FI102625B - Method and apparatus for recovering heat from the exhaust air of a paper machine or the like vacuum system - Google Patents

Description

102625

METHOD AND APPARATUS FOR HEAT RECOVERY FROM PAPER MACHINE OR SIMILAR VACUUM EXHAUST AIR

FÖRFARANDE OCH ANORDNING FÖR PREVIEW AV VÄRME UR FRÄN-5 LUFTEN I EN PAPPERSMASKINS ELLER LIKNANDES VAKUUMSYSTEM

The present invention relates to a method and apparatus for recovering heat from the exhaust air of a paper, board or pulp machine or similar vacuum system according to the preambles of the independent claims below.

The wire and press section of the paper machine mechanically dewater the web. A vacuum system is used to enhance dewatering, which maintains the vacuum required for the dewatering of suction boxes, suction rollers, and the like by sucking in the air flow required to maintain the vacuum. In addition to dewatering, vacuum is also required, for example, for transferring the web and rebuilding the felts with a wire and press section. Vacuum system exhaust air removes heat from the processes, which is sought to be recovered.

For a very long time, vacuum systems for paper, board and pulp machines have used either water ring pumps or multi-stage radial blowers. In water ring pumps, seal-water cools the exhaust air so much that utilizing it for heat recovery is not meaningful. Therefore, the exhaust air of the water ring pumps is generally directly discharged.

The so-called multi-stage radial fan. The turbo blower has an exhaust air temperature of about 120-160 ° C and a humidity of the order of 100-150 gH20 / kg dry air. The utilization of such 35 exhaust air for heat recovery is meaningful, and common practice. Typical for multi-stage radial blowers is that intake air is also drawn between the stairs 102625 2 so that the suction targets required for the highest vacuum pass through all the stairs and only the fewest vacuum units require that particular vacuum level. The exhaust air from the blower then blends the air in all the intakes and the exhaust air is led in a single stream to the heat recovery.

The state-of-the-art in vacuum blowers is represented by the smaller size and significantly cheaper single-stage, high speed radial blowers with no taps, which are smaller than multi-port radial fans. Due to their small size, these fans can be placed close to each suction point, for example on a paper machine level, and there is no need for 15 separate basement levels for the fans. When using single-stage fans, one vacuum fan is required for each vacuum level. The exhaust air from different pressure levels will not automatically mix unless combined. Exhaust air from lower pressure levels is relatively cold and is generally not worth using in heat recovery. Conversely, exhaust air flows from the highest pressure levels should be utilized.

The hot exhaust air of multi-stage radial fans 25 has previously been proposed to be utilized by directing them to the replacement air of the drying section, but today such solutions are no longer being built and the old ones have been dismantled due to numerous problems. In contrast, it is now recommended to utilize the exhaust air of radial fans to heat replacement air for paper, board and pulp machines in separate plate heat exchangers. This often requires long channel passages for conveying exhaust air from the suction points to the heat exchangers of the drying section. Transporting exhaust air 35 from multi-stage radial fans may still be profitable, but transporting different exhaust air streams from single-stage radial fans 102625 3 'to different suction points, such as a wet end of a paper machine, does not appear to be profitable.

It is therefore an object of the present invention to provide an improved method and apparatus for recovering heat from the exhaust air of vacuum blowers.

In particular, it is an object of the present invention to provide a method and apparatus for optimally utilizing the thermal energy of hot exhaust air streams from single-stage radial fans in heat recovery.

It is a further object of the present invention to provide a method and apparatus for recovering heat from the exhaust air of turbo blowers without long exhaust air transport channels.

It is also an object of the present invention to provide a method and apparatus for optimally recovering heat from exhaust air from radial fans to a variety of applications.

To achieve the foregoing purposes, the 25 methods and systems of the present invention for recovering heat from the exhaust air of a paper, board, or pulp machine or similar vacuum system are characterized in what is described in the independent claims below.

