EP2252842A1

EP2252842A1 - Heat exchanger

Info

Publication number: EP2252842A1
Application number: EP09713649A
Authority: EP
Inventors: Heikki Arola; Johnny TÖRNROOS; Jorma KALLAPERÄ
Original assignee: Loval Oy
Current assignee: Loval Oy
Priority date: 2008-02-21
Filing date: 2009-02-19
Publication date: 2010-11-24
Anticipated expiration: 2029-02-19
Also published as: EP2252842A4; DK2252842T3; FI121975B; EP2252842B1; FI20085160L; FI20085160A0; ES2733875T3; WO2009103852A1

Abstract

The invention relates to a heat exchanger (5, 6) comprising at least two separate flow channels for a flowing primary fluid and a flowing secondary fluid, which flow channels are arranged in heat exchange connection with each other in such a way that heat is transferrable between the primary fluid and the secondary fluid when they are flowing in separate flow channels. The heat exchanger (5, 6) according to the invention further comprises at least one auxiliary heater (7, 8) arranged integrally therein for raising the temperature of the primary fluid and/or the secondary fluid.

Description

HEAT EXCHANGER

BACKGROUND OF THE INVENTION

[0001] The invention relates to a heat exchanger and particularly to a heat exchanger according to the preamble of claim 1 , comprising at least two separate flow channels for a flowing primary fluid and a flowing secondary fluid, which flow channels are arranged in heat exchange connection with each other in such a way that heat is transferrable between the primary fluid and the secondary fluid when they are flowing in separate flow channels. The present invention further relates to a heat pump according to the preamble of claim 14 and particularly to a heat pump where a flowing primary fluid is circulating, the heat pump comprising a condenser for emitting heat from the primary fluid circulating in the heat pump, an evaporator for receiving heat to the primary fluid, a compressor for increasing the pressure of the primary fluid, and an expansion valve for decreasing the pressure of the primary fluid, which condenser comprises a first heat exchanger for transferring heat from the primary fluid to a secondary fluid, and which evaporator comprises a second heat exchanger for transferring heat from a heat fluid serving as the heat source to the primary fluid.

[0002] Heat exchangers in which heat is transferred between two flowing fluids are used in a plurality of applications, such as in heat pumps. The temperature of the primary fluid circulating on the primary side of the heat exchanger may be, when exiting the compressor, 125 °C, and the temperature of the secondary fluid may be 55 °C on average. The specific heat capacity of the secondary fluid is nearly constant, and so is the heat of evaporation on the primary side, so the temperature difference between the primary and secondary fluids is, in addition to the substance flows of the primary and secondary fluids, another factor affecting the magnitude of the heat flow between the fluids. However, heat consumption may often be great for short periods, for instance in winter, whereby the temperature of the secondary fluid must be raised to a higher level than what is normal. Further, authorities in many countries require that the temperature of the secondary fluid be raised to a temperature higher than 55 °C at regular intervals for hygienic reasons if hot service water is produced by means of the secondary fluid. In accordance with the prior art, raising the temperature of the secondary fluid may alternatively be implemented with a separate heater, such as a heating resistor unit, which is positioned on the flow line of the secondary fluid, separated from the heat exchanger. In other words, the use of additional heating, for example an electric flow heater, for example in connection with a heat pump, is an established solution known as such. Additional heating is used, for example, for raising the temperature of water for hygienic reasons, for increasing the heating efficiency when the capacity of a heat pump is insufficient, and as a standby heater for cases of heat pump failure. Present auxiliary heaters are separate units installed in the secondary circuit of a heat pump.

[0003] A problem with the above system is that a heat pump or another corresponding apparatus always requires a separate heater. The use of a separate heater leads to a more complex device structure and an increase in assembling and installing work.

BRIEF DESCRIPTION OF THE INVENTION

[0004] An object of the invention is thus to provide a heat exchanger with which the above problems can be solved. The object of the invention is achieved with a heat exchanger according to the characterizing part of claim 1 , characterized in that the heat exchanger further comprises at least one auxiliary heater arranged integrally therein for raising the temperature of the primary fluid and/or the secondary fluid. The object of the invention is further achieved with a heat pump according to the characterizing part of claim 14, characterized in that the first and/or the second heat exchanger is provided with an integral auxiliary heater for raising the temperature of the secondary fluid and/or the heat fluid, respectively.

