CA1149210A - Method of utilizing the enthalpy content of cooling water in a suction drying process of a paper, board or other porous web - Google Patents
Method of utilizing the enthalpy content of cooling water in a suction drying process of a paper, board or other porous webInfo
- Publication number
- CA1149210A CA1149210A CA000375918A CA375918A CA1149210A CA 1149210 A CA1149210 A CA 1149210A CA 000375918 A CA000375918 A CA 000375918A CA 375918 A CA375918 A CA 375918A CA 1149210 A CA1149210 A CA 1149210A
- Authority
- CA
- Canada
- Prior art keywords
- cooling water
- web
- drying
- temperature
- water
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 238000001035 drying Methods 0.000 title claims abstract description 70
- 239000000498 cooling water Substances 0.000 title claims abstract description 54
- 238000000034 method Methods 0.000 title claims abstract description 22
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 36
- 230000008569 process Effects 0.000 claims abstract description 10
- 238000001704 evaporation Methods 0.000 claims description 18
- 238000005086 pumping Methods 0.000 abstract description 17
- 238000009833 condensation Methods 0.000 abstract description 2
- 230000005494 condensation Effects 0.000 abstract description 2
- 230000008020 evaporation Effects 0.000 description 11
- 238000010438 heat treatment Methods 0.000 description 10
- 239000002184 metal Substances 0.000 description 8
- 230000004907 flux Effects 0.000 description 6
- 230000000875 corresponding effect Effects 0.000 description 3
- 238000001816 cooling Methods 0.000 description 2
- 238000011161 development Methods 0.000 description 2
- 230000018109 developmental process Effects 0.000 description 2
- 239000000203 mixture Substances 0.000 description 2
- 238000011084 recovery Methods 0.000 description 2
- 229920006395 saturated elastomer Polymers 0.000 description 2
- 230000001276 controlling effect Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 239000007789 gas Substances 0.000 description 1
- 239000008236 heating water Substances 0.000 description 1
- 238000009413 insulation Methods 0.000 description 1
- 238000005457 optimization Methods 0.000 description 1
Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F26—DRYING
- F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
- F26B3/00—Drying solid materials or objects by processes involving the application of heat
- F26B3/18—Drying solid materials or objects by processes involving the application of heat by conduction, i.e. the heat is conveyed from the heat source, e.g. gas flame, to the materials or objects to be dried by direct contact
- F26B3/20—Drying solid materials or objects by processes involving the application of heat by conduction, i.e. the heat is conveyed from the heat source, e.g. gas flame, to the materials or objects to be dried by direct contact the heat source being a heated surface, e.g. a moving belt or conveyor
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
- D21F5/00—Dryer section of machines for making continuous webs of paper
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
- D21F5/00—Dryer section of machines for making continuous webs of paper
- D21F5/004—Drying webs by contact with heated surfaces or materials
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
- D21F5/00—Dryer section of machines for making continuous webs of paper
- D21F5/007—Drying webs by contact with sorbent bodies
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F26—DRYING
- F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
- F26B13/00—Machines and apparatus for drying fabrics, fibres, yarns, or other materials in long lengths, with progressive movement
- F26B13/10—Arrangements for feeding, heating or supporting materials; Controlling movement, tension or position of materials
- F26B13/105—Drying webs by contact with heated surfaces other than rollers or drums
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F26—DRYING
- F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
- F26B23/00—Heating arrangements
- F26B23/001—Heating arrangements using waste heat
- F26B23/002—Heating arrangements using waste heat recovered from dryer exhaust gases
- F26B23/004—Heating arrangements using waste heat recovered from dryer exhaust gases by compressing and condensing vapour in exhaust gases, i.e. using an open cycle heat pump system
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
- Y02B30/00—Energy efficient heating, ventilation or air conditioning [HVAC]
- Y02B30/52—Heat recovery pumps, i.e. heat pump based systems or units able to transfer the thermal energy from one area of the premises or part of the facilities to a different one, improving the overall efficiency
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
- Y02P70/10—Greenhouse gas [GHG] capture, material saving, heat recovery or other energy efficient measures, e.g. motor control, characterised by manufacturing processes, e.g. for rolling metal or metal working
Landscapes
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Life Sciences & Earth Sciences (AREA)
- Microbiology (AREA)
- Textile Engineering (AREA)
- Sustainable Development (AREA)
- Drying Of Solid Materials (AREA)
- Paper (AREA)
Abstract
Abstract of the Disclosure A method of utilizing the enthalpy of cooling water used in a process for suction drying of paper, board or similar porous or fibrous webs. The wet web (4) is by means of a drying felt (5) enclosed over its whole width between two air-tight surfaces having a good thermal conductivity. The surface being in contact with the web is heated and the surface being in contact with the drying felt is cooled by cooling water so that water is evaporated from the web into the felt and condensated in the felt. After the condensation the felt is separated from the dry web. The cooling water is dis-charged from the drying process at a temperature of 60°-100°C so that the enthalpy of the discharged cooling water can be utilized economically e.g. for heat pumping. The discharge temperature of the cooling water is controlled by adjusting the inlet temperature and/or the flow rate of the cooling water supplied to the drying process.
