40-i REPORT— 1894. 



separated must be weiglied. A condenser must therefore be used, at any 

 rate occasionally, to determine the amount of steam flowing in a given 

 time. Assuming the orifice to remain unchanged, a formula can be found 

 for the weight of steam discharged per minute at any given pressure. 



VIII. Siqypvheating hy Addition of Heat. — A method has been pi"o- 

 posed by Mr. W. R. Cummins,' in which a vessel of constant volume is 

 used, completely surrounded by a steam jacket. Steam is at first blown 

 through both jacket and vessel till there is a fair sample of the steam to 

 be tested in both. Kext the jacket and vessel are cut off from the steam 

 pipe and from each other. The steam in the jacket is then heated by 

 compression (by a steam compressing pump) or by heating. The simul- 

 taneous rise of pressure and temperature in the vessel containing the 

 sample of steam are observed. "While moisture is evaporating the relation 

 of pressure and temperature will follow Regnault's law. From the 

 moment when superheating begins the pressure will increase much less 

 rapidly with the temperature. Let <, be the temperature of the sample 

 of steam initially, <., the temperature at which superheating is observed 

 to begin, i', v,^ the corresponding specific volumes of dry saturated steam, 

 X the initial dryness of the steam, V the volume of the vessel, and iv the 

 weight in pounds of steam admitted. Then 



^2 



a;=— 



where the specific volumes i'.,v, can be calculated from the observed tem- 

 peratures ^2 ^1- The method is quite accurate in principle, but it would 

 involve considerable skill to carry it out accurately, and it does not appear 

 to have been actually tried. 



IX. Chemical Methods. — These suppose that some soluble salt is added 

 to the boiler water so as to form a solution of known strength, and that of 

 all the feed supplied to the boiler part is evaporated and flows away as 

 steam, part as priming water. The former removes no salt ; the latter is 

 of the saltness of the water then in the boiler. Hence, either the decrease 

 of saltness of the boiler water or the amount of salt present in the steam 

 can be used in determining the amount of priming water in the steam. 

 Most commonly table salt (NaCl) has been used because the amount in 

 solution can be easily determined by the nitrate of silver method. In 

 other cases sulphate of soda has been used, the sulphuric acid being 

 tJetermined by precipitating with chloride of barium. It is convenient to 

 describe three variations in the way in which the chemical method is 

 applied. 



(a) Decrease of Saltness of the Boiler Water -measured. — Salt is intro- 

 duced -svith the feed till the boiler water contains 1 or 1^ per cent, of salt. 

 Then, during a test in which the amount of feed is measured, a sample of 

 the boiler water is drawn off at the beginning and end of the test. Care 

 must be taken that the level of the water in the boiler (as shown by the 

 gauge glass) is exactly the same when the two samples are taken. Let 

 'W be the weight of water in the boiler, and s, S2 the percentage of salt in 

 the two samples. The amount of salt removed from the boiler during 

 the test is W(S| — s.,)/100. Let tv be the amount of feed supplied, and x 

 the dryness fraction of the steam. Then xiv lb. of pure steam are 



' Trans. North-east Coast Inst, of Engineers and Sliiphuilders^ 1893. 



