FI59439B - FOERFARANDE FOER ATT UTNYTTJANDE AV ENTALPIINNEHAOLLET I KYLVATTNET VID EN SUGTORKNINGSPROCESS FOER PAPPER CARTON ELLER EN ANNAN POROES MATTA ELLER BANA - Google Patents

Description

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Patent, och rafistentyraltan 7 AraMcan utlajd och «tUkriftw» puMkcrad 30. OU. 81 (32) (33) (31) W «| i« uoik · »- · ^ prionm (71) Oy Tampella Ab, PO Box 256, 33101 Tampere 10, Finland-Finland (FI) (72) Jukka Lehtinen, Tampere, Suomi-Finland (FI) (7U) Oy Kolster Ab (5U) Method for utilizing the enthalpy content of cooling water in the suction-drying process of paper, board or other porous carpet or web - Förfarande för att utnyttjande av entalpiinnehället i kylvattnet vid en sugtorkningsprocess f This invention relates to a method for utilizing the enthalpy of cooling water in a paper, board or other porous or fibrous carpet or web in a suction drying process, wherein the wet carpet or web and the supporting blanket are sandwiched between two airtight and highly conductive surfaces enclosing the carpet or web. and where the surface in contact with the mat or web is subjected to heating to evaporate water from the mat or web and the surface in contact with the drying felt subjected to cooling by means of cooling water to condense volatile water from the mat or web into the drying felt, wherein the drying felt is then separated from the dried mat or web and released from the condensed water.

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In the current type of drying of paper or board, heat recovery takes place from an air-water vapor mixture obtained from inside a hood around a conventional drying cylinder or drying cylinder-blade group. Generally, the water heated by the heat from these gases comes out of the heat recovery heat exchangers at a temperature of 30-55 ° C. This water is used primarily in the same mill for the hot water needs of the wet end of a paper or board machine, e.g. shower water, and / or heating of factory halls. The temperature of this water has not been so high that it would have been worthwhile, at least in considerable quantities, to use it as district heating or as heat source water for heat pumping.

Heat pumping in connection with the drying of paper or cardboard webs has been developed e.g. in a few patents (U.S. Patent 2,933,826, SE Patent 7111908-5, DT Patent 2630853). What these inventions have in common is that the heat recovery takes place by transferring heat from said air-water vapor mixture directly to water or other flow medium flowing or boiling under reduced pressure in a heat exchanger. In any case, the recovered heat can be made to boil the water from which the steam is compressed by the compressor or compressors to the pressures required by the drying cylinder.

However, these methods have not resulted in marketed products due to the relatively small economic advantage to be achieved over the use of back-pressure steam and the operational difficulties, e.g. in the case of track breaks caused by a more tightly closed hood.

In the case of suction drying between belts or between a cylinder and a belt (FI patent 54514, US patent 4 112 586, GB patent 1502040, FI patent application 762439), the drying operation is very different from in conventional drying. The thermal energy required to evaporate the moisture of the web is introduced as saturated steam to the inner surface of the outer shell of the cylinder or to the other side of the hot strip, from where it results through a relatively thin layer of metal into the web. In the web, the heat is transferred to the depth at which evaporation occurs. The evaporated steam passes through the rest of the web and through the felt to condense with cooling water on an externally cooled cold metal surface. The cooling water leaves the drying process at a temperature below 40 ° C.

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It is clear that the enthalpy of the cooling water leaving the known suction-drying process described above could be better utilized, e.g. by means of heat pumping, if the temperature of this water could be raised. However, an obstacle to this development has been the prejudice that raising the outlet temperature of the cooling water would slow down the drying process itself. After all, in many heating devices, the heat flux is directly proportional to the temperature difference. Based on this, it could be estimated that even in the known suction drying described above, the heat flux transferred to the web to be dried through the hot metal surface would be approximately directly proportional to the temperature difference between the hot and cold metal surfaces.

Slowing down the drying process, on the other hand, would have detrimental consequences. From a practical point of view, it is important that the drying rate does not decrease much, as this would mean the need to use larger drying surfaces (more cylinders in many cases), which would be expensive.

It should be noted that raising the temperature of the heating steam from 100 ° C to 180 ° C increases the drying rate by about 6 times. Optimum steam temperature is a macroeconomic issue.

The object of the present invention is to improve the utilization of the enthalpy of cooling water in the suction drying process described above, and it is provided according to the invention so that the cooling water leaves the process at a temperature of 60 ° -100 ° C.

The invention is based on the surprising fact confirmed by experiments that in suction drying cooling water can be fed in at a temperature as high as 80 ° C and discharged at 100 ° C without causing a few percent greater drop in drying rate compared to a cooling water outlet temperature of 10-30 ° C.

