Trane SYS-APM001-EN manual System Controls, Number of chillers to operate

System Controls

high. The control point is selected to minimize over-pressurizing the system and to assure adequate flow at all critical loads.

Critical valve reset (pump pressure optimization)

Often, pumps are controlled to maintain a constant-pressure differential at a remote coil. Unless this coil serves the zone requiring the most pressure (the critical zone), the pump provides more pressure than necessary and consumes more power than necessary. Systems that have integrated airside and chiller plant control systems and contain valves with direct digital controls present an opportunity for further pump operating-cost reduction. If a system is monitored to determine the critical valve at each point in time, the pump’s operating pressure can be reset so that the critical zone’s control valve is nearly wide open.

Integrated control allows a system-level controller to determine the critical zone and reset the pump’s setpoint dynamically, therefore reducing pump operating costs.

Air-handler controllers know the position of their individual valves, which modulate to maintain the required water flow through the coil. The building automation system continually monitors the valve controllers, looking for the most open valve. The controller resets the pump setpoint so that at least one valve, the one requiring the highest inlet pressure, is nearly wide open. The result is that the pump generates only enough pressure to get the required water flow through this “critical” valve, thus saving pumping energy.

Addendum ak to ASHRAE 90.1-2007 now requires pump pressure for many chilled water systems. It will become part of 90.1-2010.

Number of chillers to operate

Some practitioners believe that operating more chillers at the same time permits each chiller to operate more efficiently. From a system perspective, this is rarely true. The chillers are not the sole energy consumers in the plant—activating a chiller also activates the condenser water pump, perhaps a chilled water pump, and tower fans. One analysis showed that if all operating chillers are equipped with variable frequency drives (VFDs), the only time it makes sense to run more chillers than necessary is if the condenser water drops below 65°F. Without VFDs on the chillers, the system never used less energy with the extra chiller operating.

88 Chiller System Design and Control SYS-APM001-EN