The basic idea
A refrigerated vehicle or container is an insulated box with a cooling machine attached. The insulation slows heat from getting in; the machine removes whatever heat does get in, plus any heat the cargo itself gives off. The result is a space that can hold a chosen temperature for the length of a trip.
Refrigerated trucks, trailers and containers are the workhorses of the cold chain. Shippers and drivers often call them “reefers”, a short form of refrigerated, and the term is used for road trailers and for ocean containers alike.
Types of refrigerated equipment
Refrigerated trucks and vans. Smaller vehicles with an insulated body and a cooling unit, used mainly for local and regional delivery of groceries, dairy, flowers and medicines. Some are divided into compartments so that more than one temperature can be carried.
Refrigerated trailers. Large insulated semi-trailers with a unit mounted on the front wall. They are common in long-distance truckload shipping of fresh produce and frozen foods. The unit typically runs on its own diesel engine, though electric and hybrid options are growing as cleaner choices, as discussed under alternative fuels.
Reefer containers. Standard-size shipping containers with insulated walls and a built-in refrigeration machine. Unlike a trailer’s unit, a container’s machine is normally electric and needs an external power supply. On a ship it plugs into outlets on the vessel; in a terminal it plugs into a rack of power points; on a truck or train it may be powered by a clip-on generator set, often called a genset. Because they fit standard handling equipment, reefer containers move easily through intermodal transport.
Rail cars and air options. Some rail cars have built-in refrigeration, and perishable goods also fly on aircraft, sometimes in insulated or cooled unit load devices. Passive options such as insulated shippers with cooling packs are used for smaller lots.
How the cooling works
Most units use a vapor-compression cycle similar to a household refrigerator. A refrigerant circulates, absorbing heat inside the box and releasing it outside. A fan blows cold air through the cargo area.
In many containers, cold supply air enters through the floor, usually through a grid of channels called T-floors. It rises through the cargo and returns at the top to the unit to be cooled again. This air circulation is what actually carries the cold to the goods. Two terms appear often:
- Set point: the temperature the operator asks the unit to maintain.
- Supply air and return air: the temperature of air leaving and re-entering the cooling unit. The difference between them gives a picture of how much heat the unit is removing.
Units may also control humidity or ventilation, and for some fruit, specialized controlled atmosphere systems alter the oxygen and carbon dioxide levels to slow ripening. Whether that is appropriate depends on the product.
What reefers cannot do
A key rule is that a reefer is designed to maintain a temperature, not to cool a warm load down. Their capacity is sized to offset heat leaking through the walls and the small heat produced by cargo and fans, not to remove a large amount of heat from a product loaded warm. Loading warm goods can leave the load in range only at the surface, or never reach the target.
For this reason good practice includes:
- Pre-cooling the product to the right temperature before loading, ideally in a cold store;
- Pre-cooling the empty unit so the walls and floor are already at temperature;
- Using covered or enclosed docks to limit time spent in warm air during loading.
Loading for airflow
Because air must circulate, how a unit is loaded matters as much as its machinery.
- Leave space at the walls, doors and ceiling according to the load line marked inside, and do not stack above it.
- Use pallets and cartons with vent holes, and align the holes so air can pass through.
- Do not block the floor channels or the return air path with plastic wrap, loose cartons or poor stacking.
- Mix cargoes carefully; products needing different temperatures or giving off gases such as ethylene (released by some fruit) may not be compatible.
Poor loading is a leading cause of spoilage even when the unit works perfectly. Reasonable pallet loads and load securing also help prevent cargo damage during transit.
Operating and checking
Before a trip, operators typically inspect the unit: fuel or power connection, door seals, drains, floors, and the unit’s own pre-trip self-test where available. During the trip, drivers or terminal staff watch the controller and alarms. Reefer containers at terminals and on ships are monitored for power and temperature, and a lost plug can warm a container surprisingly quickly. Handoffs at ports and rail yards are therefore a known risk. Port congestion is especially troublesome: a container that waits in a yard without power, or for too long, can lose its cargo.
Records of set points and readings help resolve disputes, and temperature monitoring is covered in more detail separately. Where a claim arises, the data help determine whether the unit, the loading or the product was responsible.
Fuel, energy and emissions
Refrigeration adds fuel or electricity use on top of normal transport energy. Many units are powered by a separate diesel engine on trucks and gensets, so they can contribute noticeably to freight emissions on a per-trip basis. Operators look at electric standby (plugging in at depots), efficient insulation, better units and, increasingly, electric vehicles with electric refrigeration. These choices involve trade-offs of cost, range and charging infrastructure.
Choosing equipment
Choices depend on distance, product, volume and available infrastructure:
- Short urban runs favor refrigerated vans or trucks.
- Long road hauls favor reefer trailers.
- Overseas shipments favor reefer containers, which require power at every stage.
- Small, high-value lots may use insulated passive boxes instead of active units.
Takeaway
Refrigerated transport works when four things line up: a product at the right temperature, a pre-cooled and working unit, a load arranged for airflow, and uninterrupted power or fuel. Failing at any one of these can defeat the others.