A chiller removes heat from water or another cooling fluid. Pumps carry that fluid to air-handler coils, fan coils or process equipment; the chiller transfers the collected heat to outdoor air, a water circuit or a useful heating load.
Three different ways to describe a chiller
Heat rejection
Air-cooled or water-cooled describes where condenser heat goes—not the compressor inside.
Compression technology
Centrifugal, screw, scroll and reciprocating describe how refrigerant vapor is compressed.
Bearings and lubrication
Magnetic-bearing describes rotor support. Many magnetic-bearing chillers are oil-free centrifugal machines, available in air- or water-cooled configurations.
A water-cooled magnetic-bearing centrifugal chiller combines a water-cooled condenser, centrifugal compression and magnetic shaft support. An air-cooled screw chiller combines outdoor-air heat rejection with screw compression. These labels overlap; they are not mutually exclusive equipment categories.
The vapor-compression cycle
Evaporator
Low-pressure refrigerant absorbs heat from the cooling fluid and evaporates. The chilled-water circuit and refrigerant circuit remain separate across the heat exchanger.
Compressor
Raises refrigerant vapor pressure and temperature so its heat can be rejected at the condenser. This requires energy; the condenser rejects both the collected heat and the compressor's energy input.
Condenser
Refrigerant releases heat and condenses back to liquid. Air-cooled coils use outdoor air; water-cooled condensers transfer heat to a separate water circuit.
Expansion device
Meters refrigerant and reduces its pressure before it returns to the evaporator. The cycle repeats while controls match capacity to the load.
Air-cooled versus water-cooled
Air-cooled chillers
How it works
Fans move outdoor air across condenser coils to reject heat. The chiller still supplies chilled water or another cooling fluid to the building.
Potential benefits
Avoids a cooling tower and its evaporative water use, treatment and condenser-water pumps. Often suits sites with limited water infrastructure or a preference for simpler plant support.
Trade-offs and operating needs
Hot outdoor conditions can increase compressor work and reduce available capacity. Coil cleanliness, fan noise, airflow clearances, weather exposure and freeze protection need attention.
Water-cooled chillers
How it works
Condenser heat transfers to a separate water circuit, commonly connected to a cooling tower. Chilled-water and condenser-water circuits perform different jobs.
Potential benefits
Can use favorable tower-water temperatures and serve large central plants efficiently. Indoor chiller placement can reduce outdoor equipment exposure.
Trade-offs and operating needs
Evaluate the entire plant: tower fans, pumps, water use, treatment, blowdown and water-management responsibilities. Chiller-only efficiency is not a whole-building savings guarantee.
Compressor types and magnetic bearings
Centrifugal
How it works
A rotating impeller accelerates refrigerant vapor; a diffuser converts velocity into pressure. This is dynamic compression rather than trapping a fixed volume.
Potential benefits
Well suited to many large chilled-water plants. Variable-speed and other capacity-control strategies can provide strong part-load performance within the machine's operating envelope.
Trade-offs and operating needs
Load and lift matter: lift reflects the pressure/temperature difference the machine works against. Surge limits, minimum load, staging and condenser-water control require coordinated operation.
Screw
How it works
Rotating screw elements trap refrigerant and reduce its volume. Twin-screw and other screw arrangements differ by design.
Potential benefits
Common in medium-to-large commercial and process applications, with capacity modulation through variable speed, slide valves or other design-specific methods.
Trade-offs and operating needs
Part-load efficiency depends on the unloading method and operating conditions. Many designs require oil management; noise, refrigerant and service requirements vary by model.
Scroll
How it works
Interleaved scroll elements compress refrigerant in progressively smaller pockets. Chillers often use multiple compressors or modules.
Potential benefits
Multiple compressors can stage to match changing demand. Modular arrangements can support phased installation and partial operation during service, where the system is designed for it.
Trade-offs and operating needs
Check minimum system capacity, cycling, control steps and the number of serviceable components. Multiple compressors do not guarantee system redundancy or eliminate shared-component failure risks.
Reciprocating
How it works
Pistons compress refrigerant inside cylinders. These are found in legacy chiller installations and some specialized refrigeration applications.
Potential benefits
Established service practices and cylinder unloading can make continued operation practical for suitable existing equipment.
Trade-offs and operating needs
Moving parts, vibration, noise and maintenance can be important considerations. Compare lifecycle repair and replacement options rather than assume a legacy machine should remain or be replaced.
Magnetic-bearing / oil-free centrifugal
How it works
Controlled magnetic fields support the rotating shaft without conventional bearing contact during normal operation. Oil-free centrifugal designs avoid the usual compressor lubrication circuit.
Potential benefits
Can reduce bearing friction and oil-system maintenance, and support quiet, variable-speed operation with favorable part-load performance under suitable conditions.
Trade-offs and operating needs
This is a bearing/design feature, not a separate heat-rejection category. Power electronics, controls, backup/touchdown bearings, service expertise and the operating envelope still matter. Not every oil-free design uses magnetic bearings.
Other chiller approaches
Absorption chillers
Use a heat-driven refrigerant/absorbent cycle rather than a conventional mechanical compressor. Available steam, hot water, waste heat or direct-fired input may make them useful. They still need heat rejection; water use, footprint, thermal input and specialist maintenance must be evaluated. Low compressor electricity does not mean low total energy use.
Heat-recovery chillers
Capture otherwise rejected heat for useful heating, such as a heating-water loop or indirect domestic water heating. Simultaneous useful heating and cooling demand can improve system utilization; output temperatures and seasonal load balance govern the benefit. Potable water remains separated through appropriate heat exchange.
Free cooling and modular plants
An economizer/free-cooling arrangement can use favorable outdoor conditions to reduce compressor operation, with different air- and water-side designs. Modular describes plant arrangement, not compressor type. Climate, required water temperatures, controls and design redundancy determine the result.
High-level technical overview, not equipment selection or installation instructions. Use qualified engineering, locally adopted codes and selected equipment requirements. References reviewed October 22, 2026; no manufacturer affiliation or endorsement is implied.