Single-Duct Vav Atu; System Configuration; Throttling Vav Atu - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS

SINGLE-DUCT VAV ATU

System Configuration

Figur e 2 is a schema tic of the equipment sho wn in the contr ol
dia gram Thr ottling VAV ATU in F igur e 3. The elements ar e:
1. Flow contr ol damper .
2. Damper actua tor, usuall y electr ic, bidir ectional.
3. A digital contr oller.
4. A low voltage contr ol tr ansformer.
5. A line volta ge power sour ce.
6. A digital comm unica tions b us for contr ol and BMS
functions.
7. An optional b us connection to the space w all module f or
easy access from a portable control terminal
8. A wall module .
In this pressure dependent application, the damper has
minim um and maxim um softw ar e contr ol positions. In other
applica tions, the contr oller can ha ve connections to the air f low
rings, reheat valve or electr ic heat contactor s, occupanc y sensor
input, and f an r elays.
The wall module contains an y combina tion of temper atur e
sensor, setpoint adjustment knob , after hour o ver r ide push
button, occupanc y status LED , or an LCD displa y and k eypad .
DAMPER
PRIMARY
AIR
1
DAMPER
2
ACTUATOR
3
DC
DIGITAL
CONTROLLER
8
Fig. 2. Single-Duct VAV ATU
In a single-duct VAV ATU, a temper atur e contr oller connected
to a damper actua tor contr ols the volume of cool air deli vered
to the space . A r ehea t coil ma y be ad ded , in which case the
contr oller sequences the oper ation of the damper and the v alve
or contactor . On a f all in space cooling load , the damper g oes
to minim um position bef ore an y hea t is pr ovided . A single-
duct VAV system ma y also ha ve a separ ate per imeter hea ting
system suc h as a f inned-tube r adia tor or f an coil units to sa tisfy
heating requirements.
SUPPLY
AIR
24 VOLT
POWER
TRANSFORMER
4
LINE VOLTAGE
5
POWER
DIGITAL
6
COMMUNICATIONS BUS
OPTIONAL
7
ROOM SENSOR/
T
WALL MODULE
M10520

Throttling VAV ATU

The throttling VAV ATU (Fig. 3) is the simplest and least
expensive ATU. A room controller controls the operation of
the damper actuator using PI control. The throttling VAV ATU
usually has software minimum and/or maximum damper
position limits for limiting air volume. Because the unit is
pressure dependent, volume at any given damper position varies
with the inlet duct static pressure. Maintaining a stable duct
static pressure at the end of the duct run is important for proper
operation. Proper setting of the minimum-flow limit is essential
for adequate circulation and is properly set with all boxes
operating at their minimum damper positions. At this operating
point, the duct static pressure near the fan will be lower than
at full load. If the minimum damper positions are each set
with other boxes at their maximum damper positions, when
the load decreases and the pressure drops near the fan,
inadequate circulation results at boxes near the fan.
Compromises in minimum and maximum airflow control on
pressure dependent systems result if the loads vary. Care must
be exercised in set-up and balancing to assure that all zones
get no less than minimum air flow at all loadings and that all
zones demanding maximum air flow get no less than maximum
air flow at all loadings.
The space temperature setpoint is the value set by the
occupant on the w all module . With this option, the BMS
oper ator ma y assign softw ar e limits to the set v alue . Values
may be set outside the limits b ut the contr ol pr ogram ignor es
them and oper ates at the limits set in softw ar e. Another option
frequentl y used in open and pub lic spaces is to ha ve no setpoint
knob on the module and to ha ve a single softw ar e setpoint
displayable and commandable from the BMS.
PRIMARY
AIR
24
MINIMUM SPACE
DAMPER TEMPERATURE
POSITION KNOB
SETPOINT SCHEDULE
OPEN
DAMPER
POSITION
MINIMUM
POSITION
CLOSED
LOW
SPACE COOLING LOAD
Fig. 3. Throttling VAV Air Terminal Unit.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
400
PERCENT
74
OPEN
CURRENT
24
TEMPERATURE
24 77
MAXIMUM
DAMPER
POSITION
MAXIMUM
POSITION
HIGH
M15304