30

The air aspirated from the aspirator is typically at a temperature of 35-45 ° C and a vacuum of 10-70 kPa relative to atmospheric pressure and saturated with respect to humidity at that temperature and pressure. Thus, the higher the suction vacuum, the higher the water content of the air. Correspondingly, the higher the suction vacuum, the more the air in the fan heats up. Thus, after the turbine blowers, different vacuum levels of 102625 4 provide exhaust air, the higher the temperature and humidity of the vacuum, the higher the vacuum vacuum.

5 The higher the temperature and the humidity of the exhaust air, the more useful it is in heat recovery, because firstly the exhaust energy content is higher and on the other hand it can be used to heat the heat-receiving stream to a higher temperature. When the exhaust air is moist, much of the heat transfer occurs as water vapor in the air condenses on the heat exchanger surfaces. This occurs if the temperature of the heating surface is below the dew point temperature of the extract air. The higher the dew point of the exhaust air, the higher its humidity.

15

By passing the exhaust air at different temperature and humidity levels separately through the heat recovery, it is possible to better utilize the heat transfer countercurrent principle compared to the situation where different exhaust air flows are mixed with each other prior to heat recovery.

Therefore, a typical method of the invention for recovering heat from a heat recovery tower from the exhaust air of a paper machine vacuum system is to maintain a desired different vacuum level at each of two or more suction locations, means for supplying at least two or more exhaust air streams 35 from at least two or more suction points as separate exhaust air streams to a heat recovery tower where the separate exhaust air streams are passed through different heat exchanger cells, substantially mixed with each other.

The solution according to the invention makes it possible to heat one or more heat receiving media streams with different exhaust air streams so that the temperature and humidity levels of the exhaust air and the temperature levels of the heat receiving streams are optimized with respect to heat transfer power. In this way, exhaust air streams with a temperature difference of more than 20 ° C and even more than 10 40 ° C can be optimally separately utilized.

The exhaust air from single-stage turbo blowers according to the invention can be used, for example, to heat process fresh water, to heat zero water / tap water, to heat circulating water in a paper machine room and to heat replacement air in the drying section.

Typically, different exhaust air flows are used to heat a single heat receiving stream having an inlet at the coldest 20 and an outlet at the hottest exhaust air stream. On the other hand, of course, several different heat-receiving streams can also be heated with the exhaust air streams, whereby the hottest exhaust air stream is preferably used to heat a medium stream having a higher temperature or a higher temperature.

The heat transfer in the heat recovery tower typically takes place in air / water type plate heat exchanger cells, but can also occur in air / air type plate heat exchanger cells or e.g. in process water heating: even with scrubbers or combinations of these heat exchangers.

Typically, each of the exhaust air flows through the heat exchanger tower through different heat exchanger cells. However, if desired, the heat exchanger cell, plate heat exchanger, or the like can be divided into two or more portions in the baffle and 1026256 arranged to provide one exhaust air stream through one portion and another exhaust air stream through another portion without substantially mixing the exhaust air flows.

Preferably, the various exhaust air flows are arranged to pass through a heat recovery tower, through the entire tower, or through only a portion, as parallel parallel exhaust air flows through the heat exchanger cells arranged in parallel. The exhaust air flows are preferably combined only at or below the outlet of the heat recovery tower in a separate bottom pool section.

Of course, it is possible, if desired, to direct a portion of the exhaust air streams through the heat recovery tower from the bottom up and another portion of the exhaust air streams from above downwards, whereby these exhaust air flow portions run in opposite directions to each other. Of course, it is also conceivable that the various exhaust air flows are already connected to each other already in the heat recovery tower before being passed through one of the last 20 heat exchanger cells.

The solution of the invention makes it possible to direct the hot exhaust air streams from different single-stage turbo blowers to different applications. The exhaust air flows can thus be directed, for example, in the vicinity of the wire or press section to heat recovery towers close to the intake sites. On the other hand, the exhaust air can be introduced into the existing heat-recovery towers of the drying section of the paper machine or into a separate tower in the dry section. Cold drains and exhaust air streams with the lowest moisture content, which can be up to 30%, can be left completely unused if they are not to be used. On the other hand, about 30% of the exhaust air flows are so hot and humid that heat recovery from them is very strain-35 ways.

