EP1266182A1

EP1266182A1 - A device and a method for transferring heat and use thereof

Info

Publication number: EP1266182A1
Application number: EP01918055A
Authority: EP
Inventors: Roland Lundqvist
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-03-24
Filing date: 2001-03-23
Publication date: 2002-12-18
Anticipated expiration: 2021-03-23
Also published as: SE0004378L; SE518085C2; EP1266182B1; ES2248301T3; WO2001073365A1; DE60113391D1; ATE304690T1; SE0004378D0; AU2001244926A1; DE60113391T2

Abstract

A device and a method for transferring heat, the device comprising a first member (1) for transferring heat from a first medium (2) flowing in a first circuit (3), to a second medium (4) flowing in a second circuit (5). The device further comprises a second member (16) for transferring heat from a third medium (17) to said second medium (4) and a means (18) arranged to divide the flow of the second medium (4) into a first flow part (19) flowing through the first heat transferring member (1) in a first circuit part (20) and into a second flow part (21) flowing through the second heat transferring member (16) in a second circuit part (22) and adapt the size of said first flow part (19) to the size of the flow of the first medium (2) flowing through the first heat transferring member (1) so as to reduce the loss of energy at the first heat transferring member (1) and increase the temperature of the second medium (4).

Description

A device and a method for transferring heat and use thereof

FIELD OF THE INVENTION AN D PRIOR ART

The present invention relates to a device for transferring heat, comprising a first member for transferring heat from a first medium flowing in a first circuit, to a second medium flowing in a second circuit, and a method for transferring heat, in which heat is transferred from a first flowing medium to a second flowing medium by means of a first heat transferring member, and use thereof.

The invention is applicable in heating of different mediums in several technical applications, but for illuminating, but not in any way restricting the invention, from now the application when so called boiler feed water for steam production is to be heated before it is supplied to the steam generating step of the process will be described , which step is desired within the pulp and paper industry for instance.

When producing steam for use in different process steps within the pulp and paper industry and the following use of the steam, about two thirds of the water quantity which is consumed for generation of the steam required may be reused in the state of a so called recovery condensate. Thus, the recovery condensate must be deluted by approximately 50% further water for maintaining the water quantity required for the steam production. In addition the recovery condensate as well as the added water, which together constitute the boiler feed water, have to be puri- fied by filtration so as to provide for the high degree of purity which is demanded for as regards the water which is supplied to one or more steam boilers.

Simultaneously, for energy saving purposes it is desired that the boiler feed water has such a high temperature as possible when being supplied to the steam boiler.

The requirements mentioned above are however partly incom- patible, since the filtering equipment which is usually used for purifying the boiler feed water requires that the boiler feed water has a proportionately low temperature to enable that the filtration may be performed in a satisfactory way.

According to prior art this is solved by that the recovery condensate and fresh water, and/or another suitable water, having suitable temperatures, are mixed, for example in a tank, to constitute the boiler feed water in such a way that the boiler feed water gets a temperature which is low enough as regards the fil- tering equipment mentioned above, in most cases a so called mixed bed filter, and thereafter the boiler feed water is pumped further to a heat exchanger, located upstream the mixing position in relation to the flow direction of the recovery condensate, at which heat exchanger the boiler feed water is heated by means of the recovery condensate. This means that boiler feed water gets a higher temperature before it is fed further to the steam boiler and that the recovery condensate gets a lower temperature before it is brought to the mixing tank.

However, devices and methods according to the prior art have important drawbacks in so far as a significant part of the exergy, i.e. the quality of the energy, of the recovery condensate is lost through an irreversible process as a consequence of that the flow of the recovery condensate through the heat exchanger to a considerable extent is lower than the flow of the boiler feed water, in many cases only two thirds of the flow of the boiler feed water through the heat exchanger, which is very unfavourable in the aspect of preserving the exergy. In different words this means that a certain quantity of heat is transferred from a smaller amount of recovery condensate to a larger amount of boiler feed water, which results in that the boiler feed water does not get an optimum increase of the temperature even in the case the loss of heat from the heat exchanger to the surroundings is negligible. Furthermore, it would be desired that other energy sources, such as waste heat occurring in the pulp and/or paper process, may be used for heating the boiler feed water.

