A chiller produces chilled water for air handlers, fan coils, process loads and data centres. Most chillers use the vapour compression refrigeration cycle, and they are grouped mainly by the type of compressor: centrifugal, screw or scroll. Absorption chillers are the main exception, using heat rather than mechanical work to drive the cycle.
Each type has a natural capacity range, a characteristic efficiency curve and its own maintenance needs. Understanding these differences helps owners and designers narrow the field before getting into detailed selection.
Quick comparison
| Type | Typical capacity range | Compressor action | Strengths | Limitations |
|---|---|---|---|---|
| Scroll | Small, roughly up to a few hundred kW per module | Two interleaved spirals compress gas | Simple, reliable, low cost, modular | Limited capacity per compressor, staged rather than smooth capacity unless variable speed |
| Screw | Mid-range, roughly a few hundred to over 1,500 kW | Meshing helical rotors compress gas | Robust, good at high lift, handles air-cooled duty well | Can be noisy; part-load efficiency depends on capacity control |
| Centrifugal | Large, commonly from several hundred kW to many MW | High-speed impeller adds velocity, diffuser converts to pressure | Highest full and part-load efficiency in large sizes | Surge risk at low load with high lift; larger footprint and cost |
| Absorption | Mid to large | Heat drives a lithium bromide and water cycle | Uses waste heat or steam, little electricity | Low COP, large size, needs cooling tower capacity |
Capacity ranges overlap and vary by manufacturer. In imperial terms, scroll units often cover roughly up to 200 tons, screw chillers from about 70 to 500 tons and centrifugal chillers from about 150 tons upward, with oil-free magnetic bearing centrifugals extending into smaller sizes.
Scroll chillers
Scroll compressors are hermetic, have few moving parts and are very reliable. A scroll chiller usually combines several compressors on one or more refrigerant circuits and stages them on and off to match load.
Scroll chillers dominate small air-cooled applications and modular plants, where multiple identical modules are combined. Variable speed scroll compressors smooth capacity control and improve part-load efficiency. Maintenance is relatively simple, though a failed compressor is typically replaced rather than rebuilt.
Screw chillers
Screw compressors use two meshing rotors, or a single rotor with gate rotors, to trap and compress refrigerant. They handle high pressure ratios well, which makes them a common choice for air-cooled chillers in the mid-capacity range and for heat recovery or heat pump duty where condensing temperatures are high.
Capacity control is by slide valve or variable speed. Variable speed screw chillers have much better part-load efficiency than older slide-valve units. Screw compressors are oil-flooded, so oil separation and oil management are important, and periodic bearing inspection is part of long-term maintenance.
Centrifugal chillers
Centrifugal compressors accelerate refrigerant with a high-speed impeller, then convert that velocity into pressure in a diffuser. They are the most efficient option for large water-cooled plants and are standard in big central plants, hospitals, campuses and many data centres.
Surge
At low load combined with high lift (a large difference between condensing and evaporating pressures), a centrifugal compressor can reach a condition called surge, where flow reverses momentarily. Modern controls prevent surge with variable inlet guide vanes, variable speed drives and hot gas bypass on some machines. Keeping condenser water as cold as practical at low loads reduces lift and helps.
Variable speed and oil-free designs
Variable speed drives significantly improve centrifugal part-load efficiency when condenser water temperature falls, which is common for much of the year in Canada. Oil-free magnetic bearing centrifugal compressors, offered by several manufacturers, eliminate the oil system entirely and are available in smaller capacities, often in multi-compressor chillers.
Low pressure refrigerants
Some large centrifugal chillers use low pressure refrigerants such as R-1233zd(E), which operate below atmospheric pressure in parts of the system and need purge units to remove air. Others use medium pressure refrigerants such as R-134a, R-513A or R-1234ze(E). Refrigerant choice is increasingly driven by the HFC phase-down; see refrigerant regulations and low-GWP options in Canada.
Absorption chillers
Absorption chillers use water as the refrigerant and lithium bromide as the absorbent. A heat source, such as steam, hot water or direct gas firing, drives the cycle. Electricity is needed only for small pumps.
| Absorption type | Typical COP | Heat source |
|---|---|---|
| Single-effect | Roughly 0.6 to 0.8 | Low pressure steam or hot water, often from waste heat |
| Double-effect | Roughly 1.0 to 1.4 | Higher pressure steam or direct firing |
These COPs are far lower than electric chillers, so absorption makes sense mainly where heat is cheap or free: waste heat from combined heat and power, district steam in summer, or process heat. They reject more heat per unit of cooling than electric chillers, so cooling towers and condenser pumps are larger. Crystallization of lithium bromide is a risk if controls or condenser water temperatures are not managed. See combined heat and power for commercial facilities.
Air-cooled vs water-cooled
Compressor type and heat rejection method are separate choices. Scroll and screw chillers are widely available in both air-cooled and water-cooled versions. Centrifugal chillers are almost always water-cooled, although some air-cooled centrifugal models exist. Absorption chillers are water-cooled. The trade-offs are covered in water-cooled vs air-cooled chillers.
How to narrow the choice
- Establish peak and typical loads, and how many hours the plant runs at part load.
- Decide on redundancy and how many chillers the plant should have.
- Determine whether water-cooled heat rejection is acceptable on the site.
- Identify any waste heat or steam that could drive absorption.
- Compare candidate chillers on full-load and part-load efficiency at your actual operating conditions, not only on rated values.
- Consider refrigerant, sound, footprint, service access and local service support.
- Run life cycle cost comparisons with energy modeling.
Frequently asked questions
Which chiller type is most efficient?
In large water-cooled plants, centrifugal chillers, especially variable speed models, usually deliver the best efficiency. For small and mid-size plants, variable speed screw and scroll chillers can be very competitive. Actual performance depends on operating conditions.
Are absorption chillers still used?
Yes, mainly where waste heat or low-cost steam is available, such as with combined heat and power or district steam systems.
Key takeaways
- Chillers are classified mainly by compressor type: scroll, screw and centrifugal, plus heat-driven absorption.
- Scroll suits small and modular plants, screw suits mid-size and air-cooled duty, centrifugal suits large water-cooled plants.
- Variable speed drives improve part-load efficiency for every compressor type.
- Absorption makes sense only where heat is cheap or free.
- Compare chillers at real operating conditions, ideally supported by energy modeling.
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