[0005] Preferred embodiments of the invention are disclosed in the dependent claims.

[0006] The invention is based on the idea that a heat exchanger in which heat is transferred between two or more flowing substances is integrally provided with at least one auxiliary heater for bringing heat energy directly into one or more fluids. In other words, the auxiliary heater is connected to the same entity as the heat exchanger in such a way that one or more fluids pass through the auxiliary heater before or after flowing to a heat exchange zone between the fluids or, alternatively, both before and after flowing to the heat exchange zone between the fluids. The auxiliary heater may be any heater capable of emitting heat energy to a flowing fluid. In a preferred case, the auxiliary heater is an electric heater, such as an electric resistor or the like. Alternatively, the auxiliary heater may be, for example, gas-fired, a fuel cell or any other auxiliary heater that may be used by means of a separate power or energy source. Thus, auxiliary heater may also be arranged directly or indirectly in heat exchange connection with the primary and/or secondary fluid. [0007] An advantage of the method and system according to the invention is that an auxiliary heater allows the temperature of one or more fluids to be raised with a heat exchanger to a level significantly higher than the normal operating temperature of the fluid. Thus, no separate heater needs to be installed on the flow line of the fluid at a distance from the heat exchanger. Thus, the heat pump and the rest of the apparatus can be made simpler and less space-consuming and, on the other hand, the heat exchanger itself has a significant additional feature by means of which the temperature of the primary or secondary fluid can be changed with the heat exchanger itself temporarily or when the heat exchange between the fluids is not in operation.

BRIEF DESCRIPTION OF THE FIGURES

[0008] The invention will now be described in greater detail in connection with preferred embodiments, referring to the attached drawings, of which:

Figure 1 shows schematically a heat pump according to the prior art, comprising two heat exchangers for transferring heat between fluids;

Figure 2 shows schematically a heat pump comprising, on the side of the condenser, a heat exchanger having an auxiliary heater according to the invention;

Figure 3 shows schematically a heat pump comprising, on the side of both the condenser and the evaporator, an auxiliary heater according to the invention; and

Figure 4 shows a principled view of a heat exchanger according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0009] Referring to Figure 1 , it shows schematically a heat pump according to the prior art, comprising a primary circuit 9 in the flow channel of which a primary fluid is flowing. In this application, the primary fluid means a flowing fluid circulating in connection with the heat pump 9. The heat pump ' further comprises a condenser 1 having a first heat exchanger 5 for transferring heat from the primary fluid to a secondary fluid circulating in the flow channel of the secondary circuit. The secondary fluid refers, in this application, to a fluid flowing in connection with the heat pump and receiving heat energy from the primary fluid. The heat pump further comprises an evaporator 2 comprising a second heat exchanger 6 for transferring heat to the primary fluid. The primary circuit 9 is further provided with an expansion valve 4, on the downstream side of which the primary fluid is evaporated, whereby its temperature is reduced. The primary circuit is further provided with a compressor 3 to raise the pressure of the primary fluid, whereby the primary fluid is compressed and its temperature rises. In accordance with the principled view of Figure 1 , the primary fluid is first heated in the evaporator 2, from where it flows into the compressor 3 where its pressure rises and it heats up. After this, the primary fluid flows into the condenser 1 , where it emits heat to the secondary fluid circulating in the secondary circuit, whereby the temperature of the secondary fluid 12 flowing out of the first heat exchanger 5 is higher than the temperature of the secondary fluid 11 flowing into it. At the same time, the primary fluid cools down and condenses into a liquid. After the condenser 1 , the primary fluid flows to the expansion valve 4, on the downstream side of which the primary fluid is evaporated, whereby its temperature is reduced. After this, the cycle starts from the beginning again. The heat pump solution described above is only exemplary and not intended to restrict the present invention but it serves as an exemplary embodiment in this description. It is to be further noted that all Figures 1 to 4 of the application are schematic and only illustrate principles of the invention, not detailed solutions.

