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Scroll vs Screw Chiller Compressor Selection

Reference Equipment Updated Jul 24, 2026 ยท HVAC

Why this matters

The compressor choice on a 100 to 400 ton commercial chiller drives first cost, part-load efficiency, sound-power signature, refrigerant inventory, and the future cost of a compressor swap. Scroll machines are simple, multi-circuit, and forgiving; screw machines are higher-IPLV, single-circuit per compressor, and unforgiving when oil management or VFD programming is wrong. Picking the wrong machine for a load profile means an owner pays for capacity they never modulate to or runs a small machine at full load through every shoulder hour. The selection sits between mechanical engineering and operations economics, and it is where a senior tech earns the price-book line.

Scroll basics

A scroll compressor is two interleaved spiral elements; one orbits, one is fixed. Refrigerant pockets shrink as gas moves from suction periphery to discharge center. No suction valves, no discharge valves. Tandem and trio configurations stack 2 to 4 scrolls per refrigeration circuit; multi-circuit chillers stack 4, 6, or 8 scrolls across two refrigeration circuits. Capacity steps via on/off staging of individual compressors, augmented by digital scroll modulation (Copeland Digital, Danfoss VSD) on the lead machine for the bottom 10 to 50 percent unload range.

Screw basics

A twin-screw compressor uses two intermeshed rotors (male, four lobes; female, six lobes, on a typical 4:6 profile). Refrigerant compresses continuously along the rotor length; no valves. Capacity modulation is via a slide valve that shortens the effective compression length, or via a VFD that varies rotor speed. A single screw provides 70 to 700+ tons in one machine; chillers below about 150 tons usually run a single screw, larger chillers may run two screws on a single refrigerant circuit or two circuits.

Part-load efficiency (IPLV)

Most commercial chillers run at part load 90 percent of operating hours; full-load EER matters less than IPLV. Scroll chillers with multi-circuit staging post IPLV values of 12 to 15 EER (about 0.8 to 0.95 kW/ton). VFD screws post IPLV values of 16 to 21 EER (about 0.55 to 0.75 kW/ton) because the rotor speed tracks load down to roughly 25 percent without losing isentropic efficiency. AHRI Standard 550/590-2023 publishes the test method; the IPLV number on the AHRI directory listing is the comparable apples-to-apples figure.

Service and reliability profile

Scrolls have no internal valves, no slide valve mechanism, no oil-flooded compression process. Mean time between failures (manufacturer-reported) often exceeds 100,000 hours on a properly applied scroll. A failed scroll in a tandem can sometimes be isolated and the chiller stays in service at reduced capacity. Screws have a slide valve actuator, oil management (separator, cooler, return line), and tight bearing tolerances; a single-screw chiller losing its compressor is out of service until replacement. Plan for screw machines to need a major oil/filter overhaul at 5 to 7 years and rotor inspection at 10 to 12 years.

Refrigerant charge and leak risk

Scroll chillers carry 1 to 3 lb of refrigerant per ton (multi-circuit, smaller volumes per circuit). Screws carry 2 to 5 lb per ton including the oil separator and the longer interconnecting piping. A2L refrigerant transitions (R-454B, R-32) and the older R-410A all live well in scroll; screw OEMs are moving toward R-1234ze and R-513A on the newer water-cooled screws. Per ASHRAE 15-2022 Section 7.3, the maximum permissible refrigerant concentration in occupied space drives the chiller-room ventilation and detection design; smaller per-circuit charges on scroll chillers ease this analysis.

Selection by load profile

Pick scroll multi-circuit when:

  • Load profile has many partial-load hours but is bracketed by frequent full-load peaks (typical office, retail).
  • Building owner values N+1-style redundancy at the compressor level.
  • First cost matters more than peak IPLV.
  • Service technician staffing is general HVAC, not chiller specialists.

Pick VFD screw when:

  • Load profile is dominated by 20 to 60 percent part-load (data centers, hospitals, hotels with central plant).
  • Energy code or LEED IPLV targets are tight (LEED EAc1, 90.1 above-code).
  • Plant has on-site or contracted chiller-specialist coverage.
  • First cost premium is justified by 15+ year operating savings.

References

  • AHRI Standard 550/590-2023, Performance Rating of Water-Chilling and Heat Pump Water-Heating Packages Using the Vapor Compression Cycle
  • ASHRAE Standard 90.1-2022, Section 6.4.1.1 Minimum Equipment Efficiencies, Tables 6.8.1
  • ASHRAE Standard 15-2022, Safety Standard for Refrigeration Systems
  • ASHRAE Handbook HVAC Systems and Equipment 2020, Chapter 38 Compressors
  • Carrier AquaForce 30XV/30RB and Trane Sintesis RTAF Product Engineering Guides

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