Mechanical
The Scroll Compressor: Two Spirals That Squeeze Gas
The Scroll Compressor is a positive-displacement gas compressor that traps refrigerant or air between two nested involute spirals — one fixed, one orbiting — and squeezes it in a smooth, continuous inward march to a central discharge port. Because the orbiting scroll never rotates on its own axis (it only orbits in a small circle), the machine has almost no reciprocating mass, few moving parts, and a nearly pulsation-free flow. That combination makes it the quiet, efficient heart of most modern residential air conditioners, heat pumps, and oil-free medical air systems.- Invented / patentedLéon Creux, 1905 (US 801,182); mass-produced from the early 1980s
- Orbit radius≈ 3–6 mm; orbiting scroll does NOT spin
- Typical speed3,000–3,600 rpm (50/60 Hz), or 900–7,000 rpm on an inverter
- Isentropic efficiency≈ 70–75%, ~5–10 points above equivalent reciprocating units
- Common capacity1–25 tons (3.5–90 kW) cooling; HVAC discharge ~2.5–3.5 MPa (R-410A)
- Flank clearancesingle-digit µm; scrolls machined to ~5 µm profile tolerance
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
The geometry: two involute spirals, one orbiting inside the other
The whole machine is built around a single curve — the involute of a circle, the path traced by the end of a string unwound from a cylinder. Two identical involute walls, standing on end like the wall of a snail shell, are the scrolls. One (the fixed scroll) is bolted to the housing; the other (the orbiting scroll) is nested inside it, rotated 180° so the two walls interleave with a nearly constant gap between flanks.
The key trick: the orbiting scroll orbits but does not spin. Its centerline moves around a small circle whose radius is set by the wrap geometry — the orbit radius r_o = π·r_b − t (half the wrap pitch minus the wall thickness t), typically 3–6 mm — driven by an eccentric on the motor shaft, while an Oldham coupling — a floating cross-slider — prevents it from rotating about its own axis. Every point on the orbiting scroll therefore travels the same small circle in phase. Where the two walls touch, they seal, and the touch-lines sweep smoothly along the spirals as the orbit proceeds.
This anti-rotation is why the animation shows the inner scroll wobbling in a tight circle rather than turning like a fan. If it were allowed to spin, the pockets would never form.
How gas gets squeezed: crescent pockets that shrink inward
As the orbiting scroll circles, the sealing contact points between the walls define a set of closed, crescent-shaped (lune-shaped) gas pockets. Here is the cycle, following one charge of gas:
- Suction (outer edge): As the outermost contact lines separate, a pocket opens to the suction port at the spiral's outer rim and fills with low-pressure gas.
- Sealing: Continued orbiting pinches the pocket off from suction, trapping a fixed mass of gas — one full crescent per side, so two symmetric pockets form each revolution.
- Compression (marching inward): Because each spiral wrap is tighter than the one outside it, the trapped pocket's volume shrinks as its contact lines walk toward the center. Over ~2–3 revolutions the pocket migrates inward, its volume steadily decreasing, its pressure and temperature rising.
- Discharge (center): At the innermost wrap the pocket connects to the central discharge port and vents the now high-pressure gas out through the middle of the fixed scroll.
Crucially, several pockets exist at once at different stages of compression, so suction, compression, and discharge all happen continuously and simultaneously — the source of the scroll's smooth, low-pulsation flow.
The physics and the governing numbers
A scroll is a positive-displacement machine, so its compression is set by geometry, not by throttling. The volume of a trapped pocket varies with orbit angle θ; for an ideal involute the pocket volume falls roughly linearly with wrap number, giving a fixed built-in volume ratio:
BVR = V_suction ÷ V_discharge
For that mechanical volume ratio the gas follows an adiabatic (near-isentropic) law:
P_discharge ÷ P_suction ≈ (V_suction ÷ V_discharge)^κ = BVR^κ
where κ (=cₚ/cᵥ) is ≈1.13 for R-410A and ≈1.4 for air. A scroll with a BVR of about 2.5–3.2 built for R-410A therefore develops a design pressure ratio near 3–3.5 — matching a condensing pressure of ~2.5–3.5 MPa against ~0.9–1.0 MPa suction. The ideal specific work is w = κ/(κ−1) · R·T₁ · [(P₂/P₁)^((κ−1)/κ) − 1]; real machines land at 70–75% isentropic efficiency because of flank leakage, heat transfer, and discharge-port throttling.
Sealing is by tip seals (thin plastic strips in the top of each wrap) plus a thin oil film; flank clearances are held to single-digit micrometers, which is why the scrolls are ground to profile tolerances around 5 µm.
Keeping the scrolls together: axial and radial compliance
A rigid scroll set would either bind (over-tight) or leak badly (over-loose) once thermal growth and manufacturing spread are added. Real compressors are deliberately compliant:
- Axial compliance — high-pressure discharge gas is fed to a chamber behind the fixed (or orbiting) scroll baseplate, pressing the two scrolls together with just enough force to seal the wrap tips, and letting them separate momentarily if liquid refrigerant or debris enters. This float is what lets a scroll tolerate short liquid slugs that would hydraulically lock a piston.
- Radial compliance — the eccentric drive uses a swing link or slider so the orbiting scroll can move slightly outward under centrifugal load, keeping the flanks lightly loaded against each other for sealing without over-scrubbing.
Copeland's Scroll ("Copelametic/Copeland Scroll") family, launched commercially in 1987, made these compliance schemes the standard, and they are the main reason scrolls became more reliable than the pistons they replaced. The trade-off: compliance forces and the sliding contact absolutely depend on the oil film, so oil return and refrigerant-liquid management become the dominant reliability concerns.
