ENGINEERING CMANUAL OF AUTOMATION CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
220
MIXED AIR CONTROL WITH ECONOMIZER CYCLE (VENTILATION SYSTEM ONLY)
Functional Description
OA
22
ON
ECONOMIZER DECISION.
REFER TO PREVIOUS
ECONOMIZER OPTIONS
OA MINIMUM SETPOINT
(NOTE: THE TEST AND BALANCE INITIAL
VALUE FOR PROPER VENTILATION IS 22)
M10453
10
12
11
8
CONTROL
PROGRAM
49
83
EA
MA
RA
ON
ON
55
9
5
1
4
2
3
7
6
83
55
NORMAL
PERCENT
OPEN
NC
NO
NC
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 Setpoint value for minimum ventilation
damper position.
9 Determines when OA is suitable to assist in
cooling.
10 Control program coordinates MA, minimum
ventilation, and economizer control of mixing
dampers.
FEATURES
1. The proper proportions of OA and RA are admitted to
maintain the desired MA temperature during economizer
operation periods.
2. A minimum quantity of OA, determined by the software
adjustable setpoint value, is assured at all times.
3. The OA economizer changeover program returns the OA
damper to the minimum position when OA is not suitable.
CONDITIONS FOR SUCCESSFUL OPERATION
1. Adequate mixing of OA and RA. Mixing may be obtained
using a special mixing chamber. The temperature sensor
should be in the fan discharge when possible. The fan in
a blow-through system usually provides adequate mixing.
2. An MA averaging element sensor is used on draw-through
units.
LIMITATIONS
If the manual positioning value is set to admit a large quantity
of OA and the OA temperature falls below the temperature at
which only minimum OA is required for MA temperature
control, a source of heat is necessary to maintain the MA
temperature.
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