THK TIIKItMOWXAMICAL PROPERTIES OF SUPERHEATED STEAM. 3 



Tlie results of this preliminary inquiry, which are described in a paper* (" On the 

 LAW of Flow of Saturated Steam through Small Orifices "), recently presented to the 

 Royal Society by the author, show clearly that adiabatic flow of saturated steam 

 through an orifice occurs when the orifice is drilled in a piece of plate glass, under 

 which circumstances the theory of the subject can be easily and directly applied to 

 the experimental results. 



Since the research is directly based on the experimental results of REGNAULT, it is 

 necessary to at once accept as a definition of dry saturated steam that condition of 

 steam which is obtained by draining from wet steam any entangled moisture, though 

 it must be understood that as yet this condition has not been shown to be unique for 

 any particular temperature and pressure of saturation, a point which can only be 

 settled by experiment. 



SECTION Il.Sliort Thcoi*y. 



The account of the theory here given is that given by Professor REYNOLDS in the 

 paper above quoted. Let p { be the pressure, T/ the temperature, u l the velocity, 

 and Si the dryness fraction of the steam before passing the orifice, and let the same 

 letters with suffix 2 denote corresponding quantities after passing the orifice. Also 

 let H, be the mechanical equivalent of the total heat of evaporation at pressure p lt 

 and Hj h^ the equivalent of the latent heat at the same pressure per Ib. of dry 

 saturated steam as determined from REGNAULT'S steam tables. Let H 4 and H 2 A 8 

 be corresponding quantities at the pressure jp 4 , and let Hj denote the equivalent of 

 any heat received from external sources. Let T 8 be the temperature of saturated 

 steam at the pressure p s , a quantity to be determined from tables. 



The total energy per Ib. of fluid before passing the orifice is therefore 



and after passing the orifice the same quantity is 



S(H 2 - A,) + A, + |J + K(T,' - T,), 



\\lit-re K is the mean specific heat at constant pressure between the temperatures T a 

 and T, 1 . Since the energy of motion developed in the orifice is entirely returned as 

 heat, by the law of conservation of energy we may equate the two quantities here 

 found, and we get 



SitHj - AJ + A, + I' + H, = S,(H, - A,) + A 2 + ^ + K(T, - T f ) (1). 



Now in this equation if S 3 is not equal to unity we must have T, 1 = T 2 , and in this 

 case we should have 



S^H, - A,) + A, + + H, = S^H, - A,) + h, + . . (2), 



* Not printed, but preserved for reference in the archives of the Society. 



B 2 



