Tube-type evaporator
This is a continuation of application Ser. No. 066,253 filed June 25, 1987, now abandoned. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to an evaporator comprising upwardly or vertically extending outer tubes having closed upper ends and being connected to a tube plate in a lower region of the evaporator. Inside these tubes there are other normally parallel tubes connected to another tube plate which is disposed below the said tube plate. The upper region of the evaporator is provided with means for feeding the material to be evaporated to the upper ends of the outer tubes so that the material flows down along the outer tubes forming a falling film on the outer surface of the outer tubes. An inlet and outlets are provided for the material delivering heat in a lower region of the evaporator. 
     A double-tube falling film evaporator is disclosed in German patent publication no. 737330. In the apparatus of this publication the material delivering heat is fed to a space under a lower tube plate from which it flows in an inner tube to the internal surface of an outer tube to which it delivers heat which is used for evaporating material flowing down along the external surface of the tube. One of the advantages of this construction is that the heat exchanger tube is connected to only one tube plate and thus is not subjected to stresses as the tubes in a conventional tube-type evaporator. Further, only one of the tube plates, viz. the upper one, has to face the pressure difference prevailing in the evaporator and not both the tube plates as in a conventional tube-type evaporator. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an evaporator which can be used when the steam delivering heat is contaminated such as the expansion steam from a cellulose digester. 
     In the apparatus according to the present invention, the condensate can be divided in two portions one of which, viz. the smaller portion, contains the main part of the volatiles and the other one, viz. the larger portion, is clean and can be recovered. 
     An evaporator according to the invention is characterized in that the space between the tube plates in which the ends of most of the tubes are located is divided by an intermediate wall into two compartments. Condensate is discharged from both of the compartments and one of them is connected to the inlet for the steam delivering heat. 
     The construction is clearly simpler than e.g. the twostaged lamella-type evaporator disclosed in Finnish patent specification no. 52601. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described further, by way of an example, with reference to the accompanying drawing which is a schematic vertical section through a tube evaporator. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A tube-type evaporator illustrated in the drawing comprises a vessel formed by a vertical tubular wall 1 and two convex ends 2 and 3. Two spaced apart tube plates 4 and 5 are disposed in the vessel close to the bottom end of the evaporator. A plurality of vertical tubes 6 are disposed inside the vessel with the upper ends of the tubes being closed and the lower ends being connected to the upper tube plate 4. Inside the outer tubes 6 there are other, parallel inner tubes 7 which extend towards the upper end of the outer tubes 6 and both ends of each of which tubes 7 are open with the lower ends being connected to the lower tube plate 5. A distributor 8 for liquid to be evaporated is provided in the evaporator close to its upper end. The bottom of the distributor 8 has a plurality of apertures 9 through which the material to be evaporated, e.g. lye, is evenly distributed onto the upper ends of the outer tubes 6 which serve as heat exchange surfaces and wherefrom the material flows down along the tubes and forms a thin film on the other surface of the tubes. The material to be evaporated is fed to the distributor via a pipe 10 through the wall of the vessel. 
     A space 11 between the tube plates 4 and 5 is divided into two compartments 13 and 14 by an intermediate wall 12. Heating steam is fed to inlet compartment 13 via an inlet 15. Condensate is discharged from both of the compartments via pipes 16 and 17. Steam flows at first to the outer tubes whose lower ends open into the compartment 13. The steam rises up to the top ends of the tubes 6 and condenses while delivering heat to the evaporating lye. The condensate is not totally condensed in this part of the evaporator. The portion of the steam which has not condensed is fed via inner tubes 7 to a return chamber 20 underneath the lower tube plate 5 and up the other inner tubes 7 and then down the inside of said outer tubes 6 which open in the outlet compartment 14 where the final condensation of the steam takes place. When heating material containing volatiles is used, e.g. lye steams, a major portion of the condensate is recovered clean as the steam rising up against the condensate flowing down in the heat exchanger tube 6 is the first part of the evaporator strips the condensate whereby the condensate is cleaned. The clean condensate is discharged from the evaporator via pipe16 and the contaminated condensate via pipe 17 and the gases, which have not condensed, from the compartment 14 via pipe 18. 
     The steam generated by evaporation during the mentioned condensation is discharged via an outlet 19 in the top end 3 of the vessel. 
     The invention is not limited to the embodiments described here as an example only but it can be modified and applied in various ways within the scope of protection defined by the appended patent claims.