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Free Cooling – Utilization of Low Ambient Temperature for Power Savings

With power savings focus increasing day by day, a good number of concepts/operational modes are being adopted in this direction. One such practice being adopted is Free cooling wherein lower ambient temperature air is utilized for water/Brine Solution Cooling and thereby reducing Power Consumption. This is applicable in Air-Cooled Chillers.

Free Cooling, simply put, is using cold ambient air (colder than the supply chilled water setpoint) to perform cooling rather than using the refrigeration cycle of the chiller. This is widely used in Data Center/Process applications where higher leaving chilled water temperatures is used. It is also used for Brine chilling for process applications.

Chiller with Free Cooling Coil Operation

The chiller shall have a water circuit coil (Air to water heat exchanger) integrated into the condenser coil. During normal operation, the condenser will be rejecting heat to the atmosphere with the help of a condenser fan and during the low ambient condition, the compressor will stop working and the free cooling mode will start. During free cooling mode, the chilled water will pass through the water circuit coil and taking advantage of low ambient conditions, heat exchange will happen from air to water. Condenser fans can be regulated to ensure a steady water temperature in the coil.

The unit controller shall be monitoring the ambient temperature with the help of a temperature sensor. When the ambient temperature reaches the set point, the controller shall enable the free cooling mode.

Free Cooling with a Water Circuit

Circulating water to the free cooling circuit is not recommended – particularly for comfort cooling wherein the chilled water temperature ranges between 5 to 7 * C. This is due to water likely to freeze during the low ambient condition resulting in tube burst. So, it is recommended to use Brine Solution. However, for higher chilled water temperature applications, the free cooling mode can be adopted safely.

The fan speed will be varied to maintain the chilled water temperature and the bypass valve needs to be provided to avoid low chilled water temperature in extremely low ambient conditions.

Free Cooling with Brine Solution

In applications where water is likely to freeze due to low ambient temperature, it is recommended to use, intermediate heat exchanger with a glycol circuit for free cooling.

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