Free cooling

During cool weather, the outside ambient temperature can help save energy in chilled-water systems. The low temperature of the cooling tower water supply enables free cooling of research laboratories, computer rooms, and office buildings. This free cooling is possible if the central plant incorporates a plate-and-frame heat exchanger to provide chilled water production, which means the chiller's compressor can be shut down. Free cooling can be used to save energy whenever the outside wet-bulb temperature drops below the required chilled water set-point. This energy-efficiency measure can save enough compressor electric power to pay for plate-and-frame heat exchanger installation costs in less than two years. [White, 1994]

Cooling System with Evaporative Cooling

If mechanically generated chilled water is used only to maintain a specified maximum laboratory humidity condition that cannot be achieved with indirect cooling, free-cooling system energy savings will be maximized. The chilled-water supply temperature can be set as a function of outdoor dew point to the highest value that will satisfy the laboratory conditions. The plate-and-frame free-cooling system is operated whenever the chilled-water return temperature is greater than the cooling tower's supply water temperature. This operating strategy maximizes indirect evaporative cooling opportunities and can reduce chiller operation by one-third to one-half. [Brown, 1993; Kruse, 1991]

When chilled-water supply temperatures are raised to save compressor energy and to enable free cooling to operate longer, the laboratory's air handling units (AHUs) will also operate longer or at higher speeds to maintain the required space temperatures. This can lower the energy savings from free cooling or cause an increase in energy use. Therefore, monitoring the energy use of the entire chilled water/air handling system with an EMCS can prevent negative energy impact. [White, 1994] [Knebel, 1999]


Stabilized temperature control

Blended tower water

Tower water isolation

Additional refrigerant

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