A refrigeration unit takes the heat inside the body and carries it outside. That is why its power is measured in how much heat it moves per hour, not in how cold it can get. In transport a unit is usually selected to hold pre-cooled cargo, and whether it runs with the vehicle stopped depends on its energy arrangement.
Four parts, four jobs
| Part | Job | What to check |
|---|---|---|
| Compressor | Compresses the refrigerant and drives the cycle | Model, drive, service condition |
| Condenser | Rejects heat to the outside | Airflow, fin cleanliness |
| Expansion device | Lowers the refrigerant's pressure and temperature | Service record |
| Evaporator | Takes heat from the body air | Air path, icing, fan, drain |
The functions of the parts are set out in Danfoss's explanation of the refrigeration cycle; it describes a domestic refrigerator, and a transport unit's layout is in its own manual. The same temperature symptom can come from the air path, a seal, a sensor or several parts at once.
Capacity is heat moved per hour
The Thermo King V-800 MAX table shows it: on vehicle-engine drive the 35°F (about 1.7 °C) column gives 20,500 BTU/h, roughly 6 kW of heat moved, while the -20°F (about -29 °C) column leaves 6,000 BTU/h, roughly 1.8 kW. The same unit moves less heat as the target falls, so on a distribution round with constant door openings what counts is the heat it moves under that condition, not the lowest temperature it can reach.
A setpoint is not the product temperature either: it is the air condition the control aims for, and the product's starting temperature, packaging, load density and airflow are measured separately. A unit does not make up for lost insulation, cannot reach load behind a stack the air cannot pass, and its pull-down performance on warm cargo is confirmed only by model, body and test condition.
The drive option is read from the model code. On the Thermo King V-800, 10 means a road compressor driven by the vehicle engine only, and 20 adds an electric-motor stand-by compressor in the unit; running on mains power with the engine off depends on that difference. On an independent diesel, battery or hybrid unit the energy arrangement is checked separately.
What to write into a selection
- the product's pre-cooled starting condition
- body dimensions, panels, doors, air path and load
- the capacity rating condition, kept apart from electrical or fuel input
- the setpoint and the product sensor position, recorded separately
- operation when moving, stopped, on stand-by and on power loss, against the actual model
- an acceptance test in hot and cold ambient, with door openings and a load
Prepare the specification with the eight refrigeration selection variables. During loading, blocking the air path can stop even an adequately sized unit from protecting every part of the load.
