1 Introduction
Condensation damage is a collective term for damage to cargo in a CTU from internal
humidity especially in freight containers on long voyages. This damage may
materialize in form of corrosion, mildew, rot, fermentation, breakdown of cardboard
packaging, leakage, staining, chemical reaction including self-heating, gassing and
auto-ignition. The source of this humidity is generally the cargo itself and to some
extent timber bracings, pallets, porous packaging and moisture introduced by packing
the CTU during rain or snow or packing in an atmospheric condition of high humidity
and high temperature. It is therefore of utmost importance to control the moisture
content of cargo to be packed and of any dunnage used, taking into consideration the
foreseeable climatic impacts of the intended transport.
2 Definitions
For the assessment of the proper state of "container-fitness" of the cargo to be
packed and for the understanding of typical processes of condensation damage the
most relevant technical terms and definitions are given below:
| Absolute humidity of air
|
Actual amount of water vapour in the air, measured in
g/m3 or g/kg.
|
| Condensation
|
Conversion of water vapour into a liquid state. Condensation
usually starts when air is cooled down to its dew point in contact
with cold surfaces.
|
| Corrosion threshold
|
A relative humidity of 40% or more will lead to an increasing
risk of corrosion of ferrous metals.
|
| Crypto climate in the container
|
State of relative humidity of the air in a closed container,
which depends on the water content of the cargo or materials in the
container and on the ambient temperature.
|
| Daily temperature variation in the container
|
Rise and fall of temperature in accordance with the times of day
and often exaggerated by radiation or other weather
influences.
|
| Dew point of air:
|
Temperature below the actual temperature at which a given
relative humidity would reach 100%. Example: The dew point of air at
a temperature of 30°C and 57% relative humidity (= 17.3
g/m3 absolute humidity) would be 20°C, because at
this temperature the 17.3 g/m3 represent the saturation
humidity or 100% relative humidity.
|
| Hygroscopicity of cargo
|
Property of certain cargoes or materials to absorb water vapour
(adsorption) or emit water vapour (desorption) depending on the
relative humidity of the ambient air.
|
| Mould growth threshold
|
A relative humidity of 75% or more will lead to an increasing
risk of mould growth on substances of organic origin like foodstuff,
textiles, leather, wood, ore substances of non-organic origin such
as pottery.
|
| Relative humidity of air
|
Actual absolute humidity expressed as percentage of the
saturation humidity at a given temperature. Example: An absolute
humidity of 17.3 g/m3 in an air of 30°C represents a
relative humidity of 100 · 17.3 / 30.3 = 57%.
|
| Saturation humidity of air
|
Maximum possible humidity content in the air depending on the air
temperature (2.4 g/m3 at -10°C; 4.8 g/m3 at
0°C; 9.4 g/m3 at 10°C; 17.3 g/m3 at 20°C; 30.3
g/m3 at 30°C; see figure 3.1 below).
|
| Sorption equilibrium
|
State of equilibrium of adsorption and desorption at a given
relative humidity of the ambient air and the associated water
content of the cargo or material.
|
| Sorption isotherm
|
An empirical graph showing the relation of water content of a
cargo or material to the relative humidity of the ambient air.
Usually the adsorption process is used to characterize the above
relation. Sorption isotherms are specific for the various cargoes or
materials (see figure 3.2 below).
|
| Water content of cargo
|
Latent water and water vapour in a hygroscopic cargo or
associated material, usually stated as percentage of the wet mass of
cargo (e.g. 20 t cocoa beans with 8% water content will contain 1.6
t water).
|
Figure 3.1 Absolute and relative
humidity
|
Figure 3.2 Sorption isotherms of
Sitka spruce
|
3 Mechanisms of condensation
3.1 Closed CTUs, in particular closed freight containers, packed with a cargo that
contains water vapour, will quickly develop an internal crypto climate with a
distinguished relative humidity in the air surrounding the cargo. The level of this
relative humidity is a function of the water content of the cargo and the associated
materials of packaging and dunnage, following the specific sorption isotherms of the
cargo and associated materials. A relative humidity of less than 100% will prevent
condensation, less than 75% will prevent mould growth and less than 40% will prevent
corrosion. However, this protective illusion is only valid as long as the CTU is not
subjected to changing temperatures.
3.2 Daily temperature variations to CTUs are common in longer transport routes, in
particular in sea transport, where they also depend largely on the stowage position
of the CTU in the ship. Stowage on top of the deck stow may cause daily temperature
variations of more than 25 °C, while positions in the cargo hold may show marginal
variations only.
