COLUMN ENGINEER'S NOTEBOOK ashrae.org FEBRUARY 2015 Load Building D Load Load CHW TES Tank CHWS SCENARIO 2 CHW TES Tank Building D Load Load PSV Building C Load Building B Load Building A CHWR CHWS SCENARIO 3 Building B Building C Building D Load Building A Load PSV Load PSV CHWR CHW TES Tank Load Height Above Tank Water Level ASHRAE JOURNAL Building C CHWR One commonly overlooked engineering criterion when using atmospheric TES tanks is maintaining positive gauge pressure (a pressure above atmospheric pressure) within the entire chilled water system while maintaining the top of the vented tank at atmospheric pressure. This is required to prevent the pressurized water from draining into the atmospheric TES tank, which could not only overflow the tank but also create a vacuum and draw air into the piping at its highest elevations. There are various methods to maintain positive pressure when the TES tank water level is below higher parts of the chilled water system, each with varying pumping energy penalties. One means of control is to use self-contained pressure sustaining valves (PSV). These are typically pilot-operated devices with manually adjusted setpoints. Because the setpoint is manual, it is set to create enough back pressure to pressurize the highest point in the chilled water system. For example, if the highest point in the system is 120 ft (36.6 m) above the FIGURE 3 TES tank water level, the back-pressure setpoint required to maintain 5 psig at the highest point would be 57 psid or 132 ft (40.2 m) head: Back-Pressure Setpoint = 5 psig + 120 ft 2.31 ft/psi Another means of control is using a control valve or parallel control valve arrangement. Note that the flow range will typically vary substantially from low load to peak load and the back-pressure control valve arrangement would be required to operate over the entire range. The pressure on the inlet of the valve assembly 42 Building B Load Building A Height Above Tank Water Level Maintaining Positive Pressure SCENARIO 1 Height Above Tank Water Level the chiller plant(s); * Maintain positive gauge pressure at the highest point in the chilled water system; and * Maintain positive differential pressure between the CHWS and CHWR headers. The objectives of the TES tank pumping system during TES tank discharge are: * Pump the proper amount of chilled water to supply the building loads; and * Maintain positive gauge pressure at the highest point in the chilled water system. CHWS TES tank elevation scenarios. Scenario 1: All connected loads below TES tank water level. Scenario 2: All connected loads above TES tank water level. Scenario 3: Some connected loads above TES tank water level. should be measured and set to allow positive gauge pressure at the highest point in the system being controlled. The type of control valves can vary based on the flow and back pressure required. When most of the chilled water system is above the TES tank water level, the pumping energy required for the back-pressure control can be substantial. A hydro turbine assembly used in lieu of the PSV could be used