SUMMARY OF THE I NVENTION

A first object of the present invention is to provide a device of the type defined in the introduction, which reduces the above discussed problems of such already known devices to a great extent.

This object is obtained according to the invention through that the device comprises a second member for transferring heat from a third medium to said second medium and a means arranged to divide the flow of the second medium into a first flow part flowing through the first heat transferring member in a first circuit part and into a second flow part flowing through the second heat transferring member in a second circuit part and adapt the size of said first flow part to the size of the flow of the first medium flowing through the first heat transferring member so as to reduce the loss of exergy at the first heat transferring member and increase the temperature of the second medium.

By such an arrangement of a second heat transferring member and such a means for dividing the flow of the second medium, a more favourable heating of the second medium may be achieved. In this connection, the first heat transferring member may be used with a minimum of exergy losses through adaption of the size of the flow of the second medium which passes through the first heat transferring member, to the flow of the first medium which passes through the first heat transferring member while a part of the second medium being heated by another source of heat, i.e. by a third medium deriving from for example a waste heat source, so that the second medium, in a possible mixture of the first and the second flow part, gets a higher temperature than what would be the case if only the first heat transferring member was used and the whole flow of the second medium would be brought to flow through the first heat transferring member. This may be reached in many cases although the third medium has usually a lower temperature than the first medium. In this connection the basic principle is that equal mass flows at the primary and secondary side of the first heat trans- ferring member minimizes the loss of exergy when the heat is transferred, on condition that the first and the second medium have substantially the same specific heat capacity.

According to a preferred embodiment of the invention the device includes a means connecting the first and the second circuit, arranged downstream the first heat transferring member in relation to the flow direction of the first medium, and upstream said dividing means in relation to the flow direction of the second medium, to bring the first medium and a fourth medium flowing to the intermixing means together, the first and the fourth medium being arranged to jointly constitute at least a portion of the second medium. Hereby it is possible to add a fourth medium, for example fresh water, to the first medium, for example a recovery condensate, so that the required amount of the second medium, for example a boiler feed water, is obtained and simultaneously transfer heat energy to the second medium in a very favourable way.

According to another preferred embodiment of the invention the device comprises a third member for transferring heat from said third medium to the fourth medium, which member is arranged upstream the intermixing means in relation to the flow direction of the fourth medium . The third heat transferring member makes it possible to use the third medium, for example a hot flow of waste heat from a bleachery, such as an EOP filtrate, for heat- ing the fourth medium, for example a fresh water, so that the highest temperature allowed of the second medium immediately after the intermixing medium may be obtained also during conditions when the fourth medium before heating has a proportionately low temperature and/or when the first medium before in- termixing with the fourth medium has a proportionately low temperature or a low flow.

According to a further preferred embodiment of the invention the dividing means is arranged to adapt the size of said first flow part so that this flow is substantially as big as the flow of the first medium. This embodiment is especially suitable in the case when the first and the second medium have substantially the same specific heat capacity since equal flows in combination with the same specific capacity of the two mediums will result in an optimum heat transferring, as regards the lowest possible loss of exergy, from the first medium to the second medium at the first heat transferring member.

A second object of the present invention is to provide a method of the type defined in the introduction, which method reduces the above discussed problems of already known such methods to a large extent.

This second object is obtained according to the invention through that heat is transferred from a third medium to said second medium by means of a second heat transferring member by the fact that the flow of said second medium is divided into a first flow part which is heated by the first medium at the first heat transferring member and into a second flow part which is heated by the third medium at the second heat transferring member and that the size of said first flow part is adapted to the size of the flow of the first medium flowing through the first heat transferring member so that the loss of exergy at the first heat transferring member is reduced and the temperature of the second medium is increased.