The advantages of the 102625 7 separate heat recovery towers, such as zero water, fresh water or process water towers or combinations thereof, located close to the wet end of the paper machine, can be considered as their low space requirements, short duct drains, low investment costs and high 5 energy saving potential. Profitable heat recovery from exhaust air to process, zero and recirculation water is already achieved with a single heat recovery tower.

If there is insufficient heating need for the various waters, the exhaust air or a portion thereof may be led to the heat recovery tower of the drying section and heated with exhaust air or air first to replace the drying section replacement air and then some of the aforementioned water flows. On the other hand, the heat recovery towers of the drying section can also be combined with the heat recovery of the exhaust air of the turbo blowers with the heat recovery of the hood exhaust air.

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 schematically shows a heat recovery system according to the invention, and Figures 2-7 schematically show other heat recovery systems according to the invention.

Figure 1 shows schematically and by way of example a preferred heat recovery tower 12 in accordance with the invention in connection with a wire and press section 10 of a papermaking machine for recovering heat from two different exhaust air streams 22 and 24 discharged by turbochargers 18 and 20. e.g. 45 kPa and at a second vacuum 16 respectively 65 kPa, wherein the temperature of the first exhaust air stream 22 is e.g. 102eC and the second exhaust air stream 160eC respectively. Correspondingly, the stream 22 35 has a moisture content of 95 g H2O / kg dry air and the stream 16 has a moisture content of 160 g H2O / kg dry air.

102625 8 In the heat recovery tower 12, three heat exchanger cells are superimposed on two adjacent groups of heat exchanger cells 26 and 28. Heat exchanger cells 26 ', 26' ', 26' '' and 5 heat exchanger cells 28 ', 28' ·, 28 '' 'are arranged in succession in a first group 26 such that a first exhaust air stream 22 can be conducted through the first cells 26 ^ 26111 and a second exhaust air stream through the other cells 28'-28 '' '. The exhaust air currents 22 and 24 do not mix with each other as they flow through the heat exchanger cells 10.

The heat-receiving medium stream 30, such as process, circulating or zero water, is arranged to pass through the heat exchanger cells in heat transfer contact with the exhaust air flow 22, 24. The exhaust air flows and the medium flow are not mixed when flowing through the heat recovery tower. Because of the large amount of water, the medium stream 30 is passed through three sub-streams 30 ', 30 ·', 30''1 first through a first lower temperature heat exchanger cell array 26 20 such that each sub-stream 30 ^ 30111 is passed through one heat exchanger cell 26'-26 '' '. From the first heat exchanger cell array 26, the media component streams 30'-δΟ '' 'are further conducted to higher temperature heat exchanger cells 28 ^ 28111 and therethrough out of the heat transfer tower. Outside the tower, the media component streams are combined into a single heated media stream 32.

In the heat exchanger cells, the cooled exhaust air flows 22, 24 are discharged from the heat recovery tower via the bottom pool section 34 below the tower 30. The bottom pool has; droplet separators 36 for separating condensed water.

Of course, in the system of the invention, the same heat recovery tower may have more than two sets of heat-35 exchanger cells in parallel, and there may be only one, two, or more than three separate cells in each group. Similarly, the intermediate 102625 9 material streams are divided into appropriate sections. Of course, fluxes of media having different temperatures when exiting the heat exchanger cells 28'-28 '' 'need not be combined into a single stream if they can be utilized as separate streams.

* 5

Fig. 2 shows a heat recovery arrangement of the type of Fig. 1, however, where two different media are heated with the exhaust air flows shown in the previous example. 2, the same reference numerals 10 as in FIG. 1 are used where applicable. A hotter medium, e.g., 30 kg / s, process water stream 38 at 50 ° C can be further heated in the direction of exhaust air flows in first heat exchanger cells 26 'and 28'. A colder medium, e.g., a factory hall circulating stream 40 at 25 ° C containing a significant amount of glycol, is heated due to the large amount of water divided into two partial streams 40 ', 40' 'in next-level heat exchanger cells 26 ·', 28 '' and 26111, 28111. to 42 ° C.

