Mixed Air Control - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

MIXED AIR CONTROL

Functional Description
6
20
9
Item
No. Function
1-3 Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
4 MA temperature maintained by modulating
mixing dampers.
5,6 OA and EA dampers close and RA damper
opens when fan is off, and modulate for
temperature control when fan is on.
7 Setpoint for MA temperature control.
8 Damper control and setpoint for minimum
ventilation damper position.
NOTE: This is not 22% OA or OA damper
open 22%, it is the control program
output value necessary to position
the OA damper such that the design
OA airflow is maintained.
9 OA temperature, operator information.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
EA
83
OA
63
5
AIR HANDLING SYSTEM CONTROL APPLICATIONS
RA
3
ON
NORMAL
ON
1
CONTROL
PROGRAM
22
8
OA MINIMUM
SETPOINT
FEATURES
1. The proper proportions of OA and RA, above minimum
OA setting, are admitted to prevent the MA temperature
from dropping below the desired MA temperature.
2. A minimum quantity of OA, determined by the setting
of the minimum position signal is assured at all times.
CONDITIONS FOR SUCCESSFUL OPERATION
1. Adequate mixing of OA and RA, which may be obtained
using a special mixing chamber.
2. The temperature sensor is located where the air is
thoroughly mixed. The discharge of the fan in a blow-
through system usually provides adequate mixing.
LIMITATIONS
1. If the manual positioning value is set to admit a large
quantity of OA, and the OA temperature falls below the
temperature at which MA temperature controls require
only minimum OA, a source of heat may be necessary to
maintain the MA temperature.
2. During periods of high OA temperature, 100% OA may
be undesirable.
213
2
MA
16
4
16
7
M15153