Dry Cooler and Free Cooling Units

Free cooling uses cold outdoor air to cool process water instead of running compressors. Whenever the ambient temperature is below the return-water temperature, part of the heat can be rejected without mechanical cooling; when it is cold enough, the compressors switch off completely.

A free cooling chiller such as Fusion switches automatically between three modes:

• Mechanical: in warm weather the compressors provide all of the cooling.
• Hybrid: in the transitional seasons the free-cooling coil pre-cools the return water and the compressors cover the rest.
• Full free cooling: in cold weather the fans alone meet the load and the compressors stop.

Savings depend on how many hours a year the outdoor temperature stays below the required water temperature. Processes that run around the clock with relatively warm water gain the most, because they can use free cooling for a large part of the year.

In Carnot's worked scenarios, Fusion achieves annual energy savings of up to 60%. The actual figure depends on the site's climate, water temperatures and load profile, so we calculate it for each project.

Both series use outdoor air without consuming water, but they do different jobs:

See the Fusion and Kalter pages for series details.

A dry cooler can only cool the fluid to a few degrees above the outdoor dry-bulb temperature. This difference — the approach — is the key sizing parameter: a smaller approach needs a larger heat exchanger and more airflow. Sizing therefore starts from the site's hottest design day and the warmest leaving-water temperature the process can accept.

Outdoor circuits usually run with glycol to prevent freezing in winter. Glycol reduces heat transfer and increases pumping power, so its concentration is chosen for the site's minimum temperature and included in the selection. Kalter's all-aluminium microchannel exchanger and load-based EC fans keep fan energy low when full capacity is not needed.