Variable Air Volume Atu - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

Figur e 4 shows a r eheat coil ad ded to a thr ottling VAV ATU.
In this application, the temperature controller sequences the
oper ation of the damper actua tor and the contr ol valve or r eheat
coil. The coil can be r eplaced b y single or m ultiple sta ges of
electr ic resistance hea t. The damper modula tes under PI contr ol
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 (displa yed as SPACE TEMPERA TURE in the setpoint
schedule) and the BMS operator can adjust for the deadband
between cooling and heating.
PRIMARY
AIR
PERCENT
74
OPEN
CURRENT
24
TEMPERATURE
24 24
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.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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
M15305
INDIVIDUAL ROOM CONTROL APPLICATIONS
An economical alter na tive to r ehea ting air tha t has been
cooled or reducing the reheat requirement is to reset the setpoint
of pr imar y air in the centr al fan system conditioning section.
Primary air reset limits are usually required to assure adequate
dehumidif ication.
The VAV ATU configur ations in F igur es 5 thr ough 9 ar e
pressure-independent.

Variable Air Volume ATU

The var iable air v olume ATU (F ig. 5) is a pr essur e-
independent unit w hich delivers the r equir ed air v olume to the
space r egar dless of the suppl y static pr essur e. The amount of
air deli vered is der ived fr om a space temper atur e PI loop. A
flow sensor in the suppl y airf low modula tes the damper actua tor
to contr ol air v olume . The r oom sensor r esets the airf low
setpoint as the space ther mal load c hang es. The airf low contr ol
loop can be set to maintain minim um airf low at light load
conditions, while, maxim um airf low can be set to limit f low to
meet design conditions. A single wall module can be dir ected
to multiple contr ollers to contr ol multiple ATUs with the same
or dif fering volume r atings.
401