Said measurement results due to the small effect of the cooling water temperature may seem astonishing. The reason for what has just been presented is probably the following. The rate of evaporation from the web is virtually directly proportional to the heat flux transferred from the hot surface to the web. This in turn depends on the temperature difference between the hot surface and the evaporation point inside the web. Thus, if in two different situations the temperature of the hot surface is the same and the evaporation rate and thus also the heat flux are approximately the same, the temperature of the region of the web where 59439 evaporation takes place must also be almost the same in the two different cases. This also means that in these two cases the evaporation pressure is almost the same.

The evaporated vapor passes through a portion of the web and through the felt to condense on the cold metal surface. The vapor pressure when steam enters a cold surface depends on the temperature of that surface. For example, if in one case this temperature is 29 ° C (corresponding to a cooling water temperature of about 17 ° C) and in another 87 ° C (corresponding to a cooling water temperature of 75 ° C), the vapor pressure is about 5 kPa in the former case and about 70 kPa in the latter. .

Correspondingly, in the former case, the specific volume of steam is about 6 times as large as in the latter case. In both cases, since almost the same mass flow of steam enters the cold surface through the felt, the velocity of this steam in the former case is about 6 times as high as in the latter. Steam traveling at a higher speed also experiences a greater pressure drop as it passes through a portion of the web and through the felt. Thus, it is possible (this can be shown by calculations) that the same mass flow of steam in the two different cases starts from the same evaporation pressure and arrives at different pressures on a cold metal surface to condense in it. This is explained by the test results presented earlier, in which the drying rate was very little affected by even large changes in the temperature of the cooling water.

According to the invention, the cooling water removed from the drying process at a high temperature can be used as source water for heat pumping or directly, e.g. for district heating.

When cooling water is used for heat pumping, the cooling water is removed from the drying process, preferably at a temperature of 60 ° -85 ° C. Namely, the lower limit temperature for heat pump economy is about 60 °, and the preferred upper limit temperature for drying paper and board is about 85 ° C. In this case, a portion of the removed cooling water is allowed to evaporate at low pressure in a suitable expansion unit, e.g. a packed column or cyclone, with the enthalpy of enthalpy for evaporation . This water is returned to the drying unit as cooling water. The evaporated steam is compressed by compressors to the desired pressure to be fed to the drying process.

5,59439 Steam enters the heat pump compressors at a much higher pressure than that obtained from the expansion unit of the heat pump process connected to the known suction drying process in which the cooling water leaves the drying process at a temperature below 40 ° C. Higher pressure steam, in turn, leads to better heat pump efficiency and improved economy of the entire drying process.

When cooling water is used for district heating, cooling water is introduced at a low temperature, 0 ° -50 ° C, into the drying equipment and discharged at a high temperature, preferably at a temperature of 75 ° -100 ° C. Once transported to the district heating site and completed its heating task, the water can either be returned as a low temperature to the drying unit where it re-enters as cooling water, or taken somewhere else, completely out of the drying process. In the latter case, new cooling water must be continuously introduced into the drying process.

The cooling water outlet temperature can be adjusted by changing the cooling water flow rate. If the drying rate of the web remains unchanged, which means that the heat flux through the cold strip to the cooling water remains unchanged, and if the cooling water inlet temperature is kept constant, the cooling water outlet temperature increases as the cooling water flow rate decreases. It is therefore easy to regulate the cooling water outlet temperature in a wide range of driving situations.

The invention will be explained in more detail below with reference to the accompanying drawing, which schematically shows heat pumping in connection with the suction drying disclosed in GB patent 1502040.

At the top of the drawing is a suction dryer 1 with an upper endless, airtight and highly thermally conductive metal strip 2 and a similar lower metal strip 3 running parallel in a predetermined distance. The paper web 4 to be dried is guided between the strips so that the strips completely enclose the web in said parallel area. Between the paper web and the lower strip 3, an endless drying blanket 5 supporting the web circulates. A heating box 6 is arranged on the inside of the upper strip 2, the underside of which is open towards the upper surface of the parallel part of the strip. The box is equipped with an inlet 7 for heating steam and an outlet 8 for condensate. A cooling box 9 is arranged inside the lower strip 3, the upper side of which is open towards the lower surface of the parallel part of the strip. The box is provided with an inlet 10 for cooling water and an outlet 11 for used water.

The wet web is thus enclosed between the upper steam-heated strip 2 and the lower water-cooled strip 3 as it passes through the suction dryer. As a result, the water contained in the web evaporates and passes through the web and felt and finally condenses on the surface of the lower strip, as described in said English patent publication. The web released from the water is separated from the felt after drying.

The cooling water W. is removed from the dryer at a water temperature of 60-100 ° C and passed through line 12 to the mixer 13. Condensate W from the heating box 6 is also fed to the mixer via line 14 via a condensate separator 15 and a pressure relief valve 16. However, this small amount of steam condenses immediately in the mixer upon contact with the cooling water.

