Knowledge / Maritime cargo

Cargo condensation and ventilation: reasoning with dew point

A guide connecting dew point, surface temperature, moisture sources and safe ventilation decisions.

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Cargo condensation cannot be managed by looking at relative humidity alone. The relevant questions are which air contacts which surface, how cold that surface is, and how much moisture the cargo and packaging contain. Ventilation sometimes removes water vapour; in other circumstances it brings warm, moist air to cold cargo and increases damage. A sound approach identifies the physical mechanism before selecting a control compatible with cargo-safety requirements. This article explains general heat- and mass-transfer reasoning, rather than prescribing ventilation for a particular shipment.

Relative humidity and dew point answer different questions

Relative humidity compares the current vapour condition with saturation at the same temperature. It can change when temperature changes even if no moisture enters the air. Dew point is the temperature at which cooling at a specified pressure brings the air to saturation. Comparing surface temperature with the dew point of the contacting air is therefore useful when assessing condensation. The US National Weather Service explanation sets out this distinction.

When a surface is below the relevant dew point, the adjacent air can become saturated and vapour can condense. A sensor reporting average hold temperature may not represent the temperature of cold shell plating. An outside sensor exposed to sunlight, exhaust or rain may not represent the actual ventilation inlet. Decision quality depends on measurement position and condition, as well as the psychrometric calculation.

Ship sweat and cargo sweat

Ship sweat occurs when moist hold air contacts colder ship surfaces, potentially producing droplets that fall onto cargo. Cargo sweat occurs when warm, moist air reaches cold cargo and water forms directly on it. Neither mechanism requires a leak through the hull or hatch. Distinguishing the source of wetting requires location, timing, temperature history and the pattern of damage across packages.

Containers can experience repeated moisture cycles as they warm by day and cool at night. Annex 3 of the CTU Code discusses moisture released from cargo and associated materials and condensation on a cold roof. The internal climate can differ substantially from outside conditions. IMO identifies the CTU Code as a non-mandatory code of practice, rather than a universally binding ventilation rule.

The cargo can be the moisture source

Wood, paper and many agricultural products exchange moisture with surrounding air. Such materials are hygroscopic. They tend towards an equilibrium depending on temperature and humidity, but the process takes time. The centre and surface of a thick package can respond at different rates. Reducing air humidity does not demonstrate that the entire cargo has dried immediately.

Pallets, timber dunnage and cardboard can provide substantial moisture even when the product itself is dry. Packing material exposed to rain remains inside after the container is closed. Its water can evaporate and later condense elsewhere. Prevention therefore begins before loading, with the condition of packaging, dryness of the transport unit and storage arrangements. Air exchange during transit cannot always compensate for poor initial preparation.

For non-hygroscopic metal cargoes, slow thermal response can be the principal concern. Outside air may warm rapidly while a heavy metal item remains cold. Opening doors can introduce air that condenses on the cargo. Immediate unpacking or removal of protective barriers on arrival can therefore create a post-transport damage mechanism. The moisture-control plan should extend through delivery and entry into the receiving warehouse.

What ventilation changes

Ventilation transports moisture when there is a difference in water-vapour content between incoming and outgoing air. At similar pressures, an outside dew point below the inside dew point indicates a potential to remove moisture. It does not, by itself, establish the correct operational decision. Incoming air can condense on colder surfaces, rain or spray can introduce liquid water, and the cargo can require a particular ventilation regime for other reasons.

A running fan also does not prove effective air exchange around the cargo. Air may short-circuit between inlet and outlet, leaving stagnant regions unchanged. Stowage gaps, ducts, closed vents and cargo permeability affect flow distribution. The fan's nameplate flow may not be achieved against actual system resistance or with a congested stow. Operational observations should therefore be compared with the assumptions used in the ventilation design.

Two contrasting examples

For illustration only, assume hold air at 25°C and 70 percent relative humidity, with an approximate dew point of 19°C. If the inner shell surface is at 15°C, condensation is possible there. Suppose outside air is at 10°C and 80 percent relative humidity, giving an approximate dew point of 6.7°C. Despite its higher relative humidity, that outside air contains less water vapour at comparable pressure.

