2.2.1 — Reciprocating Compressors
Reciprocating compressors use pistons moving within cylinders to compress refrigerant vapour — operating on the same principle as an automotive engine, with intake and discharge valves controlling refrigerant flow. They have been the workhorses of the refrigeration industry for over a century.
Operating Principle
Suction and Compression Strokes
- Suction stroke: the piston moves toward bottom dead centre (BDC), creating a low-pressure region; when cylinder pressure falls below suction pressure, the suction valve opens and refrigerant vapour enters
- Compression stroke: the piston moves toward top dead centre (TDC), compressing the trapped vapour; when cylinder pressure exceeds discharge pressure, the discharge valve opens and compressed vapour exits
- The clearance volume between the piston at TDC and the valve plate is minimized to maximize volumetric efficiency; trapped high-pressure gas must re-expand before new vapour can enter on the next suction stroke
Types of Reciprocating Compressors
Open-Drive
Motor is external to the compressor housing, connected by a shaft seal and coupling. Allows motor replacement without recovering the refrigerant charge and permits use of various motor types. The shaft seal is a potential leak point requiring periodic maintenance.
Hermetic
Motor and compressor are enclosed in a welded steel shell; motor is cooled by returning suction gas. Eliminates shaft seal leakage but cannot be field-serviced — the entire unit must be replaced if either the motor or compressor fails.
Semi-Hermetic (Serviceable)
Similar to hermetic but with a bolted housing that permits field service of internal components. Combines hermetic sealing with serviceability at a higher cost than fully hermetic units.
Characteristics
| Capacity Range | 0.5 to 150 tons (1.76 to 527 kW) |
| Pressure Capability | High — suitable for most refrigerants |
| Efficiency | Good at full load; decreases at part load |
| Reliability | Proven technology; numerous moving parts |
| Applications | Commercial refrigeration, residential AC, transport refrigeration, industrial |
Advantages
- Proven, well-understood technology
- Handles high pressure ratios effectively
- Available in a wide range of sizes
- Capacity control through cylinder unloading
- Relatively tolerant of small liquid slugs
Disadvantages
- Many moving parts subject to wear
- Higher vibration and noise levels
- Requires positive lubrication system
- Valves subject to wear and damage
- Pulsating discharge flow