Economizer Operation - York J Series Manual

ECONOMIZER OPERATION

Economizer operation is described in the following
sections. A final section within Economizer operation
description is on Economizer Loading. Economizer
Loading is a variation on the Economizer operation.
Its function is to override the economizer to minimize
SAT swings resulting from turning cooling/heating
stages on and off.
Criteria for Economizer Operation
Economizer operation will be used for control when:
• Economizer - ON.
• Economizer Enable - ON.
• Required sensors are reliable.
• Outside air conditions are suitable per criteria dis-
cussed in the following sections.
Criteria for Economizer Suitable Decision
There are three different methods of Economizer
Operation:
• Differential Enthalpy Method: will be self-
configured and used when sensors for Outdoor
Air temperature, Outdoor Air humidity, Return
Air temperature, and Return Air humidity are all
installed and reliable. This method is the highest
priority when sensors are available.
• Outside Enthalpy Method: will be self-configured
and used only when differential enthalpy method
is not available, and sensors for Outdoor Air tem-
perature and Outdoor Air humidity are installed and
reliable.
• Outside Temperature Method: will be self-con-
figured and used only when differential enthalpy
or outside enthalpy methods are not available, and
sensor for Outside Air temperature is installed and
reliable.
Economizer Suitable Calculations
Outside Temperature Method:
Economizer will be suitable when OAT < (SAT setpoint
+ 10°F). A 2°F hysteresis about the (SAT setpoint +
10°F) will be used. The 10°F offset represents the high-
est outdoor air temperature that the DX cooling can
reliably reduce to the SAT setpoint.
JOHNSON CONTROLS
Outside Enthalpy Method:
Economizer will be suitable when OA Enthalpy <
Outside Enthalpy Number parameter value and OAT <
(SAT setpoint + 10°F). A 2°F hysteresis about the (SAT
setpoint + 10°F) will be used. A 1 BTU/LB hysteresis
about the Outside Enthalpy Number will be used.
Differential Enthalpy Method:
Economizer will be suitable when OA Enthalpy < RA
Enthalpy and OAT < (SAT setpoint + 10°F). A 2°F hys-
teresis about the (SAT setpoint + 10°F) will be used.
A 1 BTU/LB hysteresis about the RA Enthalpy will be
used.
SAT Setpoints Used During Cooling With Econo-
mizer Operation
Under Economizer Operation, the following SAT set-
points will be used:
CV Cooling Mode:
• With a call for first stage cooling, the Economizer
First Stage Setpoint value will be used as the Econ-
omizer SAT setpoint.
• With a call for second stage cooling, Economizer
Second Stage Setpoint value will be used as the
Economizer SAT setpoint.
VAV Cooling Mode:
• With a call for first stage cooling, the VAV High
Temperature Setpoint will be used as the Econo-
mizer SAT setpoint.
• With a call for second stage cooling, the VAV Low
Temperature Setpoint will be used as the Econo-
mizer SAT setpoint.
SAT control with Economizer
The economizer control will be based on a PI, direct
acting algorithm. If economizer is "suitable" (free
cooling is available) and cooling is required (the unit
is not in satisfied state), the algorithm will be active
and modulate the economizer position (outside air and
return air dampers) in order to control SAT to the SAT
setpoint. If the economizer is not suitable, the Econo-
mizer Operation is no longer used and control reverts
to the appropriate economizer not suitable mode of
operation.
FORM 100.50-NOM7 (808)
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