[0010] A fluid refers, in the context of this application, to any flowing substance, which may be gas or liquid and the state of which may change in connection with a heat pump process, for example. The primary fluid may be, for example a known refrigerant and the secondary fluid may be for example water. A heat exchanger refers in this application to a device comprising at least two separate flow channels for a flowing primary fluid and a flowing secondary fluid, the flow channels being arranged in heat exchange connection with each other in such a way that heat is transferrable between the primary fluid and the secondary fluid while they are flowing in separate flow channels. In other words, heat can get from the primary fluid flowing in one flow channel to the secondary fluid flowing in another flow channel, or vice versa. The temperature difference between the primary and the secondary fluids thus results in transfer of heat energy between them when their flow channels are arranged in heat exchange connection with each other. Thus, the heat exchanger may be, in the solution according to the invention, a plate heat exchanger, a spiral heat exchanger or another corresponding heat exchanger where a primary fluid and a secondary fluid or a primary fluid and another fluid serving as the heat source emitting heat energy to the primary fluid flow in separate flow channels which are arranged in heat exchange connection with each other.

[0011] In heat pump processes, there often arises a need to raise the temperature of the secondary fluid temporarily above the temperature of the normal heating use planned. In accordance with the present invention, the solution according to Figure 2 may then be provided in which a first heat exchanger 5 is provided with an auxiliary heater 7 to raise the temperature of the secondary fluid. In Figure 2, the heat pump process functions in the same way as in Figure 1 but in the embodiment according to Figure 2 there is an auxiliary heater 7 integrated in the first heat exchanger 5. In this embodiment according to Figure 2, the auxiliary heater is provided on the downstream side of the first heat exchanger 5. In other words, the secondary fluid 11 flowing into the first heat exchanger 5 is first in heat exchange connection with the primary fluid, after which it flows into the auxiliary heater 7 before flowing out 12 of the first heat exchanger 5. Thus, the temperature of the secondary fluid can be raised, by means of the heat exchanger 5, to a level higher than the mutual heat transfer capacity of the fluids by means of the auxiliary heater 7. In an alternative embodiment, the auxiliary heater 7 may be arranged in the first heat exchanger 5 on the upstream side in such a way that the secondary fluid is heatable with the auxiliary heater 7 before the secondary fluid is in heat exchange connection with the primary fluid in the heat exchanger 5. A weakness of this solution is, however, that the secondary fluid gets to heat exchange connection with the primary fluid already at a raised temperature, whereby the temperature difference between the primary and the secondary fluid has been reduced, which, in turn, deteriorates or prevents heat transfer between them or even turns the heat transfer between them opposite if the temperature of the secondary fluid rises higher than the temperature of the primary fluid.

[0012] Another alternative is to provide the first heat exchanger 5 with an auxiliary heater 7 on both the downstream and the upstream side in such a way that the secondary fluid can be heated with the auxiliary heater 7 both after the secondary fluid has been in heat exchange connection with the primary fluid and before the secondary fluid has been in heat exchange connection with the primary fluid. Thus, both of these auxiliary heaters 7 can be arranged in such a way that they can be used at the same time or at different times. Thus, the first heat exchanger 5 may be provided with control means with which the operation of the auxiliary heater 7 can be controlled. These control means may operate automatically, or they may be operated manually. Manual control means allow the user to switch the auxiliary exchanger 7 on and off as desired. Automatic control means, in turn, may comprise a sensor which measures the temperature of the secondary or primary fluid, and if the temperature deviates from the target value, it switches on or correspondingly switches off the auxiliary heater if the temperature is higher than the target value. The control means may also comprise a timer by means of which the auxiliary heater may be switched on for a predetermined time when desired or at predetermined intervals. In this way, the temperature of the secondary fluid in the heat pump can be raised to over 55 °C for example once a day in accordance with the orders of the authorities.