3.3 Rising temperatures in a CTU in the morning hours will cause the established
relative humidity of the air to drop below the sorption equilibrium. This in turn
initiates the process of desorption of water vapour from the cargo and associated
materials, thus raising the absolute humidity in the internal air, in particular in
the upper regions of the CTU with the highest temperature. There is no risk of
condensation during this phase.
3.4 In the late afternoon the temperature in the CTU begins to decline with a
pronounced drop in the upper regions. In the boundary layer of the roof, the air
reaches quickly the dew point at 100% relative humidity with immediate onset of
condensation, forming big hanging drops of water. This is the formidable container
sweat which will fall down onto the cargo and cause local wetting with all possible
consequences of damage. Similarly, condensate on the container walls will run down
and may wet the cargo or dunnage from below.
3.5 The condensed water retards the overall increase of the relative humidity in the
air and thereby decelerates the absorption of water vapour back into the cargo and
associated materials. If this temperature variation process is repeated a number of
times, the amount of liquid water set free by desorption may be considerable,
although some of it will evaporate during the hot phases of the process.
3.6 A quite similar mechanism of condensation may take place if a freight container
with a warm and hygroscopic cargo, e.g. coffee in bags, is unloaded from the ship
but left unopened for some days in a cold climate. The cargo will be soaked by
condensation from the inner roof of the freight container.
3.7 Notwithstanding the above described risk of container sweat due to the daily
temperature variation, an entirely different type of condensation may take place if
cargo is transported in a closed CTU from a cold into a warm climate. If the CTU is
unpacked in a humid atmosphere immediately after unloading from the vessel, the
still cold cargo may prompt condensation of water vapour from the ambient air. This
is the so-called cargo sweat, which is particularly fatal on metal products and
machinery, because corrosion starts immediately.
4 Loss prevention measures
4.1 Corrosion damage: Ferrous metal products, including machinery, technical
instruments and tinned food should be protected from corrosion either by a suitable
coating or by measures which keep the relative humidity of the ambient air in the
CTU reliably below the corrosion threshold of 40%.
4.2 The moisture content of dry dunnage, pallets and packing material can be
estimated as 12% to 15%. The sorption isotherms for those materials show that with
this moisture content the relative humidity of the air inside the CTU will
inevitably establish itself at about 60% to 75% after closing the doors. Therefore
additional measures like active drying of the dunnage and packing material or the
use of desiccants (drying agents in pouches and other passive methods for moisture
capture) should be taken, in combination with a sealed plastic wrapping.
4.3 Fibreboard packaging and dunnage when used in association with dangerous goods
should undergo water resistance test using the Cobb method as specified in ISO
535footnote.
4.4 Mould, rot and staining: Cargoes of organic origin, including raw foodstuff,
textiles, leather, wood and wood products, or substances of non-organic origin such
as pottery, should be packed into a CTU in "container-dry" condition. Although the
mould growth threshold has been established at 75% relative humidity, the condition
"container-dry" defines a moisture content of a specific cargo that maintains a
sorption equilibrium with about 60% relative humidity of the air in the CTU. This
provides a safety margin against daily temperature variations and the associated
variations of relative humidity. Additionally, very sensitive cargo should be
covered by unwoven fabric (fleece) which protects the cargo top against falling
drops of sweat water. The introduction of desiccants into a CTU containing
hygroscopic cargo, that is not "container-dry", will generally fail due to the lack
of sufficient absorption capacity of the drying agent.
4.5 Collapse of packing: This is a side effect of moisture adsorption of usual
cardboard that is not waterproof. With increasing humidity from 40% to 95% the
cardboard loses up to 75% of its stableness. The consequences are the collapse of
stacked cartons, destruction and spill of contents. Measures to be taken are in
principle identical to those for avoiding mould and rot, or the use of "wet
strength" cardboard packaging.
4.6 Unpacking
4.6.1 Goods packed in a cold climate on arrival in a warm climate with higher
absolute humidity should be delayed until the goods have warmed up sufficiently for
avoiding cargo sweat. This may take a waiting time of one or more days unless the
goods are protected by vapour tight plastic sheeting and a sufficient stock of
desiccants. The sheeting should be left in place until the cargo has completely
acclimatized.
4.6.2 Hygroscopic goods packed in a warm climate on arrival in a cold climate with
low absolute humidity should be unpacked immediately after unloading from the
vessel, in order to avoid cargo damage from container sweat. There may be a risk of
internal cargo sweat when the cargo is cooled down too quickly in contact with the
open air, but experience has shown that the process of drying outruns the growth of
mould, if the packages are sufficiently ventilated after unpacking