ON METHODS OF DETEllMINING THE DRYNESS OK STEAM. 393 



(1) By ebullition boiler water is projected into the steam space. Part 

 falls back, but part is carried on in the steam current. The extent to 

 which wetness may be thus produced depends on the activity of the 

 ebullition, the area of the water surface, the volume of the steam space, 

 the position of the steam valve, the density of the steam, and other 

 circumstances. In certain conditions of the boiler water, it foams, and 

 the steam space is filled with vesicles. Under such circumstances the 

 meclianically produced priming may be excessively severe. 



As to priming of this kind some observations of Mr. Thornycroft on 

 a boiler with glass ends are very instructive.' He states that 'waters 

 which cause priming on boiling produce foam, consisting of a mass of 

 bubbles of various sizes. Water which is very bad produces bubbles so 

 durable as to remain a considerable time without breaking, and by them 

 the steam space of a boiler may be entirely filled. So soon as this takes 

 place, instead of simply steam leaving the boiler, the discharge consists of 

 foam, which is broken up in its rapid passage through the steam pipe.' 

 With pure water, steam retains no film of liquid for sufficient time to be 

 seen. 



(2) The steam in the boiler is subject to variations of pressure. 

 Bubbles formed under water rise to a region of less pressure. Fluctua- 

 tions of pressure arise from the intermittent demand for steam. During 

 expansions water must be formed as mist throughout the mass of the 

 steam. It is difficult, however, to suppose that any great quantity of 

 moisture is thus produced. 



(3) The steam in the steam space of the boiler, and when flowing 

 through the steam pipes, loses heat by radiation from the boiler roof and 

 the surfaces of the pipes. To this must correspond condensation of part 

 of the steam. Probably in some cases very considerable amounts of 

 moisture are produced in this way. 



Methods of Determining the Wetness of Steam. — Very difi'erent methods 

 have been tried by different observers to determine the amount of moisture 

 in steam. Some method is required sufficiently accurate for practical 

 purposes, and not involving excessively delicate measurements or com- 

 plicated apparatus. It is proposed to describe all the methods which 

 have been tried which seem at all likely to be useful, and finally to give 

 the results of some comparative trials made for the committee which 

 throw light on their relative trustworthiness. 



I. Weighing Method. — A method of direct weighing of a known 

 volume of a sample of steam has been proposed by Guzzi ^ and Knight.-* 

 A copper globe is used as a measuring vessel, which is placed in a receiver 

 connected with the boiler or steam pipe. After filling, it is taken out and 

 weighed. Let V be the volume of the globe, w the weight of wet steam 

 in it. Let {\—x)w be the moisture and xw the steam in the globe, and 

 let V be the volume in cubic feet per pound of dry steam in the same 

 conditions of pressure and temperature. Then 



(1 — x)tV=:W—~ 

 V 



x=V /vw 



The method is one obviously of excessive difficulty. 



' 'Circulation in the Thornycroft "Water Tube Boiler,' Trans. Iiist. Naval 

 Architects, 1894. 



- Revue Indvstrielle , 1878, p. 102. 



^ Journal of the Fraiiklin Institute, 1877, p. 358. 



