A transport refrigeration unit (TRU), more commonly called a reefer, is a critical component in the cold chain logistics system, ensuring that perishable goods are transported at controlled temperatures. Here's an overview of how a TRU works:
Key Components
- Refrigeration System:
- Compressor: Compresses the refrigerant, increasing its pressure and temperature.
- Condenser: Cools the high-pressure refrigerant vapor, converting it into a liquid.
- Expansion Valve: Reduces the pressure of the liquid refrigerant, causing it to cool further.
- Evaporator: Absorbs heat from the cargo area, causing the refrigerant to evaporate and cool the space.
- Power Source:
- Diesel Engine: Provides power for the refrigeration unit, common in long-haul transport.
- Electric Motor: Used when the vehicle is stationary and plugged into an external power source, also called shore power.
- Control System:
- Thermostat: Monitors and controls the temperature inside the cargo area.
- Sensors: Detect temperature and humidity levels, feeding data to the control system for adjustments.
Working Principle
- Compression:
- The refrigerant gas enters the compressor, where it is compressed. This compression increases the pressure and temperature of the refrigerant.
- Condensation:
- The high-pressure, high-temperature refrigerant gas moves to the condenser. Air flows over the condenser coils (often aided by fans), causing the refrigerant to release heat and condense into a high-pressure liquid.
- Expansion:
- The high-pressure liquid refrigerant then passes through an expansion valve, which reduces its pressure. This reduction in pressure causes the refrigerant to cool significantly, turning it into a low-pressure, cold liquid.
- Evaporation:
- The cold refrigerant flows into the evaporator coils inside the cargo area. Here, it absorbs heat from the air inside the refrigerated compartment, causing the refrigerant to evaporate into a gas. This process cools the air inside the compartment.
- Fans circulate the cooled air within the cargo area to ensure uniform temperature distribution.
- Cycle Repeats:
- The low-pressure refrigerant gas returns to the compressor, and the cycle repeats.
Temperature Control
- Thermostat and Sensors: The thermostat and sensors constantly monitor the temperature inside the cargo area. If the temperature deviates from the set point, the control system adjusts the compressor and fans to bring the temperature back to the desired level.
- Defrost System: In refrigerated transport, ice can build up on the evaporator coils, reducing efficiency. TRUs often include a defrost system that periodically removes this ice, either through electric heaters or reversing the refrigeration cycle temporarily.
Power Management
- Diesel Engine: When the vehicle is in transit, the TRU is typically powered by a diesel engine.
- Electric Power: When the vehicle is stationary, such as during loading and unloading, the TRU can be powered by an electric motor connected to an external power source. This is more efficient and reduces emissions.
Maintenance and Monitoring
- Regular maintenance of the TRU is crucial for optimal performance, including checking refrigerant levels, inspecting the compressor and condenser, and ensuring sensors and controls are functioning correctly.
- Advanced TRUs may include remote monitoring capabilities, allowing operators to track temperature and performance in real-time and address issues proactively.
Overall, a transport refrigeration unit is a sophisticated system designed to maintain precise temperature control, ensuring the safe and efficient transport of perishable goods.