Description
2~
This invention relates to a method of utilizing the enthalpy of cooling water in a suction drying process of a paper, board, or other porous or fibrous web, in which process the wet web and the drying felt carrying the web are over the whole width of the web enclosed between two air-tight surfaces having good thermal conduc-tivity, the surface which is in contact with the web is heated in order to evaporate water from the web and the surface which is in contact with the drying felt is cooled by cooling water in order to condense the water evaporating from the web to the drying felt, and the drying felt is thereafter separated from the dry web and released from the condensed water.
In present paper or board drying methods heat is recovered from the mixture of air and steam which is obtained from the hood surrounding the conventional drying cylinder or group of drying cylinders.
Usually the water which is heated by such gases flows out from the heat recovery equipment at a temperature of 30 - 55C. This water is mainly used in the same factory, for example as shower water for the wet end of the board machine, and,/or for heating the building.
Said water has not been warm enough to be utilized - at least not to an appreciable extent - as district heating water or as a heat source for heat pumping.
Developments of heat pumping in connection with drying of a paper or board web have been described, for example in some patents (US
patent 2 933 826, Swedish patent 7111908-5, German Patent 2630853).
These inventions have the common feature that heat is recovered by transferring heat from said mixture of air and steam directly into water or an other medium which is flowing or evaporating at a low pressure in a heat exchanger. In each case the recovered heat can be utilized to evaporate water, and the steam thereby generated is compressed by a compressor or compressors to pressures needed in the drying cylinder.
These methods, however, have not resulted in marketable products due to the relatively small economic advantage gained in comparison with the use of back pressure steam, and due to operational diffi-culties encountered, for example when the web is brokenl with hoods sealed more tightly than normally.
Suction drying between two bands, or between a cyiinder and a band (Finnish patent 54514, US patent 4112586, British patent 1502040, Finnish patent 59636) differs very much from conventional drying.
The heat energy needed to evaporate water from the web is brought as saturated steam to the inner surface of the cylinder shell or to the other side of the hot band, from where it passes through a relatively thin metal layer into the web. Inside the web heat pene-trates to the depth where evaporation takes p1ace. The evaporated steam passes through the rest of the web and through the felt, and condcnses onto the cold metal surface which is cooled with cooling water from the outside. The cooling water leaves the process at a temperature below 40C.
It is clear that the entha1py in the cooling water discharged from the above described, ~own, suction drying process could be better utilized, for instance in heat pumping, if the temperature of this water could be increased. This kind of development, however, has been prevented by the suspicion that the increased discharge tempe-rature of the cooling water might slow down the drying process. In many heat apparatuses the heat flux is directly proportional to the temperature difference. From this might be concluded that also in the above described suction drying process the heat flux through the hot metal surface into the web being dried would be almost directly proportional to the difference in temperature between the hot and the cold metal surfaces.
A decrease of the drying rate could lead the harmful consequences.
In practice it is important that the drying rate does not decrease much, because this would necPssitate larger drying surfaces (more cylinders in many cases), which is expensive.
~ Z3 ~
It may be noted that if the temperature of the heating steam is increased from loo& to 180 C, the drying rate becomes sixfold. The optimum steam temperature results from considera-tions involving the total economy.
The object of the present invention is to improve the utilization of the cooling water enthalpy in the above described suction drying process. According to the invention this is realized when the cooling water leaves the process at a temperature of 60 - loo&.