Most of the water is passed from the mixer 13 via line 17 and the pressure relief valve 18 to the evaporator column 19. This is a conventional, generally vacuum packed column to which the feed enters from above and from which the evolved steam leaves from above and the remaining cooled water of about 10 ° C leaves from below. This water is pumped by the pump 20 back to the cooling box 9 of the suction dryer as cooling water via a line 21. Steam is passed from the evaporation column via line 22 to the first electric motor driven press 23. Water is sprayed into the steam either before the press, inside the press or after the press, which is led from the mixer via line 24 and pump 25 so that the steam from the press is saturated. This steam is passed via line 26 to a second press 27 which operates accordingly. There are a number of presses in the series, which is determined by economic optimization. In practice, this number is usually two, possibly sometimes three. Steam is led from the last press via line 28 to the heating box 6 of the suction dryer. Steam is passed to this press from the condensate separator 15.

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The process described is otherwise closed, except that the wet web enters the dryer and that the dry web and water evaporated from the web come out of the dryer. Electric power is required for the electric motors of the presses and the motors of the water circulation pumps. Small amounts of heat escape from pipelines and equipment through thermal insulation into the environment. Thus, no supply of back pressure or other steam to the dryer is required, as in conventional cylinder-hood drying.

The economics of the process described, compared to the same drying process but without heat pumping using counter-pressure steam imported elsewhere into the drying equipment, are determined above all by the price of the electric power required for heat pumping compared to the backpressure steam without heat pumping and the additional equipment required for heat pumping. Typically, if steam at a pressure of 0.8 MPa is required for the drying itself, the price of back pressure steam is about FIM 50 / (MWh latent enthalpy of steam). If, on the other hand, the required 0.8 MPa steam is obtained by evaporating water at 60 ° C and compressing the evaporated steam to the desired pressure with a 0.83 efficiency compressor, the price of electricity is FIM 120 / MWh, the price of the required electrical power is FIM 69.0 / MWh. latent enthalpy of steam). If, on the other hand, the steam is compressed from 80 ° C volatile steam, the price of steam is FIM 45.9 / (MWh latent enthalpy of steam). Thus, on the basis of the prices shown, the drying steam generated by heat-pumping from 80 ° C water is already cheaper than the corresponding counter-pressure steam, while the heat-pumped steam from 60 ° C water is more expensive.

In the future, it is expected that the price of electricity will fall relative to fossil fuels. The heat pump breakdown described above will then become more advantageous than today.

The heat recovery described above can also be applied in a suction dryer in which another belt, preferably a heated belt, has been replaced by a rotating cylinder, as disclosed in FI patent application 762439. Likewise, the invention can also be applied when using the suction drying method described above for intermittent batch drying.

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The following table shows several examples of achieving the desired cooling water outlet temperature (outlet temperature) by adjusting the cooling water inlet temperature and flow rate. In all examples, it is assumed that a constant amount of heat of 15,000 kW is transferred to the cooling water per unit time and that the other conditions and track quality are the same.

cooling water

Example inlet temperature outlet temperature flow rate 1 50 ° C 60 ° C 359 kg / s 2 50 ° C 70 ° C 179 kg / s 3 50 ° C 80 ° C 119 kg / s 4 80 ° C 100 ° C 179 kg / s 5 0 ° C 60 ° C 60 kg / s 6 0 ° C 70 ° C 51 kg / s 7 0 ° C 80 ° C 45 kg / s 8 0 ° C 100 ° C 36 kg / s

In the examples shown, the drying speed of the web varies within only a few percent.

The drawing and the related explanation are only intended to illustrate the idea of the invention. The details of the method according to the invention can vary considerably within the scope of the claims.

Claims (4)

9,594 39 A method for utilizing the enthalpy of cooling water in a suction drying process for paper, board or other porous or fibrous carpet or web, wherein the wet carpet or web (4) and the supporting drying felt (5) are located between two airtight and highly conductive surfaces (2, 3) enclosing the mat or web over its entire width and wherein the surface in contact with the mat or web is heated (6) to evaporate water from the mat or web and the surface in contact with the drying blanket is cooled (9) by cooling water to condense water evaporating from the carpet or web into the drying felt then separated from the dried mat or web and released from the condensed water, characterized in that the cooling water Wj is removed from the drying process at a temperature of 60 ° -100 ° C. Method according to Claim 1, characterized in that the cooling water (VT) is removed at a temperature of 75 ° -100 ° C. Method according to Claim 1, characterized in that the cooling water (Wj) is removed at a temperature of 60 ° -85 ° C. Method according to Claim 1, characterized in that the outlet temperature of the cooling water (Wj) is controlled by adjusting the inlet temperature and / or the flow rate of the cooling water introduced into the drying process.