Board drying process and drier

Abstract

In order to dry boards, the boards are guided on racks through a drier and brought into contact with drying air in two stages. In a stage A with a higher drying power, the drying air is supplied at a higher temperature and with an at least average humidity, and in the other stage B, it is supplied at an average temperature and with a low humidity. The waste heat of the outgoing air from stage A is used for preheating the drying air. This process should reduce the consumption of primary energy. For that purpose, a process is known that consists of also using the outgoing air condensation heat. This has the inconvenience, however, of requiring substantially more secondary energy, as important air mass flows are required to transfer heat because of the low condensation temperature. Primary energy should be reduced by using condensation heat without substantially increasing secondary energy requirements. According to the invention, the outgoing air from stage A is supplied to stage B through a heat exchanger arranged in the rack of the drier. The drying air of stage B is supplied in counter-current through the drier with a low humidity and temperature, so that the boards are dried in stage B both by condensation heat and by radiant heat. Thus only a reduced mass flow of drying air is required to transfer the condensation heat. The consumption of primary and secondary energy is low. This invention is suitable for drying boards as for buildings and gypsum plain boards or fiber boards.

We claim:  
     
       1. A recirculated-air process for drying boards comprising the steps of: 
       (a) advancing a plurality of heated wet boards in succession along a board path in a travel direction through at least one first heating section and a drying section having a first heat exchanger in decks thereof;  
       (b) passing an incoming drying air having a first relatively low temperature and first air humidity along an air path in counterflow to the travel direction after passing through a first heat exchanger provided in decks of the drying section;  
       (c) branching a portion of the passing drying air from said first heat exchanger drying section into the heating section for encountering and heating the boards releasing a humidity, so that the drying air in said heating section has develops a humidity above that of the incoming air;  
       (d) thereafter evacuating the drying air from the heating section and conveying evacuated air in the travel direction toward the drying section, said evacuated air having a second temperature and second humidity higher than said first temperature and humidity of said incoming drying air;  
       (e) passing the evacuated air in the travel direction through the first heat exchanger in the drying section; and  
       (f) feeding a fresh drying air having a third temperature and humidity less than said first air temperature and humidity of said incoming drying air through said first heat exchanger drying section in the counterflow, so that the fresh air substantially reaches the first temperature and the first humidity of step (b) upon heat exchanging with the evacuated air in said first heat exchanger .  
     
     
       2. The process defined in  claim 1  wherein 
       said process further comprising the steps of:  
       discharging said evacuated air at the temperature from 80° to 110° C. and humidity from 0.03 to 0.1 downstream from said exchanger along said board path, and  
       discharging said incoming air at the first temperature from 150° to 300° C. and first humidity from 0.2 to 0.8 at a location upstream from in said heating section along said board path, said portion of the drying air being evacuated in step (d) at the second temperature from 120° to 200° C. and at a second humidity from 0.2 to 0.8 before entering said heat exchanger in step (e).  
     
     
       3. The process defined in  claim 2  wherein part of the portion of the evacuated air discharged from the heating section upon leaving the heat exchanger is directed to another heat exchanger downstream of said first heat exchanger in said drying section for heating said fresh air before the latter enters said first heat exchanger drying section in step (e). 
     
     
       4. The process defined in  claim 1  wherein said step (c) further comprising comprises a step of additional heating of the potion portion by a burner located along said air path in said heating section. 
     
     
       5. The process defined in  claim 1  wherein said step (a) including includes a step of preliminary heating where said plurality of boards is initially heated and dried. 
     
     
       6. A drier comprising: 
       conveying means for advancing a plurality of wet and heated boards along a board path through at least one heating station and a drying station in one direction, said heating station being located upstream from said drying station;  
       pump means for passing a fresh drying air having a first relatively low temperature and first air humidity in a second direction opposite said travel direction and located downstream from said drying station, said fresh drying air becoming an incoming drying air having a second temperature and humidity upon leaving an elongated heat exchanger located in said drying station which is formed with an elongated heat exchanger;  
       said heating station including:  
       first supply means for branching said incoming drying air into in the first heating station for encountering said plurality of boards,  
       first evacuating means for pumping said drying air out of the heating station and conveying evacuated air in the second direction towards said drying section, said evacuated air having a third temperature and second humidity higher than said first and second temperatures and humidity of said fresh drying air and said incoming drying air;  
       said drying station being provided with second evacuating means spaced upstream downstream from said heat exchanger and being in flow communication with said first supply means for evacuating the fresh drying air from said heat exchanger drying station and for passing the incoming drying air along said second direction, said fresh drying air being in counterflow with said evacuated air from said heating station in said heat exchanger, said first temperature and humidity of said fresh air being lower than said second temperature and humidity of said incoming drying air.  
     
