Compressor types in chillers and condensing units: scroll, reciprocating, screw and centrifugal
What the compressor tells you about capacity range, part-load behaviour, oil and service.
A chiller or condensing-unit quotation usually names the compressor type in one word. That word predicts a lot: the size range the equipment sits in, how it behaves at part load, whether it has an oil system to maintain and how a service technician will open it. Reading it correctly helps you compare offers that look alike on capacity.
Four compression principles
US EPA's page on refrigerant substitutes in chillers describes four compressor families and their typical capacity ranges.
Reciprocating compressors use pistons in cylinders. EPA gives chillers of this type a range of roughly 10 to 1,800 kW. They remain common in small condensing units and in some process duties.
Scroll compressors compress refrigerant between a fixed and an orbiting scroll. EPA gives the same 10 to 1,800 kW range for scroll chillers, which reach larger capacities by combining several compressors on one or more circuits.
Screw compressors, single or twin rotor, reduce the volume between rotating screws. EPA puts screw chillers at roughly 10 to 7,000 kW, air-cooled or water-cooled.
Centrifugal compressors raise pressure with an impeller. They are water-cooled machines in EPA's description, from about 200 kW to 21,000 kW.
| Type | How it compresses | Typical chiller capacity (EPA) | Usual capacity control |
|---|---|---|---|
| Reciprocating | Pistons in cylinders | about 10–1,800 kW | Cylinder unloading or compressor staging |
| Scroll | Fixed and orbiting scroll | about 10–1,800 kW | Compressor staging, sometimes one variable-speed compressor |
| Screw | Single or twin rotating screws | about 10–7,000 kW | Slide valve or variable speed |
| Centrifugal | Rotating impeller | about 200–21,000 kW, water-cooled | Inlet guide vanes, variable speed or both |
These ranges overlap, so the type alone does not decide a project. It narrows the field and tells you which follow-up questions matter.
Hermetic, semi-hermetic and open
The second word to look for describes the enclosure. A hermetic compressor seals the motor and compressor in a welded shell, typical of scroll compressors and the small appliance compressors made by ZEL in Tianjin. A semi-hermetic compressor can be opened for service; many screw machines are built this way. An open compressor has an external motor and a shaft seal, as in the Dunham-Bush LSCP open-screw package, which brings coupling alignment and seal maintenance into the scope.
Part-load behaviour is where they differ most
Most HVAC plant runs below full load most of the year, so the way a compressor unloads matters as much as its full-load figure. Reciprocating compressors unload cylinders or are switched in stages. Scroll chillers usually step capacity by switching compressors on and off, sometimes with one variable-speed compressor. Screw compressors commonly use a slide valve or variable speed. Centrifugal machines use inlet guide vanes, variable speed or both, and must avoid surge at low lift; the Xinwuxin integrated centrifugal chillers page discusses surge and oil questions directly.
Newer centrifugal designs remove oil from the compressor. The Dunham-Bush DCLC(D)M magnetic bearing family and the gas bearing DCLCDA family support the shaft without an oil system, which changes maintenance tasks and the acceptance checks you should ask for. Compare their part-load figures on the same rating basis; our chiller quotation guide explains how.
How compressor choice shows on this site
Water-cooled screw chillers are the most common chiller type in the catalogue. Compare the Hon Ming water-cooled screw family, the Zhongda water-cooled screw catalogue, the Sinuo water-cooled screw chiller and the Xinwuxin screw chillers on refrigerant, number of compressors and unloading method before price. Dunham-Bush's WCFX-RH heat recovery family shows a screw chiller arranged to recover condenser heat, while its twin-head DCLCDT ice-duty chiller shows a centrifugal machine staged for ice making.
Air-cooled machines bring ambient limits into the comparison. The Xinwuxin air-cooled screw chillers and the Dunham-Bush outdoor screw units publish defrost or ambient options that only apply to their circuit arrangement.
In refrigeration, several compressors often share one rack. The Xinwuxin parallel screw packages and the Sino King low-temperature screw family raise oil-return and evaporating-temperature questions that a single-compressor chiller does not.
Questions to ask
Ask for the compressor make, model and quantity, and how many refrigerant circuits the unit has. Ask how capacity is controlled and the minimum stable load, stated as a percentage of full load.
Ask whether the compressor is hermetic, semi-hermetic or open and which parts can be serviced on site. For machines with oil, ask for the oil type, the oil-return arrangement and the recommended oil analysis interval. For oil-free machines, ask what the bearing system needs at power loss.
Ask for the refrigerant and its pressure rating for the whole circuit; our page on refrigerants explains why the gas and the compressor should be read together.
Common questions
Is a screw chiller always more efficient than a scroll chiller? No. Efficiency depends on the selection, the condenser type and part-load control. Compare certified or tested figures under the same conditions.
Why does a quotation show several scroll compressors instead of one large one? Multiple compressors give capacity steps and some redundancy. Ask how the controller stages them and what happens if one fails.
Does an oil-free centrifugal chiller need no maintenance? It removes oil management, not maintenance. Bearings, drives, controls and water-side cleaning still need a schedule.
By HVACDeck Editorial · Updated 2026-11-04
References and scope
Technical references support the points identified in this guide. They do not certify any directory supplier.