ENGINEERING CMANUAL OF AUTOMATION CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
215
49
OA
MA
RA
EA
72.0
1
22
ON
ECONOMIZER MODE SELECTOR
OA MINIMUM SETPOINT
(NOTE: THE TEST AND BALANCE INITIAL
VALUE FOR PROPER VENTILATION IS 22)
OTHER INPUTS AND OUTPUTS
M10454
2
7
3
1
4
83
6
5
83
CONTROL
PROGRAM
1 = AUTO BASED UPON OA DB
BELOW DEGREES.
2 = MANUAL, ECONOMIZER ON.
3 = MANUAL, ECONOMIZER OFF.
ECONOMIZER
STATUS
ECONOMIZER CYCLE DECISION
Where 100% outdoor air economizer cycles are included with air
handling systems, the decision of when to switch to the economizer
mode is usually made automatically based upon the following criteria:
— The outdoor air conditions.
— The return air conditions or assumed conditions.
—The size and geographical location of the AHU.
— Cost.
— The users ability to understand control strategy and maintain
the humidity sensors.
The economizer decision does not enable or disable chiller periods
of operation. Chillers are generally enabled anytime chilled water valves
open. At economizer changeover, the OA (containing less heat than
the RA) is intended to reduce the load on the cooling coil until no
chilled water is required.
The OA sensors should generally be located at least six feet above
the ground in the shade (North side) in a perforated enclosure away
from exhausts.
Following are several popular strategies with guidelines of when
each is appropriate.
ECONOMIZER CYCLE DECISION—OUTDOOR AIR DRY BULB CONTROL
Functional Description
Item
No. Function
1OA sensor senses OA temperature.
2 Economizer decision mode selector, including OA
temperature setpoint below which the economizer
decision is invoked, and command options.
3 Economizer decision status (operator information).
4 Setpoint for minimum OA damper position.
5 Actuator positions OA and RA dampers.
6 Actuator positions EA dampers.
7 Control program coordinates occupancy,
temperature, smoke, and ventilation controls.
FEATURES
1. Outdoor air is used for cooling (or to supplement the chilled
water system) anytime the OA temperature is below the
economizer setpoint.
2. Stable, accurate, simple, electronic OA temperature sensor
makes reliable economizer decision.
3. Economizer decision may be global and broadcast over the
digital system network to other AHU systems.
4. Operator options for overriding the basic decision.
5. The test-and-balance minimum OA damper position initial
value is provided as text. If the operator adjusts the minimum
OA damper value, there is no longer a point of reference as to
what it should be without this note.
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