Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANULA OF AUTOMATIC CONTROL
INDIVIDUAL ROOM CONTROL APPLICATIONS
404
SINGLE-DUCT VAV ATU
System Configuration
Figure 2 is a schematic of the equipment shown in the control
diagram Throttling VAV ATU in Figure 3. The elements are:
1. Flow control damper.
2. Damper actuator, usually electric, bidirectional.
3. A digital controller.
4. A low voltage control transformer.
5. A line voltage power source.
6. A digital communications bus for control and
BMS functions.
7. An optional bus connection to the space wall module for
easy access from a portable control terminal
8. A wall module.
In this pressure dependent application, the damper has
minimum and maximum software control positions. In other
applications, the controller can have connections to the air flow
rings, reheat valve or electric heat contactors, occupancy sensor
input, and fan relays.
The wall module contains any combination of temperature
sensor, setpoint adjustment knob, after hour override push
button, occupancy status LED, or an LCD display and keypad.
Fig. 2. Single-Duct VAV ATU
In a single-duct VAV ATU, a temperature controller connected
to a damper actuator controls the volume of cool air delivered
to the space. A reheat coil may be added, in which case the
controller sequences the operation of the damper and the valve
or contactor. On a fall in space cooling load, the damper goes
to minimum position before any heat is provided. A single-
duct VAV system may also have a separate perimeter heating
system such as a finned-tube radiator or fan coil units to satisfy
heating requirements.
DAMPER
PRIMARY
AIR
SUPPLY
AIR
DAMPER
ACTUATOR
DIGITAL
CONTROLLER
DC
24 VOLT
POWER
TRANSFORMER
LINE VOLTAGE
POWER
DIGITAL
COMMUNICATIONS BUS
OPTIONAL
ROOM SENSOR/
WALL MODULE
3
2
1
4
5
6
7
8
T
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 wall module. With this option, the BMS
operator may assign software limits to the set value. Values
may be set outside the limits but the control program ignores
them and operates at the limits set in software. Another option
frequently used in open and public spaces is to have no setpoint
knob on the module and to have a single software setpoint
displayable and commandable from the BMS.
M10537
75
74
24 75 77
MINIMUM
DAMPER
POSITION
MAXIMUM
DAMPER
POSITION
CURRENT
TEMPERATURE
SPACE
TEMPERATURE
KNOB
SETPOINT SCHEDULE
OPEN
DAMPER
POSITION
CLOSED
MINIMUM
POSITION
MAXIMUM
POSITION
LOW
HIGH
SPACE COOLING LOAD
PERCENT
OPEN
PRIMARY
AIR
Fig. 3. Throttling VAV Air Terminal Unit.