An evaporator having a plurality of effects, a plurality of said effects including an outer housing portion disposed with its axis substantially horizontal and being generally curvilinear in a plane transverse to said axis,
- each housing portion including first and second pairs of spaced substantially vertical walls disposed in a substantially rectangular array, one of the walls of said first pair intersecting a first part of said housing portion to form therewith a vapor distributing space and a second one of the first pair of said walls intersecting a second part of said housing to form therewith a distillate collecting chamber,
- said first and second pairs of vertical walls defining an evaporating space with the upper and lower margins of said housing,
- a plurality of heat exchange tubes extending substantially horizontally through said evaporation space and between said first pair of vertical walls and opening at one end into said vapor distributing space and at their other ends into said distillate collecting space,
- a first one of said second pair of vertical walls of each effect being disposed at one end of said housing portion and the other of said second pair of walls being disposed intermediate the ends of said housing portion and extending from the upper end thereof and defining a vapor receiving space with the other end of said housing portion, an aperture formed in the lower end of the other one of said second pair of vertical walls for connecting said evaporating space to said vapor receiving space whereby vapor generated in the evaporating space of each effect is flowed downwardly and through said aperture,
- feed liquid distributing means disposed in said evaporating space and above said heat exchange tubes for distributing feed liquid as a thin film over the outer surfaces of said tubes,
- the housing portions of each effect being joined end to end in a substantially horizontal array to form a unitary housing for said evaporator,
- means for coupling the vapor distributing space of the first one of said effects to a source of steam, means for coupling the evaporating space of the last one of said effects to condensing means,
- the first one of the second pair of walls of each effect having a first opening formed therein and communicating with the vapor receiving space of the respective next higher effect for passage of vapor to the vapor distributing spaces of said effects from said respective next higher effects,
- a second opening formed in the lower end of the first one of said second pair of walls of each effect for permitting the flow of unevaporated feed liquid between effects,
- and feed liquid preheater means disposed in at least some of said effects and including a preheater housing surrounding said feed liquid preheater means and having an opening formed therein, said opening being coupled to the condensate collecting space of its respective effect for conducting uncondensed vapor exiting from the effect heat exchange tubes to said feed liquid preheater means.