20

Fig. 3 shows another heat recovery arrangement similar to Fig. 1, in which two media of different temperatures are heated with the exhaust air flows shown in the previous example. In this case, the flow of medium flows 25 is slightly different than that of Figure 2. In Figure 3, the same reference numeral as Figure 1 is applied where applicable. The hotter medium flow 44 is in this case arranged to flow only in heat exchanger cells 30'28. through countercurrent with the exhaust air flow 24, i.e. in through cell 28111 and out through cell 28 '. Thus, for example, 60 kg / s, 40 ° C process water can be heated up to 53 ° C. Correspondingly, the colder medium stream 46 is arranged to flow only through the heat exchanger cells 26'-26'11 in heat transfer connection with the colder exhaust air stream 22, upstream of the exhaust air stream 22, i.e. in through cell 26 10 'and out through cell 26'. Thus, for example, 60 kg / s, 25 ° C circulation water can be heated up to 37 ° C.

Figure 4 shows a third heat recovery arrangement 5 similar to Figure 1 in which one medium is heated. In this case, the flow of the media streams is slightly different than that of FIGS. 2 and 3. FIG. 4, where applicable, has the same reference numerals as FIG. 1. The media stream 48 is then fed into the heat recovery tower 12 at the outlet of airflow 22 at the through and upstream with the exhaust air flow 22. From the first heat exchanger cell 26 'in this group, the medium stream 15 is led at the outlet of the second hotter exhaust air stream 24 to the second heat exchanger cell group 28 and thereafter upstream with the hotter exhaust air stream 24. The medium flow is finally discharged from the heat recovery tower through the hottest heat exchanger cell 28 '. In this way, 30 kg / s, 5 ° C process water can be heated up to 56 ° C.

Figure 5 shows a heat recovery tower with three adjacent heat exchanger cell groups 26, 27, 28 instead of two heat exchanger cell assemblies 26 and 28.

In this case, the hotter exhaust air 24 may be passed through two heat exchanger cell groups 28 and 27. Then, the hot exhaust air stream entering the heat recovery tower is first passed through a heat exchanger cell array 28 where the fluid stream 30 is finally heated before exiting tower 12. From the lowest cell 28111, the exhaust air stream '. The exhaust air flow 24 is discharged from the tower via the cell 27111. The exhaust air flow 24 can simply be transferred from the cell 28111 to the cell 27 'via a channel not shown in Fig. 5262511. The arrangement shown in Fig. 5 can heat 60 kg / s, 50 ° C process water to 61 ° C.

Fig. 6 shows a solution similar to Fig. 5, however, where exhaust air is supplied to the heat exchanger cell group 27 from both the exhaust air flow 22 and the flow 24. Part 22 'of the exhaust air flow 22 passes through the heat exchanger cell group 26 and part 22' 1. Further, the exhaust air flow 24 after the cell 28 '' 'is conducted as the exhaust air flow 24' to pass through the heat exchanger group 27 from the cell 27 to the cell 271 if the temperature of the exhaust air stream 24 in the heat exchanger cell group 28 has not substantially decreased. The temperature difference DT measured in the exhaust air 15 currents 22 and 24 'by the temperature gauges 49 and 49' adjusts the control grille 50 which directs the exhaust air flow 24 'directly to the tower basin section 34 if the DT indicates the exhaust air flow 24' temperature is too low airflow 22 '' when mixed with it. However, in this case, part of the exhaust air flow 22, i.e. current 22 · ', flows through the heat exchanger array group 27.

Figure 7 further illustrates a slightly different heat-recovery system in which exhaust air flows 22 and 24 are led from the lower part of tower 12 to scrubbers 52 and 54 where 40 kg / s, 35 ° C process water 58 is heated to 41 kg / s, 63 ° C. process water 58 '.

The invention has been described above with reference to the accompanying figures. However, it is not intended to limit the invention to these exemplary embodiments and applications. On the contrary, it is intended that the invention be extensively applied within the scope defined by the following claims. Accordingly, the invention can be applied to heat recovery towers for extract air streams only, and to towers which additionally include other heat recovery elements, although other conventional heat exchangers 5 in the heat recovery towers are not shown in Figures 1-7 above.

