What Is a Scroll Pump?
A scroll pump is a positive-displacement device that moves fluid — gas or liquid — using two interleaved spiral-shaped scrolls. One scroll remains stationary (the fixed scroll), while the other orbits eccentrically without rotating. This orbital motion creates a series of crescent-shaped pockets between the two scrolls that progressively shrink from the outer edge toward the center, compressing or displacing the working fluid in the process.
Scroll pumps are widely used in vacuum generation, refrigeration, air compression, and laboratory applications. Their smooth, continuous action sets them apart from piston or diaphragm pumps.
The Two Scrolls: Fixed and Orbiting
The fundamental components of any scroll pump are:
- Fixed scroll: Bolted or mounted rigidly to the pump housing. Its spiral geometry mirrors the orbiting scroll.
- Orbiting scroll: Driven by an eccentric shaft (crankshaft), it traces a circular orbital path — it does not spin on its own axis.
- Anti-rotation mechanism: A set of pins, an Oldham coupling, or similar device prevents the orbiting scroll from rotating, ensuring it only translates in a circular orbit.
The Compression Cycle Step by Step
- Intake (suction): Gas enters at the outer periphery of the scroll set. As the orbiting scroll moves, pockets of gas are trapped between the two spiral wraps.
- Compression: The trapped gas pockets move inward toward the center as the orbit continues. The pocket volume decreases, raising gas pressure.
- Discharge: When the pocket reaches the center port, the compressed gas is discharged. In vacuum pumps, this is the exhaust; in compressors, this feeds the high-pressure line.
Multiple pockets exist simultaneously at different stages of this cycle, which is why scroll pumps deliver an exceptionally smooth, low-pulsation flow compared to reciprocating alternatives.
Key Thermodynamic Characteristics
Scroll pumps can operate under near-isothermal conditions because the compression process happens gradually over many orbital cycles. This contributes to:
- Lower gas temperatures at discharge compared to reciprocating pumps
- Higher volumetric efficiency in many operating ranges
- Reduced thermal stress on seals and tip strips
Tip Sealing and Radial Compliance
One engineering challenge is maintaining a seal between the scroll wraps without rigid metal-to-metal contact. Most scroll pumps use tip seals — thin strips of PTFE or similar polymer seated in grooves at the tip of each spiral wrap — to bridge the small gap between the wrap tip and the opposing scroll plate.
Many designs also incorporate radial compliance, meaning the orbiting scroll can flex slightly outward under pressure differences, automatically compensating for wear and maintaining the seal over the pump's service life.
Why No Oil Is Needed (in Dry Scroll Designs)
Because the compression mechanism relies on the scroll geometry rather than oil films to seal clearances, dry scroll pumps achieve clean, oil-free operation. Tip seals and compliant radial geometry handle the sealing function. This makes them ideal for cleanroom, laboratory, and medical applications where hydrocarbon contamination is unacceptable.
Summary
The elegance of scroll pump technology lies in its simplicity: two mating spirals, one fixed and one orbiting, progressively trap and compress fluid with minimal moving parts, low vibration, and no need for valves. Understanding these operating principles is the foundation for choosing, maintaining, and troubleshooting scroll pumps effectively.