Variable Air Volume Atu - Honeywell AUTOMATIC CONTROL Engineering Manual

Figure 4 shows a reheat coil added to a throttling VAV ATU.
In this application, the temperature controller sequences the
operation of the damper actuator and the control valve or reheat
coil. The coil can be replaced by single or multiple stages of
electric resistance heat. The damper modulates under PI control
at the cooling setpoint and the hot water valve modulates under
a separate PI control at the heating setpoint, with a deadband
between heating and cooling. In this example, the space
occupant can adjust the wall module temperature cooling
setpoint (displayed as SPACE TEMPERATURE in the setpoint
schedule) and the BMS operator can adjust for the deadband
between cooling and heating.
PRIMARY
AIR
PERCENT
74
OPEN
CURRENT
75
TEMPERATURE
24 75
MINIMUM SPACE
DAMPER TEMPERATURE
POSITION
SETPOINT SCHEDULE
OPEN
VALVE
ACTUATOR
POSITION
REHEAT
POSITION
CLOSED
HIGH LOW
SPACE
HEATING LOAD
HEATING SETPOINT
Fig. 4. Throttling VAV Air Terminal
Unit with Reheat Coil.
RHC
00
PERCENT
OPEN
-2.5 77
HEATING MAXIMUM
DEADBAND DAMPER
POSITION
MAXIMUM
POSITION
DAMPER
MINIMUM
POSITION
HIGH
LOW
SPACE
COOLING LOAD
DEADBAND
COOLING SETPOINT
M10538
INDIVIDUAL ROOM CONTROL APPLICATIONS
An economical alternative to reheating air that has been
cooled or reducing the reheat requirement is to reset the setpoint
of primary air in the central fan system conditioning section.
Primary air reset limits are usually required to assure adequate
dehumidification.
The VAV ATU configurations in Figures 5 through 9 are
pressure-independent.

Variable Air Volume ATU

The variable air volume ATU (Fig. 5) is a pressure-
independent unit which delivers the required air volume to the
space regardless of the supply static pressure. The amount of
air delivered is derived from a space temperature PI loop. A
flow sensor in the supply airflow modulates the damper actuator
to control air volume. The room sensor resets the airflow
setpoint as the space thermal load changes. The airflow control
loop can be set to maintain minimum airflow at light load
conditions, while, maximum airflow can be set to limit flow to
meet design conditions. A single wall module can be directed
to multiple controllers to control multiple ATUs with the same
or differing volume ratings.
405 ENGINEERING MANULA OF AUTOMATIC CONTROL