     
       7. The drier defined in  claim 6  further comprising a heater located between said pump means and said heat exchanger. 
     
     
       8. The drier defined in  claim 6  wherein said heating station further comprises a burner preheating said incoming air. 
     
     
       9. The drier defined in  claim 6  wherein said conveying means includes roller conveyors. 
     
     
       10. The drier defined in  claim 6  wherein said first heat exchanger is provided with a plurality of tubes lying parallel to said travel direction of the boards, and 
       a plurality of collectors spaced apart and connecting said tubes and running transversely to said travel direction.  
     
     
       11. The drier defined in  claim 6  wherein said conveying means includes belt conveyors. 
     
     
       12. The drier defined in  claim 6  wherein said heat exchanger which extends in decks above said conveying means. 
     
     
       13. A method of drying boards, comprising the steps of: 
       ( a )  passing boards to be dried in succession through a first drying stage A and a second drying stage B;    
       ( b )  in said first drying stage A, passing a supplied hot drying air into contact with said boards to remove moisture from said boards and produce an outgoing moist air of a higher humidity than the supplied air;    
       ( c )  in said second drying stage B, passing said outgoing air through a heat exchanger in a deck in radiation heat - exchanging relationship with said boards and collecting condensate from said heat exchanger while discharging an outgoing moist air from the heat exchanger in stage B; and    
       ( d )  simultaneously with step  ( c )  in said second drying stage B, passing a supplied drying air into convection heat - exchanging relationship with said boards.    
     
     
       14. The method defined in  claim 13  wherein said outgoing air of step ( b )  is passed through said heat exchanger in said second drying stage B in the direction of movement of said boards therethrough.    
     
     
       15. The method defined in  claim 14  wherein said supplied drying air in step ( d )  is passed through said second drying stage B in a direction opposite to movement of said boards therethrough.    
     
     
       16. The method defined in  claim 15  wherein the supplied drying air of stage B is heated before it is passed into convection heat- exchanging relationship with said boards.    
     
     
       17. The method defined in  claim 15 , further comprising the step of passing said supplied drying air of stage B and said outgoing moist air from the heat exchanger in stage B in heat exchange in another heat exchanger.  
     
     
       18. The method defined in  claim 16  wherein steps ( c )  and  ( d )  are so carried out that heat amounts transmitted to the boards by the radiation and the convection are approximately equal.    
     
     
       19. The method defined in  claim 16  wherein said outgoing air of higher humidity is produced by using the supplied drying air of stage B, after convection heat- exchange with said boards in stage B as a part of the hot drying air passed into contact with said boards in stage A as the supplied hot drying air therein.    
     
     
       20. The method defined in  claim 19  wherein said supplied hot drying air of stage A is heated by combustion.  
     
     
       21. The method defined in  claim 20  wherein a part of the drying air for stage B is passed into a preliminary drying stage for said boards.  
     
     
       22. A drier comprising: 
       
         conveying means for advancing a plurality of wet boards along a board path through at least one first station A and one second station B in a travel direction in which said station B is being located downstream from said first station A including:  
       
       
         first supply means for passing hot drying air into contact with said plurality of boards to remove moisture from said boards and produce a moist air in said first station A;  
       
       
         first evacuating means for pumping said moist air out of the first station A and for passing said moist air towards said second station B, said second station B being provided with an elongated heat exchanger, said means of station A for passing moist air towards said second station B being in flow communication with said heat exchanger of station B;  
       
       
         means for collecting condensate from said heat exchanger while discharging an outgoing air therefrom;  
       
         pump means for supplying a fresh incoming air to said station B and passing said incoming air into convection heat - exchanging relationship with said boards in a direction opposite said travel direction; and    
       
         second evacuating means being in flow communication with said first supply means of station A for passing said incoming air of said station B to the supplied hot drying air of station A.