Further advantages of and favourable features of the device according to the invention and the method according to the invention appear from the following description and the other dependent claims.

The invention also relates to the use of a device according to the invention and/or a method according to the invention for heating water for steam production.

BRIEF DESCRIPTION OF TH E DRAWI NGS

With reference to the appended drawings, below follows a more detailed description of preferred embodiments of the invention cited as examples.

In the drawings:

Fig 1 is a schematical illustration of a system for preheating of boiler feed water in accordance with the state of art,

Fig 2 is a schematical illustration of an alternative system for preheating of boiler feed water,

Fig 3 is a schematical illustration of a device according to the invention,

Fig 4 is a schematical illustration of a variant of the device according to the invention, and Fig 5 is a schematical illustration of a further variant of the device according to the invention shown in a part view.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In Fig 1 a system for preheating of boiler feed water intended for steam production is illustrated. The system is designed in ac- cordance with already known art used in the paper and pulp industry. A heat exchanger 1 is arranged for transferring heat from a recovery condensate 2 flowing in a first circuit 3, to a boiler feed water 4 flowing in a second circuit 5. The recovery condensate 2 is constituted by reused condensated steam from differ- ent steps in the process which require the use of steam. In the heat exchanger 1 , heat is transferred from the recovery condensate 2, which in a position 6 upstream the heat exchanger 1 often has a temperature in the order of 100-130°C, to the boiler feed water 4, which at a position 7 upstream the heat exchanger 1 often has a temperature in the order of 15-45°C. This means that the recovery condensate 2, by emitting of heat, is brought to a lower temperature, preferably in the order of 25-50°C at a position 8 downstream the heat exchanger 1 and the boiler feed water 4 is brought to a higher temperature, preferably in the or- der of 60-80°C, at a position 9 downstream the heat exchanger 1 .

Then the boiler feed water is pumped by means of a pump 15 to one or more boiler feed water tanks 10 and further to one or more boilers for steam production, for example soda, oil and/or bark boilers, which steam is used in different steps of the process in producing pulp and/or paper. From these process steps a part of the cooled steam is reused and brought back, as mentioned above, in the shape of the recovery condensate 2 to be used as a part of the boiler feed water 4. However, the recovered amount of recovery condensate is not enough for maintaining the required boiler feed water quantity and thus further water must be added . For this purpose fresh water 1 1 is usually used, which is mixed with the recovery condensate 2 in a mixing tank 12. To avoid salt precipitation in the following steam production the fresh water 1 1 undergoes one or more deionisation steps as regards cations and anions in an ion exchanger arrangement 13 before the fresh water 1 1 reaches the mixing tank 12.

The water flowing from the mixing tank 12 thus constitutes the boiler feed water 4, but before this may be used for steam production it has to be purified further by filtration, the remaining impurities being removed in a filtering equipment 14 and there- after heated by the recovery condensate 2 through the heat exchanger 1 as previously described. As mentioned above the pump 15 is suitably used for bringing the boiler feed water to flow to the boiler feed water tanks 10. However, it should be pointed out that for this embodiment as well as for all embodi- ments of the invention described below, if said pump 15 is used, it is emphasized that the pump often with advantage may be arranged upstream the filtering equipment 14 in relation to the flow direction of the boiler feed water 4, i.e. between the mixing position 12 and the filtering equipment 14, so as to press in- stead of sucking, the boiler feed water 4 through the filtering equipment 14 and therethrough avoid the risk of cavitation in the filtering equipment 14.

For filtration of the boiler feed water 4 in the pulp and paper in- dustry almost solely a so called mixed bed filter 14 is used. This is an efficient filtering method, but it has the disadvantage that it requires that the medium which is to be filtrated has a temperature not exceeding approximately 50°C. From this it follows that the mixture of the recovery condensate 2 and the fresh water 1 1 is not allowed to have a temperature exceeding approximately 50°C. However, simultaneously a temperature as high as possible of the boiler feed water 4 is desired in the steam production, and therefore it can be said that conflicting desires as regards the temperature of the boiler feed water 4 exist. As described above it is attained to solve this problem by using the heat exchanger 1 in accordance with the prior art.

