AQ: Frequency Inverter Direct Digital Control
Modulating Supply & Return Fans are used as a means of providing proper variable air volume (VAV) control as well as building pressurization. Many such VAV systems are still largely pneumatic with static to the downstream boxes being maintained by inlet guide vanes. To provide increased energy savings and energy comfort, these systems can be easily converted to frequency inverter fan control of the supply and return fans and Direct Digital Control (DDC) to coordinate any increased energy saving strategies. Figure 1 shows such a system.
To increase energy savings, the DDC controller can be programmed to reduce the flow from the return & supply fans for short periods of time. Coordinated with the building pressurization system, any temporary loss of space temperature may be avoided.
In Figure 1, the supply fan is controlled by the duct static pressure sensor, via the DDC, while the outside air and mixed air dampers are optimized to provide economizer control.. The return fan is modulated to stabilize building pressure at a slight positive. For simple supply and exhaust systems the building pressure and static pressure sensors may be connected directly to the frequency inverter with an internal PID controller.
Typical Energy Savings are realized from converting pneumatic (or electromechanical) control to DDC control with frequency inverter in the following ways:
- Locking inlet guide valves mechanically open to allow the frequency inverter to fully modulate the fans.
- Free cooling by accurately modulating the economizer dampers and sequencing the mechanical equipment.
- Controlling static and resetting the static pressure during short periods of time.
- Accurate building pressurization.
- Implementing other energy saving measures which include supply air reset, and night purge routines.
CONTROL CONSIDERATIONS
- Placement of the indoor static pressure sensor is important as it should provide a stable signal. Entrances, dock, and other areas where large , sudden static pressure changes may occur should be avoided.
- The outside reference static tip should be shielded from wind and rain.
- When the exhaust fan is frequency inverter controlled, consider a 2-position air damper to prevent the outside air from entering the building (infiltration) when the exhaust fan is off or a very low speeds.
- For simple VAV systems, consider using frequency inverters with built in PID controls such as the Iacdrive frequency inverters.. This minimizes hardware and installation costs. Static sensors provide a 0-10vdc control signal directly to the frequency inverter.
- Duct mounted static pressure sensor should be mounted 2/3 of the distance of the distribution system.
