Economizer Cycle Decision-Outdoor Air Enthalpy Control - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

AIR HANDLING SYSTEM CONTROL APPLICATIONS
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
ECONOMIZER CYCLE DECISION—OUTDOOR AIR ENTHALPY CONTROL
Functional Description
EA
8
OA
1 2
9
52
19.5
3
ENTHALPY
kJ PER KILOGRAM
4
NOTE: THE SYSTEM IS LOCKED OUT OF THE ECONOMIZER
83
7
83
CONTROL
PROGRAM
ECONOMIZER
ON
STATUS
ECONOMIZER MODE SELECTOR
1
1 = AUTO BASED UPON OA
15.5
ENTHALPY BELOW
2 = AUTO BASED UPON OA DB
22.0
TEMPERATURE BELOW
3 = MANUAL, ECONOMIZER ON.
4 = MANUAL, ECONOMIZER OFF.
MODE ANYTIME THE OA DRY BULB IS
27.5
ABOVE DEGREES.
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.
RA
MA
OTHER INPUTS
AND OUTPUTS
22
6
9
OA MINIMUM SETPOINT
(NOTE: THE TEST AND BALANCE INITIAL
5 VALUE FOR PROPER VENTILATION IS 22)
kJ/kg
DEGREES
M15155
ENGINEERING MANUAL OF AUTOMATIC CONTROL
216