Should it be possible to dispense with the filtering equipment, or use a filtering equipment 14b which was able to filtrate the boiler feed water 4 to the desired extent without demand for a proportionately low temperature of the boiler feed water 4, the heat exchanger 1 could be dispensed with. Such a system is illustrated in Fig 2. In this case the recovery condensate 2 in the circuit 3 is accordingly led directly to the mixing tank 12 where it is mixed with the fresh water 1 1 .

However, in many applications the system according to Fig 2 is an Utopia, since such filtering equipment 14b is not available to- day.

In Fig 3 a device according to the invention for transferring heat is illustrated. The device comprises a first member 1 for transferring heat from a first medium 2 flowing in a first circuit 3, to a second medium 4 flowing in a second circuit 5. Furthermore, the device comprises a second member 16 for transferring heat from a third medium 17 to said second medium 4 and a means 18 arranged to divide the flow of the second medium 4 into a first flow part 19 flowing through the first heat transferring member 1 in a first circuit part 20 and into a second flow part 21 flowing through the second heat transferring member 16 in a second circuit part 22. The dividing means 18 is in this connection arranged to adapt the size of said first flow part 19 to the size of the flow of the first medium 2 flowing through the first heat transferring member 1 . Furthermore, the illustrated embodiment of the device according to the invention includes a means 12 which connects the first 3 and the second 5 circuit and which is arranged downstream the first heat transferring member 1 in relation to the flow direction of the first medium 2, and upstream said dividing means 1 8 in relation to the flow direction of the second medium 4, to bring the first medium 2 and the fourth medium 1 1 flowing to the intermixing means 12 together, the first 2 and fourth 1 1 medium being arranged to jointly constitute the second medium 4.

Such a device may for example be used for solving the problems discussed in the introduction when heating boiler feed water in the pulp and paper industry. Thus, in such an application the first and the second heat transferring members are preferably heat exchangers of the counter flow type, the first medium a re- covery condensate, and the second medium boiler feed water and the fourth medium is preferably fresh water. Furthermore, an arrangement 14 for purifying and/or filtrating the second medium 4 is arranged between the intermixing means 12 and the dividing means 18. Thus, it is in this case supposed that the fil- tering arrangement 14 is a mixed bed filter. A third medium originates preferably from some other waste heat source present in the manufacturing process. A so-called EOP-filtrate from a bleachery arrangement may for example be used, since this filtrate is a residual product from the bleachery, which in some cases is available and is not used as a heat source, but is simply removed from the process through an outlet.

A required feature of the third medium 17 is that it has a temperature which exceeds the temperature of the second medium 4 so that heat may be transferred from the third medium 1 7 to the second medium 4. Although the third medium 17 could be any energy source which is able to heat the second medium 4 in a sufficient way and the second heat transferring member 16 could be of any kind, in the embodiment example the third me- dium 17 is a fluidum arranged to flow in a third circuit 23 and the second heat transferring member 16 is a heat exchanger. Below the device according to the invention illustrated in Fig 3 and the method according to the invention thereto will be exemplified by a more detailed description of the use of these for heating water intended for steam production. Some parts already described will , however, be omitted and for the description in its entirety applies that identical reference notes denotes similar or corresponding components.

