GB2079914A

GB2079914A - Drying webs

Info

Publication number: GB2079914A
Application number: GB8112135A
Authority: GB
Original assignee: Tampella Oy AB
Current assignee: Tampella Oy AB
Priority date: 1980-04-25
Filing date: 1981-04-16
Publication date: 1982-01-27
Also published as: CA1149210A; DE3115597A1; FI59439B; DE3115597C2; SE8102619L; SE449880B; FI59439C; GB2079914B

Abstract

A wet web (4) and a drying felt (5) are enclosed between two air-tight surfaces having a good thermal conductivity. The surface in contact with the web is heated and the surface 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 discharged from the drying process at a temperature of 60 DEG -100 DEG 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. <IMAGE>

Description

SPECIFICATION A method of utilizing the enthalpy content of cooling water in a suction drying process of a paper, board or other porous web.

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 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.

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 30-550C. 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 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 2 933 226, 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 difficulties encountered, for example when the web is broken, with hoods sealed more tightly than normally.

Suction drying between two bands, or between a cylinder and a band (Finnish patent 54514, US patent 4112586, British patent 1502040, Finnish patent 762439), 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 penetrates to the depth where evaporation takes place. The evaporated steam passes through the rest of the web and through the felt, and condenses onto the cold metal surface which is cooled with cooling water from the outside. The cooling water leaves the process at a temperature below 400C.

It is clear that the enthalpy in the cooling water discharged from the above described, known, 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, howevar, has been prevented by the suspicion that the increased discharge temperature 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 necessitate larger drying surfaces (more cylinders in many cases), which is expensive.

It may be noted that if the temperature of the heating steam is increased from 1000C to 1 800C, the drying rate becomes sixfold. The optimum steam temperature results from considerations 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 600--1000C.

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 800C and discharged at 1000C 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 1 OOC and 30"C.

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 and through the felt, and is then condensed on the cold metal surface. The pressure of the steam when it reaches the cold surface depends on the temperature of this surface. If this temperature is for example 290C in one case (corresponding to cooling water of about 1 70C) and 870C in another case (corresponding to cooling water of 750C), steam pressure in the first case is about 5 kPa and in the second case about 70 kPa.

Correspondingly, the specific volume of the steam in the first case is sixfold compared with the specific volume in the second case. Because the cold surface in both cases received nearly the same mass flow of steam, the speed of this stream is in the first case about six 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 surface 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 600--850C. The lowest possible temperature for economical heat pumping is about 600 C, and the preferable upper limit temperature when drying paper or board is about 850C. 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, 500--650C, 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 described above, steam is fed to the heat pumping compressors under a considerably higher pressure than what is possible to obtain from the evaporation unit of a heat pumping process in connection with the known suction drying process where cooling water leaves the drying process at a temperature below 400C. The higher pressure steam results in a better heat pumping efficiency and improved economy for the whole drying process.

When the cooling water is used for district heating, it is supplied to the drying apparatus at a low temperature, 00-5()0C, and out at a high temperature, preferably 75c1 O00C. 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 iatter case new cooling water must be continuously fed in to 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 kept unchanged, the discharge temperature of the cooling water rises when the cooling water flow rate is decreased.

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 1 502040.

The upper part of the drawing shows a suction dryer 1 comprising an upper, endless, air-tight metal band 2 with good thermal conductivity, and a similar lower metal band 3, these two 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 the said parallel zone. Between the paper web and the lower band 3 runs an endless drying felt which carries the web. On the inner side of the upper band 2 there is a heating box 6, the lower 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 the 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 surface 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 600--1000C 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 1 5 and a pressure reducing valve 1 6 to the mixer. Upon passing through the pressure reducing valve a part of the condensate evaporates. 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 1 7 and a pressure reducing valve 1 8 into an evaporation column 1 9. This is a conventional packed column operated under vacuum. It is fed from the top; the evaporating steam leaves through another conduit at the top; and the remaining water, which has cooled by about 1 00C, 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 1 5.

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 the 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 piping and the equipment 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 with 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 600C and compressing the evaporated steam to the desired pressure by a compressor with an efficiency 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 mark/MWh.If the steam is compressed from evaporating steam at 800C, the price of the steam is 45.9 mark/(MWh of latent enthalpy of steam).

The prices being as above mentioned, the drying steam obtained by heat pumping from water at 800C is already cheaper than corresponding back pressure steam, whereas steam obtained by heat pumping from water at 600C is more expensive.

In the future the price of electric power compared with fossile fuels is likely to decrease. The abovedescribed heat pumping process will then become relatively more economical than now.

The above described heat recovery process is applicable also to a suction dryer in which a rotating cylinder is substituted for one of the bands, preferably the heated band, as described in Finnish patent application 762439. 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 cooling water. In all examples it is supposed that a constant amount of heat, 1 5000 kW is transferred into the cooling water per time unit and that the other conditions and the web quality are the same.

Inlet Temperature Outlet Temperature Flow Rate Example No. of cooling water 1 503C 60-C 359 kg/s 2 50'C 70"C 179 kg/s 3 50,0 80 > C 119 kg/s 4 80-C 100-C 179 kg/s 5 OC 60-C 60 kg/s 6 O-C 70 C 51 kg/s 7 OoC 80iC 45 kg/s O-C 100'C 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 illustrate the idea of the invention. In its details the method according to the invention can vary within the scope of the claims.

Claims

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 (4) and the drying felt (5) carrying the web are over the whole width of the web enclosed between two airtight surfaces (2,3) having good thermal conductivity, the surface which is in contact with the web is heated (6) 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 (Wj) is discharged from the drying process at a temperature of 600--1000C.

2. A method according to claim 1, wherein the cooling water (Wj) is discharged at a temperature of 75C1O00C.

3. A method according to claim 1, wherein the cooling water (Wj) is discharged at a temperature of600--850C.

4. A method according to claim 1, wherein the discharge temperature of the cooling water (Wj) 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 (Wj) is supplied to the drying process at a temperature of 500--800C.

6. A method according to claim 4, wherein the cooling water (Wj) is supplied to the drying process at a temperature of 00--500C.