[0013] Figure 4 shows an embodiment of the first heat exchanger 5 according to Figure 2 in a simplified manner. In accordance with Figure 4, the first heat exchanger 5 comprises a heat exchange part 16, in which the primary fluid and the secondary fluid are in heat exchange connection with each other, flowing in separate flow channels. The first heat exchanger 5 further comprises an auxiliary heater 7, which has been arranged on the downstream side of the heat exchange part 16 in such a way that before being removed from the first heat exchanger 5, the secondary fluid flows via the auxiliary heater 7 after having been flown through the heat exchange part 16. In accordance with Figure 4, the primary fluid flows into the heat exchange part 16 of the first heat exchanger 5 via a conduit 13 and, correspondingly, the secondary fluid flows directly into the heat exchange part via a conduit 11. In the heat exchange part 16, the primary and secondary fluids flow in separate flow channels which are in heat exchange connection with each other in such a way that the primary fluid having a higher temperature emits heat to the secondary fluid having a lower temperature. After having flown through the heat exchange part 16, the primary fluid is removed from the first heat exchanger 5 via a conduit 14. The secondary fluid, in turn, flows to the auxiliary heater 7 through the heat exchange part 16 after having passed through the heat exchange part 16. In the auxiliary heater 7, the secondary fluid may, if desired, be further heated while it is flowing through the auxiliary heater 7. After having passed through the auxiliary heater 7, the secondary fluid is removed from the first heat exchanger 5 via a conduit 12. The heat exchanger 7 is preferably electrically operated and it may be connected to the power source via a wire 15. A preferred embodiment of the auxiliary heater 7 comprises electric resistors arranged in the auxiliary heater 7 and heating the secondary fluid while it is flowing through the auxiliary heater 7.

[0014] In accordance with the above, the first heat exchanger 5 may also be, in the case of Figure 4, arranged in such a way the secondary fluid flowing via the conduit 11 to the first heat exchanger 7 passes first through the auxiliary heater 7 and only after that into the heat exchange part 16. Further, the first heat exchanger 5 may comprise two auxiliary heaters 7 in such a way that the secondary fluid flows via the auxiliary heater 7 both before flowing into the heat exchange part 16 and after that.

[0015] Solutions described above for the heat exchanger 5 of the condenser part 1 of the heat pump may be applied in the same way to the heat exchanger 6 of the evaporator side 2 of the heat pump. Thus, in this second heat exchanger 6 the primary fluid receives heat energy from a heat source. The heat source may be, for example, the surrounding atmosphere, water or another flowing fluid. Further, in this application, the flowing fluid that serves as the heat source in connection with the heat pump and emits heat energy to the primary fluid circulating in the heat pump is called a heat fluid. In the embodiment of the heat pump according to Figure 3, also the second heat exchanger of the evaporator side 2 is provided with an auxiliary heater 8 in the same way as on the condenser side 1. Thus, the auxiliary heater 8 may be arranged in the second heat exchanger 6 on the downstream side in such a way that the heat fluid serving as the heat source is heatable with the auxiliary heater 8 after it has been in heat exchange connection with the primary fluid. Alternatively, the auxiliary heater 8 may be arranged in the second heat exchanger 6 on the upstream side in such a way that the heat fluid is heatable with the auxiliary heater 8 before it has been in heat exchange connection with the primary fluid. Further, if desired, the auxiliary heater 8 can be arranged in the second heat exchanger 6 on both the downstream and upstream side. The auxiliary heater 8 of the second heat exchanger 6 of the evaporator side 2 allows the temperature of the heat fluid to be raised to a level higher than what is normal. Further, the auxiliary heater 8 may prevent the heat fluid from freezing.

[0016] Although Figure 3 shows that the heat exchangers 5, 6 on both the condenser side 1 and the evaporator side 2 are provided with auxiliary heaters 7, 8, it is feasible that only the condenser side 1 or only the evaporator side 2 uses a heat exchanger 5, 6 comprising an auxiliary heater 7, 8 integrated therein. It is further to be noted that although the present invention has been described here in connection with a heat pump, the heat exchanger according to the invention, having an integrated auxiliary heater, may also be used by itself as such or as a part of another device or system requiring a heat exchanger and additional heating of a fluid. When the heat exchanger is handled as a separate device, a primary fluid means a flowing fluid emitting heat energy to a secondary fluid, which, in turn, means a flowing fluid receiving heat energy from the primary fluid. In other words, the heat exchanger according to the invention can be provided with one or more auxiliary heaters arranged integrally therein to heat the primary fluid and/or the secondary fluid. The auxiliary heater can be arranged on the downstream and/or on the upstream side of the heat exchanger. In other words, the auxiliary heater can be arranged in the heat exchanger before the primary fluid and the secondary fluid have been in heat exchange connection with each other, or after that.

