The total heat energy that must be removed from a refrigerated space to achieve and maintain target temperature, comprising transmission loads through insulation, infiltration loads from air exchange, product loads from cargo thermal mass, and internal loads from equipment operation. Accurate thermal load calculation determines whether refrigeration equipment can maintain temperature—yet transport refrigeration sizing routinely underestimates loads, particularly for multi-stop delivery operations.
Thermal Load Components
Total thermal load combines multiple heat sources:
Transmission Load (15-40% of total)
- Heat conduction through walls, floor, roof, doors
- Depends on insulation R-value, surface area, temperature differential
- Increases with thermal bridges bypassing insulation
- Solar radiation adds significant roof and wall loading
Infiltration Load (10-60% of total)
- Air exchange through door openings
- Gap leakage around seals
- Dramatically higher in multi-stop operations
- Most underestimated load component
Product Load (0-30% of total)
- Cooling incoming product from ambient to target
- Respiration heat from produce (if applicable)
- Minimal for pre-frozen courier cargo
- Significant when accepting warm product
Internal Load (5-15% of total)
- Evaporator fan motors
- Defrost cycle heat addition
- Lighting (if equipped)
- Personnel entry heat (minimal in courier operations)
Load Calculation Approach
Our Technical Formulas Reference provides thermal load calculations:
Transmission Load:
Q_transmission = U × A × ΔTWhere U = overall heat transfer coefficient, A = surface area, ΔT = temperature differential
Infiltration Load:
Q_infiltration = ρ × V × Cp × ΔT × η × nWhere n = number of door openings, η = air exchange efficiency per opening
South African Load Factors
South African operations face compounding load factors:
| Factor | Impact | Load Increase |
|---|---|---|
| Altitude (Johannesburg) | Reduced capacity | Effective +27% |
| Summer ambient (40°C) | Higher ΔT | +20-30% vs 32°C design |
| Urban heat island | Pavement radiation | +30-50% floor load |
| Multi-stop frequency | Door openings | +200-400% infiltration |
Combined, these factors can triple the thermal load compared to European long-haul assumptions used in standard equipment sizing.
Equipment Sizing Implications
Proper thermal load analysis reveals equipment requirements:
Example – 12m³ Courier Vehicle (Gauteng Summer):
- Transmission load: 1.5 kW
- Solar load: 0.5 kW
- Infiltration (30 openings): 0.34 kW average, 2.0 kW peak
- Internal loads: 0.3 kW
- Total average: 2.64 kW
- Peak (door recovery): 4.3 kW
- Altitude corrected requirement: 5.5 kW minimum
- Recommended specification: 6.5-8 kW
Standard manufacturer sizing for “delivery operations” might suggest 3.5-4.5 kW—dramatically undersized for actual South African multi-stop conditions.
Why Loads Are Underestimated
Industry systematically underestimates thermal loads because:
- Manufacturer guides assume European climate
- Door opening assumptions reflect long-haul, not courier operations
- Altitude effects ignored for inland markets
- Peak load recovery requirements overlooked
- Price competition drives minimum-viable specifications
Related Terms: Door Openings (Thermal Load), Thermal Bridge, Urban Heat Island Effect