The recovery condensate 2, i.e. condensated water steam, flows in the first circuit 3 to the first heat exchanger 1 where the recovery condensate 2 emits heat to the boiler feed water 4. Thereafter the recovery condensate is brought to flow to the mixing tank 12. With the purpose of obtaining the sufficient amount of boiler feed water also fresh water 1 1 is added to the mixing tank 12. The added fresh water quantity varies depending upon the present flow of the recovery condensate 2 which in its turn depends on to which extent steam may be reused in the process 35 for the rest. Usually an addition of fresh water 1 1 corresponding to approximately half the flow of the recovery condensate is required. The temperature of the fresh water 1 1 may vary in high degree depending upon the present season etc. Furthermore, the flow and the temperature of the recovery condensate 2 may vary, and therefore it has to be seen to that the boiler feed water 4, when leaving the mixing tank 12 for flowing in the second circuit 5, does not have a too high temperature when it reaches the mixed bed filter 14. The purpose of the mixed bed filter 14 is to purify the boiler feed water 4 before this is used for steam production and this purifying and/or filtrating step requires that the boiler feed water 4 has a temperature not exceeding approximately 50°C. This means that the boiler feed water 4 immediately after the mixing tank 12 often has a temperature in the order of 1 5-45°C. However, it should be pointed out that the intermixing means 12 does not necessarily have to include a vessel, such as a feed water mixing tank, in all applications, but in some cases it may for example be a three-way pipe coupling , with or without valves, for connecting the conduits of the first and the fourth medium to the conduit of the second medium, so as to bring the first and the fourth medium together to constitute at least a portion of the second medium. In this connection it is also pointed out that also other available and suitable mediums may be brought to the intermixing means to constitute a portion of the second medium.

The boiler feed water is pumped further to the dividing means 18 which for example is constituted by an adjustable three-ways valve, for division in the first flow part 1 9 and in the second flow part 21 . The first flow part 19 flows in the first circuit part 20 to the first heat exchanger 1 and further to the boiler feed water tank/tanks 10. In this connection it has to be mentioned that preferably all mentioned heat exchangers may be of so called counter flow type.

The second flow part 21 flows in the second circuit part 22 to the second heat exchanger 16 and further to the boiler feed water tank/tanks 10. The first flow part 1 9 of the boiler feed water 4 is thus heated by the recovery condensate 2 at the first heat exchanger 1 and in this connection the size of the first flow part 19 is adapted to the size of the flow of the recovery condensate 2 by means of the valve 18 for obtaining an optimum heat transferring , i.e. heat transferring during conservation of the exergy, to the boiler feed water 4. When considering only the first heat exchanger 1 the lowest exergy losses are obtained in heat transferring when the mass flows of the recovery condensate 2 and the first flow part 19 are equal and thus this means that in many operating conditions about two thirds of the boiler feed water 4 is brought to flow through the first heat exchanger 1 , since approximately two thirds of the boiler feed water 4 is brought back to the system as recovery condensate 2.

The second flow part 21 is heated by the waste heat source, here EOP-filtrate 23, at the second heat exchanger 16. Cer- tainly, the optimum sizes of the flow parts 19, 21 are also depending upon the supply of for example EOP-filtrate and its energy contents and thus the flows 1 9, 21 are also adapted thereto. In many operation cases there are, however, plenty of EOP-filtrate with a temperature in the order of 70-75°C, which means that through adapting of the two flow parts 1 9, 21 so that the two heat exchangers 1 , 16 are well balanced, i.e. approximately equal mass flows at the primary and secondary side (with possibly adjustment for different specific heat capacity of the mediums) of the two mediums in the respective heat exchanger 1 , 16 are present, a heat transferring may be performed to the boiler feed water 4 at the two exchangers 1 , 16 which is optimized as regards conservation of exergy in relation to what would be the case if the whole flow of the boiler feed water was brought to flow through the first heat exchanger 1 . With balanced mass flows on the primary and secondary side of the heat exchangers 1 , 16, with adjustment of the flows for a possible difference in the specific heat capacity between the medium on the primary side and the medium on the secondary side, in fact an optimum interchange of heat as regards maintaining of the exergy contents of the heat energy transferred is obtained. If the mediums have different specific heat capacity the medium which has the highest specific heat capacity should if possible have a lower flow than the other medium with lower specific heat ca- pacity for obtaining optimum heat transferring as regards the conservation of exergy. Certainly it is in this connection, however, no disadvantage if there is an excess of the third medium 17 so that this has a larger mass flow than the second flow part 21 . On the other hand then there is a possibility to divide the third medium 17 so that the flows through the second heat exchanger 16 is balanced and the excess of the third medium 17 therethrough may be used for other purposes.

