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

INDIVIDUAL ROOM CONTROL APPLICATIONS

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.
DAMPER
PRIMARY
AIR
1
DAMPER
2
ACTUATOR
3
DC
DIGITAL
CONTROLLER
8
T
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.
ENGINEERING MANULA OF AUTOMATIC CONTROL
SUPPLY
AIR
24 VOLT
POWER
TRANSFORMER
4
LINE VOLTAGE
5
POWER
DIGITAL
6
COMMUNICATIONS BUS
OPTIONAL
7
ROOM SENSOR/
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 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.
PRIMARY
AIR
24
MINIMUM
DAMPER TEMPERATURE
POSITION
SETPOINT SCHEDULE
OPEN
DAMPER
POSITION
MINIMUM
POSITION
CLOSED
LOW
SPACE COOLING LOAD
Fig. 3. Throttling VAV Air Terminal Unit.
404
PERCENT
74
OPEN
CURRENT
75
TEMPERATURE
75 77
SPACE MAXIMUM
DAMPER
KNOB POSITION
MAXIMUM
POSITION
HIGH
M10537