In one aspect of the present invention there is provided a method of utillzing the enthalpy of cooling water in a suction drying process of a paper, board, or other porous or fibrous web, in which process the wet web and the drying felt carrying the web are over the whole width of the web enclosed between two air-tight surfaces having good thermal conductivity, the surface which is in contact with the web is heated in order to evaporate water from the web and the surface which is in contact with the drying felt is cooled by cooling water in order to condense the water evaporating from the web to the drying felt, and the drying felt is thereafter separated from the dry web and released from the condensed water, characterized in that said cooling water is discharged from the drying process at a temperature of 60 - 100 C.
The invention is based on the surprising fact, proved by tests that in suction drying the cooling water can be fed into the drying process at a temperature of up to 80 & and discharged at loo& without decreasing the drying rate with more than a few percent compared with the situation where the discharge temperature of the cooling water is between lo& and 30 & .
2~
- 3a -Said test results indicating such a minor effect of the temperature of the cooling water may seem surprising- They can be explained as follows. The evaporation rate from the web is roughly directly proportional to the heat flux from the hot surface to the web. This flux depends on the difference in temperature between the hot surface and the evaporation point inside the web. If the temperature of the hot surface is the same in two different situations, and the evaporation rate, and therefore also the heat flux, are almost the same, then also the temperature at the evaporating zone of the web must be nearly the same in both cases. This means also that the evaporation pressure in these two cases is nearly equal-The evaporated steam passes through a part of the web andthrough the felt, and is then condensed on the cold metal surface. I'he pressure of the steam when it reaches the cold surface depends on the temperature of this surface, if this temperature is for example 29 C in one case (corresponding to cooling water of about 17 C) and 87 C in another case (corresponding to cooling water of 75C), steam pressure in the first case is about 5 '~Pa and in the second case about 70 }~a-Correspondingly, the specific volume of the steam in the first casets sixfold compared with the specific colume in the second case.
Because the cold surface in both cases received nearly the same mass flow of steam, the speed of this steam is in the first case about s;x times as high as in the second case. The higher velocity steam suffers a greater pressure drop in its way through part of the web and through the felt. Thus it is possible (this can be proved by calculations) that the same mass flows of steam in said two cases start from the same evaporation pressure and reach the cold metal sùrface for condensation with different pressures. This explains the above-mentioned test results where the drying rate was only little affected by the big changes in the temperature of the cooling water.
.
The cooling water discharged according to the invention from the drying process at a high temperature can be utilized as source water for heat pumping or for district heating.
When the cooling water is used for heat pumping it is preferably discharged from the drying process at a temperature of 60-85C.
The lowest possible temperature for econo`mical heat pumping is about 60C, and the preferable upper limit temperature when drying paper or board is about 85C. A part of the discharged cooling water is evaporated at a low pressure in a suitable expansion unit, for example in a packed column or a cyclone. The latent enthalpy needed for the evaporation is supplied by that part of the water which does not evaporate. This water comes out from the evaporation unit at a lower temperature, 50-65C, than- at which it was fed into the unit and the water is brought back into the drying unit as cooling water. The evaporated steam is compressed by compressors to a desired pressure for the drying process.
As desribed above, steam is fed to the heat pumping compressors under a considerably higher pressure than what is possible to ob-tain from the evaporation unit of a heat pumping process in connec-tion with the known suction drying process where cooling water leaves the drying process at a temperature below 40C. The higher pressure steam results in better heat pumping efficiency and impro~-ed economy for the whole drying process.
' ~:
- ~hen the cooling water is used for distric~ heating~ it is supplied to the drying apparatus at a low temperature, 0-50C, and out at a high temperature, preferably 75-100C. After having been used for heating at the district heating locale, the water can be either returned at a low temperature to the drying unit, where it is used again as cooling water, or it can be fed somewhere else out of the drying process. In the latter case new cooling water must be con-tinuously fed into the drying process.
The discharge temperature of the cooling water can be controlled by altering the cooling water flow rate. If the drying rate of the web remains unchanged, the heat flux through the cold band into the cooling water remains also unchanged; if the inlet temperature of the cooling water is ~ept unchanged, the discharge temperature of the cooling water rises when the cooling water flow rate is decreas-ed. Thus it is easy to control the discharge temperature of the cooling water under different operating conditions even within wide margins.
The invention will be described in more detail in the following with reference to the enclosed drawing, which shows schematically a heat pumping arrangement in connection with a suction drying process according to British patent 1502040.
