Fig. [11] represents a double still which was at one time largely employed in the colonies. It is simply an addition of the common still A to the patent still B. From time to time the contents of B are run off into A, those of A being drawn off as dunder, the spirit from A passing over into B. Both stills are heated by the same fire; and it is said that much fine spirit can be obtained by their use at the expense of a very inconsiderable amount of fuel.
Fig. 11.—Double Still.
Compound Distillation. Where stills of the form shown in Figs. [6] and [8] are used the alcohol obtained is weak. Hence it is necessary that the distillate be again itself distilled, the operation being repeated a number of times. In the better class of still, however, compound distillation is performed the mash is heated by the hot vapors rising from the still and the vapors are condensed and run back into the still greatly enriched.
Fig. 12.—Dorn’s Compound Still.
The principle of compound distillation is well shown in Dorn’s apparatus, Fig. [12]. This consists of a still or boiler A having a large dome-shaped head, on the interior faces of which the alcoholic vapors will condense. Thus only enriched vapors will pass up through goose-neck B to the mash heater D. C is a worm the end of which passes out to a compartment E through an inclined partition F. From the compartment E a pipe e leads into the still A. An agitator H is used for stirring the mash, so that it may be uniformly heated. A pipe d provided with a cock allows the mash to be drawn off into the still A. From the highest point of the compartment E a pipe M leads to condensing coil K in a tub J of cold water, having a draw-off cock I.
At the exit end of the condensing worm K the tube is bent in a U form as at L, one arm of which has a curved open-ended continuation n, through which the air in the worm is expelled. The other arm opens into an inverted jar l containing a hydrometer, for indicating the strength of the spirit. The spirits pass off through m into a receiver.