EVAPORATION 



363 



as dry demand the use of a certain amount of water for the purpose of 

 sealing the valves. 



The second classification of pumps divides them into reciprocating and 

 rotary types : the former may be direct acting or driven through a crank 

 and fly wheel.* Various forms of pumps are described below. 



Reciprocating Torpedo Wet Pump. Fig. 220 shows a form of wet-air 

 pump, of which in past times very many have been installed. The recipro- 

 cating element e may be a piston or torpedo as shown, and the valves may be 

 hinged clack valves, as at a, or preferably rubber discs on a gridiron seating, 

 as at b. The air and water from the condenser enter through c, the latter 

 being finally conducted to the ditch by way of d. This type oi pump is 

 only suited for smaller installations, since owing to the slow movement 



FIG. 220 



necessary it becomes of impossible dimensions for the large evaporating 

 units now generally installed. 



Edwards Pump. The Edwards pump (patents 18817, 1897 ; 629, 630, 

 1899) is a type which dispenses with the suction valves ; it is shown in section 

 as a wet-air pump in Fig. 221. Water from the condenser flows by way of 

 d to the reservoir c ; the conical bucket on the down stroke forces the water 

 into the barrel of the pump. As soon as the bucket rises, the entry of water 

 by way of d is closed until the bucket or piston has passed, and the water 

 which has been projected into the barrel is lifted and discharged through 

 the valves at a, passing away at e ; at g is a relief valve. The incondensible 

 gases are free to escape to the space above the water. This pump can be 

 used also as an efficient dry-air pump. As it is single acting this type is 

 usually installed as double or treble-barrelled units, when action becomes 

 almost continuous. 



Slide Valve Reciprocating Pumps. The standard type of dry-air pump 

 is a slide valve pump operating mechanically as a slide valve steam engine 

 reversed. In pumps of this nature air under compression is left in the cylinder 

 at the end of each stroke, so that the efficiency of the pump is reduced. 



* American practice quite irrationally describes a crank and fly wheel reciprocating pump as a rotative pump, 

 thus translating the mechanical means used to obtain reciprocating motion to the pump itself. The term rotary 

 is here used in its proper siijnincance. 



