If you drive, dispatch, or manage refrigerated freight, it helps to understand what the box on the front of your trailer actually does. A transport refrigeration unit, usually called a TRU or just a reefer, is the self-contained system that holds a trailer, truck body, or container at a target temperature while product moves down the road. The two names you will see most often are Thermo King and Carrier Transicold, and while the controllers and options differ, the core ideas are the same across both brands.
The single most useful thing to understand up front is this: a reefer does not make cold. Cold is not a thing you can pump into a box. What the unit actually does is move heat out of the cargo space and dump it outside. Once you understand that, everything else about how a reefer behaves starts to make sense.
What a TRU Actually Does: Moving Heat, Not Making Cold
Every reefer runs on the same basic refrigeration cycle used in your home air conditioner or refrigerator, just built tough enough to survive years of highway vibration and weather. The cycle moves a refrigerant around a sealed loop, and the refrigerant carries heat from inside the box to the outside air by changing state between liquid and gas. There are four main components.
- Compressor: The pump of the system. It takes low-pressure refrigerant gas coming back from the box and squeezes it into a hot, high-pressure gas. This is the start of the high-pressure, or discharge, side of the loop.
- Condenser: A radiator-style coil at the outside of the unit. The hot high-pressure gas flows through it, a fan pulls ambient air across it, and the refrigerant gives up its heat and condenses into a liquid. That heat you feel blowing off a running reefer is heat that came out of the cargo.
- Expansion or metering device: A small restriction the high-pressure liquid is forced through. On the far side, pressure drops sharply, which drops the refrigerant's boiling point and gets it cold. This is where the high side becomes the low side.
- Evaporator: The coil inside the cargo space. The now-cold low-pressure refrigerant boils into a gas as air from the box is blown across the coil. Boiling absorbs heat, so the air leaving the evaporator is colder than the air that entered. That cold refrigerant gas returns to the compressor as low-pressure suction, and the loop repeats.
So the flow of the box air and the flow of the refrigerant are two different loops working together. The refrigerant loop carries heat out of the trailer; the air loop spreads the cooling around the cargo. Understanding both is the key to reading what a struggling unit is telling you.
The Diesel Engine and Electric Standby
Something has to turn the compressor and run the fans, and on a trailer reefer that something is usually a small diesel engine built into the unit, with its own fuel tank separate from the tractor. The engine drives the compressor and the fans, and its speed is managed by the controller. Many units run in a high-speed and low-speed arrangement, dropping to low speed when the load only needs light maintenance cooling and stepping up to high speed when there is real heat to pull down.
Many units also offer electric standby. When the trailer is parked at a dock or yard with power available, you plug it in and an electric motor drives the system instead of the diesel. That saves fuel, cuts engine hours, and keeps things quiet where diesel noise is a problem. The refrigeration cycle itself is identical either way; only the power source driving the compressor changes.
Airflow: How Cold Air Actually Reaches the Load
Cooling the coil is only half the job. The unit has to move that conditioned air around the entire cargo space, and how the trailer is loaded matters as much as the machine. Most reefers pull return air in near the top, push it across the evaporator, and blow chilled supply air back into the box, usually along the ceiling through an air chute that runs the length of the trailer. The air sweeps down the walls, through and around the load, and back to the return.
- Product must sit on the floor rails or pallets so air can pass underneath it. Cargo sitting flat on the floor blocks that path and creates warm spots.
- Loads should not be jammed against the front bulkhead or block the return air opening, or the unit will short-cycle its own air and starve the back of the trailer.
- Air takes the path of least resistance, so gaps and loose stacking let cold air bypass the product instead of moving through it.
A perfectly healthy unit can still deliver uneven temperatures if the load blocks the airflow. That is why experienced drivers care about load pattern, not just the number on the display.
Set Point, Cool and Heat, and Defrost
The number you dial in is the set point, the temperature you want the box held at. The controller compares set point to the actual box temperature measured by sensors in the return or supply air and decides what to do. Here is the part that surprises newer drivers: a reefer can heat as well as cool. In winter, a load of produce that must stay above freezing can be threatened by cold outside air, so the unit switches to heat mode to hold the set point. Heat is often produced by reversing or rerouting the refrigerant flow, or with electric heaters on some units, but the goal is the same: hold the set point from either direction.
Because the evaporator coil runs colder than the box, moisture in the air freezes onto it as frost over time. A frosted coil chokes airflow and kills capacity, so the unit runs a periodic defrost cycle, briefly warming the coil to melt the frost, draining it out, then returning to normal operation. Defrost is normal and expected. A unit that defrosts far too often, or ices up between defrosts, is telling you something is wrong.
Units also run in one of two general operating modes. In continuous operation the engine runs the whole time, cycling between cool, heat, and idle to hold a tight temperature, which suits sensitive fresh product. In start-stop or cycle-sentry operation the unit shuts the engine off once the set point is satisfied and restarts only when temperature drifts or the battery needs charging, which saves fuel on frozen and less sensitive loads.
Why a Reefer Cannot Rescue a Warm Load
This is the most important operational fact, and the one that causes the most claims: a transport reefer is designed to maintain a pre-cooled load, not to chill a warm one. These units are sized to hold temperature and offset heat leaking through the walls, not to pull thousands of pounds of product down from ambient. If you load warm cargo and expect the reefer to bring it to temperature on the road, you will be disappointed and the product may be compromised. Product should be brought to its target temperature before it is loaded, and the trailer itself should be pre-cooled so the unit only has to hold the line.
All of this depends on the machine actually running at full capacity, and that is where maintenance earns its keep. A low refrigerant charge, a dirty or blocked condenser, a worn belt, a failing sensor, a weak defrost, or a tired engine all quietly rob capacity. On a mild day you may never notice, but on a hot afternoon with a full load the unit runs out of margin exactly when you need it most. Regular service and honest inspection are what keep a reefer holding temperature when conditions get hard.
Northeast Refrigeration Services provides independent mobile TRU repair on Thermo King and Carrier Transicold units across Vermont, New Hampshire, Maine, New York, Massachusetts, and Connecticut. If your unit is not holding temperature or you just want it checked before your next haul, call us at (802) 243-9373.