The upper part of the drawing shows a suction dryer 1 comprising an upper, endless, air-tight metal band 2 with good thermal conduc-tivity, and a similar lower metal band 39 these ~wo bands running in parallel over a certain zone. The paper web 4 to be dried is led between the bands so that it is completely enclosed by the bands within ~he said parallel zone. Between the paper web and the lower band 3 runs an endless drying felt which carries the web. On the 9~
.
inner side of the upper band 2 there is a heating box 6, the lowèr side of which is open against the upper surface of the band in the parallel zone. The box is provided with an inlet 7 for the heating steam and with outlets 8 for the condensate. On the inner side of the lower band 3 there is a cooling box 9, the upper side of which is open against the lower surface of the band in the parallel zone.
The box is provided with an inlet 10 for ~he cooling water and with an outlet 11 for the used water.
When the wet web passes through the suction dryer, the web is thus enclosed between a steam heated upper band 2 and lower water-cooled band 3. Therefore, the water contained in the web evaporates, passes through the web and the felt, and is finally condensed on the sur-face of the lower band, as is explained in said British patent publication. After drying, the web is separated from the felt.
Cooling water Wj is discharged from the dryer at a temperature of 60-100C and led by a pipe 12 to a mixer 13. The condensate Wa is led by a pipe 14 from the heating box 6 through a condensate separator 15 and a pressure reducing valve 16 to the mixer. Upon passing through the pressure reducing valve a part of the conden-sate e~aporates. This small amount of steam is condensed in the mixer immediately when it comes into contact with the cooling water.
From the mixer 13 most of the water is led through a pipe l7 and a pressure reducing valve 18 into an evaporation column 19. This is a conventional packed column operated under vacuum. It is fed from the top; the e~aporating steam leaves through another conduit at the top; and the remaining water, which has cooled by about 10C, leaves through the bottom. This water Wj is pumped through a pipe 21 by means of a pump 20 back to the cooling box 9 of the suction dryer as cooling water. From the evaporation column steam is led through a pipe 22 to a first compressor 23, which is operated by an electric motor. Either before, inside, or after the compressor water is sprayed into the steam so that the steam leaving the ',~
compressor is saturated. This water is pumped by means of a pump 25 from the mixer through a pipe 24. The steam leaving the first column passes through a pipe 26 to a second compressor 27, which operates similarly. The number of compressors in series depends on economical optimization. In practice this number is usually two, sometimes possibly three. From the last compressor steam passes through a pipe 28 to the heating box 6 of the suction dryer. This steam is mixed with steam passing through pipe 30 from the blow-through re-circulation compressor 29. This compressor receives steam from the condensate separator 15.
The above described process is closed, except that the wet web passes into the dryer, and the dry web and the evaporated water pass out of tlle dryer. Electric power is needed for the electric motors of the compressors and for the motors of the water circulation pumps.
There are minor heat losses from the pir)ing and the equipmant through heat insulation into the surroundings Contrary to conventional cylinder-hood drying, this drying process does not need any supply of back pressure steam or any other steam from the outside.
The economy of the above described process, compared with the same drying process without heat pumping and using back pressure steam from sources outside the drying apparatus, is mainly dependent on the price of electric power needed for heat pumping compared w;th the price of back pressure steam, and on the capital cost of the additional equipment required by the heat pumping. For example, if the steam of 0.8 MPa is needed for the actual drying, the price of back pressure steam is about 50 mark/(MWh latent enthalpy of steam). If the required steam of 0.8 MPa is obtained by evaporating water at 60C and compressing the evaporated steam to the desired pressure by a compressor with an efficieny of 0.83, the price of the required electric power is 69.0 mark/(MWh of latent enthalpy of steam) provided that the price of electric power is 120 rnark/MWh.
If the steam is compressed from evaporating steam at 80C, the price of the steam is 45.9 mark/(MWh of latent enthalpy of steam).
2~
The prices being as above mentioned, the drying steam obtained by heat pumping from water at 80C is already cheaper than correspond-ing back pressure steam, whereas steam obtained by heat pumping from water at 60C is more expensive.
tn the future the price of electric power compared with fossile fuels is likely to decrease. The above-described heat pumping pro-cess will then become relatively more economical than now.
The above described heat recovery process is applicable also to a suctlon dryer in which a rotating cylinder is substituted for one of the bands, preferably the heated band, as described in Finnish patent no 59636~ The invention can also be applied when using the above described suction drying method for serial batch drying.
