Natural and Artificial Draught

Definition

Draught is the small pressure difference that causes air to enter the furnace and flue gases to flow through the boiler system and exit through the chimney or exhaust. It is usually measured in millimeters of water column (mm WC) or Pascals. A natural draught is produced by the density difference between hot flue gases and cooler outside air, while an artificial draught is produced by mechanical means such as a fan, blower, or steam jet to force or draw air and gases through the system.

Main Content

1. Natural Draught

• Natural draught is developed due to the temperature difference between the hot gases inside the chimney and the colder outside air. Hot gases are lighter, so they rise upward through the chimney and create suction at the furnace end.
• It does not require any external power for operation, which makes it simple, economical, and reliable for small installations. The draught increases with chimney height and with a greater temperature difference between flue gases and atmospheric air.

Natural draught is commonly used in small boilers, domestic furnaces, fireplaces, and low-capacity steam plants where the required air flow is modest. The pressure difference created is limited, so the height of the chimney becomes very important. For example, a tall chimney at a thermal power station can create a stronger natural draught than a short chimney because the weight difference between the inside hot gases and the outside air column becomes more significant.

2. Artificial Draught

• Artificial draught is produced by mechanical devices like fans, blowers, or injectors to supply air to the furnace or remove flue gases from it. It is used when a strong, controlled, and adjustable airflow is required.
• It can be classified into forced draught, induced draught, and balanced draught depending on whether the air is pushed into the furnace, gases are pulled out, or both methods are combined.

Forced draught uses a fan to push air into the combustion chamber under pressure, improving fuel burning and enabling better control of combustion. Induced draught uses an exhaust fan to suck flue gases out of the furnace, creating a negative pressure inside the combustion chamber. Balanced draught uses both forced and induced draught fans together, giving excellent control and preventing smoke leakage. Artificial draught is widely used in modern thermal power plants, industrial boilers, and large furnaces because it can handle high heat load and maintain stable combustion even when natural draught is insufficient.

3. Comparison and Importance of Draught Systems

• Natural draught is simple, low-cost, and requires no power, but it is limited in strength and depends on weather conditions, chimney height, and flue gas temperature.
• Artificial draught provides greater flexibility, stronger air supply, better combustion control, and higher efficiency, but it requires power and additional equipment.

In real applications, the choice of draught system depends on boiler size, fuel type, required combustion rate, plant location, and economic considerations. For instance, coal-fired power plants generally use artificial draught because large quantities of air and high combustion rates are needed. On the other hand, small heating systems may rely on natural draught since they need less complicated arrangements. Proper draught design is important because inadequate draught can cause incomplete combustion, smoky exhaust, heat losses, and poor boiler performance, while excessive draught may carry away heat too quickly and reduce efficiency.

Working / Process

1. In natural draught, fuel burns in the furnace and produces hot flue gases; these gases become lighter than the surrounding air and rise through the chimney.
2. The rising gases create a low-pressure region at the furnace entrance, which allows fresh air to enter naturally and support further combustion.
3. In artificial draught, a fan or blower either forces air into the furnace, removes gases from it, or does both, thereby maintaining continuous and controlled airflow for efficient combustion.

Advantages / Applications

• Natural draught is simple, inexpensive, and does not need external power, so it is suitable for small boilers, fireplaces, and domestic heating systems.
• Artificial draught gives better combustion control, higher efficiency, and faster steam generation, making it ideal for large boilers, industrial furnaces, and power plants.
• Artificial draught can also reduce smoke and improve fuel utilization, while natural draught is preferred where maintenance and operating cost must be kept minimal.

Summary

• Natural draught is produced by chimney action due to density difference between hot gases and outside air.
• Artificial draught is produced by fans or blowers to force or remove air and gases.
• Draught is essential for proper combustion, boiler efficiency, and removal of flue gases.
• Natural draught is simple but limited, while artificial draught is stronger and more controllable.
• Important terms to remember: draught, chimney effect, forced draught, induced draught, balanced draught, flue gases, combustion, fan, blower.