A reefer trailer keeps a load cold by holding a temperature, not by making one. Thick foam insulation slows outside heat, a refrigeration unit moves cold air around the load, and its own fuel tank keeps that unit running whether the tractor is on or off.
That's why product needs to go in already at temperature. The reefer is built to keep cold product cold, not to pull heat out of warm product.
Does a reefer cool down a warm load?
No. Reefer units are designed to hold product temperature, not to pull down product that's already warm. Thermo King and Carrier both say so in their loading guidance: the unit isn't built to remove "field heat," meaning the heat product carries in from the field, the warehouse or the sun.
Think of it like a good cooler. A cooler keeps cold drinks cold all afternoon, but dump in warm ones and you get lukewarm water. Same idea, just a lot bigger and with a diesel engine.
So the rule is simple: product should be loaded cold. If a shipper hands you freight that's clearly warm, the unit may struggle to bring it down, and the air coming back off the load will tell you something is off. More on that below.
What keeps the cold in: the box itself?
The first system is the trailer body. Reefer walls, roof and floor are filled with polyurethane foam insulation, which is what slows heat moving in from outside.
That foam slows outside heat to a crawl. Compare it to a standard dry van, which isn't built or insulated to hold a set temperature. A dry van is built to keep rain off freight, not to hold a temperature.
Good insulation is why a reefer can ride through a hot afternoon or sit parked for a stretch without the load drifting. It doesn't stop heat completely, it just slows it a lot, which is what lets the unit keep up.
What is that box on the front of the trailer?
That's the refrigeration unit, mounted on the nose of the trailer. On most trailers it runs on its own diesel engine, separate from the tractor. Electric and hybrid units are out there too, and they're being offered more now, but diesel units are still the most common.
The unit has two main jobs: make cold air and push it through the trailer. It also pulls air back in off the load, cools it again and sends it out. That loop runs over and over for as long as the unit is on.
Because it has its own engine, the tractor doesn't have to be running. You can shut the truck down and the load stays cold.
How does the air move through the load?
The second system is airflow, and it's the one most often wrecked by bad loading.
- Cold air leaves the unit and travels along the ceiling, often through a duct called an air chute.
- It runs to the rear of the trailer.
- It drops down through and around the load.
- It returns along the floor to the unit's return intake, where it gets cooled again.
That loop only works if air can actually get through. Loading tight against the front bulkhead (the wall behind the unit) or the rear doors chokes it. So does stacking above the load line, the mark that shows how high you can load without blocking the air above the freight.
Thermo King and Carrier both recommend leaving clearance at the front and rear and keeping the load below the load line. Ignore that and you get hot spots, pockets of product the cold air never reaches.
Why can the reefer run for days without the tractor?
The third system is fuel. The reefer has its own fuel tank mounted under the trailer, and it feeds only the refrigeration unit. That's why the unit can run whether the tractor is on or off.
So a driver can be at a dock, in a shop or parked for a break and the load keeps getting cold air. The limit is simply that tank. Keep an eye on it, because a unit that runs dry stops cooling no matter how good the insulation is.
The tank and engine are separate from your tractor's fuel and engine, so don't mix up the two when you're fueling.
What should I check before and during loading?
A few habits protect the load, and they come straight from the unit makers' guidance:
- Pre-cool the empty trailer before loading, unless the shipper says otherwise. Shipper instructions come first.
- Check that the product is already at temperature. The unit won't fix a warm load.
- Leave clearance at the front bulkhead and rear doors, and keep freight below the load line.
- Watch discharge and return air. Discharge (also called supply) air is air leaving the unit. Return air is air coming back off the load. A big gap between the two can point to warm product or restricted airflow.
- Shut the unit off while the doors are open. With the unit running and the doors open, it pulls in warm, moist air that can frost or ice up the evaporator coil, the part that makes the air cold. An iced coil moves less air and cools worse.
Specific temperature settings and procedures come from the shipper and receiver, so follow their paperwork. Food-safety rules for hauling temperature-controlled freight also exist in both the US and Canada, and the details vary, so check what applies to your loads.
What's the bottom line?
A reefer holds cold, it doesn't make it. The foam box slows heat, the airflow loop carries cold through the load, and the under-trailer tank keeps it all running.
Get the product loaded cold, the trailer pre-cooled, and the air path clear, and the unit does its job for days. Skip any of those and you're asking it to do something it wasn't built for.