Air-cooled screw chillers offer high performance, particularly at partial load. Their compressors typically are modulating (slide valve or VFDs), rather than stepped, which yields more accurate control. Advantages of air-cooled chillers include:

• Positive-displacement compression with no surge.

• No cooling towers.

• Avoidance of city water costs.

• Avoidance of water waste.

• Avoidance of chemical costs.

• In the event of a natural disaster, no water issues.

• No condenser pumps and piping.

• Reasonable maintenance costs.

• Saved mechanical-room space.

• Simpler control (no tower bypass) and operation (no tower freezing) in cold climates.

Table 1 compares two air-cooled chillers: one based on 1998 efficiency levels, the other one of today’s most efficient models. Modern air-cooled chillers can perform at full-load efficiencies up to 17-percent higher and part-load efficiencies up to 37-percent higher.