A transport refrigeration unit is the self-contained refrigeration system mounted on a trailer, rigid box body or shipping container, holding the cargo space at a set temperature independent of the tractor unit hauling it. It carries its own power source, its own compressor and its own controls, because the load inside has to stay in range whether the vehicle is moving on a motorway, queued at a border, or parked overnight at a depot with the engine off.
Diesel, electric standby and cryogenic units
A diesel unit runs its own small engine, independent of the vehicle's main engine, burning fuel purely to drive the refrigeration compressor for as long as the load needs cooling. An electric standby unit adds the option to plug into mains power at a depot or distribution centre, running the compressor off grid electricity instead of diesel whenever the vehicle is stationary and connected, which cuts engine idling at loading docks and in urban delivery zones where idling diesel units are increasingly restricted. A cryogenic unit dispenses with a mechanical compressor entirely: liquid carbon dioxide or liquid nitrogen is injected directly into the cargo space from an onboard tank, cooling the box as it changes phase, which runs near silent and needs no engine at all, at the cost of carrying a finite charge that has to be refilled rather than fuel that can be topped up anywhere.
Capacity sizing
A unit's cooling capacity has to be matched to the box it serves: its volume, its insulation standard, and how often the doors open along the route. A single-drop line-haul run between two points needs far less capacity than a multi-drop urban delivery route covering the same box volume, because every door opening on a multi-drop run lets warm air in and forces the unit to recover, over and over across the shift. Undersizing a unit for a multi-drop route shows up as a load that never quite reaches temperature on a busy day, even though the same unit would hold range easily on a single long haul.
Pulldown limits
A transport refrigeration unit is sized to hold a temperature already achieved, not to pull a warm load down to temperature during transit. Loading product that is already at the correct temperature before the doors close is standard practice for exactly this reason: most units cannot recover a fully warm load to setpoint within a normal transit window, and trying forces the compressor to run flat out for hours with no guarantee the core of a dense pallet load ever catches up before delivery. Pulldown capacity and holding capacity are different specifications, and a unit rated to hold a load easily may still be the wrong choice if a shipper is counting on it to cool a load that was never chilled before loading.
Multi-temperature and single-temperature boxes
Some trailers run a single unit holding one setpoint across the whole box, which suits a load of one product travelling to one destination. Others are built as multi-temperature trailers, with internal bulkheads splitting the box into separate compartments, each fed by its own evaporator off a shared or split refrigeration system, so a single vehicle can carry frozen, chilled and ambient product on the same run without cross-contaminating temperatures. Multi-temperature trailers cost more to run and maintain, with more seals, more evaporators and more setpoints to check on the pre-trip inspection, but they let a distributor consolidate several smaller drops of different product types onto one vehicle instead of running separate single-temperature trailers for each.
Maintenance and pre-trip checks
A pre-trip inspection covers the points most likely to fail mid-route: refrigerant level and signs of a leak, belt condition, fuel level on a diesel unit, door seal condition, and the setpoint and data logger confirmed and recording before the vehicle leaves the yard. The check exists because a unit running unattended on the road has no one to notice a slow refrigerant leak or a slipping belt until the temperature record already shows a problem, and by then the load may already be compromised. Fleets running temperature-sensitive freight treat the pre-trip check as a fixed step before departure, not an occasional one, in the same way a route is never dispatched without confirming the unit is set to the right temperature for what it is carrying.
Position in the road chain
Transport refrigeration units are the equipment behind refrigerated road transport, mounted on the trailers and rigids that move chilled and frozen freight between cold rooms and cold stores at each end of a route. A unit rated for standard chilled and frozen work is not built for the far colder bands covered by dedicated cryogenic shipping equipment, which runs on a different cooling principle entirely. Fleet operators, third-party logistics providers and food and pharmaceutical distributors are the main users, specifying unit type and capacity route by route rather than fitting one standard unit across an entire fleet.