EVAPORATION 371 



spray heads are spaced t went}'- five feet apart, the spraj^ heads being spaced 

 thirteen feet centre to centre. 



A comparison of the two systems favours the spray system, both as regards 

 first cost and renewals, the chief disadvantage being the annoyance following 

 on the frequent choking up of the nozzles. 



In either system the loss of water is from 3 to 5 per cent., which has to be 

 supplied from outer sources. 



Entrainment. — By this term is meant the carr\dng forward of material 

 into the vapour pipes and its consequent loss. Three causes are at work : — 

 I. Material is splashing into the head boxes of the vessels. 2. Material 

 creeps up the sides of the vessels due to capillarity. 3. Hollow drops or 

 bubbles are formed, and when the forward velocity of the current of vapour is 

 such that it exerts on the bubble a pressure equal to its weight the latter floats 

 and is carried forward. This process is referred to as vesicular transference. 

 It is in the last cell of the evaporators that these influences are mostly at 

 work. They may be reduced to a minimum by the devices indicated below. 



Splashing losses may be avoided by giving a liberal height to the vessel 

 and by placing hoiizontal guard plates in the body of the vessel. These 

 guard plates may conveniently take the form of a ring, with its opening 

 covered by an overlapping disc. This means is claimed in Vivien and 

 Dujardin's patent (22S6 of 18S4). 



Losses due to vesicular transference are best avoided b}' shock obtained 

 by abrupt changes in direction, by a sudden decrease in velocit}^ obtained 

 by enlargement of the vapour pipe, or by a combination of these means. 

 In Figs. 228 and 229 are shown two methods as applied in the vapour pipes. 

 The Hodek ralentisseur, a standard European model, is indicated in Fig. 230. 

 It combines decrease of velocity with the passage of the vapour through 

 screens. Not dissimilar in action 10 the Hodek is the arrangement of 

 Stillman {Fig. 231), shown in U.S. patent 484831, 1892. It is largely used 

 in Hawaii, and is indicated in section in Fig. 231, as located in the bod}^ of 

 a vessel. It is made up of three horizontal plates, each carrying a number 

 of two-inch tubes, a tube in one plate being opposite a blank in others. 

 A similar arrangement may be located as vertical baffles in a horizontal 

 length of pipe, and in this case the chamber takes the form of two opposed 

 pyramidal vessels. Types of centifugal separators are indicated in Figs. 232 

 and 233, the former being due to McNeil. 



What is perhaps the most commonly used arrangement is indicated in 

 Fig. 234, and this is due to Vivien and Dujardin, being claimed in patent 

 2286 of 1884. 



Finally a somewhat different device, based on a well-kno\\Ti form of oil 

 separator, is indicated in Fig. 235. In this the direction of flow ma}^ equally 

 well be opposite to that sho\\Ti. 



Capillary losses are found mainly in the vapour pipes after the bubbles 

 have burst, and hence all the devices indicated above must be efflciently 

 dramed. Very often the drain pipe is led into, and terminates in, the vapotu: 

 space of the effect. The downward flow of the liquid is opposed then to the 

 rapid forward flow of the vapours, and a considerable quantity of material may 

 be carried forward to the condenser. To avoid this the drain pipes ma}^ dip 

 below the surface of the hquid, or an inverted syphon seal may be used. The 

 writer, however, beheves that the most satisfactory results are obtained by 

 draining the save-alls into a receptacle external to the vessel. This receptacle 



