The present invention concerns an evaporator comprising two heat exchangers, the liquid to be evaporated being disposed to flow downward from above in heat exchange with the heating steam in a first heat exchanger.

In modern distilling technology, in particular in that associated with seawater desalination, the so-called multiple-effect falling film evaporator has gained increasingly extensive use. In a multiple-effect distilling installation of this type, the liquid to be distilled runs on the heat exchange surface of a heat exchanger as a thin film from the upper part of the evaporator to the lower part, partially evaporating at the same time, as the heating steam is condensed on the other side of the heat exchange surface. The heat exchanger can be composed in many ways, such as vertical tubes, horizontal tubes or plates. The unevaporated liquid runs down to the bottom of the evaporator and is therefrom pumped with a pump into the upper part either of the same or the next distilling stage, -whence it once more runs down along the heat exchanger. In a multiple-effect installation, this arrangement necessitates a great number of pumps, which, as they are operating close to cavitation circumstances, give rise to extra costs and need of maintenance.

The object of the invention is to teach an improvement in present evaporator designs known in the art. The more detailed object of the invention is to provide an evaporator in which the use of conventional pumps can be avoided in transporting the liquid to be evaporated. The other objects of the invention and the advantages gainable with its aid will be apparent from the disclosure of the invention.

The objects of the invention are achieved by means of an evaporator which is mainly characterized in that the evaporator comprises a second heat exchanger, composed of substantially vertical flow channels, and that said second heat exchanger has been disposed to transport the unevaporated liquid upward from below.

The evaporator of the invention is applied in a multiple-effect distilling installation in that the liquid that has run down into the lower part of a given distilling stage runs to the upper part of the next distilling stage through substantially vertical channels which are in heat exchange with the heating steam of this distilling stage.

It is thus understood that one characteristic embodiment of the invention is the combination of a horizontal tube falling film evaporator (HTFFE) and on the other hand of a vertical tube rising film evaporator (VTRFE) in such manner that the vertical evaporator transports the liquid from the lower part of the horizontal evapor¬ ator to the upper part of the next horizontal evaporator.

The invention is described in detail, referring to certain advan¬ tageous embodiments of the invention, depicted in the figure of the drawing attached, to which the invention is not, however, meant to be exclusively confined.

The figure of the drawing shows in schematic elevational view, two successive distilling stages of multiple-effect distilling instal¬ lation in which stages an evaporator according to one embodiment of the invention has been used.

The distilling stage 10 of the multiple-effect distilling instal¬ lation depicted in the figure consists of horizontal heat exchange tubes 12 placed substantially in the space 11, said tubes communi¬ cating by one end with space 13 and by the other end with space 14. The space 14 contains substantially vertical heat exchange tubes 20 which communicate at the lower end with the lower part of the space 11 through the tube 21 and at the upper end, through the tube 16 with a liquid distributor means 15' in the upper part of the next distilling stage 10 - . From the upper part of the stage 11 leads a vapour passage 17 to the vapour space 13' of the next distilling stage 10 ₊ - , which further communicates with the heat exchange tubes 12' of this distilling stage.

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The distilling installation operates in the manner that the liquid to be distilled flowing from the preceding distilling stage 10 . through the tube 16 , is divided through the distributing means 15, which may be of any design known in the art, e.g. a perforated tube, to the outside of the heat exchange tubes 12, where it runs as a thin film downward from above. The heating steam flows into the space 13 and therethrough into the tubes 12, in which it condenses to distillate, accumulates on the bottom of the space 14 and escapes therefrom into the tube 19. The uncondensable gases entrained with the steam are voided through the pipe 18 into the air removal system. As it condenses in the tubes 12, the vapour imparts its latent heat to the liquid film on the outside of the tubes 12, part of which liquid film is evaporated, and the vapour flows through the passage 17 into the next distilling stage 10 . to serve there as heating steam. The unevaporated liquid running to the bottom of the space 11 escapes into the tube 21 and thence through the water well 22 into the tubes 20. Because the tubes 20 are in heat exchange with the same steam that has heated the tubes 12, the vertical tubes 20 operate as heat exchangers, the liquid flowing in them evapor- ating partly, forming vapour bubbles and ultimately a powerful vapour flow which carries unevaporated liquid along with it. In this manner the liquid ascends to the upper part of the next distilling stage 10 . , which may be located at a considerably higher elevation than would be implied merely by the differential pressure between the distilling stages in its effect as a hydrostatic head. Thus, by this arrangement the unevaporated liquid is pumped without recourse to conventional pumps from one distilling stage to the next.

Naturally, the embodiment of the figure is not the only possible application. For instace, the tubes 20 could equally well be placed in the vapour space 13. Furthermore, part of the liquid flowing in them could be returned to the same distilling stage. Likewise, the number of ascending tubes and their diameter may vary widely according to the circumstances.

The falling film heat exchanger 12 and the rising film heat exchanger 20 may also be designed in anyt other way known in the art such as

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vertical tubes or plate heat exchangers.