It's Hot…But Your Favorite Frozen Treat is Ice Cold

This summer has been brutally hot across much of the country. Temperatures have pushed well past 100 degrees in places that are no strangers to summer heat, with several cities setting new all-time records. In July, Billings, Montana reached 111°F, Miles City climbed to 115°F, and Salt Lake City hit 109°F.

Those are the kinds of temperatures where the asphalt radiates heat, your steering wheel becomes something you approach with caution, and every air-conditioning unit in town seems to be working overtime.

Yet somewhere in that same heat, a refrigerated shipping container may be sitting in direct sunlight with a load of frozen food inside that has not softened one bit.

Your box of fudgesicles or pint of mint chocolate chip are frozen inside.

That is an impressive thing when you stop and think about it. A steel container exposed to outside temperatures above 110°F may be maintaining an interior temperature below zero, creating a temperature difference of more than 120 degrees between the world outside and the cargo inside.

So how does a steel box sitting in the sun manage to beat the heat?

The answer starts with an important distinction. A refrigerated shipping container, commonly referred to as a reefer, does not really create cold. Like any refrigeration system, its job is to remove heat.

Heat is constantly trying to enter the container through the roof, walls, floor, and doors. Warm air enters whenever the doors are opened, and even the cargo itself can introduce additional heat if it is loaded above its intended transport temperature. The reefer's job is to remove that heat faster than the outside environment can put it back in.

At its core, the system operates on the same basic refrigeration cycle used in your refrigerator at home or the air-conditioning system in your house. Refrigerant moves through a compressor, condenser, expansion device, and evaporator, transferring heat from inside the container to the outside environment.

The compressor increases the pressure and temperature of the refrigerant. The condenser then releases that heat outside the container. From there, the refrigerant passes through an expansion device, which reduces its pressure and temperature before it enters the evaporator. Inside the evaporator, the refrigerant absorbs heat from the air circulating through the container before returning to the compressor to begin the process again.

The system repeats that cycle continuously.

What makes the process particularly interesting during extreme heat is that the reefer is not simply keeping hot outside air away from the cargo. It is actively collecting heat from inside the container and rejecting it into an environment that may already be more than 100 degrees.

That is a difficult job, and the refrigeration machinery could not do it effectively on its own.
The construction of the container itself plays an equally important role.

A typical dry freight container is primarily designed around strength, weather resistance, and cargo protection. A reefer has to do all of those things while also creating a highly insulated thermal envelope.

The walls, roof, floor, and doors are insulated to slow the transfer of heat between the outside environment and the cargo space. The insulation does not stop heat completely, but it significantly reduces the rate at which heat can move into the container.

That gives the refrigeration system a fighting chance.

If you tried to maintain frozen temperatures inside an ordinary steel shipping container sitting in 110-degree heat, the refrigeration unit would be battling heat gain almost continuously. The insulation in a reefer slows that heat transfer enough that the refrigeration system can remove the incoming heat and maintain the desired temperature.

Another major difference becomes obvious the moment you step inside a reefer and look at the floor.

Instead of the wood flooring commonly found in standard dry containers, refrigerated containers typically use an aluminum T-bar floor. The channels running along the length of the floor are not there simply for structural support. They are an essential part of the air circulation system.

Conditioned air is discharged beneath the cargo and travels through those floor channels before moving upward and around the load. As the air absorbs heat, the warmer return air travels back toward the refrigeration unit, where that heat is removed before the air is circulated through the container again.

This is also why loading a reefer correctly is so important. A refrigerated container can have a perfectly functioning refrigeration unit and still experience temperature problems if the cargo blocks the airflow. If the floor channels are obstructed or the load is packed too tightly in the wrong areas, conditioned air may not reach the entire cargo space evenly.

In other words, maintaining temperature inside a reefer is not simply about producing cold air. It is about moving that air where it needs to go.

That becomes even more important when you consider the range of products transported in refrigerated containers.

We tend to think of reefers as large freezers, and certainly many of them are used to transport frozen products. Meat, seafood, frozen foods, and yes, fudgesicles may travel at temperatures well below freezing.

But a great deal of reefer cargo is not frozen at all.

Fresh produce, flowers, dairy products, certain pharmaceuticals, and other temperature-sensitive goods may need to remain at carefully controlled temperatures above freezing. In those cases, getting too cold can be just as damaging as getting too warm.

Some cargo introduces an additional challenge because it is still biologically active after harvest.

Fruits and vegetables continue to respire during transportation, producing heat, moisture, and gases. Depending on the commodity, the container may need to manage not only temperature but also ventilation and humidity.

More advanced reefer systems can go a step further by controlling the atmosphere inside the container, including oxygen and carbon dioxide levels, to slow the ripening process and help extend the life of certain products during long-distance transportation.

At that point, calling it a refrigerated shipping container almost seems too simple.

It is really a mobile, controlled environment designed to maintain very specific conditions while traveling thousands of miles.

And that brings us to one of the most impressive parts of the entire system. A reefer has to do all of this while living the life of a shipping container.

It may begin its trip at a packing facility, travel by truck to a marine terminal, be lifted onto a vessel, cross an ocean, be moved onto rail or another truck, and eventually arrive at a distribution center hundreds or thousands of miles from where it started.

Along the way, it can encounter intense heat, freezing temperatures, salt air, vibration, repeated lifting, and constant movement.

Depending on where it is in that journey, the refrigeration unit may receive power from several different sources. At a terminal or aboard a vessel, it can be connected to an electrical supply. During road or rail transportation, a generator set can provide power when another electrical source is not available.

Meanwhile, electronic controllers and temperature sensors continuously monitor the internal environment and regulate the refrigeration system.

Modern reefer containers can maintain remarkably precise temperatures considering the conditions in which they operate. From the outside, they still look like steel boxes. Behind that appearance, however, is a carefully engineered combination of refrigeration, insulation, airflow, electronics, and structural design.

Most of us never think much about that system because, when it works, there is nothing to notice.

We walk into a grocery store, open the freezer door, and expect the ice cream to be frozen. We expect fresh produce to look fresh, seafood to remain properly chilled, and temperature-sensitive products to arrive in usable condition.

The cold chain operates quietly in the background, and refrigerated shipping containers are one of the pieces that make it possible.

At W&K Container, Inc., we spend a lot of time thinking about shipping containers, so we probably notice some things that most people do not. Reefers are one of those things. They may look simple from the outside, but there is a lot of engineering hidden inside those insulated walls and beneath that aluminum floor.

So, the next time it is hot enough outside that frying an egg on the sidewalk does not sound completely ridiculous, remember that somewhere nearby there may be a steel shipping container sitting in that same heat with a load of frozen food inside.
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We're curious. What's your favorite ice cream treat?