Mechanicaldraft

Mechanicaldraft, often written as mechanical draft or mechanical-draft, is the use of mechanical means—most commonly fans or blowers—to create and regulate the flow of combustion air into a furnace and the exhaust of flue gases through a stack. It is used to supplement or replace natural buoyancy (natural draft) in boilers, furnaces, cement kilns, and other equipment, allowing more precise control of combustion conditions independent of ambient weather.

Two principal configurations are forced draft and induced draft. In forced-draft systems, a blower pushes combustion air into the furnace, increasing pressure and ensuring adequate oxygen supply. In induced-draft systems, a fan exhausts flue gases from the furnace, pulling air and gas through the system and maintaining a slight negative pressure in the furnace. Some installations use a balanced-draft arrangement, employing both a forced-draft fan and an induced-draft fan to control pressure and gas flow.

Applications and benefits include improved combustion control, the ability to operate under varying loads or at high altitude, and often better emissions performance. Mechanical draft is common in large-scale boilers for electricity generation, waste-to-energy plants, cement kilns, and metallurgical furnaces. However, these systems consume additional power, require ducting and control equipment, and can introduce noise and maintenance considerations.

Design considerations involve selecting appropriate fan types (such as centrifugal or axial), estimating required pressure and flow (often expressed as pressure in inches of water gauge or pascals), and implementing control strategies (variable-speed drives or dampers). Draft levels are monitored with gauges and sensors to maintain safe and efficient operation. In contrast to natural draft, mechanical draft provides active, adjustable control over furnace air and flue-gas flow.