The following table illustrates several examples of how a desired discharge (outlet) temperature is obtained by controlling the inlet temperature and flow rate of the coo1ing water. In all exam-ples it is supposed that a constant amount of heat, 15000 kW is transferred into the cooling water per time unit and that the other conditions and the web quality are the same.
Example No. Inlet Temperature¦ Outlet Temperature¦ Flow rate of cooling water .. . _ 1 50C 60C 359 kg/s 2 50C 70C 179 kg/s
This invention relates to a method of utilizing the enthalpy of cooling water in a suction drying process of a paper, board, or other porous or fibrous web, in which process the wet web and the drying felt carrying the web are over the whole width of the web enclosed between two air-tight surfaces having good thermal conduc-tivity, the surface which is in contact with the web is heated in order to evaporate water from the web and the surface which is in contact with the drying felt is cooled by cooling water in order to condense the water evaporating from the web to the drying felt, and the drying felt is thereafter separated from the dry web and released from the condensed water.
In present paper or board drying methods heat is recovered from the mixture of air and steam which is obtained from the hood surrounding the conventional drying cylinder or group of drying cylinders.
Usually the water which is heated by such gases flows out from the heat recovery equipment at a temperature of 30 - 55C. This water is mainly used in the same factory, for example as shower water for the wet end of the board machine, and,/or for heating the building.
Said water has not been warm enough to be utilized - at least not to an appreciable extent - as district heating water or as a heat source for heat pumping.
Developments of heat pumping in connection with drying of a paper or board web have been described, for example in some patents (US
patent 2 933 826, Swedish patent 7111908-5, German Patent 2630853).
These inventions have the common feature that heat is recovered by transferring heat from said mixture of air and steam directly into water or an other medium which is flowing or evaporating at a low pressure in a heat exchanger. In each case the recovered heat can be utilized to evaporate water, and the steam thereby generated is compressed by a compressor or compressors to pressures needed in the drying cylinder.
These methods, however, have not resulted in marketable products due to the relatively small economic advantage gained in comparison with the use of back pressure steam, and due to operational diffi-culties encountered, for example when the web is brokenl with hoods sealed more tightly than normally.
Suction drying between two bands, or between a cyiinder and a band (Finnish patent 54514, US patent 4112586, British patent 1502040, Finnish patent 59636) differs very much from conventional drying.
The heat energy needed to evaporate water from the web is brought as saturated steam to the inner surface of the cylinder shell or to the other side of the hot band, from where it passes through a relatively thin metal layer into the web. Inside the web heat pene-trates to the depth where evaporation takes p1ace. The evaporated steam passes through the rest of the web and through the felt, and condcnses onto the cold metal surface which is cooled with cooling water from the outside. The cooling water leaves the process at a temperature below 40C.
It is clear that the entha1py in the cooling water discharged from the above described, ~own, suction drying process could be better utilized, for instance in heat pumping, if the temperature of this water could be increased. This kind of development, however, has been prevented by the suspicion that the increased discharge tempe-rature of the cooling water might slow down the drying process. In many heat apparatuses the heat flux is directly proportional to the temperature difference. From this might be concluded that also in the above described suction drying process the heat flux through the hot metal surface into the web being dried would be almost directly proportional to the difference in temperature between the hot and the cold metal surfaces.
A decrease of the drying rate could lead the harmful consequences.
In practice it is important that the drying rate does not decrease much, because this would necPssitate larger drying surfaces (more cylinders in many cases), which is expensive.
~ Z3 ~
It may be noted that if the temperature of the heating steam is increased from loo& to 180 C, the drying rate becomes sixfold. The optimum steam temperature results from considera-tions involving the total economy.
The object of the present invention is to improve the utilization of the cooling water enthalpy in the above described suction drying process. According to the invention this is realized when the cooling water leaves the process at a temperature of 60 - loo&.
In one aspect of the present invention there is provided a method of utillzing the enthalpy of cooling water in a suction drying process of a paper, board, or other porous or fibrous web, in which process the wet web and the drying felt carrying the web are over the whole width of the web enclosed between two air-tight surfaces having good thermal conductivity, the surface which is in contact with the web is heated in order to evaporate water from the web and the surface which is in contact with the drying felt is cooled by cooling water in order to condense the water evaporating from the web to the drying felt, and the drying felt is thereafter separated from the dry web and released from the condensed water, characterized in that said cooling water is discharged from the drying process at a temperature of 60 - 100 C.
