Natural, Forced, Induced and Balanced Draft

Boiler Draft Systems

Draft is the small static pressure differential ($\Delta P$, measured in Pascals [$\text{Pa}$], millimeters of water [$\text{mm } H_2O$], or inches of water column [$\text{in. } H_2O$]) that causes combustion air and flue gases to flow through the furnace, heat exchangers, and stack.

Natural vs. Mechanical Draft

1. Natural Draft: Generated by the buoyancy difference between hot flue gases inside a vertical chimney and cold, dense ambient air outside:

Where $H$ is stack height, $g$ is gravity, and $\rho$ is gas density. Limited in modern boilers because heat recovery equipment (economizers, air heaters, baghouses) creates high gas-side pressure drops exceeding natural draft capacity. 2. Forced Draft (FD): A high-pressure fan forces ambient air through the windbox, burner air registers, and furnace. The furnace operates under positive pressure. Requires a gas-tight welded membrane casing to prevent hot, toxic combustion gases from escaping into the boiler room. 3. Induced Draft (ID): An ID fan located at the boiler outlet exhausts flue gases up the stack, maintaining a negative pressure throughout the boiler setting. ID fans must handle hot, abrasive, corrosive flue gas, with materials and cooling selected for their actual service. 4. Balanced Draft: Combines both a Forced Draft fan and an Induced Draft fan operating in closed-loop control:

• The FD fan pushes combustion air through the burner registers.
• The ID fan draws flue gases through convection passes, economizer, and air preheater.
• The Golden Rule of Balanced Draft: The ID fan speed and dampers are modulated to maintain the specified furnace-pressure setpoint, commonly slightly negative in conventional balanced-draft units below atmospheric pressure.
• Rationale: Maintaining a slightly negative draft ensures that hot combustion gases, fly ash, and lethal carbon monoxide ($CO$) cannot leak outward through inspection doors or casing joints, while simultaneously minimizing the infiltration of cold excess air that degrades thermal efficiency.

Fan Aerodynamics & Profiles

• Backward-Curved (Backward-Inclined) Centrifugal Fans: Blades curve away from the direction of rotation. Can provide high efficiency and a non-overloading region in the specified power curve. Actual motor loading depends on the fan curve, speed, density and system resistance; the blade shape does not guarantee prevention of motor overload.
• Radial (Paddle-Wheel) Centrifugal Fans: Flat blades project radially from the hub. Rugged radial designs can suit particle-bearing gas, but erosion, buildup, balance and maintenance remain important. Select and operate against the actual duty and curve.
• Forward-Curved (Squirrel-Cage) Fans: Deliver large volume at low speeds, but possess an overloading horsepower curve; used primarily in small HVAC blowers, with selection constrained by the actual duty and power curve.
• Axial Flow Fans: Propeller-type fans handling massive gas volumes at low static pressures; variable-pitch rotor blades allow rapid load following in large utility power plants.

Read pressure and flow together

A fan supplies the pressure difference needed to move gas through the system resistance. Dampers, ductwork, heat-transfer banks and pollution controls all contribute pressure loss. A dirty gas pass or collector can increase resistance and reduce available flow at a given fan condition. A running fan and open damper do not independently establish the required airflow. Use the designed pressure/flow indications and proofs.

For an idealized natural-draft example with stack height 30 m, outside-air density 1.2 kg/m³ and average flue-gas density 0.8 kg/m³, buoyancy draft is 30 × 9.81 × (1.2 − 0.8) = 117.72 Pa. Actual available draft is reduced by flow losses and affected by gas temperature and ambient conditions. Do not compare Pa directly with kPa without converting: 1 kPa is 1,000 Pa.

Fan affinity relationships under similar conditions give flow approximately proportional to speed, pressure to speed squared and power to speed cubed. A 10% speed increase would give about 1.10³ = 1.331 times power in that ideal similarity comparison. Real system curves, density, controls and mechanical ratings constrain the result. These laws do not authorize overspeed or operation beyond the fan/motor envelope.

During rounds, observe abnormal vibration, noise, bearing condition indicators and draft trends from a safe location. Trips and startup sequences must preserve the required furnace-pressure and airflow conditions. Do not restart an ID or FD fan after a fault without evaluating the documented cause and its effect on the boiler sequence.

Source notes

Official scope and technical references checked 2026-10-17: Reference 1; Reference 2; Reference 3.