45
ENTHALPY SETTINGS
When the OA temperature, enthalpy and dew point are be-
low the respective setpoints, the outdoor air can be used for
economizing. Figure 55 shows the new single enthalpy
boundaries in the W7220. There are 5 boundaries (setpoints
ES1 through ES5), which are defined by dry bulb tempera-
ture, enthalpy and dew point.
Refer to Table 18 for ENTH CURVE setpoint values.
The W7220 calculates the enthalpy and dew point using
the OA temperature and humidity input from the OA enthal-
py sensor. When the OA temperature, OA humidity and OA
dew point are all below the selected boundary, the economiz-
er sets the economizing mode to YES, economizing is
available.
When all of the OA conditions are above the selected
boundary, the conditions are not good to economize and the
mode is set to NO.
Fig. 55 shows the 5 current boundaries. There is also a
high limit boundary for differential enthalpy. The high limit
boundary is ES1 when there are no stages of mechanical
cooling energized and HL (high limit) when a compressor
stage is energized.
Table 18 provides the values for each boundary limit.
Fig. 55 — Single Enthalpy Curve and Boundaries
Table 18 — Single Enthalpy and Dual Enthalpy High Limit Curves (EN Units)
TEMPERATURE
ENTHALPY
RA TEMP
ABSOLUTE HUMIDITY
ECONOMIXING
AVAILABLE
NOT AVAILABLE
ES5 ES4 ES3 ES2 ES1 HL
DUAL ENTHALPY
HIGH LIMIT
SINGLE ENTHALPY
P2 (T,RH)
P1
(T,RH)
R
A
H
U
M
(
%
R
H
)
ENTHALPY
CURVE
TEMP
DRY BULB (°F)
TEMP
DEWPOINT (°F)
ENTHALPY
(btu/lb/da)
POINT P1 POINT P2
TEMP(°F)
HUMIDITY (%
RH)
(°F)
HUMIDITY (%
RH)
ES1 80.0 60.0 28.0 80.0 36.8 66.3 80.1
ES2 75.0 57.0 26.0 75.0 39.6 63.3 80.0
ES3 70.0 54.0 24.0 70.0 42.3 59.7 81.4
ES4 65.0 51.0 22.0 65.0 44.8 55.7 84.2
ES5 60.0 48.0 20.0 60.0 46.9 51.3 88.5
HL 86.0 66.0 32.4 86.0 38.9 72.4 80.3
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