Method and equipment for drying a pulp web using hot air of different temperatures
View Patent ↗The invention relates to a method for drying a fibre-based pulp web ( 2 ), and to a drying box ( 1 ) used in the method. According to the invention, the drying box ( 1 ) contains drying zones ( 9 a - 9 e ), to each one of which hot drying air can be blown independently so that the web can be dried in the various zones at different temperatures. The temperature of the first zone ( 9 a ) can thus be lower than that of one or more subsequent zones ( 9 b - 9 e ) to prevent the water in the incoming wet web from boiling. The last zone in the drying box ( 1 ) can be a separate cooling zone ( 11 ), wherein the web is cooled by indoor air from outside the box. The temperatures can be adjusted by adjusting, with the aid of valves ( 8 ), the pressure of the steam, which heats the drying air that is fed into the various zones ( 9 a - 9 e ), on the basis of the temperatures measured by the sensors ( 23 ) for the various zones and of the basis weight and moisture measurements carried out on the web ( 2 ) that goes into the box ( 1 ) and comes out of the box.
1. A method of drying a web of cellulose pulp obtained from pulping or bleaching through removal of water from the web, the method comprising the steps of:
passing the web through a series of at least two drying zones in a drying box to dry the web at different temperatures in the respective zones,
blowing a first independent supply of drying air of a first temperature into a first drying zone in the drying box, to dry the web in said first temperature,
blowing a second independent supply of drying air of a second temperature into a second drying zone in the drying box, to dry the web in said second temperature, said second temperature being higher than said first temperature,
passing the web through a cooling zone in the drying box, subsequent to the drying zones in the path of travel of the web, and
blowing an independent supply of cooling air into the cooling zone at a temperature lower than the temperature of the drying air blown into the immediately preceding drying zone,
the web being routed from an inlet port to an outlet port of the drying box through a number of superimposed reciprocating runs.
2. The method according to claim 1 , wherein the temperature is measured by sensors located in the various drying zones of the drying box, and that on the basis of the measurement results, the temperature of the drying air fed into the various drying zones is adjusted.
3. The method according to claim 1 , wherein the moisture and the basis weight of the web going into the drying box is measured, and that on the basis of the measurement results, the temperature of the drying air fed into one or more drying zones is adjusted.
4. The method according to claim 1 , wherein the moisture of the web coming out of the drying box is measured, and that on the basis of the measurement results, the temperature of the drying air fed into one or more drying zones is adjusted.
5. The method according to claim 1 , wherein the temperature of the first drying zone in the web's direction of travel is adjusted to a lower level than that of one or more subsequent zones.
6. The method according to claim 1 , wherein the drying air is blown from nozzles located above and below the pulp web.
7. The method according to claim 6 , wherein the drying air blown from the nozzles located below the web carry the web as it travels through the drying box.
8. The method according to claim 1 , wherein air with a temperature in the range of 150°-200° C. is blown into the one or more subsequent zones.
9. The method according to claim 1 , wherein the drying zones comprise the horizontally superimposed web runs.
10. The method according to claim 1 , wherein the drying box comprises a partition wall separating the cooling zone from the preceding drying zones.
11. The method according to claim 10 , wherein the cooling zone is the lowermost zone in the drying box.
12. The method according to claim 1 , wherein indoor air from the outside of the drying box is used as cooling air in the cooling zone.
13. The method according to claim 1 , wherein the moisture of the web is measured before the drying box, and the measurement is used for selective adjustment of the temperature of the drying air conducted to the first drying zone.
14. The method according to claim 1 , wherein the moisture of the web coming out of the drying box is measured, and the measurement is used for selective adjustment of the temperature of the drying air conducted to the last drying zone.