Abbreviations; Air Terminal Unit Control; Variable Air Volume Atu; Pressure-Dependent And Pressure-Independent Atu - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

ABBREVIATIONS

The following abbr eviations ar e used:
BMS — Building Management System
ATU — Air Terminal Unit
VAV — Var iable Air Volume
ZEB — Zer o Ener gy Band
DDC — Direct Digital Control
CAV — Constant Air Volume

AIR TERMINAL UNIT CONTROL

Air ter minal units (A TUs) r egula te the quantity and/or
temper atur e of conditioned air deli ver ed to sa tisfy the
temper atur e requir ements of a r oom or space . ATUs ar e
classified b y air handling system design and ar e availa ble in
man y configur ations. F or inf ormation on the air suppl y to ATUs,
refer to the Air Handling System Contr ol Applica tions section
and the Building Airflow System Contr ol Applica tions section.
ATU contr ols can be as basic as a r oom sensor contr olling a
damper or as comple x as a r oom sensor and an airf low sensor
oper ating a damper and a r ehea t coil valve. In all cases, an
individual r oom contr ol applica tion contr ols the en vironment
of the r oom or space . Figur e 1 shows basic indi vidual r oom
contr ol with an ATU.
CONDITIONED
AIR
FAN
DAMPER
ACTUATOR
ATU
CONTROLLER
GRILLE
WALL
MODULE
SPACE 1
Fig. 1. Basic Individual Room Control.
Table 1. AHU Operating Modes and Associated ATU Airflow Settings.
Mode
Smoke Purge Mode
Thermal or IAQ Pre-Purge
Preoccupancy Cool Down
ENGINEERING MANUAL OF AUTOMATIC CONTROL
TO OTHER
SPACES
ATU
CONTROLLER
GRILLE
WALL
MODULE
SPACE 2
C2399-1
Command
All associated boxes to 82%
maximum airflow.
All the boxes to 50% maximum
airflow.
Perimeter boxes to 50%
maximum airflow and the interior
boxes to 80% maximum airflow.
INDIVIDUAL ROOM CONTROL APPLICATIONS

VARIABLE AIR VOLUME ATU

A var iable air v olume (VAV) ATU contr ols the cooling of a
space b y var ying the amount of conditioned air supplied r
than c hang ing the temper atur e of the conditioned air . Most VAV
systems pr ovide ener gy savings b y reducing the load on the
central fan when cooling loads are less than design and avoid
reheating cooled air .
A system designed to r educe overall system airf low as ATUs
shut do wn offer s the greatest ener gy savings because system
fans and f an motor s oper ate at lighter loads. These par agraphs
descr ibe the oper ation of VAV ATUs in single-duct and dual-
duct air handling systems.
PRESSURE-DEPENDENT AND
PRESSURE-INDEPENDENT ATU
A Pr essur e-dependent ATU is affected b y chang ing duct sta tic
pr essur es. It ma y have mechanical minim um and maxim um
airf low limits.
A pr essur e-independent ATU automa tically adjusts to duct
static pr essur e chang es because it contains airf low sensor s and
contr ollers to compensa te for pr essur e chang es. When used with
DDC systems, the pr essur e-inde pendent ATU ma y be
programmed to change the airflow delivered based on the
oper ating mode . Table 1 descr ibes some modes with the
associated command and comments on the reasoning.
AHU was sized with diversity such that the total box
maximum airflows exceed the fan design airflow.
Take advantage of the fan motor power cube root
relationship to airflow (fan runs twice as long at one
eighth the motor power).
Reduce the fan energy per cubic meter of air delivered
by half, match the fan-to-chiller load for better chiller
efficiency and for better dehumidification.
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