Now consider steel cargo remaining at 8°C while air at 25°C and 70 percent relative humidity enters the space. The incoming air's approximate 19°C dew point is well above the steel temperature, so ventilation can cause condensation directly on the cargo. These examples assume near-standard sea-level pressure, representative readings and no additional moisture source. They establish the physical direction of the effect, not an instruction to open ventilation for any actual cargo.

The dew points are rounded psychrometric estimates. Decisions only a few tenths of a degree from a boundary are sensitive to measurement uncertainty and differences between surfaces. One side of the vessel may receive sunlight while the other stays cold. Critical surface temperatures and representative air measurements are therefore more informative than a single overall average.

Estimate the scale of moisture removal

Assume a hypothetical ventilation system passes 1,000 kg of dry air per hour, and the outgoing humidity ratio exceeds the incoming ratio by 3 grams of water per kilogram of dry air. An ideal mass balance then indicates removal of 3 kg of water per hour. This is not a guaranteed drying capacity. Airflow and humidity difference can vary, while surface transfer or diffusion through the material can become the limiting process.

The example challenges the assumption that brief ventilation will rapidly dry a large wet cargo. Even at an ideal constant rate of 3 kg per hour, removing 300 kg of accessible water would require 100 hours. In reality the driving difference may fall, extending the process. Whether drying creates cracking, quality deterioration or unwanted reactions is another question. The objective is the product's suitable carriage condition, not necessarily the driest achievable environment.

Barriers and desiccants need a defined moisture budget

A vapour barrier can reduce exchange between sensitive cargo and surrounding air, but its seams, penetrations and handling damage determine the effective enclosure. Sealing wet packaging inside the barrier traps a moisture source. Similarly, a desiccant has finite capacity that depends on its material, exposure conditions and time. Counting sachets without estimating the enclosed moisture load does not establish protection.

For an original planning example, suppose a sealed package is assumed to release 150 g of moisture from its internal materials over the journey. A proposed drying arrangement with only 100 g of effective absorption under those conditions cannot balance that assumed release, even before leakage is considered. This is a conceptual budget, not a product-sizing recommendation. Real selection requires manufacturer data, packaging permeability, cargo compatibility and suitable safety margins.

Cargo-safety requirements take priority

Generic condensation controls do not override hazards involving self-heating, flammable gas, toxic atmospheres, fumigation or oxygen depletion. For solid bulk cargoes, consult the applicable IMSBC cargo schedule and the vessel's approved procedures. Supplying or restricting air can have different consequences for fire and gas safety. General dew-point reasoning cannot displace those cargo-specific provisions.

Ventilation also does not prove that a hold or container is safe to enter. Atmospheric assessment, entry authorization, rescue arrangements and competent personnel remain separate requirements. Nobody should enter an unsafe space merely to obtain a humidity reading. Considering safe sampling and remote measurement when designing the monitoring arrangement prevents moisture control from introducing another hazard.

Records, diagnosis and common errors

A useful record places inside and outside measurements, critical surface temperatures where available, weather, vent or fan status and the reason for changes on one time line. A note stating that ventilation occurred does not explain whether it helped. Calibration, contamination and measurement range also need attention. Unsynchronized clocks can suggest a false relationship between otherwise accurate readings.

Damage investigation should distinguish condensation from hatch leakage, seawater ingress and cargo already wet at loading. A salt trace or single photograph does not establish the complete cause. Package distribution and the voyage's temperature history provide stronger context. Common mistakes include treating high outside relative humidity as invariably bad, assuming warm air always dries, and accepting fan operation as proof of an effective control.

Simple temperature-difference rules arise from particular cargo and voyage experience. They should not be elevated into universal physical laws, especially when dew-point and surface-temperature data are available. The final decision should explain which moisture is being moved where, while preserving the cargo's specific safety limits.

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