The built-in volume ratio: greatest strength and key limitation
The fixed BVR is a double-edged sword. It gives efficient, valveless compression at the design point — but the real system pressure ratio changes with weather and load. When the compressor's mechanical ratio doesn't match the system's demanded ratio, you get:
- Over-compression — at low condensing pressure (mild weather) the scroll compresses past the needed discharge pressure, then the gas blows down at the port, wasting work.
- Under-compression — at high condensing pressure the pocket opens to discharge before reaching it, and high-pressure gas back-flows into the pocket, again wasting work.
Engineers attack this several ways: vapor injection (economizer / EVI ports partway along the wrap) to boost capacity and shift the effective ratio in cold-climate heat pumps; variable-speed inverter drives (900–7,000 rpm) that modulate capacity 10–100% while staying near best efficiency; and Copeland's Digital Scroll, which axially lifts the scrolls apart for part of each cycle to unload capacity smoothly. The classic failure mode remains flooded-start slugging and oil dilution: liquid refrigerant migrating to the sump overnight can wash out the oil film at start-up, so systems use crankcase heaters and hard-shutoff logic. Running a fixed-speed scroll backward (reversed rotation from a miswired 3-phase supply) makes it pump the wrong way and can destroy it in minutes — a notorious field pitfall.
Where scrolls are used — and what they replaced
Scroll compressors dominate the 1–25 ton (3.5–90 kW) band. Real-world homes for them include:
- Residential & light-commercial HVAC: the outdoor unit of nearly every modern split air conditioner and heat pump (Copeland Scroll, Danfoss/Sanhua, Daikin, LG, Mitsubishi) uses one; R-410A and increasingly R-32/R-454B units.
- Automotive electric A/C: battery-EV cabin cooling and battery thermal loops use compact, high-speed, oil-managed electric scrolls (e.g., in Tesla, Toyota, and many EV platforms) because they run smoothly from a variable-frequency inverter.
- Oil-free air: dry, lubricant-free scroll air compressors (Atlas Copco SF, Anest Iwata, Hitachi) supply clean air for hospitals, dental clinics, labs, and food packaging, where oil contamination is unacceptable — typically 0.5–5.5 kW, ~0.7–1.0 MPa.
- Refrigeration & transport: supermarket racks and reefer units.
They largely displaced small reciprocating (piston) compressors in this range because they run quieter (~65–72 dBA), have fewer wearing parts, no suction/discharge valves to fatigue, and give 5–10 points better efficiency. Above ~30–50 tons, screw compressors take over; for very high flow at low ratio, centrifugal machines win.
| Attribute | Scroll compressor | Reciprocating compressor |
|---|---|---|
| Moving parts | ~half as many; no suction/discharge valves | Pistons, rings, wrist pins, suction & discharge reed valves |
| Flow character | Continuous, near pulsation-free; multiple pockets compress at once | Intermittent, pulsating; one charge per cylinder per revolution |
| Isentropic efficiency | ≈ 70–75% (higher at design point) | ≈ 60–70%, drops with valve losses & re-expansion |
| Noise & vibration | Low (~65–72 dBA); smooth torque | Higher; inertial imbalance from reciprocating mass |
| Part-load behavior | Excellent with inverter/digital modulation; no re-expansion loss | Fixed built-in ratio-free but valve losses grow at part load |
| Weak point | Fixed built-in volume ratio → over/under-compression off-design; liquid-slug tolerant only via axial compliance | Valve fatigue, cannot swallow liquid slugs (hydraulic lock) |
Frequently asked questions
Does the orbiting scroll spin or just orbit?
It only orbits — every point travels a small circle of typically 3–6 mm radius — but it never rotates about its own axis. An Oldham coupling (a floating cross-slider) constrains it against self-rotation. If it were allowed to spin, the sealing contact lines that form the crescent pockets would never establish, so no compression would occur.
Why don't scroll compressors need suction and discharge valves?
Because compression is purely geometric. Each pocket automatically opens to suction at the outer rim, seals as it migrates inward, and opens to the central discharge port at the end of its travel — the geometry itself does the timing. Eliminating reed valves removes a major fatigue/failure source and the valve throttling losses that hurt reciprocating-compressor efficiency.
What is the 'built-in volume ratio' and why does it matter?
It's the fixed ratio of a pocket's sealed suction volume to its volume at the discharge port, set entirely by the spiral geometry (often ~2.5–3.2). It fixes the design pressure ratio via P₂/P₁ ≈ BVR^κ. When the system's demanded ratio differs from this fixed value you get over- or under-compression, which wastes work — the main reason variable-speed and vapor-injection scrolls exist.
Can a scroll compressor tolerate liquid refrigerant?
Better than a piston can, but only briefly. Axial compliance lets the two scrolls float apart momentarily to pass a small liquid slug or debris instead of hydraulically locking. Sustained liquid flooding still washes out the oil film and can cause slugging damage, so systems use crankcase heaters, accumulators, and controlled start-up logic to keep liquid out of the sump.
How efficient are they compared with piston compressors?
A well-matched scroll reaches roughly 70–75% isentropic efficiency, about 5–10 percentage points above an equivalent small reciprocating compressor. The gains come from having no valve losses, minimal re-expansion (there's no clearance volume being re-expanded each stroke), continuous multi-pocket flow, and low friction — plus much lower vibration and noise (~65–72 dBA).
What most commonly kills a scroll compressor in the field?
Loss of the oil film — from liquid-refrigerant dilution at flooded start, poor oil return in long-line-set systems, or overheating from low charge/high superheat. A second classic is reversed rotation: a miswired 3-phase supply spins a fixed-speed scroll backward, so it pumps the wrong direction, overheats, and can be destroyed within minutes if not caught.