COLUMN ENGINEER'S NOTEBOOK to recover a majority of the additional pumping energy. This allows the potential energy from the hydraulic back pressure to drive a reaction turbine, which, in turn, can assist in driving the main distribution pumps. This technology should be considered when the back-pressure head is greater than 50 ft (16 m). Figure 3 shows various elevation scenarios that can be encountered when connecting atmospheric TES tanks in chilled water distribution networks. Scenario 1 shows that when the chilled water TES tank water level is above the entire chilled water piping system, no additional back-pressure control is required since the atmospheric pressure on the TES tank is pressurizing the piping system. Scenario 2 illustrates when all or most of the chilled water piping system is above the TES tank water level. This can happen when the TES tank is below grade or the central plant and TES tank are located at a lower elevation than the buildings served. Under this scenario, it is typically preferred to control back pressure at a single location near the TES tank. The pressure drop from this back-pressure device will result in additional pumping head anytime the system is operating and must be taken into account when calculating pump head. Scenario 3 is the most energy efficient when only some of the connected chilled water distribution system is higher than the TES tank water level. It is optimal to only control back pressure in those buildings where the piping is higher than the TES tank water level, thus reducing the total system pumping energy required to control back pressure. Summary Chilled water thermal storage can provide many energy cost benefits when implemented in chilled water systems. Placing an atmospheric chilled water thermal energy storage tank in a chilled water system will have several engineering ramifications on the hydraulic performance of the system. Understanding these hydraulic implications and designing the optimum solutions for addressing these implications will improve both control and energy performance. References 1. Hyman, L. 2011. Sustainable Thermal Storage Systems: Planning, Design, and Operation. New York: McGraw Hill. 2. ASHRAE. 2013. District Cooling Guide. www.info.hotims.com/54425-36 FEBRUARY 2015 ashrae.org ASHRAE JOURNAL 43