Cold Chain Guides
Why a Refrigerated Body Must Be Pre-Cooled Before Loading
A refrigeration unit holds temperature; it does not create it. Pre-cooling the body and the cargo is what separates a stable run from a slow drift.
Why a Refrigerated Body Must Be Pre-Cooled Before Loading
There is a sentence in the operating instructions for these units that is worth quoting before anything else, because it resolves most arguments about refrigeration performance in a single line: the unit is designed to hold temperature, not to cool cargo that is already above temperature.
That is not a limitation of one product. It is how transport refrigeration works. A unit selected for a body of a given size has a cooling capacity matched to the heat that body will gain during a route — through the insulation, through door openings, through the cargo's own respiration and through solar load. It does not have the surplus capacity to also remove the sensible and latent heat of a load that arrived warm.
Understanding why pre-cooling is required turns a rule that drivers follow reluctantly into a step they can reason about.
Two Different Jobs
Pulling a body down from ambient and holding it at setpoint are different jobs with different loads.
Pulling down means removing heat from the structure itself: the insulation, the internal linings, the floor channels, the evaporator coil and the air in the box. A body that has been standing in a yard at 35°C contains a substantial amount of stored heat, and that heat has to leave before the box can hold anything.
Holding means replacing heat that leaks in, at a rate set by the insulation quality, the temperature difference across the walls, the number of door openings and the ambient condition. That is a much smaller load, and it is the load the unit was selected against.
Starting a route with a warm body asks the unit to do both jobs at once, on a vehicle that is already moving and already drawing power for traction. The result is a pull-down that never quite finishes, a compressor that runs continuously at high output, and a box temperature that sits above setpoint for the first part of the route — often the part where the cargo is most vulnerable.
What Pre-Cooling Actually Does
Pre-cooling removes the stored heat from the body before any cargo is loaded, so that when the doors open the unit only has to deal with the heat that enters through the opening.
The operating instruction is specific: set the temperature for the compartment, then run the system for 30 to 60 minutes before loading, and longer where circumstances allow. Two details in that instruction matter as much as the duration.
The first is that the setpoint must be set during the pre-cool, not after. Running the unit without a setpoint does not pre-cool anything; it just circulates air.
The second is that the doors should stay closed during the pre-cool and be opened only when loading actually begins. A pre-cool followed by twenty minutes of open doors on a warm dock has achieved nothing except a compressor cycle.
The Cargo Has Its Own Temperature
Pre-cooling the body solves half the problem. The other half is the cargo, and it is the half that is more often neglected.
Cargo arrives with heat in it. For fresh produce that heat is substantial and it is continuing to be generated — respiring produce releases heat continuously, which is why the load does not simply equalise and stop. For chilled meat, dairy or prepared food, the heat is stored sensible heat from a warm processing or staging area. For frozen goods, the risk is different: cargo that has partially thawed during staging carries latent heat that must be removed before the product returns to its intended state, and that removal is far more demanding than holding a frozen setpoint.
The instruction is therefore that cargo must be pre-cooled before loading. Where a load arrives above its target temperature, the honest option is to bring it down in a cold store before it goes on the vehicle, or to accept on the loading documentation that it was loaded out of specification.
That last point is not bureaucracy. The final cargo temperature check before departure, and the note on the loading sheet when it deviates, is what protects both the carrier and the customer when a claim is made three days later. A deviation recorded at loading is an operational fact. The same deviation discovered at delivery looks like a refrigeration failure.
Humidity and Frost: The Hidden Cost of Skipping It
There is a second consequence of loading warm cargo that shows up as a mechanical fault rather than a temperature fault.
Warm cargo in a cold box releases moisture. That moisture condenses on the evaporator coil, and the coil then frosts faster than it would on a dry load. The unit responds as designed by entering defrost more often, and during each defrost cycle it is not cooling. A body that would otherwise hold temperature comfortably begins to drift, and the driver experiences it as a unit that cannot keep up — when in fact the unit is being asked to manage a moisture load it was never sized for.
This is also why high box humidity before loading is worth avoiding. The instruction attached to the operating procedure is explicit that pre-cooling both the body and the cargo reduces the risk of the evaporator frosting frequently and affecting box temperature. Frosting is not an evaporator defect; it is usually a loading practice showing up downstream.
Doors Are the Largest Single Leak
Even with a correctly pre-cooled body and correctly pre-cooled cargo, the loading operation itself is where most of the heat enters.
Two practices reduce that entry substantially. The first is to stop the unit before opening the cargo doors, so that cold air is not being actively pushed out of the box while the doors stand open. The exception is a vehicle reversed into a sealed cold dock, where the unit can run with the doors open because the dock itself is a controlled environment.
The second is to load quickly and to close the doors as soon as loading finishes. A body that is pre-cooled and then left open while paperwork is completed has simply been re-warmed at the customer's expense.
Pre-cool the body, pre-cool the cargo, and keep the doors shut except while loading. Three steps, and they resolve more temperature complaints than any service visit.
Capacity quoted for a unit describes a holding condition at a stated ambient, not a pull-down condition. That distinction is covered in why a TRU loses cooling capacity.
If the cargo needs two temperature bands in one body, the same pre-cooling logic applies per zone, and the configuration question is addressed in multi-temperature configuration.
None of this makes a unit more powerful. What it does is put the unit in the condition it was selected for, which is the only condition in which its published capacity means anything.
Capacity figures are quoted at a stated ambient and box temperature, so they describe a holding condition rather than a pull-down condition. The HD-380 product page sets out its rated capacities at both 0°C and -20°C box temperature, along with the body volumes the unit is intended for.