The invention is based on the surprising fact, proved by tests that in suction drying the cooling water can be fed into the drying process at a temperature of up to 80 & and discharged at loo& without decreasing the drying rate with more than a few percent compared with the situation where the discharge temperature of the cooling water is between lo& and 30 & .
2~
- 3a -Said test results indicating such a minor effect of the temperature of the cooling water may seem surprising- They can be explained as follows. The evaporation rate from the web is roughly directly proportional to the heat flux from the hot surface to the web. This flux depends on the difference in temperature between the hot surface and the evaporation point inside the web. If the temperature of the hot surface is the same in two different situations, and the evaporation rate, and therefore also the heat flux, are almost the same, then also the temperature at the evaporating zone of the web must be nearly the same in both cases. This means also that the evaporation pressure in these two cases is nearly equal-The evaporated steam passes through a part of the web andthrough the felt, and is then condensed on the cold metal surface. I'he pressure of the steam when it reaches the cold surface depends on the temperature of this surface, if this temperature is for example 29 C in one case (corresponding to cooling water of about 17 C) and 87 C in another case (corresponding to cooling water of 75C), steam pressure in the first case is about 5 '~Pa and in the second case about 70 }~a-Correspondingly, the specific volume of the steam in the first casets sixfold compared with the specific colume in the second case.
Because the cold surface in both cases received nearly the same mass flow of steam, the speed of this steam is in the first case about s;x times as high as in the second case. The higher velocity steam suffers a greater pressure drop in its way through part of the web and through the felt. Thus it is possible (this can be proved by calculations) that the same mass flows of steam in said two cases start from the same evaporation pressure and reach the cold metal sùrface for condensation with different pressures. This explains the above-mentioned test results where the drying rate was only little affected by the big changes in the temperature of the cooling water.
.
The cooling water discharged according to the invention from the drying process at a high temperature can be utilized as source water for heat pumping or for district heating.
When the cooling water is used for heat pumping it is preferably discharged from the drying process at a temperature of 60-85C.
The lowest possible temperature for econo`mical heat pumping is about 60C, and the preferable upper limit temperature when drying paper or board is about 85C. A part of the discharged cooling water is evaporated at a low pressure in a suitable expansion unit, for example in a packed column or a cyclone. The latent enthalpy needed for the evaporation is supplied by that part of the water which does not evaporate. This water comes out from the evaporation unit at a lower temperature, 50-65C, than- at which it was fed into the unit and the water is brought back into the drying unit as cooling water. The evaporated steam is compressed by compressors to a desired pressure for the drying process.
As desribed above, steam is fed to the heat pumping compressors under a considerably higher pressure than what is possible to ob-tain from the evaporation unit of a heat pumping process in connec-tion with the known suction drying process where cooling water leaves the drying process at a temperature below 40C. The higher pressure steam results in better heat pumping efficiency and impro~-ed economy for the whole drying process.
' ~:
- ~hen the cooling water is used for distric~ heating~ it is supplied to the drying apparatus at a low temperature, 0-50C, and out at a high temperature, preferably 75-100C. After having been used for heating at the district heating locale, the water can be either returned at a low temperature to the drying unit, where it is used again as cooling water, or it can be fed somewhere else out of the drying process. In the latter case new cooling water must be con-tinuously fed into the drying process.
The discharge temperature of the cooling water can be controlled by altering the cooling water flow rate. If the drying rate of the web remains unchanged, the heat flux through the cold band into the cooling water remains also unchanged; if the inlet temperature of the cooling water is ~ept unchanged, the discharge temperature of the cooling water rises when the cooling water flow rate is decreas-ed. Thus it is easy to control the discharge temperature of the cooling water under different operating conditions even within wide margins.
The invention will be described in more detail in the following with reference to the enclosed drawing, which shows schematically a heat pumping arrangement in connection with a suction drying process according to British patent 1502040.
The upper part of the drawing shows a suction dryer 1 comprising an upper, endless, air-tight metal band 2 with good thermal conduc-tivity, and a similar lower metal band 39 these ~wo bands running in parallel over a certain zone. The paper web 4 to be dried is led between the bands so that it is completely enclosed by the bands within ~he said parallel zone. Between the paper web and the lower band 3 runs an endless drying felt which carries the web. On the 9~
.
inner side of the upper band 2 there is a heating box 6, the lowèr side of which is open against the upper surface of the band in the parallel zone. The box is provided with an inlet 7 for the heating steam and with outlets 8 for the condensate. On the inner side of the lower band 3 there is a cooling box 9, the upper side of which is open against the lower surface of the band in the parallel zone.
