ox METHODS OF DETERMINING THE DRYNESS OF STEAM. 405 



generated, and (l—x)tv lb. of priming water carried over. The mean, 

 sallness of the priming water is (s, +80) 12. Hence 



{l—x)^os^+S2 W/ _ v 



-^00 2~~lW' ''^ 



, 2Wsi— So 



The method deals with the whole amount of steam produced, and not 

 with mere samples of it. On the other hand, the disadvantages of the 

 method are : (1) It measures only priming water mechanically projected 

 into and carried by the steam, and not moistness produced in other ways. 

 (2) Great exactness of analysis is necessary, tlie difference of saltness 

 being a small quantity. (3) The feed, being continuously supplied at one 

 part of the boiler, tends to produce, in spite of the circulation, a non- 

 uniform distribution of saltness throughout the mass of water in the 

 boiler. If the saltness is not uniform, the difference of saltness of the 

 samples gives a quite erroneous result. If there is salt in the feed water, 

 it can be allowed for. This method was first used at Mulhouse in 1859 ; 

 much later it was revived, and is often called Brauer's method. 



(/3) Percentage of Salt in the Steam determined. — At intervals during 

 a test, samples of the boiler water are taken, and at the same time some 

 steam is condensed in a small surface condenser. Equal quantities of the 

 water samples are mixed together, and also equal quantities of the con- 

 densed steam. Let s, be the percentage of salt in the averaged sample 

 of boiler water, and Sj that in the averaged sample of condensed steam. 

 If a; is the dryness fraction of the steam, each pound contains x lb. of 

 steam and l—x lb. of water of the saltness of the boiler water. 



{l—x)si=S2 



x=l- 



The disadvantages of this method are (1) that very small portions of the 

 steam are tested, and, as the mechanical action producing priming is 

 probably irregular, it is very uncertain whether they fairly represent the 

 average condition of the steam ; (2) the accurate determination of s, can 

 only be made if the boiler water is uniformly salt ; (3) the amount of salt 

 in the steam is very small compared with that in the boiler water, so that 

 the accurate determination of s, is difficult. 



(y) Fschei-'s Method. — A quite different method of considerable interest 

 has been proposed by Mr. R. Escher, of Zurich. • Mr. Escher considered 

 the case of a boiler fed with impure water. Of the salts in solution 

 some are thrown down as incrustation, others remain in solution. In 

 the boiler water during working there is a steady concentration of these 

 soluble salts, which tends towards a fixed limit. At this limit the priming 

 removes as much salt in the form of concentrated boiler water as the feed 

 brings iu. 



Let s be the saltness of the feed water in pounds per pound of water, 

 and k, the saltness at any given time of the boiler water. Then each 



» Civilingenieur, 1879, p. 51. 