[0017] In implementing the product according to this invention, a preferred manufacturing technique common to these two separate devices, i.e. a plate heat exchanger and an electric flow heater, can be utilized, for example furnace or vacuum soldering. Thus, their essential manufacturing stage, i.e. assembly taking place by soldering, can be combined into one work stage. This results in significant advantages: the total manufacturing costs of the devices are reduced; the devices require less space, which is advantageous for the space utilization and costs of the final product, such as a heat pump; installation work is more inexpensive because the connection between the devices is implemented as early as in connection with the manufacturing process, which saves both material and work; the heat insulation required by both devices can be implemented in a simpler manner and with lower costs.

[0018] The heating elements used in the heater can be permanently attached to the structure in a soldering process, or the structure is implemented in such a way that the heating elements are replaceable. When a separate auxiliary heater is used, both ways are known as such and are commonly used.

[0019] In a preferred way of implementation, direct heat transfer between the heat exchanger and the auxiliary heater, which reduces the power factor of the heat pump and is thus an undesirable factor, can be made insignificant by positioning between them a structure made of a metallic material shaped appropriately and having poor heat conductivity, such as stainless steel. When vacuum soldering is used, a vacuum can be permanently generated within this structure, which vacuum further reduces heat transfer via said insulating structure.

[0020] The product according to the invention may also be efficiently utilized for protecting the condenser of the air/water heat pump against freezing. The evaporator of the heat pump taking its energy from air gathers ice generated of air humidity on its heat exchange surfaces. A common way to remove ice from the evaporator is to turn the process of the heat pump opposite for some time, whereby the functions of the evaporator and the condenser are interchanged. In this case, temperatures lower than the freezing point of water may be generated for short periods in the condenser of the actual process. This may result in the freezing of water in the secondary circuit in the condenser, particularly in areas where the water flow rate is low. Freezing may, in turn, break down the condenser and cause thus significant damage.

[0021] This freezing problem can be prevented with the structure according to the invention by switching the auxiliary heater on and by reversing the flow direction of the secondary circuit for the duration of the melting process, preferably in such a way that the flow direction is reversed shortly before the melting process starts. The heat energy generated by the auxiliary heater raises the temperature of the water going to the condenser in such a way that no freezing can take place.

[0022] Further, although the operation of the heat exchanger has above been described in connection with a heat pump, the heat exchanger according to the invention may also be used separately as such. Thus, all the above aspects also relate to such heat exchangers that are not related to heat pumps. In other words, the invention may be applied to any heat exchangers irrespective of its object of use. [0023] It is obvious to a person skilled in the art that as the technology advances, the basic idea may be implemented in a plurality of ways. The invention and its embodiments are thus not restricted to the above examples but may vary within the scope of the claims.

Claims

1. A heat exchanger (5, 6) comprising at least two separate flow channels for a flowing primary fluid and a flowing secondary fluid, which flow channels are arranged in heat exchange connection with each other in such a way that heat is transferrable between the primary fluid and the secondary fluid when they are flowing in separate flow channels, characterized in that the heat exchanger (5, 6) further comprises at least one auxiliary heater (7, 8) arranged integrally therein for raising the temperature of the primary fluid and/or the secondary fluid.

2. A heat exchanger (5, 6) according to claim 1, characterized in that the auxiliary heater (7, 8) is directly or indirectly electrically operated, gas-fired or a fuel cell.

3. A heat exchanger (5, 6) according to claim 1 or 2, characterized in that the auxiliary heater comprises one or more heating elements installed in the auxiliary heater fixedly or replaceably.

4. A heat exchanger (5, 6) according to claim 2 or 3, characterized in that the auxiliary heater (7, 8) comprises one or more electric resistors for heating the primary fluid and/or the secondary fluid in connection with the heat exchanger (5, 6).

