- Chillers produce cold water for air conditioning or process cooling applications.
- They are often found in medium to large commercial and industrial facilities.
- Types of chillers include centrifugal, screw, scroll, reciprocating and hybrids.
Source: Lawrence Berkeley National Laboratory
Chillers produce cold water for air conditioning or process cooling for medium to large commercial and industrial facilities. Commercial applications include office buildings, schools, hospitals, hotels, supermarkets, large retail stores and strip malls. Industrial applications include food processing, pulp and paper processing, electronics, printing, brewing and plastics manufacturing.
The operating principle of a chiller is similar to conventional air conditioning because the cooling is provided by refrigerant evaporation. However, most chillers use water as the primary cooling medium rather than air.
Chilled water circulates through air handler cooling coils that serve the space to be conditioned, or through heat exchangers within a particular segment of a manufacturing process to remove heat and then discharge it to the outside air through a cooling tower. The ability to chill a particular fluid depends on the properties of refrigerants, which are liquids capable of absorbing heat at comparatively low temperatures of evaporation. Since these liquids absorb heat when they evaporate, they produce a cooling effect on the surrounding area.
Refrigerants are becoming more environmentally friendly. Hydrochlorofluorocarbon (HCFC) refrigerants are being replaced with hydrofluorocarbons (HFC) because of their much lower ozone-depleting effect. More than 80 percent of centrifugal chillers now being manufactured use HFC-134a.
Chillers commonly use mechanical compression (centrifugal, screw, scroll, or reciprocating) to achieve refrigerant phase change, though there are also absorption chillers that take advantage of waste heat. The type of chiller chosen is dependent on the amount of cooling required; usually expressed in refrigerant tons (RT) or tons. Centrifugal chillers range from 200 to 6,000 tons, and dominate the market with more than a 70 percent share. Centrifugal chillers are the most efficient, with efficiencies of 0.6 kW/ton and lower. They usually last for 20 to 30 years.
Chillers using reciprocating compressors are generally found in the 15 to 200 ton range, with efficiencies in the 0.8 to 1.0 kW/ton for water-cooled units and 1.0 to 1.5 kW/ton for air-cooled designs. These chillers only last for 12 to 15 years. Screw type or rotary compressors are typically above 70 tons and can reach capacities of up to 1,000 tons. Efficiencies for water-cooled screw compressors range from 0.55 to 0.75 kW/ton, while air-cooled units are considerably less efficient at 1.1 to 1.3 kW/ton. Scroll compressors dominate the smaller chiller sizes below 25 tons. The U.S. Department of Energy provides guidelines on selecting high-efficiency chillers.
There are also gas-driven centrifugal compressors, though they occupy only a small percentage of the available market. Natural gas engine-driven centrifugal chillers include a variable-speed power source that enables compressor capacity control down to 35 percent of load. Waste heat can be recovered to drive desiccant or absorption systems. Some types of natural gas chillers can have high efficiencies if they recover waste heat; a 200-ton system has a coefficient of performance (COP) of 2.6; removing 2.6 units of heat for each unit of energy consumed.
How chillers work
More than 90 percent of all chillers operate on the principle of vapor compression, where cooling is produced when heat from the indoors is absorbed as the liquid refrigerant converts to a vapor. When the vapor reaches the compressor, its temperature and pressure is increased. It then circulates to the condenser (coil), where heat is rejected and the vapor condenses to a liquid. The liquid refrigerant is metered into the evaporator via a thermal expansion valve. The refrigerant enters the evaporator as a liquid-rich, vapor/liquid mixture and the evaporation portion of the cycle is repeated.
Absorption systems have evaporators and condensers similar to conventional compression systems, but use thermal energy to boil a solution to liberate the refrigerant rather than electrical energy for compression. Absorption systems usually have higher operating and capital costs, but can be used for applications where waste heat is available or where the customer does not want to use refrigerants. An absorption chiller keeps the water at a pressure so low the water boils at the cooling temperature; eliminating the need for conventional refrigerants. Water-based lithium bromide or ammonia solutions are used as the refrigerant.
Mechanical vapor compression systems rely on a compressor to produce the necessary pressure difference between vaporizing and condensing levels, whereas absorption systems use an absorber generator for this purpose. Absorption chillers are much less efficient with COP ratings of 0.6 to 1.0.
Combination chiller systems are available that may have lower energy consumption and higher efficiencies than stand-alone systems. Some take advantage of waste heat and cheaper fuels. They usually combine a turbine engine (steam or gas) to drive a compression chiller (gas or electric), using waste heat to drive an absorption chiller. Combination chillers may have reduced reliability and are more difficult to maintain because of their increased complexity.
When considering a chiller replacement, identify potential cooling load reductions prior to sizing the new system. Cooling requirements may have changed because of energy efficiency upgrades.
Cooling load conditions and peak times are also important issues to consider when sizing a new or replacement chiller. For example, a facility that needs 500 tons at peak load 30 days out of the year, but only needs 250 tons for the remainder of the year, would operate at a low efficiency at this partial load. In this case, a chiller that performs well in the 50 percent range is the best choice.
Chillers that are over 10 years old are good candidates for replacement because there may be refrigerant replacement issues to address and newer models are significantly more efficient. The older types of refrigerants (Class 1 ozone depleting) are no longer produced because of negative environmental impacts. As a result, they are becoming more expensive and difficult to obtain due to strict regulations on handling and storage.