Emerson E2 Digital Economizer Control, Analog Economizer Control, Dehumidification Control

11.6.5 Economizer Control

Economizer dampers on AHUs are used to bring out- side air into the building for use in cooling. When tem- perature and humidity conditions are favorable, the economization dampers are opened, and outside air is allowed to flow into the AHU. Economization is generally used by the AHU just as a cool stage would be in Tem- perature Control; if cooling is needed, and conditions are favorable for economization, the dampers will open and economization will begin. If more cooling is needed, the cooling stages would then cycle on as normal.

The E2 supports control of both two-position (digital) and variable-position (analog) economizer dampers.

11.6.5.1Economization Enable

Before the AHU Control application may open econo- mization dampers, it must first determine if the outdoor air conditions are favorable for economization. There are six possible ways the AHU Control may do this:

1.Enthalpy Switch - An enthalpy switch is a digital device that is pre-set to detect when the tempera- ture and humidity levels are favorable for econo- mization. When the conditions are favorable, this switch sends an OK (ON) signal to the AHU Control application. Otherwise, the switch sends a NOT OK (OFF) signal, and economization is disabled.

2.Dewpoint Setpoint - A dewpoint probe measur- ing the dewpoint of the outside air is compared to a setpoint. If the outside air dewpoint is less than the setpoint, economization is enabled. If it is higher than the setpoint, economization is dis- abled.

3.Calculated Enthalpy - The AHU Control appli- cation calculates the outside air enthalpy by read- ing the value of a relative humidity sensor and an outdoor air temperature sensor. If the enthalpy is lower than the setpoint, economization is enabled. If the enthalpy is greater than the set- point, economization is disabled.

4.Dewpoint Fail-Safe - This is similar to method #2, except an outdoor temperature sensor value is compared to the setpoint instead of a dewpoint probe’s value. This comparison is a poor substi- tute for actual dewpoint readings and is recom- mended for use as a fail-safe only. When possible, use humidity or dewpoint sensors.

5.Temperature Comparison - The AHU Control application simply compares the temperature of the inside air with the temperature of the outside air. If the outside air is cooler than the inside air,

economization is enabled.

6.In vs. Out Enthalpy - This strategy requires indoor and outdoor humidity sensors and also indoor and outdoor temperature sensors. The enthalpy of the outdoor air is calculated and com- pared to the enthalpy of the indoor air. If the out- door air enthalpy is less than the indoor air enthalpy, economization is enabled. Otherwise, economization is disabled.

You may choose a different economization checking method for use in summer and winter months.

11.6.5.2Economization Lockout Features

In addition to the methods listed above, there are two economization lockout features that apply to all AHUs using economization.

Max Outside Air Humidity

The Max Outside Air Humidity is the highest allow- able humidity level for the outside air. If the outside rela- tive humidity is higher than this setpoint, economization will not be allowed to occur.

Minimum Supply Temp

The Minimum Supply Temp is a user-defined setpoint that locks out economization if the supply air temperature falls below a minimum supply temperature setpoint. This feature ensures that the air coming from outside is not too cold.

11.6.6 Digital Economizer Control

Control of digital or two-position economizers is rela- tively simple. When conditions are favorable for econo- mization, the dampers will be opened when cooling is needed. If not favorable, the dampers will be closed.

11.6.7 Analog Economizer Control

For variable-position dampers, economization is gen- erally used by the AHU just as a cool stage would be in Temperature Control. If cooling is needed, and conditions are favorable for economization, the dampers will open and economization will begin. If more cooling is needed, the cooling stages would then cycle on as normal.

The position of the analog economizer damper is important only for the purposes of keeping the mixed air temperature (a combination of the outdoor air coming through the dampers and the return air temperature) near the user-specified setpoint. The damper is controlled using PID control.

11.6.8 Dehumidification Control

Dehumidification control uses the AHU’s existing cool stages (and a separate dehumidification device such as a desiccant wheel, if available) to remove moisture from the

11-22 • E2 RX/BX/CX I&O Manual 026-1614 Rev 4 5-JAN-2013