The box is provided with an inlet 10 for ~he cooling water and with an outlet 11 for the used water.
When the wet web passes through the suction dryer, the web is thus enclosed between a steam heated upper band 2 and lower water-cooled band 3. Therefore, the water contained in the web evaporates, passes through the web and the felt, and is finally condensed on the sur-face of the lower band, as is explained in said British patent publication. After drying, the web is separated from the felt.
Cooling water Wj is discharged from the dryer at a temperature of 60-100C and led by a pipe 12 to a mixer 13. The condensate Wa is led by a pipe 14 from the heating box 6 through a condensate separator 15 and a pressure reducing valve 16 to the mixer. Upon passing through the pressure reducing valve a part of the conden-sate e~aporates. This small amount of steam is condensed in the mixer immediately when it comes into contact with the cooling water.
From the mixer 13 most of the water is led through a pipe l7 and a pressure reducing valve 18 into an evaporation column 19. This is a conventional packed column operated under vacuum. It is fed from the top; the e~aporating steam leaves through another conduit at the top; and the remaining water, which has cooled by about 10C, leaves through the bottom. This water Wj is pumped through a pipe 21 by means of a pump 20 back to the cooling box 9 of the suction dryer as cooling water. From the evaporation column steam is led through a pipe 22 to a first compressor 23, which is operated by an electric motor. Either before, inside, or after the compressor water is sprayed into the steam so that the steam leaving the ',~
compressor is saturated. This water is pumped by means of a pump 25 from the mixer through a pipe 24. The steam leaving the first column passes through a pipe 26 to a second compressor 27, which operates similarly. The number of compressors in series depends on economical optimization. In practice this number is usually two, sometimes possibly three. From the last compressor steam passes through a pipe 28 to the heating box 6 of the suction dryer. This steam is mixed with steam passing through pipe 30 from the blow-through re-circulation compressor 29. This compressor receives steam from the condensate separator 15.
The above described process is closed, except that the wet web passes into the dryer, and the dry web and the evaporated water pass out of tlle dryer. Electric power is needed for the electric motors of the compressors and for the motors of the water circulation pumps.
There are minor heat losses from the pir)ing and the equipmant through heat insulation into the surroundings Contrary to conventional cylinder-hood drying, this drying process does not need any supply of back pressure steam or any other steam from the outside.
The economy of the above described process, compared with the same drying process without heat pumping and using back pressure steam from sources outside the drying apparatus, is mainly dependent on the price of electric power needed for heat pumping compared w;th the price of back pressure steam, and on the capital cost of the additional equipment required by the heat pumping. For example, if the steam of 0.8 MPa is needed for the actual drying, the price of back pressure steam is about 50 mark/(MWh latent enthalpy of steam). If the required steam of 0.8 MPa is obtained by evaporating water at 60C and compressing the evaporated steam to the desired pressure by a compressor with an efficieny of 0.83, the price of the required electric power is 69.0 mark/(MWh of latent enthalpy of steam) provided that the price of electric power is 120 rnark/MWh.
If the steam is compressed from evaporating steam at 80C, the price of the steam is 45.9 mark/(MWh of latent enthalpy of steam).
2~
The prices being as above mentioned, the drying steam obtained by heat pumping from water at 80C is already cheaper than correspond-ing back pressure steam, whereas steam obtained by heat pumping from water at 60C is more expensive.
tn the future the price of electric power compared with fossile fuels is likely to decrease. The above-described heat pumping pro-cess will then become relatively more economical than now.
The above described heat recovery process is applicable also to a suctlon dryer in which a rotating cylinder is substituted for one of the bands, preferably the heated band, as described in Finnish patent no 59636~ The invention can also be applied when using the above described suction drying method for serial batch drying.
The following table illustrates several examples of how a desired discharge (outlet) temperature is obtained by controlling the inlet temperature and flow rate of the coo1ing water. In all exam-ples it is supposed that a constant amount of heat, 15000 kW is transferred into the cooling water per time unit and that the other conditions and the web quality are the same.
