ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
216
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.
CONDITIONS FOR SUCCESSFUL OPERATION
Local weather data analysis needed to determine the optimum
changeover setpoint. The analysis need only consider data when
19.5
OA
MA
RA
EA
15.5
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
ECONOMIZER
STATUS
ENTHALPY
kJ PER KILOGRAM
M15155
4
7
5
9
3
6
83
8
1 2
83
CONTROL
PROGRAM
1 = AUTO BASED UPON OA
ENTHALPY BELOW kJ/kg
22.0
2 = AUTO BASED UPON OA DB
TEMPERATURE BELOW DEGREES
3 = MANUAL, ECONOMIZER ON.
4 = MANUAL, ECONOMIZER OFF.
NOTE: THE SYSTEM IS LOCKED OUT OF THE ECONOMIZER
MODE ANYTIME THE OA DRY BULB IS
ABOVE DEGREES.
9
52
27.5
the OA is between approximately 16°C and 25.5°C, and during
the occupancy period.
NOTE: The dry bulb economizer decision is best on small
systems (where the cost of a good humidity sensor cannot
be justified), where maintenance cannot be relied upon,
or where there are not frequent wide variations in OA
RH during the decision window (when the OA is
between approximately 16°C and 25.5°C).
SPECIFICATIONS
A global economizer function shall be provided to switch all
AHUs from OA cooling to minimum OA based upon an OA
temperature setpoint. Software shall also be provided to allow
the user to override the decision from being based upon OA dry
bulb (with an appropriate commandable setpoint), to manually
lock the system into or out of the economizer mode.
ECONOMIZER CYCLE DECISION—OUTDOOR AIR ENTHALPY CONTROL
Functional Description
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