How best to ventilate to avoid moisture damage to cargo

The air at sea is very humid; it contains a large quantity of water vapour. When this humid air meets relatively colder surfaces, the moisture condenses on the surface as either cargo sweat or ship’s sweat. What is cargo sweat?
 

Cargo sweat is condensation that can form on exposed surfaces of cargo. It occurs when large amounts of warm, moist air are introduced into a hold containing substantially colder cargo.
 

What is ship’s sweat?
 

Ship’s sweat is condensation that forms on the vessel structure. It occurs when the air within the hold is, made warm and moist by the cargo, comes into contact with cold surfaces as the vessel moves into cooler climates. Cargo can be damaged as a result of overhead drips, by direct contact with sweat that forms on the ship’s side, or by condensed water that gathers in the bottom of the hold.

‘Sweating’ or condensation is a problem many shipping companies need to overcome to ensure cargo isn’t affected by ‘moisture damage’. This is because a significant number of cargo claims are made due to ‘moisture damage’ - particularly claims involving bagged or bulk agricultural products. Claimants typically allege that failure by the ship to ventilate correctly resulted in the development of condensation (‘sweat’), causing the cargo to deteriorate.
 

Why ventilation is important
 

If cargo spaces are not appropriately ventilated, the temperature in the hold will increase. It’s particularly problematic when a ship sails from a colder to a warmer region. As the deck is exposed to sunlight, it heats. Through conduction, the air temperature in the hold also begins to rise. Increases in seawater temperatures also have the same effect on the shipside plating and, subsequently, on the hold temperature.

To avoid claims, and to safeguard crew, shipping companies can use new features in SPOS 9.2, which include the relative humidity and dew point in the daily weather forecast. This enables onboard teams to know how best to ventilate based on the conditions.

 

Knowing whether or not to ventilate
 

The decision to ventilate or not comes down to four influencing factors:

1. Saturation: The amount of water vapour the air may contain

2. Relative humidity: The actual amount of water vapour in the air, compared with the saturation or air at the same temperature or pressure

3. Dew point temperature: When an isolated volume of air cools, and the temperature falls, the relative humidity increases. Once the temperature decreases, to the point where saturation occurs, and water begins to condense.

4. Dew point measurement: Theoretically, all decisions regarding cargo ventilation should be based on dew point temperatures, comparing the dew point of the ambient air with the dew point of the air inside the hold. The dew point temperature inside a cargo space is more problematic. One of the simplest methods is to use a "whirling psychrometer", swinging the instrument inside the hold until the wet bulb temperature has stopped falling and remains steady.


Once the influencing factors are known, the rules for ventilation are simple.
 

#1 Dew point Rule

VENTILATE if the dew point of the air inside the hold is higher than the dew point of the air outside the hold.

DO NOT VENTILATE if the dew point of the air inside the hold is lower than the dew point of the air outside the hold.

#2 Three Degree Rule
 

In many instances, it is impracticable to measure the hold dew point temperatures accurately, or at all. In such cases, ventilation requirements may be estimated by comparing the average cargo temperature at the time of loading with the outside air temperature several times a day. Ventilation may then be carried out on the following basis;

VENTILATE if the dry bulb temperature of the outside air is at least 3°C cooler than the average cargo temperature at the time of loading.
DO NOT VENTILATE if the dry bulb temperature of the outside air less than 3°C cooler than the average cargo temperature at the time of loading, or warmer.

#3 Traveling between warm tropical and cold winter zones
 

The rule is to ventilate when travelling from a warm tropical zone to a cold winter zone.

"HOT TO COLD, VENTILATE HOLD, COLD TO HOT, VENTILATE NOT"

 

Want to know more and discover how one shipping company is already using SPOS to help prevent issues caused by high humidity and sweating?

Download "Keeping Cargo Safe During Transit: How to Prevent Moisture Damage with SPOS"

 

Download Case Story

Avoiding moisture damage is a key challenge for merchant shipping companies. Discover how best to ventilate, based on conditions, to help overcome this issue.