Honeywell AUTOMATIC CONTROL Engineering Manual page 227

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.
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 81°F. 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 economizer control software selectable
options; an automatic OA enthalpy based
economizer decision, an automatic OA dry
bulb based economizer decision (for use if the
AIR HANDLING SYSTEM CONTROL APPLICATIONS
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 (including
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 29 Btu
per pound of dry air (Line 3), and the dry bulb setpoint is
75°F (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.
6. Line 3 represents the dry bulb value, to the right of which
the cooling coil could not handle the sensible load, no
matter how dry the air is. This is the high temperature
economizer cutout line used with the enthalpy and OA/
RA enthalpy comparison economizer decision.
ENTHALPY
CONTROL
SELECTS RA
3
ENTHALPY
CONTROL
SELECTS OA
2
OA DB
CONTROLLER
SELECTS OA
217 ENGINEERING CMANUAL OF AUTOMATION CONTROL
4
5
75°F DB, 54% RH
1
ENTHALPY
CHANGEOVER
LINE, 29 BTU/LB
6
HIGH OA DB
CUT-OUT
7
OA DB
C2565-1
CONTROLLER
SELECTS RA