Calculations for the fan speed controller using dynamic fan speed control are included, along with explanations for what is being attempting with typical sequences. The specific values will have to be worked out for specific zones as they will exist in actual buildings.
First, it is important to note that diffusers are somewhat better mixers of zone air when the room temperature approaches set point. In other words, when the room is near set point, as zone load lightens (in either heating or cooling mode) and the discharge air temperature from the terminal unit approaches room set point, you really do not need the constant fan volume or the high velocities as designed for full cooling or full heating conditions. Another way to describe this is that as room conditions change (maybe we could call this room diversity) we can adjust the fan operation to take advantage of the energy savings that the ECM allows us.
At full cooling, the fan will be set at 100% of the cooling fan volume. This air volume will be calculated as the volume necessary to satisfy the cooling load in the zone.
At full heating, the fan will be set at 100% of the heating fan volume. Unlike normal series fan powered terminal unit calculations, this fan volume does not have to be the same as the cooling fan volume, but rather will be calculated as that air volume necessary to satisfy the heating load in the zone. Heating airflow can be higher or lower than cooling fan airflow as the climate and building design dictate, as well as other parameters such as type of perimeter diffuser, height of ceiling, curtain wall, and the U factor of the glass. Discharge air temperature should be limited to 78 – 83o F. for optimum diffuser operation and room air mixing when a very good curtain wall is present.
DEADBAND FAN VOLUME
When the room is at set point, the deadband fan volume should be about 50% of the cooling fan volume for typical fan powered applications. Specific applications may vary. The equation for calculating the actual fan volume as the damper modulates to its minimum set point during the cooling sequence is attached. The equation for the heating calculation is also attached, but this equation assumes either a modulating water valve or SCR’s on the electric heater. If there is no modulating heat, the heating fan volume should be constant, or can adjust with each stage of heat to maintain the desired output air temperature from the terminal unit, 78-83 o F.
Source: Nailor Product Bulletin, May 31, 1999