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Economizers
Economizer

A packaged rooftop unit’s air-side economizer is a mechanical device intended to reduce energy consumption and perform other functions such as regulating the amount of ventilation airflow being introduced to the building and controlling the building static pressure. Air-side economizers can save energy in buildings by using cool outside air as a means of cooling the indoor space instead of consuming energy by running compressors. When the outside air is sufficiently cool, no additional mechanical air-conditioning is needed. This portion of the air-side economizer control design is frequently called "free cooling." At other times it is possible to conserve energy by using a minimal amount of compressor capacity to further cool the outside air, when it would take a greater amount of compressor capacity to condition the air being re-circulated from the space. This control process is referred to as “Integrated Economizer” operation. When the outside air conditions are not suitable for free cooling or integrated economizer operation, the economizer dampers are positioned to provide only the required amount of ventilation airflow.

The economizer controls must be able to decide when it is appropriate to provide free cooling or integrated economizer operation versus minimum ventilation airflow. This “economizer changeover” control can be based on a number of different inputs:

  • The outdoor dry-bulb temperature
  • The outdoor enthalpy
  • A combination of outdoor dry-bulb and dew-point temperatures
  • By comparing the outdoor dry-bulb to the return air dry-bulb temperature
  • By comparing the outdoor enthalpy to the return air enthalpy
  • Or a combination of these criteria

Which method is used can depend on the level of sophistication of the specific economizer controller, what methods are allowed by ASHRAE 90.1 or local building codes, or owner/engineer preference.

As the volume of air being introduced into the building changes during economizer operation an equal amount of air must also be relieved from the building envelope. Barometric relief or power exhaust (link to PE page) devices are available as options on most units when an economizer is installed.

Carrier economizer features:

  • Incorporates a gear-driven parallel blade design with a direct-mount spring return actuator that will close upon loss of power. No linkage rods or arms used
  • Standard with either an outdoor air temperature sensor or an outdoor air enthalpy sensor
  • Field installed sensors are available for additional control strategies such as: differential temperature, enthalpy, dew-point, differential enthalpy or CO2 (DCV) control
  • Available as a factory-installed option or field installed accessory kit (varies by unit model)
  • Equipped with low-leakage dampers
  • Capable of introducing up to 100% outdoor air

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