364 



CHAPTER XVIII 



If at this moment communication between the two faces of the piston be made 

 air will pass over from one side to the other. This scheme is known as the 

 equalization of pressure, and the valve establishing communication as a 

 " flash port." It was first suggested by Welner, and is contained in Burchart 

 and Weiss's patent (3551 of 1882.) The expressions giving the efficiency of 

 pumps with and without equalization of pressure are : (i) Without : — 



I — — 



I — — 



Efficiency == p ; (2) With : — Efficiency — p , where a is the volume 



-L — a' 



Fig. 221 



of the dead space per unit volume of cylinder, and p is the pressure 

 reached in the vessel being evacuated referred to atmospheric pressure as 

 unity. 



A type of this pump, as made by Wegelin & Hiibner, is shown in 

 Fig. 222. There are three valves, a, b, c, known as the distributing, equalizing 

 and delivery valves. The valve a allows air to enter or depart and serves 

 to connect the suction or discharge to the pan or atmosphere ; the delivery 

 valve c on the valve a is for the escape of air and to prevent air returning 

 to the pump ; the equalizing valve b is for the purpose of connecting the 

 channels d when the piston is at the end of the stroke. At this moment the 

 valve a is nearly central, and the discharge and suction ports are closed. 

 The equalizing valve b now makes connection between the two faces of the 

 piston b}^ means of the channel d, and equalizes the pressure on either side 

 of the piston. The valve b now closes and a opens, and as the piston moves 

 from right to left air is drawn into the vacuum formed. The equalizing 



