DDC vs. Conventional Pneumatic Controls

Sold-state sensors and controllers used in DDC systems have considerable energy-efficiency advantages over conventional pneumatic systems. Substantial advantages are realized in calibration and maintenance, but the critical value lies in the accuracy and reliability of the DDC systems. These features can yield operational energy savings of 15% and greater when compared to the conventional pneumatic system. The inherently precise positioning of valves and dampers with EMCS control loops and blocks are responsible for these energy savings. For instance, to realize the energy-savings opportunity offered by VAV fume hoods, an 8:1 turndown ratio of the exhaust air flow is required. Pneumatic airflow systems typically lose accuracy at 25% of their span, resulting in the capability of only a 4:1 turndown ratio. Solid-state DDC systems provide the degree of precise air-flow measurement and control that enables the operation of these VAV systems.

DDC systems can accurately modulate VAV boxes, control dampers, valves and other mechanical equipment with electronic operators. However, pneumatic operators are still used for certain components in air-handling systems. When a large amount of torque is required to position large air-handling unit dampers and control valves, cost-effective, reliable electronic operators may not be available. Control air systems give the system designer the option of using the combined powerful control capabilities of a DDC system with inexpensive, reliably operating conventional pneumatic valve and damper control systems. [Ruys, 1990]