Evaporator

The heat exchanger inside the refrigerated cargo space where liquid refrigerant absorbs heat from the air, evaporating into gas and lowering the cargo temperature. The evaporator is where cooling actually happens—and its design, sizing, airflow distribution, and defrost management determine whether a transport refrigeration system maintains uniform frozen temperatures throughout the cargo space or creates dangerous temperature gradients.

Evaporator Function

The evaporator performs critical functions:

1. Receives cold liquid refrigerant from expansion valve
2. Absorbs heat from cargo space air
3. Evaporates refrigerant (liquid → gas phase change)
4. Delivers low-pressure gas to compressor suction
5. Distributes cooled air throughout cargo space

The evaporator’s ability to absorb heat determines system cooling capacity. Undersized evaporators limit performance regardless of compressor capacity.

Evaporator Design Considerations

Surface Area

• Larger surface area = more heat transfer capacity
• Coil design (fin density, tube diameter) affects performance
• Trade-off between capacity and airflow resistance

Airflow

• Fan-forced air circulation essential for uniform cooling
• Poor airflow creates temperature gradients (warm spots)
• Cargo stacking affects air distribution
• Proper air chute design directs cooled air throughout cargo

Operating Temperature

• Evaporator temperature typically 8-12°C below cargo setpoint
• For -18°C cargo, evaporator operates at -26°C to -30°C
• Lower evaporator temperature = faster ice accumulation = more frequent defrost

Ice Accumulation Problem

In frozen transport, evaporators continuously accumulate ice:

• Humid air enters during door openings
• Moisture contacts cold evaporator surface
• Ice forms on coils and fins
• Ice insulates coils, reducing heat transfer
• Airflow restricted as ice builds

Without periodic defrost, evaporator efficiency degrades progressively until cooling fails entirely.

Multi-Stop Operations Impact

Multi-stop delivery creates accelerated evaporator stress:

• Each door opening introduces humid ambient air
• 30-40 openings per route = massive moisture infiltration
• Ice accumulates faster than long-haul transport
• More frequent defrost cycles required
• Demand-based defrost essential for courier operations

Timer-based defrost systems designed for long-haul transport (2-4 door openings/day) are inadequate for multi-stop courier operations.

Evaporator Placement and Airflow

Evaporator position affects cooling performance:

Ceiling-Mounted (Common)

• Cold air descends naturally
• Good air distribution in empty/light loads
• May create stratification in full loads
• Requires proper air return path

Front-Wall Mounted

• Horizontal airflow through cargo
• Requires T-bar floor for return air
• Better performance with dense loads
• Common in larger transport applications

Airflow Obstructions

• Cargo blocking air chutes creates hot spots
• Product stacked against evaporator reduces efficiency
• Proper load planning essential for temperature uniformity

South African Evaporator Considerations

South African conditions affect evaporator performance:

Humidity Variation

• Coastal areas (high humidity): Faster ice accumulation
• Inland (lower humidity): Slower ice accumulation
• Summer conditions: More moisture per door opening

Altitude Effects

• Lower air density at Johannesburg altitude
• Reduced convective heat transfer
• Fan airflow patterns affected
• May require larger evaporator surface area

Ambient Temperature

• Higher summer temperatures increase thermal load
• Evaporator works harder to achieve temperature differential
• Extended compressor runtime

Evaporator Maintenance

Proper maintenance ensures evaporator performance:

• Regular defrost system verification
• Drain line cleaning (blocked drains cause ice buildup)
• Fan motor inspection
• Coil cleaning (dirt reduces heat transfer)
• Air chute integrity verification

Related Terms: Condenser, Defrost Cycle, Compressor