138. Force Pumps. Force pumps are used on deep wells and in forcing water into storage tanks. They should be kept oiled; they should be operated with an even stroke, and the packing in them should be renewed if they leak air. In force pumps, the valves differ in their arrangement from suction pumps (Fig. 69).
139. Compressed-Air Pumps. Compressed-air pumps consist of a tank for storing the compressed air—a pump to force air into the tank and cylinders equipped with valves. These act automatically. Whenever an outlet pipe is opened, the extra pressure of air from the storage tank raises the water from the well or cistern (Fig. 70). Air should be kept in the pressure tank.
Fig. 70. Compressed-air pump system.
When this arrangement is used, open and close faucets slowly, not with a jerk. Fig. 70-a shows plumbing where such a system is used.
140. Water Filters. Water filters are devices for straining minute particles out of water. They are made of sand, charcoal or porcelain, kisselguhr and other materials. They are without value unless they are kept clean. A dirty filter is worse than none. Almost the only way to clean them is to sterilize them or put new material in them. Only with expert care can filters be made effective for removing disease germs. A dirty filter may prove a menace. Filters are valuable for removing coarse dirt from the water.
Fig. 70-a. System of plumbing with compressed-air tank.