How it Works – Refrigerated Trailers

Reefer units are often misunderstood, and this misunderstanding can lead to misguided performance expectations. Today, we will outline some facts that will help us better understand how reefer units work and clarify their operation.

One of the most important elements to keep in mind when thinking about reefers is that they are designed to maintain temperature of the product inside them, not cool. Although it may happen if given enough time, reefers are not designed to for example, bring a field load from upwards to 100°F ambient temperature down to 45°F storage temperature. Reefers are solely designed to remove any increased heat, keep temperature of products stable, and control humidity by condensing moisture in the air.

Common ways that heat enters reefer trailers is through the walls (trailers are insulated to prevent this). Heat can also flow around gaps in door openings, through cracked door seals, through trailer floors, or out of the ceiling. A steel bolt or a structural member will act as a conductor if it passes from outside the trailer to inside. Most newer reefer trailers have internal structures for rub-rails and E-tracks that are separated and insulated from the outside walls.

To accomplish the items that are mentioned at the beginning of this post (remove increased heat, keep product temperatures stable, and control humidity), reefers require 4 components:

– Thermal integrity in the trailer to prevent the inflow of additional heat
– Sufficient BTU capacity to remove the expected amount of heat
– Sufficient airflow
– Sufficient air velocity to move the air through the trailer, over and through the load. 

Regular scheduled reefer maintenance and proper loading and stacking procedures will assist in succeeding the 4 above components. We know what the typical occurrences are: loads are often too warm when they are loaded, drivers don’t shut their reefers off before opening doors, and debris often blocks airflow. These occurrences often greatly increase the potential for temperature issues upon delivery.

As a review, the idea of refrigeration is to remove heat, and thus, maintain temperature of products; refrigeration systems are closed, and have many parts that assist in helping the unit maintain temperature. The major parts are:

The Compressor – in a reefer unit, the compressor is driven by a small engine. The compressor draws gaseous refrigerant in and compresses it. The pressure inside the compressor liquefies the gas, and the now-liquid refrigerant gives off heat to the body of the compressor, and ultimately to the air.

The Condenser – the liquid from the compressor is still relatively warm, so it is pumped into a condenser; the condenser is a heat exchanger. Warmth flows from the liquid to the walls of the tubing, to fins on the tubing. The fins present more surface area for cooling outside air drawn through the condenser by a fan. Similar to the way a radiator cools an engine!!

The Evaporator – The evaporator is located in the trailer. The refrigerant, having given up much of its heat in the condenser, has turned into a cool liquid under pressure. It now flows through a metering valve into the evaporator. The metering valve controls the amount of refrigerant released into the evaporator, acting like a throttle to control the amount of cooling. It also works to help maintain backpressure in the high-pressure part of the system, which runs from the compressor to the evaporator.

In the evaporator, the refrigerant rapidly expands, once again becoming a gas. As it does, it absorbs a great amount of heat from its surroundings. Those surroundings are finned coils, which help transfer heat from air flowing over the fins to the refrigerant. Air from inside the trailer is blown over the evaporator. The refrigerant gas, now under low pressure, is drawn back to the compressor where the cycle starts again. 

The trailer air, now cooled by giving up some of its heat to the evaporator, circulates back into the trailer to keep the cargo cool.

This is a very basic description of how a reefer system works to maintain air temperature. Like any component of a truck or trailer, it requires regular maintenance to ensure it operates as designed. Maintenance is relatively straightforward, and as with all other past suggestions, UWC strongly recommends regular preventative maintenance to avoid costly breakdown repair bills! Units should be checked regularly for oil leakage at both the engine and the compressor. Belt and hoses should also be checked regularly. Standard units call for oil and filter changes at approximately 1,500 hours, or about once every few months. This depends on how much the reefer operates when the truck is idle, and is best to also check with the manufacturers recommendations.
Along with mechanical maintenance, be sure to check your unit regularly to make sure air passages are clear and free of debris; check the channels on the floor for cleanliness, and remember to periodically remove the forward bulkhead to make sure airways are clean. It is also a good idea to routinely check the evaporator for any paper or scraps that may affect cooling.