Compressor

The mechanical heart of any refrigeration system, compressing low-pressure refrigerant gas from the evaporator into high-pressure, high-temperature gas for the condenser, driving the entire refrigeration cycle. Compressor technology, sizing, and efficiency directly determine whether a transport refrigeration system can maintain frozen temperatures under actual South African operating conditions—or whether it struggles, wastes fuel, and fails during peak thermal loads.

Compressor Function in Refrigeration

The compressor performs essential functions:

Draws refrigerant from evaporator at low pressure
Compresses gas, increasing pressure and temperature
Discharges high-pressure gas to condenser
Creates pressure differential that drives refrigerant circulation
Enables heat rejection by raising refrigerant temperature above ambient

Without adequate compression, refrigerant cannot release heat to atmosphere, and cooling cannot occur.

Compressor Types in Transport Refrigeration

Type	Mechanism	Efficiency	Noise	Best Application
Reciprocating	Piston in cylinder	70-80% isentropic	Higher	Low-temp, variable loads
Scroll	Interleaved spirals	85-92% isentropic	Lower	AC, moderate refrigeration
Rotary	Rotating elements	Moderate	Low	Small capacity units

Reciprocating Compressors

Traditional technology, well-understood
Effective at high pressure ratios (frozen temperatures)
More moving parts, more maintenance
Handles wide operating range well
Common in transport refrigeration

Scroll Compressors

Higher efficiency at design point
70% fewer moving parts than reciprocating
Smoother, quieter operation
Less effective at extreme pressure ratios
Growing adoption in transport applications

Compressor Sizing and Altitude

Compressor capacity is rated at sea level. At Johannesburg altitude (1,750m):

Air density reduced 18%
Compressor volumetric efficiency decreased ~12%
Effective capacity reduced significantly
Systems must be oversized to compensate

Altitude Capacity Example:

Compressor rated: 5.0 kW at sea level
Altitude correction: -12% per 1,000m
At 1,750m: 5.0 kW × (1 - 0.12 × 1.75) = 3.95 kW actual

Required capacity: 4.0 kW
Available capacity: 3.95 kW
Result: Marginal—system runs at maximum continuously

Equipment specified without altitude correction operates at the edge of capability, causing:

Extended compressor runtime
Inability to recover from door openings
Accelerated wear and reduced lifespan
Temperature excursions during peak loads

Fixed-Speed vs Variable-Speed Compressors

Traditional fixed-speed compressors cycle on/off:

Full power when running, zero when off
3-4× startup current surge
Mechanical stress from cycling
Temperature fluctuation between cycles

Variable-speed compressors modulate continuously:

Speed matches actual cooling demand
No startup surges
Smoother temperature control (±0.5°C vs ±3-5°C)
20-40% energy savings
Extended equipment life

Variable-speed technology costs more initially but delivers superior performance and lower lifecycle cost—particularly valuable in multi-stop delivery with varying thermal loads.

Compressor Energy Consumption

Compressors dominate TRU energy consumption:

70-80% of total refrigeration energy
Directly driven by diesel engine (belt-drive) or electric motor
Fuel consumption proportional to compressor load
Efficiency improvements yield direct fuel savings

Energy Impact Calculation:

Compressor power: 3 kW average
Operating hours: 2,500/year
Fuel equivalent: 3 kW × 2,500 hrs = 7,500 kWh
At 10 kWh/L diesel: 750 L/year compressor fuel

20% efficiency improvement: 150 L/year savings
At R18/L: R2,700/year per vehicle

Compressor Maintenance Indicators

Warning signs of compressor problems:

Extended runtime to maintain temperature
Inability to reach setpoint
Unusual noise or vibration
Oil pressure abnormalities
Elevated discharge temperature
Increased fuel consumption

Addressing issues early prevents catastrophic failure during delivery operations.

Related Terms: Transport Refrigeration Unit (TRU), Evaporator, Condenser

Compressor

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