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Airflow in reefer containers: circulation and ventilation explained

Read time: 4 minutes

Ventilation and circulation are frequently get mixed-up. While fresh air intake may seem like a good idea to optimise conditions inside containers, I will show in this article why this is not necessarily the case. In fact, ventilation harms refrigeration, dehumidifying efficiency and will increase fuel consumption and emissions.

Air circulation under pressure of mechanical fans is the airflow inside the container. The air is pushed through and around the cargo. The aim of air circulation is

  • reach and maintain the correct temperature
  • to distribute fresh air
  • to reduce the relative humidity
  • etc.

The air circulation in a container is approx 5,400 m3/hour at full fan speed and approx 2,350 m3/hour at low fan speed (both at 60 Hz). The volume of a 40′ high cube container is approx 65 m3 (up to the maximum load limit).

Ventilation

Ventilation is replacing the air inside the container by air from the outside surrounding environment. The fresh air intake capacity of most containers varies from 0 to 180 m3/hour (50 Hz). Fresh air, i.e. ventilation, is introduced to remove accumulated respiration gasses, such as carbon monoxide and ethylene, emitted by the cargo as a result of the ripening process.

Each cargo has its own specific respiration characteristics. Fruit, for instance, can be divided in climacteric and non-climacteric. Climacteric fruit continues to ripen after harvest; non-climacteric stops ripening after harvest. As a result, climacteric fruit has a high level of respiration. Pear, mango, avocado and banana are examples of climacteric fruit. Non-climacteric fruit includes citrus, grape, cherry and water melon.

Fresh air affects the efficiency of the refrigeration installation. The evaporator cooling coil removes humidity from the air inside the container. Fresh air means that the evaporator cooling coil must also remove humidity for the freshly introduced air. At temperatures below 10 degrees centigrade this results in more frequent defrosting periods, resulting in more frequent interruption of the circulation fans. In other words: less fresh air intake means that

  • the evaporator cooling coils needs to be defrosted less often
  • circulation is interrupted less frequently

The evaporator fans too are affected by fresh air.

Suffice to say that the respiration gasses emitted by non-climacteric fruit is much lower than climacteric fruit. As a result the need for ventilation of non-climacteric fruit is far less.

Limitation of ventilation in containers

In addition, fresh air intake renders compliance with Controlled Atmosphere conditions impossible. Considering the negative effects of fresh air intake, ventilation should be determined on the basis of the specific characteristics of the cargo.

 

Reefer air flow

Airflow in containers (courtesy MSC)

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