A chiller is a refrigeration system designed to cool a fluid—typically water or a water/glycol mixture—by removing heat through a vapor-compression or absorption refrigeration cycle. The cooled fluid, commonly referred to as chilled water, is then circulated through a building’s hydronic distribution system to provide comfort cooling or process cooling. Chillers serve as the central cooling plant in most large-scale commercial, institutional, and industrial HVAC systems.
Technical Details and Specifications
Chiller capacity is measured in tons of refrigeration (TR), where 1 ton equals 12,000 BTU/hr (3.517 kW). Units range from small modular systems of 15 to 20 tons up to large centrifugal machines exceeding 2,000 tons. Leaving chilled water temperature is typically set between 40°F (4.4°C) and 45°F (7.2°C), though specific applications may require different setpoints.
Chillers fall into two primary categories based on their refrigeration cycle:
- Vapor-compression chillers: These use a mechanical compressor and include subtypes such as centrifugal, screw, scroll, and reciprocating compressors. Common refrigerants include R-134a, R-513A, R-1233zd(E), and R-410A.
- Absorption chillers: These use a thermal energy source (steam, hot water, or direct-fired gas) and a refrigerant-absorbent pair, most commonly water and lithium bromide.
Chillers are further classified by their method of heat rejection as either air-cooled or water-cooled. Water-cooled chillers reject heat through a cooling tower and generally achieve higher efficiency, with full-load COP values ranging from 5.0 to 7.0 or higher. Air-cooled chillers, which reject heat directly to ambient air, typically operate at a COP of 2.8 to 3.5 but eliminate the need for cooling tower infrastructure.
Efficiency is also evaluated using the Integrated Part Load Value (IPLV), which accounts for performance across varying load conditions and better reflects real-world energy consumption.
Applications
Chillers are the backbone of cooling systems in office buildings, hospitals, universities, data centers, hotels, and manufacturing facilities. They supply chilled water to air handler units (AHUs), fan coil units (FCUs), and radiant cooling panels. Chillers also support district cooling plants, where a centralized facility serves multiple buildings through an underground chilled water distribution network. In industrial settings, chillers provide precise temperature control for process cooling in pharmaceutical, food and beverage, and plastics manufacturing operations.
Related Standards and Codes
Chiller performance and efficiency are governed by several key standards:
- ASHRAE Standard 90.1: Establishes minimum energy efficiency requirements for chillers based on type, capacity, and condenser cooling method.
- AHRI Standard 550/590: Defines the testing conditions and rating procedures for water-chilling and heat pump water-heating packages using the vapor-compression cycle.
- ASHRAE Standard 15: Addresses safety requirements for refrigeration systems, including refrigerant detection and ventilation in mechanical rooms.
Practical Significance
Chillers often represent the single largest energy-consuming component in a commercial building, accounting for 25% to 40% of total electricity use. Selecting the correct chiller type, sizing it properly, and implementing staging strategies for multi-chiller plants are critical decisions that directly affect operating costs, system reliability, and occupant comfort. Modern variable-speed chillers and advanced controls have significantly improved part-load efficiency, making proper specification and maintenance essential to long-term building performance.