The importance of cargo compartment ventilation

1) How does water vapor in the air condense into water droplets?

Ventilation is a major measure taken by crew members for cargo storage, and some people believe that ventilation is better than not being able to handle it, which is not always correct. Cargo compartment ventilation is not simple. In different situations, there are differences between necessary, unnecessary, prohibited, and impossible ventilation, and even slow (breathable) and accelerated ventilation.

To correctly understand and truly grasp the timing of ventilation, it is necessary to take some time to first understand the objective natural phenomenon of invisible water vapor in the air condensing into visible water droplets (commonly known as dew condensation or sweat on crew members). At a certain temperature, the water vapor content (i.e. absolute humidity, which will be slightly reduced later) in one cubic meter of air reaches a saturation state when it reaches a certain amount, and then condenses into water droplets.

In the above saturated state, we call the relative humidity at that time 100%; If its water vapor content is only half of that at saturation, its relative humidity is 50%, thus different relative humidity can be derived. It should be noted that if the temperature is higher (or lower), the water vapor content in the air needs to be slightly higher (or lower) in order to reach saturation and form dew. Therefore, after loading cargo at ports with high temperatures and relative humidity, the ship will sail. If it is not ventilated along the way and reaches waters with very low temperatures, the water vapor in the cabin air is likely to condense into small droplets on the cold hull plate. On the contrary, if the cargo is loaded at a port with very low temperatures and sails without ventilation along the way to a port with high humidity and relative temperature, if the cabin is suddenly opened or strongly ventilated, the water vapor in the outside air will condense into small droplets on the cold surface of the cargo. Since at a certain temperature, when the water vapor content per unit volume of air reaches a certain amount, that is, when the relative humidity is 100%, it will condense into water droplets; On the contrary, when a certain amount of water vapor in a unit volume of air drops to a certain temperature, it will condense into water droplets, which is called the dew point temperature. The dew point temperature can be read from the instrument, or can be found in the table using dry and wet temperatures as two parameters.

2) What is the purpose of cargo compartment ventilation?

There are three purposes to prevent cargo damage and the safety of people, ships, and cargo, namely:

·Dehumidification, specifically reducing relative humidity, means avoiding condensation or sweating, which can cause certain goods to become damp, moldy, overheat, spontaneously ignite, and accelerate spoilage;

·Cooling, its effect is the same as "dehumidification";

·Eliminate hazardous gases and provide fresh air to prevent explosions and poisoning accidents.

3) What are the reasons for the complexity of cargo compartment ventilation?

Cargo compartment ventilation can generally have a good effect, but sometimes it can actually have a bad effect. Some complex factors are difficult for onshore personnel to experience, such as:

A. If ventilation can reduce temperature and humidity, it is best, but sometimes when humidity is reduced, it will increase temperature, or when temperature is reduced, it will increase humidity; Generally, goods are afraid of temperature and humidity. Fortunately, most of them are afraid of humidity and not too afraid of heat, but excessive heat is not enough. When it is not possible to take into account both, they can only be dehumidified first. In addition, many goods are more likely to heat up after being wet, so it is even more important to consider dehumidifying first. Only a few goods are afraid of heat and humidity, so they can be ventilated and cooled at the same time, or even sprayed with water to cool down. Therefore, different types of goods, weather zones, and ports should be treated differently in terms of ventilation, which is one of the reasons for complexity.

B. There are different temperatures in various parts of the cargo hold, which not only affect each other but also rise and fall due to different factors. If the temperature changes with the temperature outside the cabin during ventilation and the heat emitted or absorbed by the cargo inside the cabin; The surface temperature of the goods varies with the temperature at the loading port, the characteristics of the goods, and their water content; The temperature of the ship's shell plate varies with the temperature of the seawater; The deck temperature varies with temperature, sunlight, waves, snow, and ice formation; In the temperature of the bulkheads and bottom plates, those adjacent to the fuel tank are subject to changes in oil temperature, while those separated from the engine room are subject to changes in engine room temperature. Therefore, when warm and humid air enters the cabin, some places in the cabin will condense dew, while others will not condense, with some coming first and then. This is the second reason for complexity.

C. In theory, cargo compartment ventilation is equivalent to two different temperature and humidity air exchange places, but in reality, the ventilation process is not so simple. The low-temperature and dry air outside the cabin enters the cabin and pushes out the high-temperature and high humidity air outside the cabin, which is a great good thing. However, if the entry is too fast and too strong, two different temperature and humidity air meet in a narrow path, nearby places will instead condense water droplets, and good things become bad things. This is the third reason for complexity. In short, cargo that is generally afraid of moisture only needs ventilation when the dew point temperature of the outside air is lower than the surface temperature of the cargo, which is also a basic condition for ventilation.