Trane SYS-APM001-EN manual Low-ambient operation, Energy efficiency

Primary System Components

Low-ambient operation

Air-cooled chillers are often selected for use in systems with year-round cooling requirements that cannot be met with an airside economizer. Air- cooled condensers have the ability to operate in below-freezing weather, and can do so without the problems associated with operating the cooling tower in these conditions. Cooling towers may require special control sequences, basin heaters, or an indoor sump for safe operation in freezing weather.

For process applications, such as computer centers that require cooling year- round, this ability alone often dictates the use of air-cooled chillers.

Energy efficiency

Water-cooled chillers are typically more energy efficient than air-cooled chillers. The refrigerant condensing temperature in an air-cooled chiller is dependent on the ambient dry-bulb temperature. The condensing temperature in a water-cooled chiller is dependent on the condenser-water temperature, which is dependent on the ambient wet-bulb temperature. Since the design wet-bulb temperature is often significantly lower than the dry-bulb temperature, the refrigerant condensing temperature (and pressure) in a water-cooled chiller can be lower than in an air-cooled chiller. For example, at an outdoor design condition of 95°F [35°C] dry-bulb temperature, 78°F [25.6°C] wet-bulb temperature, a cooling tower delivers 85°F [29.4°C] water to the water-cooled condenser. This results in a refrigerant condensing temperature of approximately 100°F [37.8°C]. At these same outdoor conditions, the refrigerant condensing temperature in an air-cooled condenser is approximately 125°F [51.7°C]. A lower condensing temperature, and therefore a lower condensing pressure, means that the compressor needs to do less work and consumes less energy.

This efficiency advantage may lessen at part-load conditions because the dry- bulb temperature tends to drop faster than the wet-bulb temperature (see Figure 4). As a result, the air-cooled chiller may benefit from greater condenser relief. Additionally, the efficiency advantage of a water-cooled chiller is much less when the additional cooling tower and condenser pump energy costs are considered. Performing a comprehensive energy analysis is the best method of estimating the operating-cost difference between air- cooled and water-cooled systems.

6 Chiller System Design and Control SYS-APM001-EN