The thermal energy that must be removed from cargo to achieve and maintain target temperature, encompassing both sensible heat (temperature reduction) and latent heat (phase change), plus any ongoing heat generation from product respiration or chemical reactions. Product load varies dramatically depending on whether cargo is pre-frozen (minimal load) or requires cooling (significant load)—a distinction that determines whether transport refrigeration can maintain temperature or is overwhelmed.
Types of Product Load
Sensible Heat (Temperature Reduction) Heat required to change product temperature without phase change:
Q_sensible = m × Cp × ΔT
Where:
m = mass of product (kg)
Cp = specific heat capacity (kJ/kg·K)
ΔT = temperature change required (K)Latent Heat (Phase Change) Heat required for freezing/thawing:
Q_latent = m × Lf
Where:
Lf = latent heat of fusion (~334 kJ/kg for water)Freezing product releases enormous latent heat—transport refrigeration cannot perform this function.
Respiration Heat (Living Products) Fresh produce continues metabolic activity:
- Generates heat requiring continuous removal
- Rate depends on product type and temperature
- Not applicable to frozen goods
Pre-Frozen vs Warm Product
| Condition | Product Load | TRU Capability |
|---|---|---|
| Pre-frozen at -18°C | Minimal (maintain only) | Adequate |
| Slightly warm (-12°C) | Moderate (pulldown required) | May struggle |
| Partially thawed (0°C) | High (refreezing required) | Cannot achieve |
| Fresh/unfrozen (+4°C) | Extreme | Completely inadequate |
Critical Understanding: Transport refrigeration units maintain temperature—they are not designed to actively cool warm product or freeze unfrozen product. The thermal mass and limited refrigeration capacity of transport equipment cannot overcome significant product loads.
Why Product Temperature Matters at Pickup
When we collect frozen goods for delivery:
Properly Frozen (-18°C or colder):
- Minimal sensible heat load
- System maintains temperature easily
- Recovery from door openings manageable
- Product arrives in original condition
Marginally Frozen (-12°C):
- 6K pulldown required
- Extended compressor runtime
- Slower recovery from openings
- Risk of not reaching -18°C before delivery
Partially Thawed (above -5°C):
- Latent heat of refreezing required
- Transport refrigeration cannot achieve
- Product continues warming
- Delivery at compromised temperature
This is why we verify product temperature at pickup.
Product Load Calculation Example
Scenario: Transport 500 kg frozen meat at -18°C in 35°C ambient
Product Load (if pre-frozen at -18°C):
- No temperature change required
- Load = 0 (maintenance only)
Product Load (if at -12°C, needs pulldown to -18°C):
- Specific heat (frozen meat): ~1.7 kJ/kg·K
- Temperature change: 6K
- Q = 500 kg × 1.7 kJ/kg·K × 6K = 5,100 kJ
Time to cool with 4kW excess capacity:
t = 5,100 kJ / 4 kW = 1,275 seconds = 21 minutes
Product Load (if at 0°C, requires freezing):
- Sensible cooling (0°C to -18°C): 500 × 1.7 × 18 = 15,300 kJ
- Latent heat of freezing: 500 × 334 × 0.7 (water content) = 116,900 kJ
- Total: 132,200 kJ
Time with 4kW excess capacity:
t = 132,200 kJ / 4 kW = 33,050 seconds = 9.2 hours
Reality: Not achievable during delivery routeProduct Load and Route Planning
Product load affects route feasibility:
- Pre-frozen product: Standard routes achievable
- Warm product: Reduced stop count, shorter routes
- Mixed temperatures: Segregate by temperature, plan accordingly
The “Cold Enough” Fallacy
Some operators accept product at marginal temperatures:
- “It’s almost frozen, it’ll be fine”
- “The truck will cool it down on the way”
- “A few degrees won’t matter”
Reality:
- Transport refrigeration has limited pulldown capacity
- Multi-stop thermal loads consume available capacity
- Product arriving warm delivers warm
- Customer receives compromised product
Professional operations verify temperature at pickup and refuse product that transport refrigeration cannot maintain.
Related Terms: Thermal Load, Pre-Cooling, Temperature Excursion