Fig. 359.
It may be said that however slowly an ordinary air pump with foot and bucket valves may be running, the pressure in the condenser has to be sufficiently above that in the pump to lift the foot valves, overcome the inertia of the water, and drive the water up through the valves into the barrel where the water is dealt with mechanically. The higher the speed of the older type of pump the greater is the pressure required to overcome these resistances owing to the very short space of time available, and as any increase of pressure in the condenser is accompanied by a corresponding increase of back pressure in the low pressure cylinder, hence the absence of the valves referred to allows a higher speed of the plunger. The elimination of the foot valves it is claimed gives from 1⁄2 to 1 inch better vacuum.
Fig. 360.
Another advantage claimed for this pump is that clear air inlets are maintained—see Figs. [359] and [360]. Under ordinary working conditions, when the bucket descends and the ports open, there is no obstruction between the condenser and the pump; the air has a free entrance while immediately afterward the water is injected into the barrel at a high velocity. Thus, instead of obstructing the entrance of the air, the water tends to compress that already in the barrel, and to entrain or carry in more air with it.
The bucket or piston is a hollow casting with water grooves instead of packing rings.
The valve seat is constructed with a rib between each valve and a lip around the outer edge, so that each valve stands in its own water and is separated from the others. This forms a ready means of testing the relative tightness of each valve.
The cast iron working barrel is lined with brass.
The pump rod is Tobin bronze, and valve plate and valves of composition. These pumps are either single, twin, triplex, and are steam, electric or belt driven, for stationary, marine or sugar plantation service.