However, in some cases it may be necessary to forego the opti- mum heat transferring and allow a somewhat larger first flow part 19. This depends on that even if the operation condition is such that an increase of the first flow part 19 results in an increase of the exergy loss, this increase will still lead to a further decrease of the temperature of the recovery condensate 2 at a position downstream the first heat exchanger 1 in relation to the flow direction of the recovery condensate 2. In some situations this is required, since otherwise the temperature in the mixing tank 12, and thereby in the mixed bed filter 14 would be too high.

Furthermore, the device illustrated in Fig 3 has a means 24 arranged downstream the first heat transferring member 1 in relation to the flow direction of the first flow part 19, and downstream the second heat transferring member 16 in relation to the flow direction of the second flow part 21 , to connect the first 20 and the second 22 circuit part. The connection means 24 may be a pipe which connects the circuit parts 20, 22 so that communication therebetween is obtained, such as illustrated in Fig 3, or it may be a common pipe of the two circuit parts 20, 22 for bringing the two flow parts 19, 21 to flow in this pipe on the way to the boiler feed water tanks 10, as illustrated in Fig 5 for instance. The management of the flow parts 1 9, 21 downstream the heat exchangers 1 , 16 and possibly mixing of these for equalisation of temperature for instance and the number of boiler feed water tanks 1 0, may be varied to provide the current needs which are present for feeding the steam generating process step. In this connection it should be mentioned that in the boiler feed water tanks 1 0 the temperature of the boiler feed water is increased by means of steam received from the following steam generating step, which steam has a temperature of approximately 120-140°C, until the boiler feed water boils with the purpose of forcing oxygen present in the boiler feed water away. The part of this heating of the boiler feed water which may be performed by means of the device according to the invention, before the boiler feed water reaches the boiler feed water tanks, thus saves steam and therefore fuel. In this way the process may accordingly be performed in a way which involves lower costs.

In Fig 4 a variant of the device according to the invention is il- lustrated. Hereafter the components and the methods which are characteristic for this specific embodiment will in first hand be described. In other respects it is referred to the embodiment according to Fig 3. The device comprises a third member 25 for transferring heat from the third medium 17 to the fourth medium 1 1 , which member is arranged upstream the intermixing means 12 in relation to the flow direction of the fourth medium 1 1 . Furthermore, the third heat transferring member 25 is arranged at the third circuit 23 downstream the second heat transferring member 16 in relation to the flow direction of the third medium 17. Between the second 16 and the third 25 heat transferring member a means 26 is arranged by which means a desired part of the flow of the third medium 17 may be brought to flow to a position 27 at the third circuit 23 downstream the third heat transferring member 25 in relation to the flow direction of the third medium 17 without flowing through the third heat transferring member 25. In an analogous operation mode and application as described with reference to the embodiment in Fig 3, the embodiment in Fig 4 means that the EOP-filtrate 17, if required, may be used to heat the fresh water 1 1 . In fact the matter stand in a way that the highest temperature allowed, as regards the performance of the mixed bed filter, is desired to achieve in the mixing tank 12. During certain periods, for example in the winter time, the fresh water may be proportionately cold and looked at over the year the temperature may vary in the interval from some few °C to approximately 25°C. After passing through the second heat exchanger 16 the EOP-filtrate has a temperature in the order of 25-50°C. The temperature variation depending on the size and the temperature of the present second flow part 21 , and the flow and the temperature of the EOP-filtrate upstream the heat exchanger 16. In this connection the EOP-filtrate 17 thus has a temperature sufficient to be used for heating the fresh water 1 1 too. To be able to control the heating of the fresh water 1 1 the abovementioned by-pass means 26, including an adjustable three-ways valve and a pipe 29, is arranged so that, if required, a portion of or the whole EOP-filtrate may flow in the third circuit 23 without passing through the third heat exchanger 25.

