Refrigerated Cargo Systems

Marine refrigerated cargo systems maintain precise temperature and atmospheric conditions for perishable goods during ocean transport. These systems range from self-contained reefer containers to integrated ship refrigeration plants serving multiple cargo holds. Design requirements balance thermodynamic efficiency with the unique challenges of the marine environment including salt air corrosion, vessel motion, and limited power availability.

Cargo Temperature Requirements

Different perishable commodities require specific temperature ranges for optimal preservation during transit.

Temperature uniformity across the cargo space typically must remain within ±0.5°C for sensitive products and ±1.5°C for standard frozen cargo.

Reefer Container Systems

Refrigerated containers (reefers) are self-contained units with integral refrigeration machinery powered by vessel electrical supply or generator sets.

System Components:

1. Refrigeration unit: Typically vapor-compression cycle with scroll or reciprocating compressor, 5-15 kW electrical input
2. Evaporator section: Forced-air circulation, 800-1200 m³/hr airflow capacity
3. Condenser section: Air-cooled with axial fans, designed for 50°C ambient
4. Microprocessor controller: Setpoint accuracy ±0.3°C, data logging capability
5. Insulated container body: 80-120 mm polyurethane foam, U-value 0.25-0.35 W/m²K

Standard 40-ft reefer containers provide approximately 60 m³ of cargo capacity with refrigeration capacity ranging from 7,000 to 12,000 W at -20°C cargo temperature and 38°C ambient conditions.

Air Distribution:

Proper airflow pattern prevents warm spots and maintains cargo quality. Bottom-air delivery systems supply cold air through T-bar floor decking, flowing upward through the cargo and returning at the top. Supply air velocities range from 0.5 to 1.5 m/s depending on cargo sensitivity. Cargo must be loaded with proper air channels to ensure circulation.

Integrated Ship Refrigeration Plants

Conventional refrigerated cargo vessels utilize centralized refrigeration machinery serving multiple insulated cargo holds through distributed cooling coils.

System Architecture:

• Compressor room: Multiple reciprocating or screw compressors, 200-800 kW total capacity per plant
• Refrigerant: Ammonia (R-717) or R-134a, selected for efficiency and safety
• Evaporator coils: Direct expansion or brine-cooled, installed in cargo hold ceilings
• Brine circulation (if used): Calcium chloride or propylene glycol, secondary loop to cargo spaces
• Insulated holds: 150-300 mm polyurethane or polystyrene, K-value 0.020-0.025 W/mK

Each cargo hold typically receives independent temperature control with dedicated expansion valves and evaporator sections. Modern systems employ variable-speed compressors and electronic expansion valves for improved efficiency and temperature stability.

Controlled Atmosphere Systems

Certain perishable cargoes require modified atmospheric composition in addition to temperature control to extend shelf life and prevent ripening or spoilage.

Atmospheric Parameters:

• Oxygen reduction: 2-5% O₂ (from normal 21%) slows respiration and ripening
• Carbon dioxide addition: 3-10% CO₂ suppresses microbial growth
• Nitrogen enrichment: Balance gas to displace oxygen
• Ethylene removal: Catalytic or absorption systems for ethylene-sensitive produce

Controlled atmosphere (CA) containers and holds incorporate gas-tight construction, atmospheric monitoring sensors, and nitrogen generators or CO₂ injection systems. Typical nitrogen generators use pressure swing adsorption (PSA) technology producing 95-99% N₂ purity at rates of 20-100 m³/hr.

Heat Load Calculations

Accurate heat load calculation ensures adequate refrigeration capacity for maintaining cargo temperature.

Heat load components:

1. Transmission load (Q_trans): Heat gain through insulated walls

   • Q_trans = U × A × ΔT
   • U = overall heat transfer coefficient (W/m²K)
   • A = surface area (m²)
   • ΔT = temperature difference (K)

2. Product load (Q_prod): Heat removal from cargo cooling

   • Q_prod = m × c_p × ΔT / t
   • m = cargo mass (kg)
   • c_p = specific heat (kJ/kgK)
   • t = pull-down time (s)

3. Respiration load (Q_resp): Heat generated by fresh produce

   • Typically 500-2000 W per 1000 kg for fruits and vegetables

4. Air change load (Q_air): Heat from door openings and ventilation

   • Calculated based on air exchange rate and enthalpy difference

5. Equipment load: Fan motors, lighting, defrost heaters

Total refrigeration capacity must exceed peak heat load by 15-25% safety factor for adequate performance under extreme conditions.

System Design Considerations

Power Supply:

Reefer containers operate on 440V 3-phase 60Hz or 380V/440V 3-phase 50Hz vessel power. Voltage variation tolerance is ±10%. Power consumption ranges from 3-15 kW per container depending on setpoint and ambient conditions. Vessel electrical systems must provide adequate generation capacity and distribution infrastructure.

Defrost Strategy:

Evaporator coil frosting occurs during low-temperature operation. Automatic defrost cycles use electric resistance heaters (2-6 kW) or hot gas bypass every 6-12 hours. Defrost duration is 15-45 minutes with termination based on coil temperature reaching 10-15°C.

Monitoring and Alarms:

Modern systems provide continuous data logging of supply air temperature, return air temperature, setpoint, defrost cycles, and alarm conditions. Remote monitoring via satellite communication enables shore-based oversight. Critical alarms include high/low temperature deviation, compressor failure, and power loss.

Marine Environment Adaptations:

Salt air corrosion requires enhanced protective coatings on condensers and electrical components. Vibration-resistant mounting prevents refrigeration line fatigue. Condensate drainage must function properly during vessel pitch and roll conditions.

Regulatory Standards

Marine refrigerated cargo systems must comply with multiple international standards:

• IMO (International Maritime Organization): Guidelines for carriage of perishable cargoes
• SOLAS (Safety of Life at Sea): Chapter II-2 fire safety in machinery spaces
• Classification societies: ABS, DNV, Lloyd’s Register rules for refrigeration machinery
• ISO 1496-2: Thermal container specifications and testing
• ATP Agreement: International carriage of perishable foodstuffs requirements

Equipment certification verifies performance capabilities, electrical safety, and refrigerant system integrity for marine service conditions.

Energy Efficiency Optimization

Reducing refrigeration energy consumption lowers operational costs and environmental impact.

Efficiency measures:

• Variable-speed compressors matching load requirements
• High-efficiency evaporator and condenser coil designs
• Optimized insulation thickness (economic analysis of material cost vs. energy savings)
• Thermal storage using phase-change materials for load shifting
• Heat recovery from condenser reject heat for vessel services
• Proper cargo pre-cooling before loading reduces pull-down load

Modern reefer containers achieve energy efficiency ratio (EER) values of 1.5-2.0 W/W at standard rating conditions, while optimized ship plants can exceed 2.5 W/W with screw compressor technology and enhanced heat exchangers.

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