ENGINEERING MANUAL OF AUTOMATIC CONTROL
INDIVIDUAL ROOM CONTROL APPLICATIONS
402
Fig. 5. Variable Air Volume Single-Duct Air Terminal Unit.
Figure 5 displays the primary supply air temperature and static
pressure. These values ar e data points in the AHU digital
contr oller and not ph ysical points a t the VAV box. They will
var y when measur ed at the VAV boxes. When a BMS user looks
at a VAV box display, it is usuall y because a pr oblem exists.
The fan system suppl y air temper atur e and sta tic pr essur e ar e
necessary data for problem analysis.
Variable Air Volume ATU with Electric Reheat
The VAV ATU with electr ic reheat in F igur e 6 adds r eheat to
Figur e 5. Figure 6 also shows four bo xes on one displa y. The
cooling PID varies the box airflow from maximum to minimum
as the cooling load drops to maintain setpoint. If the cooling
load dr ops fur ther , the space temper atur e dr ops thr ough a dead
band until the heating setpoint is reached, at which point the
heating unit is contr olled to maintain the hea ting setpoint. When
electric reheat is used, the heating PI may be modified with
software heat anticipa tion to ad vance the hea ter sta ges off
command to minimize wide swings in space temperature which
would result from simple temperature sensed control.
Although F igur es 3, 4, 5, and 6 sho w several inputs, outputs,
and setpoints, man y more par ameter s are available and
necessar y to allow technicians to ada pt the standar d pr ogram
to a unique r oom and set of equipment. These ad ditional
parameters, usually addressable and commandable from the
BMS or portable terminal, include the cooling loop PI gains,
the heating loop PI gains, reheat airflow setpoint, unoccupied
space temperature setpoint, standby temperature setpoint,
temper atur e sensor calibr ation offset, bypass push-b utton sta tus,
bypass push-b utton o ver ride time in min utes, smoke pur ge mode
% of maximum airflow setpoint, smoke pressurization mode
% of maximum airflow setpoint, and unoccupied mode % of
maximum airflow setpoint.
NOTE: On-off electr ic heat contr ol often r equir es a minim um
air velocity above that of the minimum cooling airflow
to pr event excessive heating element temper atur es.
Constant Air Volume ATUs
The constant air v olume (CAV) ATUs ar e similar to the
pr essure independent VAV units, except the y contr ol at a single
air volume setpoint. They usuall y include r eheat coils, an
associated room sensor that controls the heating, and are often
found connected to VAV AHUs which also ha ve VAV boxes
and the associa ted var ying duct sta tic pr essur es. CAV units ar e
used wher e the r oom suppl y air v olume m ust be maintained a t
a constant v alue.
Induction VAV ATU
The induction VAV ATU (Fig. 7), uses induced r etur n air as
the initial r eheat medium. Induction VAV ATUs ar e usuall y
installed a bove the ceiling and dr aw retur n air fr om the plen um
created b y a false ceiling. The contr oller r esets the pr imar y air
airflow control setpoint in unison to opening the return air
damper s per the oper ation cycle shown in F igure 7. As the
dampers open, more return air is induced into the unit and
recirculated into the space.
M15306
24
13
0.35
52
0.165
1.445 0.543 24
MINIMUM
AIRFLOW
SETPOINT
(m
3
/s)
kPa
MAXIMUM
AIRFLOW
SETPOINT
(m
3
/s)
CURRENT
AIRFLOW
(m
3
/s)
CURRENT SPACE
TEMPERATURE
SETPOINT
CURRENT SPACE
TEMPERATURE
AIRFLOW
ZERO
ADJUSTABLE
MINIMUM
AIRFLOW
ADJUSTABLE
MAXIMUM
AIRFLOW
LOW
HIGH
SPACE COOLING LOAD
PERCENT
OPEN
PRIMARY
AIR
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