 and the second heating circuit C.sub.2, the amount of refrigerant charge may be continuously adapted, thereby improving the efficiency of the heat pump 1. The skilled person will understand that applying three-way valves T.sub.1, T.sub.2 according to the first preferred embodiment also allows the distributor 12 to redistribute refrigerant charge. Nevertheless, the second embodiment having a plurality of shut-off valves S.sub.1, S.sub.2, S.sub.3, S.sub.4 is preferred. Firstly, shut-off valves are less susceptible to leaking than three-way valves. Secondly, shut-off valves may be easily controllable, especially when they are embodied as solenoid valves. Thirdly, it provides the option to combine more than two lines, as will be elucidated in more detail in the third preferred embodiment shown in FIG. 4.

[0052] In the first, second and third embodiment according to the invention, the distributor 12 preferably comprises a controller 6 that is configured to redistribute the refrigerant charge from the first heating circuit C.sub.1 to the second heating circuit C.sub.2 or vice versa, in dependency of a level of subcooling of the refrigerant in at least one of the first heating circuit C.sub.1 and the second heating circuit C.sub.2. In this context, subcooling is defined as a temperature difference between the condensing temperature and a temperature of the refrigerant.

[0053] The controller 6 may be configured to adjust the refrigerant charge in the active heating circuit at a preset optimum level of subcooling. This optimum subcooling may be selectively set to correspond to one of: the optimum coefficient of performance, and thus the minimum energy use on the one hand, and the maximum heating capacity obtainable on the other hand.

[0054] Preferably, the subcooling of at least the most critical heating circuit, which is typically the heating circuit comprising the condenser with the lowest volume, is taken into account by the controller 6. After all, the skilled person will acknowledge that the volume of refrigerant is far more critical in a relatively small heat exchanger, relative to a larger heat exchanger. For example, referring to a situation as shown in FIGS. 2A-2C, the condenser 41 may be part of a small plate heat exchanger, whereas the condenser 42 may be defined by a large coil arranged in or wrapped around a heated water tank 11. When wrapped around the heated water tank 11, the length may be multiple tens of meters.

[0055] The controller 6 is preferably configured to determine at least one of: [0056] a level of subcooling in the first heating circuit C.sub.1 by calculating a temperature difference between a condensing temperature at the first condenser 41 and a temperature of the refrigerant leaving said first condenser 41; and [0057] a level of subcooling in the second heating circuit C.sub.2 by calculating a temperature difference between a condensing temperature at the second condenser 42 and a temperature of the refrigerant leaving said second condenser 42.

[0058] The skilled person will acknowledge that the condensing temperature at one of the first condenser 41 or the second condenser 42 may be determined directly or indirectly. On the one hand, a direct measurement may be provided by a first temperature sensor 15, 17, 19 arranged near an entrance in or shortly upstream of the condenser 41, 42, and a second temperature sensor 16, 18, 20 arranged near an output in or shortly downstream of the condenser 41, 42. On the other hand, in a direct measurement, said condensing temperature may be derived from a condensing pressure, that may be determined by a pressure sensor in a vapour line leading to said condenser, or even via a temperature obtained in a further heating circuit that is heated by the condenser.

[0059] The controller 6 may be configured to at least one of: [0060] redistribute refrigerant charge from the first heating circuit C.sub.1 to the second heating circuit C.sub.2 if the subcooling in the first heating circuit C.sub.1 is above a pre-determined upper threshold temperature difference; [0061] redistribute refrigerant charge from the second heating circuit C.sub.2 to the first heating circuit C.sub.1 if the subcooling in the first heating circuit C.sub.1 is below a pre-determined lower threshold temperature difference; [0062] redistribute refrigerant charge from the second heating circuit C.sub.2 to the first heating circuit C.sub.1 if the subcooling in the second heating circuit C.sub.2 is above a pre-determined upper threshold temperature difference; and [0063] redistribute refrigerant charge from the first heating circuit C.sub.1 to the second heating circuit C.sub.2 if the subcooling in the second heating circuit C.sub.2 is below a pre-determined lower threshold temperature difference.

