Effective partial-load condition strategies
Because buildings don’t always (or typically ever) run at their peak design demand, taking advantage of a variety of common technologies and systems can help minimize the energy and cost expenditure of any facility’s daily operations.
Series temperature sensors, pressure sensors, flow measuring stations, flow meters, control valves, modulating dampers, variable frequency drives (VFDs), and electronically commutated motors (ECM) for fans and pumps are just a few examples of strategies that can be employed to bridge the gap between the peak mechanical, electrical, plumbing (MEP), and fire protection design and actual systems usage.
While a detailed description of the intricacies of VFD design is outside the scope of this article, it has fundamental pertinence due to its influence on system flow reduction and system pressure modulation. The VFD adjusts motor speed, which reduces the motor’s power consumption in reaction to system flow, applicable to major pumps and main fans of a building’s HVAC systems. Rather than a full horsepower motor, an “inverse duty” motor with a VFD would be trimmed to operate at reduced horsepower.
Theoretically, the power savings can be calculated from the aforementioned affinity law for power. Figure 1 illustrates VFD motor speed at a reduced system flow and head pressure. This control application trims down multiple subsystems in the building. Today’s VFDs are applicable to most major pumps and fans and have become a common addition to any large-capacity building.
Suzan X. Sun-Yuan is a senior associate and lead mechanical engineer with Environmental Systems Design. She has experience with super-tall buildings, food labs, and central plant designs and upgrades. Mohsen Aghai is a senior associate and lead electrical engineer with Environmental Systems Design. He has experience with super-tall, mixed-used, commercial, and governmental buildings as well as hospitals and central plant designs and upgrades.