Monday, April 7, 2008

New York City Rooftop Water Tanks & Pneumatic Systems

Many New Yorkers do not think about how they get water in high-rise buildings for drinking, bathing, and mechanical uses, such as cooling towers and supplying HVAC equipment. Rooftop water tank systems, left, started being used in the late 1800s and early 1900s. They combined with constant-speed pumps that operated by a level switch in the tank and when the level in the tank would approach a pre-determined height, the pumps would either turn on to pump more water to the
tank or turn off because the tank was full. In winter the tanks have internal heaters to prevent freezing. In the 1950s, pneumatic pressure tank systems, left, started replacing many roof tank systems.

Building owners now use state-of-the-art variable-speed control in choosing water pressure systems or booster systems. This significantly cut energy bills from the old 50's constant speed systems that had little to no controls. Variable-speed water pressure systems use a transducer to sense pressure and automatically adjust the speed of the pump in order to maintain a constant discharge pressure regardless of demand or flow.

As city water mains age, their ability to deliver water pressure to buildings is reduced, which is why most multi-story buildings need a booster pump system to pressurize water on upper floors. Typically, a pressure of 40 psi at the top of a building is ideal. Multistage centrifugal and turbine pumps, right, are generally used for high head applications. The pumps’ multistage design affords high efficiency on low-, medium- and high-flow systems. The diagram at left is of a pneumatic tank pump system. (Source: PMEngineer, Paul Larson P.E.)

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