SE449880B - PROCEDURE FOR UTILIZATION OF THE ENTALPIIN CONTENT IN THE COOL WATER BY A SOFT DRYING PROCESS FOR A PAPER, CARTON OR OTHER POROS MATTA OR BANA - Google Patents

Description

449 '880 for heating factory buildings. This water has not been hot enough to be used - at least to a greater extent - as district heating water or heat source water for heat pumping. _ Warm-up in connection with the drying of a paper or board web has admittedly developed e.g. in some patents (U.S. Patent 2,933,826, SE Patent 7111908-5; DE Patent 2630853). Common to these inventions is that heat is recovered by transferring heat from the said mixture of air and water vapor in a heat exchanger directly to water or some other medium, which flows or evaporates under low pressure in the heat exchanger. In any case, with the recovered heat, water can be evaporated, whereby the steam formed is pressed in one or more compressors to a pressure suitable for the drying cylinder. However, these methods have not led to commercial products due to the relatively small economic advantage in relation to the consumption of back pressure steam and due to the operational difficulties which occur, for example, when the web is subjected to breakage in connection with a hood, sealed more frequently than normal.

Suction drying between two belts or between a cylinder and a belt (F1 patent 545l #, US patent b1l2586, GB patent l502Dü0, F1 patent application 762Ä39) takes place completely differently from ordinary drying. The heat energy for evaporating water from the web is supplied as saturated steam to the inner surface of the outer shell of the cylinder or to one side of the hot strip, from where the heat energy is conducted to the web through the relatively thin metal layer. In the web, the heat penetrates to a depth where evaporation takes place. The evaporated steam passes through the rest of the web and the blanket and is condensed on a cold metal surface which is cooled from the outside and by means of cooling water.

The cooling water departs from the drying process at a temperature below MOOC.

It is clear that the enthalpy content of the cooling water which departs from the known suction drying process described above can be used better, for example in heat pumping, if the temperature of the water could be raised. However, this development has been prevented by such a prejudice that a higher temperature of the cooling water at its departure from the process would make the drying process itself slower. In several heaters, the heat flow is directly proportional to the heat difference. Therefore, it could be assumed that even with the known suction drying described above, the transient heat flow through the hot metal surface to the web to be dried would be almost directly proportional to the heat difference between the hot and the cold metal tooth 449 880 s The slower drying process would in turn have harmful consequences.

In practice, it is important that the drying speed does not drop much, because this would require larger drying surfaces (often several cylinders), which would be expensive.

If the temperature of the heating steam is raised from 100 ° C to 18 ° C, the drying rate will be about six times. The optimal steam temperature is a total economic issue. The object of the present invention is to make better use of the enthalpy content in the cooling water in the suction drying process described above. According to the invention this is achieved by diverting the cooling water from the process at a temperature of 600-100 ° C.

The invention is based on the surprisingly proven fact by test that the cooling water during suction drying can be introduced even at a temperature of 8006 and diverted at 100 ° C without lowering the drying rate by more than a few percent compared to a situation where cooling ~ the water is diverted at a temperature between 100 and 3006.

The above-mentioned measurement results, which show a smaller effect on the temperature of the cooling water, may seem surprising. The reason for the described relationship is probably the following. The evaporation rate from the web is practically directly proportional to the heat flow from the hot surface to the web. This in turn is due to the temperature difference between the hot surface and the evaporation point inside the web. Thus, if the temperature of the hot surface is the same in two different situations and if also the evaporation rate and thus the heat flow is almost the same, the temperature of the area of the web where the evaporation takes place must also be almost the same in the two cases mentioned. This also means that the evaporation pressure in these two cases is almost the same.

The evaporated steam goes through a part of the web and through the felt to condense on the cold metal surface. The pressure of the steam upon its arrival on the cold surface depends on the temperature of this surface. If this temperature is, for example, Z9 ° C (corresponding cooling water of approx. 1700) or 87 ° C (corresponding cooling water of 7500), the corresponding pressure is approx. 5 kPa resp. ca. 70 kPa. In the former case, the specific volume of the steam is about 6 times as large as in the latter case. Since the cold surface in both cases has an almost equal volume flow of steam through the felt, the velocity of this steam 449,880 in the former case is about 6 times as great as in the latter case. The steam flowing at a greater speed also undergoes a greater pressure loss on its way through a part of the web and through the felt. Thus, it is also possible (this can be proved by means of calculations), that the same volume flow of steam in these two different situations starts from the same evaporation pressure and comes to the cold metal surface with different pressures to be condensed there. This explains the previously mentioned test results, where even large changes in the temperature of the cooling water only insignificantly affected the drying speed. The cooling water, which according to the invention is diverted from the drying process at a high temperature, can be used for heat pumping or directly for, for example, district heating.

