AHU – Economizers

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An economizer uses outside air to reduce the refrigeration requirement. A logic circuit maintains a fixed minimum of ventilation outside air. The air side economizer is an attractive option for reducing energy costs when the climate allows. The air-side economizer takes advantage of cool outdoor air to either assist mechanical cooling or, if the outdoor air is cool enough, provide total system cooling. It is necessary to include some method of variable volume relief when air-side economizers are employed, to exhaust the extra outdoor air intake to outdoors. The relief volume may be controlled by several different methods, including fan tracking (operating the supply and return fans to maintain a constant difference in airflow between them,) or relief air discharge dampers which modulate in response to building space pressure. The relief system is off and relief dampers are closed when the air-side economizer is inactive. In systems with large return air static requirements, return fans or exhaust fans may be necessary to properly exhaust building air and take in outside air.

Advantages of Air-Side Economizers
• Substantially reduces compressor, cooling tower, and condenser water pump energy requirements
• Has a lower air-side pressure drop than a water-side economizer
• Has a higher annual energy savings than a water-side economizer
• Reduces tower makeup water and related water treatment

Disadvantages of Air-Side Economizers
• Humidification may be required during winter operation
• Equipment room is generally placed along the building’s exterior wall
• Installed cost may be higher than that for a water-side economizer if the cost of providing the exhaust system requirements exceeds the costs of piping, pump, and heat exchanger

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