Modern Technology & Industry Standards

The Evolution of Compressor Technology

For decades, the reciprocating compressor was the workhorse of the HVAC industry. Reciprocating compressors use a piston-and-cylinder arrangement (much like a car engine) to compress refrigerant vapor. While reliable, reciprocating compressors have inherent limitations: they have many moving parts subject to wear, they generate significant vibration and noise, and they operate at a fixed speed, cycling on and off to match varying loads.

Modern residential and light commercial systems have largely moved away from reciprocating compressors in favor of scroll compressors and, increasingly, inverter-driven variable-speed compressors.

How a Scroll Compressor Works

The scroll compressor is the most common compressor type in residential HVAC systems today. It uses two interleaving spiral-shaped scrolls (sometimes called involutes) to compress refrigerant. One scroll is fixed (the stationary scroll), and the other orbits around it (the orbiting scroll) without rotating. The orbiting scroll is driven by the motor shaft through an eccentric bearing.

As the orbiting scroll moves, it creates crescent-shaped pockets of gas between the two scrolls. These pockets form at the outer edge of the scrolls (the suction intake) and progressively shrink as they move toward the center. By the time the gas pocket reaches the center discharge port, it has been compressed to the desired high pressure.

Key advantages of scroll compressors over reciprocating compressors:

• Fewer moving parts: A scroll compressor has approximately 70 percent fewer moving parts than a reciprocating compressor of equal capacity. Fewer parts means less wear, longer life, and higher reliability.
• Smooth, continuous compression: Unlike a reciprocating compressor that produces pulses of compressed gas (one pulse per piston stroke), a scroll compressor provides nearly continuous gas flow with minimal pulsation. This results in quieter operation and less vibration.
• Higher volumetric efficiency: Scroll compressors do not have suction and discharge valves that create re-expansion losses. This gives them higher volumetric efficiency, meaning they move more refrigerant per revolution.
• Tolerance of liquid slugging: If a small amount of liquid refrigerant enters a scroll compressor, the orbiting scroll can briefly separate from the fixed scroll (a feature called axial compliance), allowing the liquid to pass through without catastrophic damage. A reciprocating compressor, by contrast, can suffer immediate valve or connecting rod damage from liquid slugging.
• Lower noise and vibration: The smooth orbital motion produces far less vibration than the reciprocating action of a piston compressor, resulting in quieter operation.

Inverter-Driven Compressors

An inverter-driven compressor is a variable-speed compressor controlled by a Variable Frequency Drive (VFD). Instead of running at a single fixed speed and cycling on and off to match the load, an inverter-driven compressor can modulate its speed continuously across a wide range, typically from about 20 percent to 120 percent of its rated capacity.

The inverter (VFD) works by converting the incoming AC power to DC, then reconstructing it as AC at whatever frequency is needed to achieve the desired compressor speed. Standard AC power in North America is 60 Hz, which drives a standard compressor motor at a fixed speed. By varying the frequency from, say, 15 Hz to 75 Hz, the inverter can run the compressor at speeds ranging from roughly 25 percent to 125 percent of the speed it would achieve at 60 Hz.

Benefits of Inverter-Driven Compressors

Inverter-driven (variable-speed) compressors offer significant advantages:

1. Better energy efficiency: Instead of running at full capacity and cycling off when the load is met (which wastes energy during startup and creates temperature swings), a variable-speed compressor runs continuously at a lower speed that closely matches the actual load. Operating at lower speeds is inherently more efficient because the compression ratio is lower and there are no energy-wasting startups.

2. Better comfort: Because the compressor runs continuously at a matched speed, indoor temperatures remain much more stable. There are no noticeable temperature swings between the thermostat "on" and "off" points. The air coming from supply registers is consistent rather than alternating between full blast and nothing.

3. Better humidity control: This is a particularly important advantage. A variable-speed compressor running at low speed pushes air across the evaporator coil more slowly (assuming the blower also modulates, which it typically does in these systems). Slower airflow means the air stays in contact with the cold coil surface longer, allowing more moisture to condense out. The result is significantly better dehumidification, especially during mild, humid weather when a single-speed system would short-cycle and provide poor dehumidification.

4. Lower noise levels: At low speeds, both the compressor and the indoor blower motor operate much more quietly than at full speed. For much of the year, the system runs at reduced speed, providing a quieter indoor and outdoor environment.

5. Longer equipment life: Eliminating frequent on/off cycling reduces mechanical stress on the compressor, contactors, and other components. Smooth, continuous operation with gentle ramp-up and ramp-down extends equipment life.

6. Reduced inrush current: Inverter-driven compressors start slowly and ramp up gradually, eliminating the high inrush current (locked rotor amps) associated with single-speed compressor starts. This reduces stress on the electrical system and may allow the use of smaller wiring and breakers in some cases.

Multi-Speed vs. Variable-Speed

It is worth distinguishing between multi-speed (or multi-stage) and true variable-speed compressors:

• Two-stage compressors can operate at two fixed speeds: typically 67 percent capacity and 100 percent capacity. They offer some of the benefits of variable speed but with less precise load matching.
• Variable-speed (inverter) compressors can operate at any speed within their range, providing infinitely adjustable capacity. This is the technology identified on the exam as "inverter-driven" or "VFD-controlled."