Economizer

The Economizer module generates two coded signals as internal variables (EnthalpyOverride and AbsHumOverride) with a possible value of False or True, representing the absolute humidity comparison and the enthalpy comparison between outside air and return air conditions. The internal variables are used by the Control module.

The economizer mode override reduces the load on a cooling coil by selecting an airflow to be cooled that has the least total heat (enthalpy). Enthalpy is a measure of the total (latent and sensible) heat content of the air. Whenever outside air enthalpy is greater than return air enthalpy, only a minimum amount of outside air is allowed to enter the fan system. Enthalpy and absolute humidity control are based on a calculation using dry-bulb temperature and relative humidity.

During the warm season of the year, the total heat content of both return air and outside air is used to determine which source of air is the most efficient to use for cooling.

The module uses internal inputs for:

• return air temperature (RAT);

• return air relative humidity (RAH);

• outside air temperature (OAT);

• and outside air relative humidity (OAH).

General

The module uses 4 internal inputs, respectively return air temperature and humidity and outside air temperature and humidity. With the information from these sensors, the module calculates the enthalpy values as well as the absolute humidity values of the return and outside air. With the parameter AtmPressure, the atmospheric pressure can be adjusted and used in the calculation method. The module provides two internal digital outputs as override outputs, which are used in the Control module.

Enthalpy Comparison for Economizer Functions

During the warm season of the year, the total heat content of both return air and outside air is used to determine which source of air is the most efficient to use. Whenever outside air enthalpy is greater than the return air enthalpy, only a minimum amount of outside air is allowed to enter the air handling unit during the cooling season. The economizer mode is based on a comparison between the total heat content or the absolute humidity and uses dry-bulb temperature and relative humidity sensors for the calculation of the total heat. With this economizer function, the cooling load can be reduced during warm periods of the year. The difference in enthalpy values must have a higher value than the value of parameter EnthalpyDiff before the override function becomes active.

Absolute Humidity Comparison for Dehumidification Function

Absolute humidity comparison is used for dehumidification purposes in the Control module. The difference in absolute humidity values must have a higher value than the value of the parameter AbsHumDiff before the override function becomes active.

Switch-over Delays

Both enthalpy- and absolute humidity overrides are activated after a delay defined by the parameter SwitchoverDel. This delay prevents fast and often switching between both condition situations.

Note: It is recommended to set the parameter SwitchoverDel (by default, 300 s) to a rather high value, to prevent chattered switching.