The periodic process of melting ice accumulation from evaporator coils to restore heat transfer efficiency, temporarily suspending cooling while electric heaters or hot gas warm the coils. Defrost cycles are necessary for frozen transport but consume significant energy and interrupt temperature maintenance—making defrost control strategy a major determinant of system efficiency and operating cost.
Why Defrost is Necessary
In frozen transport refrigeration:
- Evaporator coils operate below freezing (typically -25°C to -30°C)
- Humid air contacts cold coils, moisture condenses and freezes
- Ice accumulation insulates coils, reducing heat transfer
- Efficiency degrades progressively as ice builds
- Eventually, airflow blocked entirely
Without periodic defrost, evaporator ice accumulation would render refrigeration ineffective within hours of operation.
Defrost Cycle Operation
During a typical defrost cycle:
- Compressor stops (no cooling)
- Electric heaters energize (or hot gas diverts to evaporator)
- Ice melts from coil surfaces
- Meltwater drains from evaporator
- Heaters de-energize when coils clear
- Compressor restarts, cooling resumes
Defrost cycle parameters:
- Duration: 15-30 minutes typical
- Heater power: 1.5-3.0 kW
- Frequency: Every 4-12 hours (timer-based) or as-needed (demand-based)
Defrost Control Strategies
| Strategy | How It Works | Efficiency |
|---|---|---|
| Timer-based | Fixed intervals regardless of ice | Poor (60% waste) |
| Demand-based | Sensors detect actual ice buildup | Good (20-30% savings) |
| Adaptive | AI learns optimal timing | Best (additional savings) |
Timer-Based Defrost Waste
Our Technical Formulas Reference documents defrost cycle energy waste:
Timer-based systems defrost on fixed schedule regardless of actual ice accumulation:
- Defrost cycles run whether needed or not
- 60% of defrost cycles unnecessary under typical conditions
- Each unnecessary cycle wastes energy and interrupts temperature stability
Annual Waste Calculation:
Unnecessary cycles: 60% × 500 cycles/year = 300 cycles
Energy per cycle: 2.5 kW × 0.42 hrs = 1.04 kWh
Annual waste: 313 kWh ≈ 31 L diesel equivalent
Cost: ~R560/year per vehicleMultiply by fleet size and equipment lifetime for total waste from timer-based defrost.
Multi-Stop Defrost Challenges
Multi-stop delivery creates accelerated ice accumulation:
- Frequent door openings introduce humid ambient air
- Moisture-laden air contacts cold evaporator
- Ice accumulates faster than long-haul transport
- More frequent defrost cycles required
Timer-based systems set for long-haul intervals may not defrost frequently enough for courier operations, causing efficiency degradation. Demand-based systems adapt automatically to actual conditions.
Defrost Timing Considerations
Defrost cycle timing affects temperature stability:
- Cargo temperature rises during defrost (no cooling)
- Recovery time required after defrost completes
- Defrost during delivery stop prevents customer temperature complaints
- Pre-delivery defrost ensures optimal temperature at handoff
South African Defrost Factors
South African conditions affect defrost requirements:
Humidity Variation
- Coastal areas (Durban, Cape Town): Higher humidity, more ice accumulation
- Inland (Johannesburg): Lower humidity, less frequent defrost needed
- Timer-based systems can’t adapt to regional differences
Summer Moisture
- Afternoon thunderstorms increase humidity
- Rapid ice accumulation after rain
- Demand-based systems respond; timer systems don’t
Altitude Effects
- Lower air density at altitude affects moisture content
- May reduce ice accumulation rate in Gauteng
- Potential for timer-based over-defrost
Related Terms: Demand-Based Defrost, Timer-Based Defrost, Evaporator
Related Articles: The Defrost Cycle Dictatorship