If the cooling water is used for heat pumping, it is advantageously diverted from the drying process at a temperature between 60 ° and 85 ° C. The lowest temperature for profitable heat pumping is about 60 ° C and the highest favorable temperature for drying paper and board is about 85 ° C.

In this case, a part of the removed cooling water is allowed to evaporate under low pressure in a suitable expansion unit, for example in a column with filler bodies or in a cyclone, while the latent enthalpy content necessary for evaporation is obtained from the part of the water which does not evaporate and which water comes out of the expansion unit at a lower temperature, 500-65 ° C, than at which it was introduced. This water is returned to the drying unit as cooling water. The evaporated steam is pressed with compressors to a desired pressure to be led into the drying process. As described above, the steam to the heat pumping compressors comes under significantly higher pressure than can be obtained from the expansion unit of a heat pumping process in connection with the known suction drying process, where the cooling water is diverted from the drying process at a temperature below ÜOOC. The steam under higher pressure results in a better efficiency for the heat pumping and a more economical drying process.

If the cooling water is used for district heating, it is introduced into the dryer at a low temperature, 00-50 ° C, and diverted at a high temperature, preferably 75 ° ~ 100 ° C. After reaching the district heating object and used for heating, the water can either be carried at a low temperature back to the drying unit where it is re-introduced as cooling water, or from the drying process to another place. In the latter case, fresh cooling water must still be added to the drying process. 449 880 The cooling temperature of the cooling water can be controlled by changing the flow volume of the cooling water. If the drying rate of the web remains unchanged, it means that the heat flow through the cold belt to the cooling water remains unchanged, and if the inlet temperature of the cooling water is kept unchanged, the discharge temperature of the cooling water rises when the cooling water flow volume decreases. Thus, it is easy to control the cooling water discharge temperature even to a greater extent under different operating conditions.

The invention is described in more detail in the following with reference to the accompanying drawing, which shows schematically heat pumping in connection with suction drying according to GB-patemæt15020Å0. The upper part of the drawing shows a suction dryer 1, which has an upper endless, airtight metal strip 2 with good thermal conductivity and a liquid-lower metal strip 3. The strips run parallel over a certain distance. The paper web Å to be dried is inserted between the belts so that they enclose the web completely between them over said separate area. Between the paper web and the lower belt 3 runs a web n carrying endless drying felt 5. On the inner side of the upper belt 2 a heating surface 6 has been arranged, the lower side of which is open towards the upper surface of the belt within the parallel area. The box is provided with an inlet 7 for heating steam and with an outlet 8 for condensate. On the inner side of the lower belt 3 a cooling box 9 has been arranged, the upper side of which is open towards the lower surface of the belt in the parallel area. The drawer is provided with an inlet 10 for cooling water and with an outlet 11 for consumed water.

When the wet web runs through the suction dryer, it is thus enclosed between the upper, steam-heated belt 2 and the lower, water-cooled belt 3. This evaporates the water from the web and passes through the web and felt and finally condenses on the lower belt. '' as described in the British patent specification. After drying, the dried web is separated from the blanket.

The cooling water Wj is removed from the dryer at a temperature of 600-100 ° C and passed through a line 12 to a mixer 13. Through a line 1e condensate wa is also introduced into the mixer from the heating box 6 through a condensate separator 15 and a pressure reducing valve 16. A part of the condensate evaporates as it passes through the pressure reducing valve. In any case, this small volume of steam condenses in the mixer immediately after it has come into contact with the cooling water. 449 880 Most of the water is fed from the mixer 13 through a line 17 and a pressure reducing valve 18 to an evaporation column 19.

This is a conventional, usually vacuum-operated column with filler bodies, which are fed from above and from which the formed steam departs at the top.

