THE EVAPORATION OF THE JUICE TO SYRUP. 



Multiple Evaporation. In modern factories evaporation is always 

 performed in multiple evaporators. The principle on which these are con- 

 structed has already been indicated. The first multiple evaporator was 

 invented in 1834 by Nbrbert Killieux, of Louisiana, and was a double effect 

 horizontal submerged tube apparatus. Apparatuses conforming to his original 

 design and only differing in mechanical details are still constructed. 



In Fig. 157 is shown a diagrammatic sketch of a vertical submerged tube 

 triple effect apparatus. Each body consists of an upright cylindrical vessel of 

 height rather more than twice the diameter ; near to the bottom and at about 

 one-third the height are placed two tube plates : these plates cany a number of 

 tubes, the space between the tube plates and external to the tubes being filled 

 with steam or vapour from a preceding effect ; the vapours pass over from the 

 juice in one vessel to the calandria of the next by means of the vapour pipe c. 

 In each vapour pipe is often placed a cylindrical baffle plate d for the purpose 

 of intercepting any juice carried over in entrainment : any juice so intercepted 

 is returned by the pipe e to the vessels. The juice circulates from vessel to 

 vessel by the pipes /under the influence of a difference in pressure or aided 

 by a circulating pump ; the concentrated juice from the last body is pumped 

 out against atmospheric pressure ; very often this pump is worked off the 

 main vacuum pump. The water of condensation in the calandrias of the 

 vessels passes out through the pipe '; generally steam traps of various con- 

 structions are used or the water may be removed by a small pump. In some 

 designs the water of condensation is passed from vessel to vessel, affording a 

 slight economy of heat. The pipe h is for the purpose of introducing acid or 

 alkali to cleanse the tubes of scale and j is a small pipe carrying the incon- 

 densible gases from the second to the third vessel and thence to the pump. 



The method of operation is as follows : juice is allowed to flow into the 

 vessels from the supply tank until a little above the level of the upper tube 

 plate ; the vacuum pump is then started, and a vacuum of about 25 inches, 

 corresponding to a pressure of about 5 inches, is obtained in the last body of the 

 apparatus, and the water injection cock opened. Exhaust steam at a pressure 

 of about 5 Ibs. per square inch is admitted to the calandria of the first body ; 

 the juice boils and vapour passes over into the calandria of the second body, 

 into which juice from the first body has already passed; the tubes in the 

 second body act as a surface condenser to this vapour. The condensation of 

 this latter creates a partial vacuum in the first body, and by its condensation 

 parts with its latent heat to the juice in the second body, causing this juice to 

 boil. A similar process takes place between the second and third bodies, and 

 so on. 



In this way 1 Ib. of steam could evaporate an infinite quantity of water ; 

 for reasons explained later on, the practical working limit is generally granted 

 to be reached with quadruple evaporation. The majority of apparatuses are, 

 however, triple effects ; but quintuple plants have been constructed. 



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