5. A heat exchanger (5, 6) according to any one of claims 1 to 4, characterized in that a structure preventing heat transfer has been arranged between the heat exchanger (5, 6) and the auxiliary heater (7, 8) to minimize heat transfer between the heat exchanger (5, 6) and the auxiliary heater (7, 8).

6. A heat exchanger (5, 6) according to claim 5, characterized in that the structure preventing heat transfer comprises a vacuum.

7. A heat exchanger (5, 6) according to any one of the preceding claims, characterized in that the auxiliary heater (7) is arranged in the heat exchanger (5, 6) on the downstream side in such a way that the secondary fluid is heatable with the auxiliary heater (7) after the secondary fluid has been in heat exchange connection with the primary fluid.

8. A heat exchanger (5, 6) according to any of the preceding claims 1 to 6, characterized in that the auxiliary heater (7) is arranged in the heat exchanger (5, 6) on the upstream side in such a way that the secondary fluid is heatable with the auxiliary heater (7) before the secondary fluid has been in heat exchange connection with the primary fluid.

9. A heat exchanger (5, 6) according to any one of preceding claims 1 to 6, characterized in that the heat exchanger (5, 6) is provided with an auxiliary heater (7) both on the downstream and the upstream side in such a way that the secondary fluid is heatable with the auxiliary heater (7) both after the secondary fluid has been in heat exchange connection with the primary fluid and before the secondary fluid has been in heat exchange connection with the primary fluid.

10. A heat exchanger (5, 6) according to any of preceding claims 1 to 9, characterized in that the auxiliary heater (7) is arranged to raise the temperature of the secondary fluid to a temperature higher than the normal operating temperature.

11. A heat exchanger (5, 6) according to any one of preceding claims 1 to 10, characterized in that the auxiliary heater (8) is arranged to raise the temperature of the primary fluid to a temperature higher than the normal operating temperature.

12. A heat exchanger according to any one of preceding claims 1 to 11, characterized in that the heat exchanger (5, 6) and/or the auxiliary heater (7, 8) comprises control means for switching on the power of the auxiliary heater and/or switching the auxiliary heater on and off automatically or manually.

13. A heat exchanger (5, 6) according to any one of preceding claims 1 to 15, characterized in that the heat exchanger (5, 6) is a plate heat exchanger, a spiral heat exchanger, a tube heat exchanger or another corresponding heat exchanger in which the primary fluid and the secondary fluid flow in separate flow channels arranged in heat exchange connection with each other.

14. A heat pump where a flowing primary fluid is circulating, the heat pump comprising a condenser (1) for emitting heat from the primary fluid circulating in the heat pump, an evaporator (2) for receiving heat to the primary fluid, a compressor (3) for increasing the pressure of the primary fluid, and an expansion valve (4) for decreasing the pressure of the primary fluid, which condenser (1) comprises a first heat exchanger (5) for transferring heat from the primary fluid to a secondary fluid, and which evaporator comprises a second heat exchanger (6) for transferring heat from a heat fluid serving as the heat source to the primary fluid, characterized in that the first and/or the second heat exchanger (5, 6) is provided with an integral auxiliary heater (7, 8) for raising the temperature of the secondary fluid and/or the heat fluid, respectively.

15. A heat pump according to claim 14, characterized in that the auxiliary heater (7, 8) is directly or indirectly electrically operated, gas-fired or a fuel cell.

16. A heat exchanger (5, 6) according to claim 14 or 15, characterized in that the auxiliary heater comprises one or more heating elements installed in the auxiliary heater fixedly or replaceably.

17. A heat exchanger (5, 6) according to claim 15 or 16, characterized in that the auxiliary heater (7, 8) comprises one or more electric resistors for heating the heat fluid and/or the secondary fluid in connection with the heat exchanger (5, 6).

18. A heat exchanger (5, 6) according to any one of preceding claims 14 to 17, characterized in that the a structure preventing heat transfer has been arranged between the heat exchanger (5, 6) and the auxiliary heater (7, 8) to minimize heat transfer between the heat exchanger (5, 6) and the auxiliary heater (7, 8).