In Fig 5 a part view of an alternative embodiment of the device according to the invention comprising an additional member 30 for transferring heat from an additional medium 31 to the second medium 4. This variant may be applied in both the embodiment according to Fig 3 and the embodiment according to Fig 4. The additional heat transferring member 30 is arranged downstream the second heat transferring member 16 in relation to the flow direction of the second flow part 21 . In the previously discussed application of the invention, by means of this embodiment, thus the other flow part 21 of the boiler feed water 4 may be heated further through heat transferring from the additional medium 31 , preferably a sometimes present heat source in the shape of a fluidum originating from so-called blowing of one or more steam generating boilers, at the additional heat transferring member 30. From the blowing a fluidum with a temperature of approximately 140°C is received which makes that the second flow part 21 with a temperature in the order of 60-65°C at a position be- tween the second heat exchanger 16 and the additional heat exchanger 30, may be heated further before it flows further to the boiler feed water tank/tanks 10. Also other heat sources, such as flue gases, could be used for transferring heat to the second medium 4 at the second heat transferring member 16 and/or the additional heat transferring member 30. In the case it would be achieved that the second flow part 21 gets a higher temperature than the first flow part 19, for example through the fact that the third medium 17 has a proportionately high temperature, the additional heat transferring member 30 should instead be arranged downstream the first heat transferring member 1 in relation to the flow direction of the first flow part 19 with the purpose to further heat the first flow part 19 of the boiler feed water 4.

Of course it is obvious for the man skilled in the art to modify the device according to the invention as well as the method according to the invention when the idea of the invention is disclosed. Above a number of variants have already been suggested and furthermore it may be mentioned that the embodiment according to Fig 4 could be designed with a closed third circuit 23. In such a case, an interstep fluidum could flow in the third circuit 23, preferably a very clean such fluidum, through which interstep fluidum heat would be transferred from the third medium, for example EOP-filtrate, to the second medium, for example boiler feed water, and possibly to a fourth medium, for example fresh water. For this purpose one additional heat transferring member in the shape of for example a heat exchanger arranged at the third circuit 23 for transferring heat from the third medium, the EOP-filtrate, for instance, to the interstep fluidum, is suitably used. The advantage with such an embodiment is that the risk that contaminated fluidum will reach the clean boiler feed water is reduced. The fact is that it has to be secured to the utmost possible extent that the third medium, the EOP-filtrate for instance, which is usually very unclean, does not come into the boiler feed water pipe in the event of a possi- ble damage in for example a heat exchanger.

It will also be added as regards all the described embodiments of the invention that in combination with the components illustrated in Figs 3-5 the controlling and regulating equipments 32, 33 are suitably used for controlling the dividing means 18 and regulating the two flow parts 19, 21 of the second medium 4, and for controlling the by-pass means 26 and regulating the amount of the third medium 17 which is by-passed the possible third heat transferring member 25 instead of through the same.

Claims

1 . A device for transferring heat, comprising a first member (1 ) for transferring heat from a first medium (2) flowing in a first cir- cuit (3), to a second medium (4) flowing in a second circuit (5), characterized in that the device comprises a second member (16) for transferring heat from a third medium (17) to said second medium (4) and a means (18) arranged to divide the flow of the second medium into a first flow part (1 9) flowing through the first heat transferring member (1 ) in a first circuit part (20) and into a second flow part (21 ) flowing through the second heat transferring member (16) in a second circuit part (22) and adapt the size of said first flow part (19) to the size of the flow of the first medium (2) flowing through the first heat transferring mem- ber (1 ) so as to reduce the loss of exergy at the first heat transferring member (1 ) and increase the temperature of the second medium (4).

