36
The W 7220 calculates the enthalpy and dew point using
the OA tempera ture and humidity input from the OA
entha lpy sensor. W hen the OA temperature, OA humidity
and OA dew point are all be low the selected boundary, the
economizer sets the economizing mode to YES,
economizing is available.
When all of the OA conditi ons are above the selec ted
boundary, the conditions are not good to economize a nd
the mode is set to NO.
Fig. 48 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 8 provides t he va lues for each boundary limit.
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
)
C12015
Fig. 48 -- Single Enthalpy Curve and Boundaries
Table 8 – Single Enthalpy and Dual Enthalpy High Limit Curves (EN Units)
Enthalpy
Curve
Temp.
D r y --- B u l b ( _F)
Temp.
Dewpoint (_F)
Enthalpy
(btu/lb/da)
Point P1 Point P2
Temp. (_F) Humidity %RH Te m p. (_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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