Container rain

The steel walls of shipping containers are great conductors of heat. As the temperature drops outside, likely with nightfall, the metal surfaces of the container quickly cool. If they cool enough, they will reach what is known as the ’dew point’, a temperature where water vapour becomes a liquid again. The warm air (holding water vapour) trapped in the container does not cool at the same rate as the air outside. When it comes into contact with the cold metal walls, it condenses from a vapour to a liquid, forming water droplets on the metal.

As the warmer, moister, air rises most condensation occurs on the ceiling. At this point, it will either drip onto the contents stored in the container or run down the walls. In areas where the is a high contrast between day and night temperatures, the effect is more pronounced.

There are two main general factors to address when minimizing the risk of condensation in a container. First, reduce the amount of water vapour in the container in the first place. Secondly, allow the warm moist air to escape. These two fundamental issues can be dealt with in a number of ways.

• Inspect it for visible moisture. Before loading the container, check the walls and ceilings and ensure that they are not damp, and wipe away any liquid that is present.
• Place desiccants in the container. Desiccants are hygroscopic substances, the opposite of humectants, which induce and sustain dry environments. In simpler terms, they are materials which absorb moisture out of the air. A common example would be silica gel which is often found product packaging to keep goods dry (the ’DO NOT EAT’ packets…). They can be installed in a number of forms, but are commonly contained in strips on the walls or bags on the floor of containers.
• Consider your environment. If your container will be stored in areas where there is a large diurnal (day/night) temperature range, you take more precautions against ’container rain’.
• Careful with what you put in the container. Materials, such as fresh wood, can have particularly high moisture contents. This means that even the pallets that your items are loaded on, can cause more condensation. Materials which do contain water should be used whenever possible. In terms of avoiding ’container rain’, plastic is a better choice than wood. If wood must be used, try to dry it out fully beforehand and make sure it has a low moisture content. It can make a big difference.
• Insulation, Insulation, Insulation. Insulating the walls of the container can stop the temperature of the walls dropping to dew point, and therefore stopping condensation from occurring.
• Ventilation, Ventilation, Ventilation. Containers are often fitted with ventilation units, or even just simple vents to reduce the build-up of warm air in a container.
• Dehumidifiers and air con units. Although expensive to fit and run, air conditioning and dehumidifiers are effective at keeping stable, dry conditions for the most moisture sensitive of container contents. It is worth keeping in mind however they do require a source of electricity, either from a reefer point or generator, and do run the risk of failing.