2. A device according to claim 1 , characterized in that the de- vice includes a means (12) connecting the first (3) and the second (5) circuit, arranged downstream the first heat transferring member (1 ) in relation to the flow direction of the first medium (2), and upstream said dividing means (18) in relation to the flow direction of the second medium (4), to bring the first medium and a fourth medium (1 1 ) flowing to the intermixing means (12) together and in that the first (2) and the fourth (1 1 ) medium are arranged to jointly constitute at least a portion of the second medium (4).

3. A device according to claim 2, characterized in that the device comprises a third member (25) for transferring heat from said third medium (17) to the fourth medium (1 1 ) which member is arranged upstream the intermixing means (12) in relation to the flow direction of the fourth medium (1 1 ).

4. A device according to any preceding claim, characterized in that the third medium (17) is arranged to flow in a third circuit (23).

5. A device according to any preceding claim, characterized in that the third medium (17), before transferring of heat from this to the second medium (4), has a lower temperature than the first medium (2) has before transferring of heat from this to the second medium (4).

6. A device according to claim 3 and according to claim 4 or 5, characterized in that the third heat transferring member (25) is arranged at the third circuit (23) downstream the second heat transferring member (16) in relation to the flow direction of the third medium (17).

7. A device according to claim 6, characterized in that a means (26) is arranged between the second (16) and the third (25) heat transferring member by means of which means (26) a desired part of the flow of the third medium (17) is brought to flow to a position (27) at the third circuit (23) downstream the third heat transferring member (25) in relation to the flow direction of the third medium (17) without flowing through the third heat transferring member.

8. A device according to any preceding claim, characterized in that the dividing means (18) is arranged to adapt the size of said first flow part (19) so that this flow is substantially as big as the flow of the first medium (2).

9. A device according to any preceding claim, characterized in that the device has a means (24) arranged to connect the first (20) and the second circuit part (22) downstream the first heat transferring member (1 ) in relation to the flow direction of the first flow part (1 9) and downstream the second heat transferring member (16) in relation to the flow direction of the second flow part (21 ).

10. A device according to any of claims 2-9, characterized in that an arrangement (14) for purification and/or filtration of the second medium (4) is arranged between the intermixing means (12) and the dividing means (18).

1 1 . A device according to any preceding claim, characterized in that the device comprises an additional member (30) for transferring heat from an additional medium (31 ) to the second medium (4).

12. A method for transferring heat, in which heat is transferred from a first flowing medium (2) to a second flowing medium (4) by means of a first heat transferring member (1 ), characterized in that heat is transferred from a third medium (17) to said second medium (4) by means of a second heat transferring member (16) through the fact that the flow of said second medium (4) is divided into a first flow part (1 9) which is heated by the first medium (2) at the first heat transferring member (1 ) and into a second flow part (21 ) which is heated by the third medium (17) at the second heat transferring member (16) and in that the size of said first part flow (19) is adapted to the size of the flow of the first medium (2) flowing through the first heat transferring member (1 ) so that the loss of exergy at the first heat transferring member (1 ) is reduced and the temperature of the second medium (4) is increased.

13. A method according to claim 12, characterized in that the first medium (2), after passing through the first heat transferring member (1 ), and a fourth medium (1 1 ) are brought together to jointly constitute at least a portion of the second medium (4).

14. A method according to claim 13, characterized in that heat is transferred from said third medium (17) to the fourth medium ( 1 1 ) at a third heat transferring member (25).

15. A method according to claim 14, characterized in that the third medium (1 7) is brought to flow in a circuit (23) from the second heat transferring member (16) to the third heat transferring member (25).

16. A method according to any of claims 12-15, characterized in that the size of the first flow part (19) is adapted so that this flow is substantially as big as the flow of the first medium (2).

17. A method according to any of claims 12-16, characterized in that heat is transferred from an additional medium (31 ) to the second medium (4).

18. A use of a device according to any of claims 1 -1 1 for heating water for steam production.

19. A use of a method according to any of claims 12-17 for heating water for steam production.