Example No. Inlet Temperature¦ Outlet Temperature¦ Flow rate of cooling water .. . _ 1 50C 60C 359 kg/s 2 50C 70C 179 kg/s
3 50C 80 C 119 kg/s
4 80C 100 C 179 kg/s 0C 60C 60 kg/s 6 0C 70C 51 kg/s 7 0C 80C 45 kg/s 8 0C 100C 36 kg/s In the above examples the drying rate of the web varies only a few percent.
The object of the drawing and the description is only to il1ustrate the idea of the invention. In its details the method according to the invention can vary within the scope of the claims.
The object of the drawing and the description is only to il1ustrate the idea of the invention. In its details the method according to the invention can vary within the scope of the claims.
Claims (6)
1. A method of utilizing the enthalpy of cooling water in a suction drying process of a paper, board, or other porous or fibrous web, in which process the wet web and the drying felt carrying the web are over the whole width of the web enclosed between two air-tight surfaces having good thermal conductivity, the surface which is in contact with the web is heated in order to evaporate water from the web and the surface which is in contact with the drying felt is cooled by cooling water in order to condense the water evaporating from the web to the drying felt, and the drying felt is thereafter separated from the dry web and released from the condensed water, characterized in that said cooling water is discharged from the drying process at a temperature of 60° - 100°C.
2. A method according to claim 1, wherein the cooling water is discharged at a temperature of 75° - 100°C.
3. A method according to claim 1, wherein the cooling water is discharged at a temperature of 60° - 85°C.
4. A method according to claim 1, wherein the discharge temperature of the cooling water is controlled by controlling the inlet temperature and/or the flow rate of the cooling water that is supplied into the drying process.
5. A method according to claim 4, wherein the cooling water is supplied to the drying process at a temperature of 50°- 80°C.
6. A method according to claim 4, wherein the cooling water is supplied to the drying process at a temperature of 0° - 50°C.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
FI801337A FI59439C (en) | 1980-04-25 | 1980-04-25 | FOERFARANDE FOER ATT UTNYTTJANDE AV ENTALPIINNEHAOLLET I KYLVATTNET VID EN SUGTORKNINGSPROCESS FOER PAPPER CARTON ELLER EN ANNAN POROES MATTA ELLER BANA |
FI801337 | 1980-04-25 |
Publications (1)
Publication Number | Publication Date |
---|---|
CA1149210A true CA1149210A (en) | 1983-07-05 |
Family
ID=8513438
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
CA000375918A Expired CA1149210A (en) | 1980-04-25 | 1981-04-22 | Method of utilizing the enthalpy content of cooling water in a suction drying process of a paper, board or other porous web |
Country Status (5)
Country | Link |
---|---|
CA (1) | CA1149210A (en) |
DE (1) | DE3115597A1 (en) |
FI (1) | FI59439C (en) |
GB (1) | GB2079914B (en) |
SE (1) | SE449880B (en) |
Families Citing this family (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
DE69635682T2 (en) * | 1995-09-18 | 2006-10-19 | Minnesota Mining And Manufacturing Company, St. Paul | Drying system for a coated substrate |
FI104100B1 (en) | 1998-06-10 | 1999-11-15 | Valmet Corp | Integrated paper machine |
Family Cites Families (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB1502040A (en) * | 1975-12-30 | 1978-02-22 | Tampella Oy Ab | Method of drying a cardboard or a paper web and drying device for applying this method |
-
1980
- 1980-04-25 FI FI801337A patent/FI59439C/en not_active IP Right Cessation
-
1981
- 1981-04-16 GB GB8112135A patent/GB2079914B/en not_active Expired
- 1981-04-16 DE DE19813115597 patent/DE3115597A1/en active Granted
- 1981-04-22 CA CA000375918A patent/CA1149210A/en not_active Expired
- 1981-04-24 SE SE8102619A patent/SE449880B/en not_active IP Right Cessation
Also Published As
Publication number | Publication date |
---|---|
FI59439B (en) | 1981-04-30 |
DE3115597A1 (en) | 1982-08-05 |
FI59439C (en) | 1981-08-10 |
GB2079914A (en) | 1982-01-27 |
SE8102619L (en) | 1981-10-26 |
SE449880B (en) | 1987-05-25 |
GB2079914B (en) | 1985-01-30 |
DE3115597C2 (en) | 1987-05-21 |
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