The inside story of a reefer: how air moves

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One of the most common sources of disputes in the fresh produce industry involves temperature management during transportation. Understanding how refrigerated trailers function—and how airflow, loading practices, equipment condition, and monitoring affect product quality—can help reduce risk and prevent costly claims. Originally researched and written by Karen Davidson, Editor of The Grower publication, this article provides practical guidance for those involved in transporting fresh fruits and vegetables.

The reefer download shows a flat 36°F for the entire run, pickup to delivery.

You open the trailer at the dock, pull a clamshell from three rows deep, slide in a probe, and it reads 48°F. The rejection email is already in your inbox.

So which number is wrong?

Neither.

Why doesn’t the reefer set point match the product temperature?

Short answer: they’re measuring two different things, and the reefer was never built to tell you what the pulp is doing. Part of the trouble is a vocabulary problem. Four different temperatures get treated as one number on the dock, and they are not the same:

1. Set point: What you dialed in: an instruction, not a measurement.
2. Supply air: The chilled air leaving the evaporator, on its way into the box.
3. Return air: The air coming back to the unit after it has moved through the load.
4. Product temperature: What a probe reads when you push it into the pulp.

The reefer manages air. It can’t read the middle of a pallet three rows deep, and most units are built to keep cold products cold, not chase down heat the shipper sent in warm. So the trailer holds set point while a warm pallet in the nose takes hours to catch up, and hot spots build in dead zones the return-air sensor never sees.

That’s why one reading, stripped of context, sends you after the wrong problem. When air and pulp disagree, check the load, the precool, and the airflow before anyone blames the box.

How does air actually move through a refrigerated trailer?

Airflow is where most of the trouble hides, so it helps to picture the loop before worrying about what any sensor reads inside it. Cold air leaves the unit at the front top of the trailer, runs the length of the ceiling, drops over the load, picks up heat, and returns along the floor to the evaporator. Then the cycle starts again.

That loop only works, though, if the load gives the air somewhere to go, and a few common loading habits can shut it down:

• Pallets jammed against the walls choke the sides and pinch off the downward path.
• Product blocking the floor channels chokes the return and starves the evaporator of warm air to work on.
• A load stacked too tall crowds the ceiling and keeps supply air from reaching the rear. Any one of those turns the back of the trailer into a dead zone, which is where the air chute earns its keep. A good chute carries cold air past the front pallets, and delivers it to the door end before it runs out of push.

Without one, or with a shortened one, the rear of the load drifts warm for the entire trip while the temperature log still looks clean.

What are the most common reasons a sensor reads warmer or colder than expected?

Once the airflow picture is clear, the next question is what trips it up on real loads. Most discrepancies trace back to a short list of very fixable causes.

• The trailer wasn’t precooled: The GCCA calls precooling a baseline practice, and for good reason. Loading cold products into a warm box transfers heat straight into the pallets before the doors close. Set point alone does not confirm precool. Someone has to verify the air temperature in the nose and the tail before the first pallet rolls in.
• The load blocks its own airflow: Even a precooled trailer fails if the pallets strangle the loop. Products need gap space from the walls and a stack height that does not crowd the ceiling. Pallets touching the walls conduct outside heat into the product. Wall loading wrecks temperature uniformity, which is why the back corners so often read warm.
• The unit is in the wrong mode: Good loading still loses if the reefer is run the wrong way. Sensitech recommends continuous mode for fresh perishables because it makes small adjustments and holds a tighter band. Fuel saver or cycle sentry means that temperatures may drift between cycles, creating hot spots or top freezing on sensitive cargo.
• The trailer itself is the problem: Sometimes the equipment is the weak link. A torn air chute, a blocked return, a bad door seal, or a damaged wall panel will quietly undo everything else you did right. Don’t overlook this factor: FDA rules require transportation equipment to maintain a certain temperature.
• Something happened on the road: Finally, when the hardware checks out, look at the trip itself. Sharp spikes, light events, and off-route door openings point to handling rather than mechanics. Extra stops, a long lunch with the doors cracked, a curious receiver who peeked early. Those fingerprints show up in the data when you have the right visibility.

Where should I place a temperature monitor and how do I interpret it?

Finding the root cause is only useful if you are collecting the right data in the first place. Sensitech’s anchor recommendation is the rear pallet, right side facing the door, at eye level. The door end runs warmest, the spot is repeatable, and a receiver can retrieve the device without tearing the load apart.

How can I turn temperature alerts into a repeatable response process?

A good alert tells you a pallet is warming up. It doesn’t tell you who to call, what to say, or whether the driver can still do anything about it. That silence between the ping and the fix is where loads are lost, while the sensor and the software sit back, satisfied they did their jobs.

Five steps keep that from happening:

1. Write the SOPs before you need them: Put precooling, loading inspection, operating mode selection, and monitor placement into documented procedures your team follows on every load. An SOP you invent at 2:00 a.m. during a claim is not a sound SOP.
2. Assign every alert type an owner and a next action: A temperature excursion, a door-open event, and a late-stage spike should each have a name attached—and a specific first move. Ambiguity is how products spoil in a parking lot while four people debate who calls the driver.
3. Investigate with context, not just a number: Pull the lane, commodity, sensor placement, loading photos, reefer mode, door-open log, and trailer condition before drawing conclusions. A 48°F spike means one thing on a hot Phoenix dock and something very different on a mountain pass at night.
4. Close the loop in writing: Document what happened, what the team did, and whether it worked. Patterns only show up when you can compare last Tuesday’s load to the one from three months ago—without relying on anyone’s memory.
5. Let the system do the escalating: A workflow engine built on triggers, conditions, and actions can route the right alert to the right person, escalate when nobody responds, and skip the ones that do not need a human. Knowing why the reefer temperature and the pulp drifted apart only matters if the response happens fast enough to save the load.

Key Takeaway

For DRC members, temperature-related disputes often hinge on questions of handling, documentation, and accountability.

By understanding how refrigerated trailers function and by implementing consistent procedures for loading, monitoring, and responding to temperature events, companies can reduce risk and strengthen their position when issues arise.

If you have any questions about the article and would like to learn more, our team at the DRC is here to assist you. We value your inquiries and are eager to provide support. Click here to proceed.