How it works - Refrigerated Trailers - Part II

In a post back in February, we introduced the topic of how refer units work that outlined some facts what will help us better understand and clarify their operation.

This lead us to ask the question, exactly how do reefer units work? So, we did some research to find some more technical explanations.

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.

Understanding exactly how your reefer unit works will allow you to be better equipped to take care of it. Remember, take care of your equipment, and it will in turn take care of you! Do you have any comments or more details to add to our post? Please share!!!


An on a final note, we hope you have a very cool St. Patrick’s Day!!!!

References:

Understanding reefers. Paul Abelson, technical editor. May 2001. Landline: The Business Magazine for Professional Truckers. Accessed on March 17, 2011.