High-rise
continued from page 57
poses of distributing water, or waste outside pressure added or subtracted; 144 to get the pressure in square inch- water, efficiently. Sanitary, waste, Pressure = Depth x Density es: and storm systems rely on gravity Water has a density of 62.4 pounds 62.4 lbs/Cu Ft 144 Sq In = whenever physically possible. per cubic foot. If this is a perfect 0.43333333 lbs/Sq In (per foot of Gravity is relentless. Pumps are not. cube, one foot high, it would exert a depth) pressure of 62.4 pounds per square See Figure 2. Therefore, we deter- Static pressure foot. One square foot is equal to 144 mine that a column of water, one foot At atmospheric pressure, with no square inches, so we divide 62.4 by high, exerts a pressure of 0.433 PSI. This is called static pressure. It is the result of the force of gravity applied to the column of water. Gravity is relentless and considered to be constant although gravitational fields vary slightly. This variation has no effect for our purposes. The weight of water also varies depending upon its density, which is a factor of its temperature. Water is
The weight of water also varies depending upon its density, which is a factor of its temperature. Water is considered to be non- compressible, so atmospheric pressures have little or no effect on its density.
considered to be non-compressible, so atmospheric pressures have little or no effect on its density. We consid- er the density to be 62.42 pounds per cubic foot, because this is an average density at "atmospheric" tempera- tures and varies little within the nor- mal range of domestic cold water sys- tems. Pressure concerns in high-rise applications Pressure considerations are evident in our calculations for domestic booster pumps and other aspects of our designs. High-efficiency fixtures normally require 35 PSI minimum to "accurately dose" the small amount of water used for flushing. This does not mean that the fixtures will not operate with less PSI but to really be sure we are getting the precise amount of water that we designed to, we need 35 PSI. When we bear in mind that we are allowed to supply a maximum of only
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Page 58/Plumbing Engineer October 2013
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