Nuts and Bolts of Chillers

By Patricia T. Foley

Chillers are the main component of central system air conditioning units installed in medium- and large-sized structures, such as office buildings, shopping centers, hospitals, sport complexes, schools and colleges, government installations, and factories. In simple terms, chillers remove heat from warm water that enters the unit; and return cold water to the central air conditioning system.

Chillers use a refrigerant or working fluid in a two-phase refrigeration cycle. There are two main categories of chillers: mechanical vapor compression units, which use mechanical compression to increase the pressure and temperature of the refrigerant, and absorption units that use a thermal or chemical process to produce the refrigeration effect necessary to provide chilled water.

Why Copper is Used in Chillers

The widespread use of copper in air conditioners -- from window units to large chillers -- reflects copper's favorable combination properties: excellent heat transfer capability, easy formability, corrosion resistance, and moderate strength.

Copper conducts heat up to 8 times better than other metals, such as steel and aluminum. Copper tube is relatively easy to manufacture and form. Seamless and welded tube products are available in a wide range of sizes and a variety of configurations, from smooth-bore to enhanced-surface tube with complex patterns on both the inner and outer surfaces. Copper tube can be easily bent, fitted, and joined.

Copper resists corrosion and requires little maintenance, a very important consideration in a closed system with a 20-30 year life expectancy. Copper has the strength required to withstand fluid pressures in large air conditioning systems.

To a building owner, the key difference between mechanical and absorption chillers is the type of energy used. Mechanical compression chillers typically rely on electrical energy, while absorption units require a source of heat, for example, natural gas, hot water, or steam. Absorption units are installed when the available heat source is more economical than electricity. They account for a very small portion of total installed capacity.

There are three basic types of mechanical compression chillers.

Common Chiller TypeSize Range (Tons)Full-Load Efficiency (kW/ton)
Centrifugal 100 - 1500+

0.49- 0.68

Screw 40 - 1100
water-cooled 0.61 - 0.70
air-cooled 1.1 - 1.3
Reciprocating 1 - 400
water-cooled 0.8 - 1.0
air-cooled 1.4 - -1.6
Source: E Source, Space Cooling Technology Atlas, reproduced in
  • Reciprocating chillers use pistons and cylinders for compression and are most cost effective at small loads. They account for the bulk of the market under 200 tons of refrigeration capacity. Because of their small size, these units are less energy efficient than other designs.
  • Screw compressors generally use two intermeshed screws for compression. They are not as cost effective as either the small reciprocating units or the large centrifugal chillers. Most of the applications are in the range of 100 to 300 tons of cooling capacity, but even here they make up only a small portion of the market. Their energy efficiency is between that of reciprocating and centrifugal units.
  • Centrifugal chillers are the most widely used. A centrifugal compressor rotates at high speed, compressing the refrigerant by centrifugal force. Because the design, they are well suited for large cooling loads, and they comprise over 95% of chillers with cooling capacity in the 200- to 10,000-ton range. Centrifugal units are most energy efficient chillers, with ratings in the range of 0.49-0.68 kW/ton.

Chillers include a compression, evaporator and condenser along with ancillary equipment to move air. They are major pieces of equipment, and cost between $125,000 and $400,000 for units with cooling capacity in the 500- to 850-ton range. Often a large building will use several chillers. For example, prior to its tragic destruction on 9/11, the World Trade Center in New York City utilized seven centrifugal chillers. Five new units were installed in 1993 to replace those damaged in the first World Trade Center bombing. These new chillers each provided 2,000 tons of cooling capacity.

Many of the residential air conditioning manufacturers also serve the chiller market, including Carrier Corp., York International, The Trane Co., Dunham-Bush, Inc., and McQuay International.

Chillers are copper intensive, with individual units containing 3,000 - 6,000 lbs of copper. The US chiller tube market uses some 20 - 25 million lbs of copper products. Wolverine is the US principal supplier, and Wieland Metals is also active.