Container Rain
Container rain is condensation that forms on the ceiling of a cargo container and drips onto the cargo causing moisture damage.
This can result in a container being rejected by a receiver at the port costing the exporter dearly.
An atmospheres humidity is relative to its temperature. This is because cold air can hold less moisture than warm air. A 1m cube of air at 25°C which contains 20g of moisture may have a relative humidity of 80%rH (relative humidity). That means it could potentially hold another 20% (4g) of moisture before becoming saturated (100%rH).
However, drop the cubes temperature to 17°C and the relative humidity will rise to 100%rH even though it contains the same amount of moisture. This is because at the lower temperature the air can no longer hold as much moisture.
When this saturated air comes into contact with a surface colder than itself the moisture will condense out onto it.
Put this in the context of a logistics operation and it means that when a container is loaded with cargo in a tropical port at an atmosphere of 90%rH and 32°C it will hold about 33g of moisture per m³.
As a 20ft container has a volume of around 33m³ this equates to about 1.089 litres of water held in the air of the container.
When it arrives in a European port with an atmosphere of 0°C and 55%rH the air will be able to hold just 5g of moisture. This results in 28g of moisture condensing out of every 1m³ of air inside the container, which is just under 1 litre of moisture for a 20ft and 2 litres for a 40ft container.
The objective is to maintain a humidity below which condensation forms, and if does form, to have a system that will react quick enough to absorb the excess moisture prior to droplets forming.
1kg of Superdry container desiccant can absorb up to 20g of moisture per hour so can make a significant difference to the atmosphere during the day and night cycles of temperatures increasing and decreasing within a container.
It acts as a buffer absorbing excess moisture and helps to maintain the equilibrium between the humidity of the air and the container's contents.