Centrifugal and Screw Chillers

In a typical commercial building, chillers consume more electricity than any other single energy-consuming device, except for an occasional extremely large fan. Thus, inefficient chillers can waste significant amounts of electricity, and even modest improvements in efficiency may yield substantial energy savings and attractive paybacks.

However, it’s important to select a chiller (and its associated efficiency) carefully. Choosing a chiller that’s most efficient at full or part load, according to standard ratings, might be counterproductive because the ratings don’t measure the efficiency of the overall cooling system. To maximize cost-effectiveness, we recommend analyzing the entire chilled-water system as well as exercising care in specifying the efficiency of the chiller itself—it may be wiser to invest in a less-efficient chiller and instead spend more on efficient auxiliary equipment and improved operating strategies. (See the Chiller Terminology sidebar for more information.)

Chiller terminology

Tons: One ton of cooling is the amount of heat absorbed by one ton of ice melting in one day, which is equivalent to 12,000 Btu per hour (h), or 3.516 kilowatts (thermal).

Chiller performance is certified by the Air-Conditioning, Heating, and Refrigeration Institute (AHRI), a manufacturer trade organization, according to its Standard 550/590-2003: Performance Rating of Water-Chilling Packages Using the Vapor Compression Cycle. Various efficiency metrics are commonly used for chillers.

Full-load efficiency: This metric is the efficiency of the chiller at peak load and at AHRI standard conditions, measured in kilowatts (kW) per ton. This metric is a specific case of the broader metric, power input per capacity—sometimes called the “kW/ton rating”—which can be measured at any given set of rating conditions. A lower kW/ton rating indicates higher efficiency.

Part-load efficiency: This is the efficiency of the chiller at part load, measured in kW/ton by either integrated-part load value (IPLV) or nonstandard part-load value (NPLV), depending on the particular AHRI part-load test conditions. Both give the efficiency of the chiller averaged over four operating points according to this formula:


  • A = kW/ton at 100 percent load
  • B = kW/ton at 75 percent load
  • C = kW/ton at 50 percent load
  • D = kW/ton at 25 percent load

Coefficient of performance (COP): The ratio of the cooling capacity output power to the total power input at any given set of rating conditions, expressed as watts of output per watts of input.

Energy-efficiency ratio (EER): This metric is frequently used for smaller chillers; it is the ratio of the cooling capacity to the total power input at any given set of rating conditions, expressed as Btu per watt-hour. Use these conversion factors to relate COP, EER, and kW/ton:

  • COP = 0.293 EER; EER = 3.413 COP
  • kW/ton = 12/EER; EER = 12/(kW/ton)
  • kW/ton = 3.516/COP; COP = 3.516/(kW/ton)

AHRI standard conditions: Standard reference conditions at which chiller full-load performance is measured. For water-cooled chillers, this means a constant flow rate of 2.4 gallons per minute (gpm) for water leaving the evaporator at 44° Fahrenheit (F) and a constant flow rate of 3.0 gpm for water entering the condenser at 85°F.

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