Claims (18)

A method for recovering heat with a heat recovery tower or equivalent from the air of a papermaking, cardboard or cellulose machine or corresponding vacuum system, wherein two or more suction points or suction point groups are maintained in each desired deviation the vacuum air removed by the vacuum pump or the suction point group removed from the vacuum pump or the corresponding, and from two or more suction points removed from said two or more suction points or suction point groups by means of at least two different vacuum pumps, are conducted as separate exhaust streams to the heat recovery tower, which said separate exhaust streams are passed through different heat exchanger elements without substantially mixing with each other. A method according to claim 1, characterized in that at least two different exhaust air streams are passed through the entire heat recovery tower as parallel exhaust streams. Method according to claim 1, characterized in that at least part of the separate exhaust air streams are passed in the heat recovery tower through several from the exhaust stream seen successive heat exchanger elements, in which different heat exchanger elements heat is transferred to different heat receiving medium. 35 4. A method according to claim 1, characterized in that at least one heat receiving medium stream is caused in the heat recovery tower to flow through at least two of different air streams, through which heat exchangers element, so that heat is transmitted to the heat receiving medium stream from different streams of air. 5 5. A method according to claim 1, characterized in that two heat-receiving medium currents are caused to flow in the heat recovery tower through heat exchanger elements, which are passed through different exhaust air streams, so that heat is transferred to two heat-receiving medium streams. Process according to claim 1, characterized in that heat is recovered in heat exchanger elements one with a plate heat exchanger to a liquid medium stream, such as process water or backwater, or a gaseous medium stream, such as replacement air for a hood. Process according to claim 1, characterized in that heat is recovered in the heat recovery tower with a scrubber in process water or the equivalent. Method according to claim 1, characterized in that two exhaust streams are led to the heat recovery tower, the temperature of which is at least 90 ° C and whose temperature difference is at least about 15 ° C, preferably at least about 40 ° C. An arrangement for recovering heat from the exhaust of a paper, cardboard or cellulose machine or equivalent vacuum system, comprising: - at least two vacuum pumps, such as single turbo fans, or parallel coupled turbo fan groups to provide a vacuum suction or suction vacuum system point groups, 35. a heat recovery tower having at least two heat exchanger elements or the equivalent for recovery of heat from the vacuum streams of the vacuum pumps, 102625 19 5. means for conducting a heat-receiving medium through the heat recovery tower, characterized in that the arrangement comprises - means for conducting at least two separate air streams for at least two separate vacuum pumps or parallel coupled vacuum pump groups as separate extract air streams to the value recovery tower or through at least a portion thereof. Arrangement according to claim 9, characterized in that the heat recovery tower comprises - a first heat exchanger element and means for conducting a first exhaust air flow through, - a second heat exchanger element and means for conducting a second exhaust air flow through, and a 20 means for conducting an air passage. medium consecutively through the first and second heat exchanger elements in heat exchange contact in turn with the first and second exhaust air streams. 11. Arrangement according to claim 9, characterized in that at least part of the heat exchanger elements are plate heat exchanger. Arrangement according to claim 9, characterized in that a drip separator is arranged in the heat recovery tower after the heat exchanger elements as seen in the direction of the exhaust pipe. Arrangement according to claim 9, characterized in that the arrangement comprises means for combining the separate exhaust air streams after the heating recovery tower or its final part. 102625 20 Arrangement according to claim 9, characterized in that the arrangement comprises - one or more so-called two air streams. common heat exchanger elements; and 5. means for conducting a selected exhaust air stream through the common heat exchanger element. Arrangement according to claim 9, characterized in that the heat recovery tower further comprises conventional heat exchanger means for recovering heat t.e. from the exhaust of a hood. Arrangement according to claim 9, characterized in that the vacuum pumps are single-stage turbo fans. 15 Arrangement according to claim 9, characterized in that the heat recovery tower has a bottom pool part and outlet duct which is common to different exhaust streams. Arrangement according to claim 9, characterized in that at least two heat exchanger elements are formed from a common element structure by means of a partition or partitions, which prevent the air streams from being mixed with each other. 25