Principles Of Ventilation in Marine Cargo Holds

The following knowledge is shared by Jiaxing Heyuan Supply Chain Co.,Ltd, a professional freight forwarding company. The principle of Marine cargo hold ventilation is based on air flow and exchange, which can be realized mainly in the following ways:

Principle of natural ventilation
1. Differential air pressure ventilation
Principle: When the ship is sailing, there is a wind pressure difference between the outside air and the cargo hold. The wind blows to one side of the ship, reducing the air pressure outside the cargo hold on that side, while the air pressure is relatively high on the other side. At this time, if the cargo hold is equipped with a vent, the air will enter the cargo hold from the side of the high air pressure vent, and then discharge from the side of the low air pressure vent, so as to achieve air circulation and exchange.
Example: In the open sea, when the wind speed is large, the wind pressure differential ventilation effect is significant. For example, a ship traveling at a speed of 15 knots, when the external wind speed is about 10 knots and the wind direction has a certain Angle with the ship's course, the wind pressure difference can effectively promote the flow of air in the cargo hold, timely discharge of hot and humid air, and introduce fresh air.

2. Hot pressure differential ventilation
Principle: The air in the cargo hold is usually higher than the outside air due to the respiratory action of the cargo, microbial activity or the introduction of external heat. Hot air is less dense and moves upward. When the cargo hold is provided with a vent at the top and an air intake at the bottom, the hot air will be discharged from the top vent, and the outside cold air will flow in from the bottom air intake, forming a natural air convection to achieve the purpose of ventilation.
Example: In the cargo hold of grain, the air temperature in the cargo hold rises due to the heat generated by the respiration of the grain. Under suitable weather conditions, hot differential pressure ventilation can continuously discharge the hot air in the cargo hold, maintain the appropriate temperature and humidity environment in the cargo hold, and prevent grain mildew.

Principle of mechanical ventilation
1. Pressure ventilation
Principle: Through the fan installed in the cargo hold ventilation system, the outside air is forced into the cargo hold. The pressure generated by the fan causes the air to overcome the resistance in the ventilation duct and the cargo hold, and is evenly distributed in all corners of the cargo hold. This ventilation method can rapidly increase the amount of air in the cargo hold and improve the ventilation efficiency, especially for areas requiring rapid air exchange or where the cargo hold structure is more complex and natural ventilation is difficult to reach.
Example: In the cargo hold of volatile and hazardous gas cargo, such as the cargo hold of petrochemical products, pressure ventilation can quickly push fresh air into the outside world, dilute the concentration of harmful gases, and ensure the safety of cargo and crew.

2. Suction ventilation
Principle: The fan is used to generate suction at the top of the cargo hold or other positions, and the air in the cargo hold is pumped out, so that a certain negative pressure is formed in the cargo hold. Under the action of negative pressure, fresh air from the outside world will automatically flow into the air intake of the cargo hold, so as to achieve air renewal. Suction ventilation can effectively exhaust the dirty air in the shipping cabin, and it is more suitable for the transportation of goods that emit odors, dust or harmful gases.
Example: In the cargo hold transporting coal, the suction ventilation can timely extract harmful gases such as methane volatilized by coal and dust, improve the air quality in the cargo hold and reduce safety hazards.

3. Mixed ventilation
Principle: Combined with the characteristics of pressure ventilation and suction ventilation, the compression fan and suction fan are set up in the cargo hold ventilation system. By reasonably controlling the operation of the two fans, the direction, speed and volume of air flow in the cargo hold can be more accurately adjusted to meet the ventilation needs of different goods under different transportation conditions.
Example: For the cargo hold of a large container ship, due to the variety of cargo types and complex storage methods, the hybrid ventilation can flexibly adjust the ventilation mode according to the cargo characteristics and ventilation requirements in different areas, so as to ensure that the cargo in the entire cargo hold can be well ventilated and protected.