[0064] The temperature difference between the lower threshold temperature difference and the upper threshold temperature difference may, for a heat pump for domestic use having a subcooling of about 5K, be in the range of 0.2-3 C., preferably in the range of 0.5-2 C., and more preferably in the range of 1-1.5 C. The lower threshold temperature difference is in the range of 0.5-1.2 C., and/or the upper threshold temperature difference is in the range of 0.3-1.2 C. The skilled person will acknowledge that industrial heat pump may have a significantly larger subcooling than 5 K, e.g. a subcooling of 10 K and above, and consequently the temperature ranges may differ.

[0065] The heat pump 1 may, according to a third preferred embodiment, comprise one or more than one further condenser 43 arranged in a further heating circuit C.sub.3, wherein the first heating circuit C.sub.1, the second heating circuit C.sub.2 and the one or more than one further heating circuit C.sub.3 share the common line 21 passing through the evaporator 2 and the compressor 3. In FIG. 4, the common line 21 also passes through the expansion valve 5. The distributor 12 is configured to redistribute a refrigerant charge from at least one of the first heating circuit C.sub.1, the second heating circuit C.sub.2 and the one or more than one further heating circuit C.sub.3 to at least one other of the first heating circuit C.sub.1, the second heating circuit C.sub.2 and the one or more than one further heating circuit C.sub.3. In FIG. 4, only one further heating circuit, i.e. a third heating circuit C.sub.3, is shown, but the skilled person will understand that applying shut-off valves S.sub.1, S.sub.2, S.sub.3, etc., instead of three-way valves T.sub.1, T.sub.2 provides the opportunity to apply multiple further heating circuits in addition to the first heating circuit C.sub.1 and the second heating circuit C.sub.2.

[0066] FIG. 5 shows a schematic view of a heat pump 1 according to a fourth preferred embodiment of the invention. This fourth embodiment is closely related to the second preferred embodiment shown in FIGS. 3A-3C but differs relative to this second preferred embodiment in that the first heating circuit C.sub.1 and the second heating circuit C.sub.2 now each comprise a dedicated expansion valve 5, 5.sub.2.

[0067] The first shut-off valve S.sub.1 is defined by a second expansion valve 5.sub.2 of the one or more than one expansion valve, wherein this second expansion valve 5.sub.2 is arranged downstream of the second condenser 42 and upstream of the combiner 14.

[0068] The second shut-off valve S.sub.2 is defined by a first expansion valve 5.sub.1 of the one or more than one expansion valve, wherein this first expansion valve 5.sub.1 is arranged downstream of the first condenser 41 and upstream of the combiner 14.

[0069] The common line 21 that extends between the combiner 14 and the branch 13 now lacks an expansion valve 5 as applied in the second preferred embodiment. Instead, the common line 21 now comprises the evaporator 2 and the compressor 3. A four-way reversing valve 7 is optionally also arranged in the common line 21. Replacing the single expansion valve 5 in the common line 21 of the second preferred embodiment for dedicated expansion valves 5.sub.1, 5.sub.2 in the first heating circuit C.sub.1 and the second heating circuit C.sub.2, respectively, allows the expansion valves 5.sub.1, 5.sub.2 to also fulfil the functionality of the shutoff-valve S.sub.2, S.sub.1 in said respective heating circuit C.sub.1, C.sub.2. In other words, the expansion valve 5.sub.1, 5.sub.2 may also serve as valves V that are controllable by the controller 6 of the distributor 12 to thereby redistribute refrigerant charge between heating circuits of the heat pump 1.

[0070] Although they show preferred embodiments of the invention, the above-described embodiments are intended only to illustrate the invention and not to limit in any way the scope of the invention. Accordingly, it should be understood that where features mentioned in the appended claims are followed by reference signs, such signs are included solely for the purpose of enhancing the intelligibility of the claims and are in no way limiting on the scope of the claims. Furthermore, it is particularly noted that the skilled person can combine technical measures of the different embodiments. For example, the fourth embodiment of FIG. 4 having three heating circuits C.sub.1, C.sub.2 and C.sub.3 may alternatively apply three (not shown) dedicated expansion valves 5.sub.1, 5.sub.2 and 5.sub.3. After all, the skilled person will acknowledge that applying dedicated expansion valves 5.sub.1, 5.sub.2 in each of the heating circuits is not limited to two heating circuits. The scope of protection is defined solely by the following claims.