Cooling - Chillers Chillers are a type of cooling equipment that produces chilled water to cool air. The chilled water is distributed throughout the building by pipes. The two major categories of chillers are water-cooled and air-cooled. Water-cooled chillers use water to transport away the heat rejected in their condensers. The water, called condenser water, is cooled in a cooling tower. Air-cooled chillers have condensers that are cooled with ambient air.
In large facilities, the equipment used to produce chilled water for HVAC systems can account for up to 35% of a facility's electrical energy use. If replacement is determined to be the most cost-effective option, there are some excellent new chillers on the market.
The most efficient chillers currently available operate at efficiencies of 0.50 kilowatts per ton (kW/ton), a savings of 0.15 to 0.30 kW/ton over most existing equipment. When considering chiller types and specific products, part-load efficiencies must also be compared. If existing chiller equipment is to be kept, there are a number of measures that can be carried out to improve performance.
Consider chiller replacement when existing equipment is more than ten years old and the life-cycle cost analysis confirms that replacement is worthwhile. New chillers can be 30% to 40% more efficient than existing equipment. First-cost and energy performance are the major components of life-cycle costing, but refrigerant fluids may also be a factor. Older chillers using CFCs may be very expensive to recharge if a refrigerant leak occurs (and loss of refrigerant is environmentally damaging).
An excellent time to consider chiller replacement is when lighting retrofits, glazing replacement, or other modifications are being done to the building that will reduce cooling loads. Conversely, when a chiller is being replaced, consider whether such energy improvements should be carried out; in some situations those energy improvements can be essentially done for free because they will be paid for from savings achieved in downsizing the chiller (see Integrated Building Design). Be aware that there can be lead times of six months or more for delivery of new chillers.
Electric chillers use a vapor compression refrigerant cycle to transfer heat. The basic components of an electric chiller include an electric motor, refrigerant compressor, condenser, evaporator, expansion device, and controls. Electric chiller classification is based on the type of compressor used: common types include centrifugal, screw, and reciprocating. The scroll compressor is another type frequently used for smaller applications of 20 to 60 tons. Hydraulic compressors are a fifth type (still under development).
Both the heat rejection system and building distribution loop can use water or air as the working fluid. Wet condensers usually incorporate one or several cooling towers. Evaporative condensers can be used in certain (generally dry) climates. Air-cooled condensers incorporate one or more fans to cool refrigerant coils and are common on smaller, packaged rooftop units. Air-cooled condensers may also be located remotely from the chillers. New Chillers Chillers have been significantly reengineered in recent years to use new HCFC and hydrofluorocarbon (HFC) refrigerants. New machines have full-load efficiencies down to 0.50 kW/ton in the 170- to 2,300-ton range. Some have built-in refrigerant containment, are designed to leak no more than 0.1% refrigerant per year, and do not require purging.
Other important energy efficiency improvements in new chillers include larger heat transfer surfaces, microprocessor controls for chiller optimization, high-efficiency motors, variable-frequency drives, and optional automatic tube-cleaning systems. To facilitate replacement, new equipment is available from all manufacturers that can be unbolted for passage through conventional doors into equipment rooms. Many positive-pressure chillers are approximately one-third smaller than negative-pressure chillers of similar capacity.
Thermal energy storage may be added when replacing chillers and may enable the use of smaller chillers. Although this strategy does not save energy per se, operating costs may be reduced by lowering electrical demand charges and by using cheaper, off-peak electricity. Thermal storage systems commonly use one of three thermal storage media: water, eutectic salts, or ice. Volumes of these materials required for storage of 1 ton-hour of cooling are approximately 11.4, 2.5, and 1.5 cu ft (0.33, 0.07, and 0.04 cu m), respectively.
Multiple chiller operations may be made more efficient by using unequally sized units. With this configuration, the smallest chiller can efficiently meet light loads. The other chillers are staged to meet higher loads after the lead chiller is operating close to full capacity. If an existing chiller operates frequently at part-load conditions, it may be cost-effective to replace it with multiple chillers staged to meet varying loads.
Double-bundle chillers have two possible pathways for rejecting condenser heat. One pathway is a conventional cooling tower. The other pathway is heat recovery for space heating or service-water heating. Candidates for these chillers are facilities in cold climates with substantial hours of simultaneous cooling and heating demands. Retrofitting existing water heating may be difficult, because of the low temperature rise available from the heat-recovery loop.
Steam or hot water absorption chillers use mixtures of water/lithium-bromide or ammonia/water that are heated with steam or hot water to provide the driving force for cooling. This eliminates global environmental concerns about refrigerants used in vapor-compression chillers. Double-effect absorption chillers are significantly more efficient than single-effect machines. See Absorption Cooling section.
Specifying and procuring chillers should include load-reduction efforts, careful equipment sizing, and good engineering. Proper sizing is important in order to save on both initial costs and operating costs. Building loads often decrease over time as a result of conservation measures, so replacing a chiller should be accomplished only after recalculating building loads. |
