Flow Tracking Control System - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

FLOW TRACKING CONTROL SYSTEM

Figure 4 provides a simplified hardware schematic of a
variable volume supply and return fan system with flow stations
in the supply and return to control the return fan volume to
match the supply fan volume less any exhaust volume. A volume
differential is maintained between the supply and return to
pressurize the building. There may be only one such system in
a building or there may be many. The supply fan static pressure
setpoint, and thus the output volume, may be reset by the VAV
EA
RA FAN
STATUS
OA
MA MA
DAMPER TEMPER-
ATURE
ENGINEERING MANUAL OF AUTOMATIC CONTROL
RETURN FAN
CT VSD
RA
RA FAN
RA
TEMPER-
SPEED
FAN
ATURE
SUPPLY FAN
VSD
HEATING COOLING SA SA FAN
VALVE VALVE FAN SPEED
SA FAN
STATUS
Fig. 4. HVAC Flow Tracking.
INDOOR AIR QUALITY FUNDAMENTALS
terminal box requirements that the fan supplies. Typically there
are 30 to 70 VAV terminal units per fan system. The sample
VAV terminal unit shown is a fan powered unit but the units
could be non-fan powered. Without an air flow station in the
outdoor air, this system might not deliver the minimum air flow
at all fan volumes, because the mixing box negative static
pressure and therefore the OA airflow varies with the supply
fan load. If mixed air temperature is used to calculate the
percentage of outdoor air, as the outdoor temperature
approaches 10 C the margin of error increases because of the
narrowing differential between outdoor air and mixed air.
RA
F
RA FLOW
TEMPERATURE
SIGNAL
SA
F
CT
SA SA
TEMPER- PRESSURE
ATURE
SA FLOW
SIGNAL
165
OA
VAV TERMINAL BOX
F
REHEAT
AIR FLOW
HOT WATER
FLOW DAMPER
VALVE
ROOM
TEMPER-
ATURE
M10486