The rest of the water, which is cooled by approx. i0 ° C, departs at the bottom. This water Wj is pumped with a pump 20 such as cooling water back to the suction box 9 of the suction dryer through a line 21. From the evaporation column the steam is passed through a line 22 to an electric motor driven compressor 23. Either before the compressor, inside the compressor or after the compressor is injected into the steam water, which is supplied from the mixer through a line Zk and a pump, so that the steam coming from the compressor is saturated. This steam is fed by a line 26 to another compressor 27 which operates in a corresponding manner. The number of compressors in the series is determined economically optimally. In practice, the number is usually two, possibly sometimes three. From the last compressor the steam is passed through a line 28 to the heating box 6 of the suction dryer. To the heating steam also comes steam from a steam circulation compressor 29 through a line 30. To this compressor steam is led from the condensate separator 15.

The process described is closed, except that the wet web enters the dryer and the dry web and the evaporated water come out of the dryer.

Electric power is needed for the electric motors of the compressors and for the motors of the circulation pumps. Smaller amounts of heat are lost through the thermal insulation of the pipelines and devices to the surroundings. Unlike standard cylinder hood drying, the dryer does not need to be fed with back pressure or other steam.

The economic advantages of the described process in comparison with saw ". Drying process without heat pumping and with back pressure steam which is started to the drying device from the outside, depends above all on the price of the electric power needed for heat pumping compared to the price of back pressure steam required without heat pumping. For example, if steam with a pressure of 0.8 MPa is needed during the actual drying, the price of back-pressure steam is about 50 mk / (Mwh latent enthalpy content in the steam). If the necessary pressure of 0.8 MPa is obtained by to evaporate water with a temperature of 60 ° C and compress the evaporated steam to the desired pressure with a compressor whose efficiency is 0.83, the cost of the required self-power is 69.0 mk / (Mwa latent enthalpy content in the steam), provided that the price on electric power is 120 mk / Mwh.0m the steam, on the other hand, is compressed from evaporating with the temperature 80 ° C, 4490880 the cost of the steam is ë5,9 mk / (MWh latent e ntalpi content in the steam).

According to the above-mentioned prices, the drying steam obtained by heat pumping from water with a temperature of 8000 is already cheaper than the corresponding back-pressure steam, while from water with a temperature of 6000 heat-pumped steam is more expensive. In the future, the price of electricity can be assumed to fall in comparison with fossil fuels. Then the heat pump described above will be more economical than today.

: The heat recovery described above can also be applied in a suction dryer, where the second belt, preferably the heated belt, has been replaced with a rotating cylinder according to FI patent application 762Å39. Furthermore, the invention can be applied when using the suction drying method described above for stepwise drying.

The following table shows several examples of how the cooling water's desired drainage temperature (outlet temperature) is obtained by regulating the cooling water inlet temperature and the flow volume. In all examples, it is assumed that a constant amount of 15,000 kw of heat is transferred to the cooling water per unit of time and that the other conditions and the quality of the track remain unchanged.

Example Cooling water inlet temperature outlet temperature flow volume 1 5o ° c VV 6o ° c 359 kg / S 2 5o ° c 7u ° c 179 kg / S 3 5o ° c a0 ° ç 119 kg / SA .sn ° c 1oo ° c 179 kg / S o ° c 6o ° c 'eo kg / S 6 o ° c 7o ° c 51 kg / s 7 o ° c ßo ° c M5 kg / 5 8 o ° c 1oo ° c 36 kg / s In the above example, the drying speed of the web varies only a few percent.

The drawing and the description are only intended to illustrate the invention.

In detail, the method according to the invention can vary considerably within the scope of the claims.

Claims (6)

449 880. Claims: A method of utilizing the enthalpy content of cooling water in a suction drying process for a paper, board or other porous mat or web, the wet mat or web (Å) and a drying blanket (5) supporting the same being located between two airtight surfaces (2,3) with good thermal conductivity, which surfaces enclose the carpet or web over its entire width between them and whereby a surface in contact with the carpet or web is heated (6) to evaporate the water from the carpet or web and the with the drying blanket in contact with the contact surface, cool (9) by means of cooling water. ) is derived from the drying process at a temperature of 600-1000 ° C. 0 2. A method according to claim 1, characterized in that the cooling water (Wj) is diverted at a temperature of 75-100 ° C. 3. A method according to claim 1, characterized in that the cooling water (Wj) is diverted at a temperature of 60-8500. H. A method according to claim 1, characterized in that the cooling water (Wj) drainage temperature is controlled by controlling the temperature and / or the flow rate of the cooling water introduced into the process. ' 5. A method according to claim H, characterized in that the cooling water (Wj) is introduced into the drying process at a temperature of 50-8000. 6. A method according to claim 1, characterized in that the cooling water (Wj) is introduced into the drying process at a temperature of 0-5000.