TUBE-TYPE EVAPORATOR

Abstract: A tube-type falling film evaporator which comprises vertical outer tubes (6) having closed upper ends and being connected to an upper tube plate (4) disposed in the bottom portion of the evaporator and other parallel tubes (7) disposed inside said outer tubes (6) and connected to a lower tube plate (5) under said tube plate (4). The space (11) between the tube plates (4, 5), in which the ends of the outer pipes are located, is divided by an intermediate wall (12) in two compartments (13, 14). Heating steam is fed to the first compartment (13). Clean condensate is recovered from the first compartment (13) and the contaminated condensate from the other compartment (14).

Description: 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 top portion 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 outlet are provided for the materials delivering heat in a lower region of the evaporator. A double-tube, falling film evaporator is disclosed in DE-C-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. 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 position, is clean and can be recovered. An evaporator according to the invention is characterised 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 in 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 two-staged lamella-type evaporator disclosed in Fi-A-52601. The invention will be described further, by way of example, with reference to the accompanying drawing which is a schematic vertical section through a tube evaporator. 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 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 outer 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 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 3. The steam arrives at the top ends of the tubes 6 and condenses while delivering heat to the evaporating lye. The condensate flows down onto the lower tube plate 5. The steam is not totally condensed in this portion of the evaporator. The portion of the steam which has not condensed is fed via inner tubes 7 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 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 in the first portion of the evaporator strips the condensate whereby the condensate is cleaned. The clean condensate is discharged from the evaporator via pipe 16 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. Outlet means (not shown) are provided in the vessel to enable any liquor which is not evaporated to be drained off plate 4.

Claims: An evaporator comprising upwardly or vertically extending outer tubes (6) having closed upper ends and being connected to a first tube plate (4) disposed in a lower portion of the evaporator; and other inner tubes (7) disposed inside said outer tubes (6) and connected to a second tube plate under said first tube plate (4); and feed means (8, 9, 10) in an upper region of the evaporator for feeding the material to be evaporated to the upper,outer ends of the outer tubes (6) in such a way that the fed material flows down along the outer tubes (6) and forms a descending film or layer on the external surfaces of the outer tubes (6); and, an inlet (15) and outlets (16, 17) in lower regions of the evaporator for the material delivering heat, characterised in that a space (11) between the first and second tube plates (4, 5), with which the ends of the outer pipes (6) communicate is divided by an intermediate wall (12) into two compartments (13, 14) from which condensate is dischargable with one of said compartments (13) being connected to the inlet (15) for the material delivering heat. An evaporator as claimed in claim 1 characterised in that the other of said compartments (14) is connected to a gas outlet (18). An evaporator as claimed in claims 1 or 2, characterised in that each compartment (13, 14) has a condensate discharge outlet (16, 17). An evaporator as claimed in any of claims 1 to 3, characterised by the feature that the inner and outer tubes (7, 6) and tube plates ( 4, 5) are located within a vessel (1, 2, 3).

Inventors: HAVUKAINEN, REINO
Applicants: A. AHLSTROM CORPORATION
Application year: 1987
Main classification: B01D1
IPC codes: F28D7, B01D1