19. A heat exchanger (5, 6) according to claim 18, characterized in that the structure preventing heat transfer comprises a vacuum.

20. A heat pump according to any one of claims 14 to 19, characterized in that the auxiliary heater (7) is arranged in the first heat exchanger (5) on the downstream side in such a way that the secondary fluid is heatable with the auxiliary heater (7) after the secondary fluid has been in heat exchange connection with the primary fluid.

21. A heat pump according to any one of preceding claims 14 to 19, characterized in that the auxiliary heater (7) is arranged in the first heat exchanger (5) on the upstream side in such a way that the secondary fluid is heatable with the auxiliary heater (7) before the secondary fluid has been in heat exchange connection with the primary fluid.

22. A heat pump according to any one of preceding claims 14 to 19, characterized in that the first heat exchanger (5) is provided with an auxiliary heater (7) both on the downstream and the upstream side in such a way that the secondary fluid is heatable with the auxiliary heater (7) both after the secondary fluid has been in heat exchange connection with the primary fluid and before the secondary fluid has been in heat exchange connection with the primary fluid.

23. A heat pump according to any one of preceding claims 14 to 22, characterized in that the auxiliary heater (7) is arranged to raise the temperature of the secondary fluid to a temperature higher than the normal operating temperature.

24. A heat pump according to any one of preceding claims 14 to 23, characterized in that the auxiliary heater (8) is arranged in the second heat exchanger (6) on the downstream side in such a way that the heat fluid is heatable with the auxiliary heater (8) after it has been in heat exchange connection with the primary fluid.

25. A heat pump according to any one of preceding claims 14 to 23, characterized in that the auxiliary heater (8) is arranged in the second heat exchanger (6) on the upstream side in such a way that the heat fluid is heatable with the auxiliary heater (8) before it has been in heat exchange connection with the primary fluid.

26. A heat pump according to any one of preceding claims 14 to 23, characterized in that the second heat exchanger (6) is provided with an auxiliary heater (8) both on the downstream and the upstream side in such a way that the heat fluid is heatable with the auxiliary heater (8) both after it has been in heat exchange connection with the primary fluid and before it has been in heat exchange connection with the primary fluid.

27. A heat pump according to any one of preceding claims 14 to 26, characterized in that the auxiliary heater (8) is arranged to raise the temperature normal of the heat fluid to a temperature higher than the operating temperature.

28. A heat exchanger according to any one preceding claims 14 to 26, characterized in that the heat pump or the heat exchanger (5, 6) or the auxiliary heater (7, 8) comprises control means for switching on the power of the auxiliary heater and/or switching the auxiliary heater on and off automatically or manually.

29. A heat exchanger according to any one of preceding claims 14 to 28, c h a r a c t e r i z e d in that the first and/or the second heat exchanger (5, 6) is a plate heat exchanger, a spiral heat exchanger or another corresponding heat exchanger where the primary fluid and the secondary fluid or the primary fluid and the heat fluid flow in separate flow channels arranged in heat exchange connection with each other.

30. A method for manufacturing a heat exchanger, the heat exchanger comprising at least two separate flow channels for a flowing primary fluid and a flowing secondary fluid, the flow channels being arranged in heat exchange connection with each other in such a way that heat is transferrable between the primary fluid and the secondary fluid while they are flowing in separate flow channels, characterized in that the heat exchanger is provided with an auxiliary heater at the same stage with the manufacture of the heat exchanger.

31. A method according to claim 30, characterized in that the auxiliary heater is arranged in the heat exchanger at the assembly stage of the heat exchanger.

32. A method according to claim 30 or 31, characterized in that the auxiliary heater is arranged in the heat exchanger at the soldering stage of the heat exchanger.

33. A method according to claim 31 or 32, c h a r a c t e r i z e d in that the auxiliary heater is arranged in the heat exchanger in connection with the furnace or vacuum soldering of the heat exchanger.

34. Use of a heat exchanger according to any one of preceding claims 1 to 13 for preventing a condenser of a heat pump from freezing during the melting process of an evaporator.