Emerson 2 user manual Economizer Cooling Optional, Heating Optional, Enthalpy Sensor, Actuator

Operation

5.2Economizer 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.

5.2.1Enthalpy 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

	70
	65
(DB°F)	60
			Enthalpy Control
	55		Operating Range
Temperature	50
	45
	40
	10	20	30	40	50	60	70	80	90

Outdoor Relative Humidity (%RH)

5.2.2Actuator

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.

5.3Heating - 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.

12