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How To Design And Install Plumbing
Compressing only the original pressure contained in the tank is insufficient. The slightest draw on the distribution system will spend the increased pressure very quickly. There is also the possibility of the tank becoming water-logged. The air compressor is the unit installed to overcome water distribution difficulties. It usually is controlled manually.
When the system is put into operation the volume of air in the tank is increased by the air compressor to a pressure of one-half the minimum required. Admitting water to the tank increases the pressure proportionately by displacing a portion of the tank's air content. This permits a more extended draw on the distribution system.
A pressure range above the minimum pressure required is necessary. The pressure range usually is from 20 to 40 pounds. The air pressure switch governs this phase by starting and stopping the pumps. For example, suppose a pressure of 70 pounds is required to supply the water needs of a building. The centrifugal pump would operate until a predetermined maximum pressure of 100 pounds was attained. The pressure range would then be 30 pounds. This would permit the system to deliver water until the 30 pounds of pressure was spent. The installation would not require further use of the pumps until the system was again under the minimum pressure of 70 pounds.
The air compressor need be operated only when a part of the air contained in the tank has been eliminated.
Distribution Piping. The design of the distribution piping for the pneumatic air pressure system of water supply is identical with that described in the preceding chapter.
OVERHEAD FEED SYSTEM
An overhead system of water supply is an installation which distributes water to the plumbing fixtures by gravity, Fig. 217. It is probably the oldest method of water distribution and dates back to the early Roman aqueducts. It is commonly used in municipalities where the source of water is taken from a drilled well, because a pneumatic pressure system for municipal use would be impractical.
Overhead feed systems of water distribution were the first practical installations used in buildings of extreme height. Although the overhead supply is becoming obsolete, many architects and building designers still favor it. Many large buildings are supplied with water by this method.
There are several disadvantages in an overhead system of water supply. The stored water is subject to contamination because the tank is of the open type. The installation requires more material and labor, thus increasing the cost, and the equipment is distributed over the entire building, utilizing valuable space.
