The return fan
speed is controlled to maintain the differential airflow rate between the supply and return fans
at a set point, which is determined by the required building static pressure (Taylor 2014).
All SDVAV AHUs in building A, B, C have return fan
VFD tracking the speed of supply fans.
The second reason is that the return air is highly stratified, even after passing through the return fan
, so a measurement would be meaningless.
The ability to monitor all digital inputs such as damper-receipt proof contacts, safeties, or supply-fan run interlocks to the return fan
over the building automation system through the VFDs I/O.
If it has a return fan
that is controlled by airflow tracking, (4) control the fan with zero offset.
Heat off--disable heating coil Signal cool--enable cooling coil Cool off--disable cooling coil Controller Return on--turn All behaviors-- return fan
on communication to all LRU's enabled.
Like most buildings, the Portland World Trade Center ran the return fan
motors at full power and used dampers to regulate airflow as needed.
So, the return fan
simply needs to be controlled like a supply air fan on a supply air VAV system: maintain enough negative static pressure in the return air duct to allow all return air VAV boxes to meet their airflow setpoints.
Work consists of replacing existing duplex submersible condensate pumps with duplex control panel and float switch with new high temperature condensate pumps and controls, provide new motor and variable frequency drive on the existing return fan
on AHU-1, direct digital controls, and associated mechanical and electrical work.
3 Return fan
fault 08/22 Return fan
fixed at 30% speed - half normal speed.
For Return Fan
systems using Direct Pressure control, the sequence is the same except the relief damper is controlled independently off of building pressure.