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Item
No.
Function
1
Sensor senses OA temperature.
2
Sensor senses OA humidity.
3
OA enthalpy calculated from OA temperature
and humidity.
4
Economizer mode decision selector, including
OA enthalpy setpoint below which the
economizer decision is invoked, and
command schedule.
5
Economizer decision status (operator
information).
6
Setpoint for minimum OA damper position.
7
Actuator positions OA and RA dampers.
8
Actuator positions EA dampers.
9
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 enthalpy is below
the economizer setpoint.
2. OA enthalpy considers total heat and will take advantage
of warm dry low enthalpy OA and will block out cool
moist OA, thus saving more energy than a dry-bulb based
economizer loop.
3. Economizer decision should be global and broadcast over
the digital system network to other AHU systems.
4. Operator options for overriding the basic decision and
for selecting OA DB economizer changeover during
periods of humidity sensor maintenance or failure..
5. The test-and-balance minimum OA damper positioned
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
1. A high quality RH sensor with at least 3% accuracy
should be selected.
2. Periodic maintenance of the humidity sensor is provided.
3. An estimate of the typical return air enthalpy is needed
to determine the optimum changeover setpoint.
NOTE: This OA enthalpy changeover decision is
generally recommended unless the system falls
into the range where OA dry bulb or OA/RA
enthalpy comparison should be considered.
LIMITATIONS
A high dry-bulb limit setpoint should be included to prevent
the enthalpy decision from bringing in air too warm for the
chilled water coil to cool down.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
SPECIFICATIONS
A global economizer program function shall be provided to
switch all AHUs from OA cooling to minimum OA based upon
an OA enthalpy calculation setpoint, except the system shall be
locked out of the economizer mode anytime the OA DB is higher
than 27.5 C. Software shall also be provided to allow the user
to switch, with an appropriate commandable setpoint, the
decision to be based upon OA dry bulb or to lock the system
into or out of the economizer mode.
NOTE: The preceding graphic is an example of a major
benefit of digital control systems. This graphic
implies several enhancements over a simple
economizer decision. It gives the user four econ-
omizer control software selectable options; an
automatic OA enthalpy based economizer
decision, an automatic OA dry bulb based econ-
omizer decision (for use if the OA RH sensor is
bad), a manual economizer ON command (for
use if the chiller plant is not ready to run for any
reason), and a manual economizer OFF
command (for use if the outdoor air is poor or a
sensor malfunctions in the summer).
This enhanced scheme has the same program
inputs (OA temperature, OA humidity) and the
same output (the economizer decision status) as
a basic OA enthalpy decision program. The
enhancements are all software. After initial
development and testing of the program (in-
cluding the graphic development) this program
can be cataloged, selected, and subsequently
loaded on other projects for no appreciable
additional cost over the basic program.
PSYCHROMETRIC ASPECTS
1. The following chart shows a comparison between an OA
enthalpy and an OA DB economizer decision. For
comparison, the enthalpy changeover setpoint is 67.5 kJ
per kilogram of dry air (Line 3), and the dry bulb setpoint
is 24 C (Line 4).
2. If the changeover decision is based upon enthalpy, the
system will be in the economizer mode anytime the
outdoor air lies in the area below Line 3 and to the left of
Line 6.
3. If the changeover decision is based upon dry bulb
temperature, the system will be in the economizer mode
anytime the outdoor air lies to the left of Line 4.
4. Area 1 contains high enthalpy, low temperature air that
would be used for free cooling if the decision was based
upon OA dry bulb. This excess enthalpy would burden
the chiller unnecessarily.
5. Area 6 contains low enthalpy, high temperature air that
would be not used for free cooling if the decision was
based upon OA dry bulb. This low enthalpy would reduce
the chiller load.
217
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