HOW THE AIR-PUMP WORKS.
Fig. 33a.
In [Fig. 33a], steam enters from the boiler at the nipple 35, and fills the steam-space between the heads of the main piston-valve 15, 16, maintaining a constant pressure of steam there while the pump is at work. The upper head of the main valve being of greater area than the lower one, the tendency of the pressure is to raise the valve. A downward movement of the valve is provided for by a separate single-headed piston-valve 20, working in a cylinder above the main valve. The reversing-rod 12 operates a slide-valve 13, which regulates the admission and release of steam for the third piston.
In the cylinder shown in the engraving, the main valve is down, so that steam is passing into the lower end of the main cylinder. Two small ports can be seen close to the piston-head 16, one above the other. The upper port is open, and is the admission port; the lower port, which is closed by the small piston, is for exhausting the steam. The main piston 7 is on its upward stroke, and the upper exhaust port seen above the piston-valve 15 is open, while the steam port immediately below it is closed by the valve-piston in the same way that the exhaust port is closed at the other end. When the main piston 7 shall reach near the top of its upward stroke, the plate 10 will strike on the projection on the reversing-rod, pushing up the slide-valve 13. The upper edge of this slide-valve will cut the steam off the passage a, and open the passage b to the exhaust. This takes the steam away from the piston 20, and allows piston 15 to move upward, closing the exhaust-port, and opening the upper steam-port. The same movement makes the piston 16 close its steam-port, and open the exhaust. Piston 7 now begins to travel downward; and, when it reaches nearly to the bottom of the cylinder, the plate 10 catches the knob on the end of the reversing-rod, and pulls down the slide-valve 13 to the position it holds in the engraving. Steam then rushes through the passage a, and makes the piston 20 push down the main valve. That completes the circle of the operations in the steam cylinder.
HOW THE AIR-END OPERATES.
Fig. 33b.
The operation of the air part of the pump is very simple. While the main piston ([Fig. 33b)], which is on the same rod as the piston of the steam cylinder, is moving upward, it is forcing the air out of the upper end of the cylinder up under the discharge-valve 32, and away through the proper passages to the main reservoir. At the same time the lower end of the cylinder is being filled with air drawn through the lower receiving-valve 34. During the downward stroke of the piston, the air will be delivered through the valve 33, and the upper part of the cylinder filled by air received through the upper valve 34.