Refrigeration compressor technology operating at a single constant speed determined by electrical supply frequency (50Hz in South Africa = approximately 3,000 RPM), requiring wasteful on/off cycling to match variable cooling demands—a 1960s design that transport refrigeration manufacturers continue installing despite 25-35% efficiency penalties compared to modern variable-speed alternatives.
How Fixed-Speed Compressors Work
Fixed-speed compressors run at maximum capacity whenever energized, then shut off completely when the thermostat reaches setpoint. The system cycles between full-power operation and complete shutdown, unable to modulate output to match actual cooling requirements. This binary operation—100% or 0%—fundamentally mismatches the variable thermal loads encountered in multi-stop delivery operations.
The Efficiency Problem
When cooling demand drops—during cool weather, overnight operation, or after cargo unloading reduces thermal mass—a fixed-speed compressor still runs at maximum capacity for short bursts. These brief high-power cycles are inherently inefficient for several reasons:
- Startup surge: Compressors draw 3-6x running current during startup, creating energy spikes with every cycle
- No partial-load efficiency: Running at full speed when 30% capacity would suffice wastes 70% of potential efficiency
- Short run times: Brief operation prevents reaching optimal steady-state efficiency
- Excessive cycling: Variable loads cause 30-50 daily start/stop cycles versus 6-8 for properly matched systems
Research shows fixed-speed systems in variable-load applications consume 25-35% more energy than variable speed compressors performing identical cooling work.
Equipment Wear Implications
Each start/stop cycle subjects compressor components to mechanical and thermal stress. Compressor cycling at 30-50 times daily—compared to near-continuous operation with variable-speed—represents 400-600% increase in wear-inducing events. This dramatically shortens equipment lifespan, turning 15-20 year systems into 8-10 year replacements.
Why Fixed-Speed Persists in Transport Refrigeration
Fixed-speed compressors cost R2,000-3,000 less than variable-speed equivalents. Fleet buyers focused on purchase price rather than total cost of ownership choose cheaper fixed-speed systems, then spend R30,000+ more over equipment lifetime on fuel and maintenance. TRU manufacturers profit from both higher fuel consumption (diesel-powered systems) and increased maintenance requirements from excessive cycling wear.
The Variable-Speed Alternative
Modern variable-speed compressors using inverter drives modulate from 1,500-4,500 RPM based on actual cooling demand. Running slower during light loads achieves higher Coefficient of Performance while eliminating cycling losses. The 8-18 month payback period makes variable-speed economically superior for any operation planning beyond short-term thinking.
Related Terms: Variable Speed Compressor, Compressor Cycling, Energy Efficiency (Cold Chain)
Related Articles: Technical Formulas Reference – Variable Speed Compressor ROI