Emerson 2 - Manual (Page 18)

Operation
12
5.2 Economizer Cooling - Optional
The Economizer mode uses outside air to cool the room when the outside air temperature is cool
enough to allow for economical cooling. Components include the damper, spring return direct drive
actuator, enthalpy sensor and actuator control board with an integral mixed air sensor. Economizer
operation is enabled when the outside conditions meet required temperature and humidity as set on
the enthalpy sensor. If outdoor conditions are favorable on a call for cooling, the damper actuator will
open the Economizer damper. The damper position will be controlled based on the mixed air tempera-
ture in the evaporator section. The factory set mixed air temperature target is 50-56°F (10-13°C).
The amount of damper opening depends on the cooling capacity of the outside air; the cooler the out-
side air, the less that is needed to cool the room, and so a smaller damper opening is required. As the
outside air temperature rises, the damper will open wider since more of that air is required to keep
the room at the same temperature.
The Economizer and compressorized cooling will never operate at the same time. If outdoor conditions
meet the required enthalpy sensor setpoint during a cooling cycle, the unit will automatically shut
down the compressor and begin using outside air.
5.2.1 Enthalpy Sensor
The enthalpy sensor responds to the total heat content of the outdoor air to provide changeover from
compressorized cooling to free-cooling operation. The changeover point is factory set (at the “D” posi-
tion). This setting will ensure rated capacity when the outdoor conditions allow for Economizer cooling.
Figure 5 Enthalpy control setting
5.2.2 Actuator
The Economizer actuator is mounted in the lower part of the Economizer section below the electric
box. It is accessible by opening the lower front panel. The actuator is a 24 VAC, spring return device
that will fully close the Economizer damper. The actuator has internal controls that allow it to oper-
ate based on signals from the enthalpy control and mixed air sensor.
5.3 Heating - Optional
On units supplied with optional resistance heating, a call for heating closes the heat relay and ener-
gizes the evaporator fan and resistive elements.
10 20 30 40 50 60 70 80 90
40
45
50
55
60
65
70
Outdoor Relative Humidity (%RH)
Te
mp
eratu
re (DB°F
)
Enthalpy Control
Operating Range