Parallel Fan Atu; Series Fan Atu - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS
kPa
0.35 13
0.165
MINIMUM MAXIMUM
AIRFLOW SETPOINTS (m
OPEN
MAX
PRIMARY
AIRFLOW
POSITION
INDUCTION
DAMPER
ZERO
CLOSED
The induction VAV ATU maintains mor e air motion in the
space a t lower loads than a thr ottling VAV ATU. The r ehea t
coil is optional, and ma y be in the induced air or suppl y air.
Parallel F an ATU
The par allel fan ATU (F ig. 8) replaces the induction VAV
ATU damper s with a small centr ifugal fan to r ecirculate r etur n
air a t a constant v olume after pr imar y air decr eases. The fan
acts as the f irst sta ge of reheat after pr imar y flow is at minim um
or can be s witched on a t a lo w pr imar y flow thr eshold w hile
pr imar y flow goes to zero. The r eheat coil is optional.
PLENUM AIR
PRIMARY
VAV BOX
AIR
0.575 0.465
CURRENT
AIRFLOW
3 3
/s)
(m /s)
MIN
FULL
HEATING
LOAD
HEATING SETPOINT
Fig. 7. Induction VAV Air Terminal Unit.
RHC
24 00
58
PERCENT OPEN
0 0
DEADBAND
(ADJUSTABLE)
The oper ation c ycle in F igur e 8 shows tha t when space
temper atur e is below the hea ting setpoint, the pr imar y air f low
is at a minim um, and the f an is on. As the hea ting load g oes to
zero, the r eheat valve closes, then the f an stops. After the space
temperature rises through a deadband, as the cooling load
incr eases, the pr imar y air f low incr eases to maxim um.
Ser ies F an ATU
The ser ies fan ATU (F ig. 9) deliver s a constant v olume of
dischar ge air with a v ar iable volume of AHU suppl y air. In this
ATU, the fan is on contin uously dur ing occupied hour s. Pr imar y
air is modula ted to meet space demand f or cooling . As pr imar y
air damper modula tes closed, mor e plen um air is dr awn in to
maintain a constant disc har ge volume. Optional r eheat can be
sequenced as required.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
404
WALL MODULE
TEMPERATURE
COOLING
SETPOINT
24 24
-1.5
HEATING
DEADBAND
FULL
COOLING
LOAD
COOLING SETPOINT
M15308