6.3 Draft Systems

6.3 Draft Systems

Quick Answer: Draft is the pressure difference that causes flow of air and combustion gases. The three types Arkansas often tests are natural, forced, and induced. Forced = fan supplying the furnace. Induced = fan between boiler and chimney pulling gases out. Natural = chimney effect (hot column vs cold outside air). Balanced draft uses both push and pull. Dampers control flow; breeching is the duct from boiler to stack.

What “Draft” Means

In boiler language, draft is not a cold breeze from an open door. It is the pressure difference that causes air and gases of combustion to flow through the fuel bed or burner, across heating surfaces, through the breeching, and up the chimney. Without draft (natural or mechanical), the fire starves for oxygen and products of combustion cannot leave the furnace.

Draft is measured in inches of water column (not psi) with a draft gauge. Furnace draft is often a slight negative pressure on induced/natural systems, or carefully controlled near zero/slight negative on balanced systems, or slightly positive on some forced-draft package arrangements. Exact setpoints come from manufacturer data — your job is to know what creates the pressure difference and which fan does what.

Three Types Often Tested (Arkansas Sample Focus)

Arkansas sample questions and standard operator texts repeatedly test this trio:

Memorize the short Arkansas-friendly lines:

• Forced draft — a fan supplying air to the furnace.
• Induced draft — a fan between the boiler and the chimney that pulls the gases of combustion out.
• Natural draft — produced by the chimney because hot gases inside are lighter than cooler outside air.

Natural Draft — The Chimney Effect

Natural draft depends on a column of hot, light gases inside the chimney compared with a column of cooler, denser outside air of the same height. The density difference creates a pressure difference that pulls air through the boiler. Factors that strengthen natural draft:

• Taller stack (longer hot column)
• Hotter flue gas (lower density inside the stack)
• Clean, unrestricted gas path (no soot-choked breeching)

Factors that weaken natural draft:

• Cold stack on light-off
• Low fire / cool flue gas
• High outside air temperature (smaller density difference)
• Dirty tubes, closed dampers, or blocked breeching

Many modern package boilers do not rely on natural draft alone; fans provide reliable air regardless of weather and load. Natural draft still appears on exams and on older or simple installations.

Forced Draft

A forced-draft fan is on the supply side. It forces air through burners, windboxes, or grates into the furnace. Advantages:

• Positive, controllable air supply independent of stack temperature
• Better mixing and higher heat release rates in compact packages
• Works well with low chimneys

Because the fan is pushing, furnace pressure may be neutral to slightly positive depending on design and outlet restriction. Observation ports and casings must be maintained; a positive furnace can push hot gas or flame toward the operator if ports are open carelessly. Forced-draft package boilers are extremely common in Arkansas commercial plants.

Induced Draft

An induced-draft fan sits in the gas path after the boiler heating surface — typically in or near the breeching/stack connection — and extracts flue gas, creating a negative pressure (suction) in the furnace that draws air in through burners or dampers. Advantages:

• Keeps the furnace under suction so leaks tend to pull room air in rather than push fire out
• Helps overcome resistance of economizers, air heaters, and long breeching
• Common on larger industrial boilers and many multi-pass designs

If the ID fan fails or trips while fuel continues, combustion products and incomplete combustion hazards rise quickly — another reason air-flow and fan interlocks exist on automatic burners.

Balanced Draft

Balanced draft uses both a forced-draft fan and an induced-draft fan. The FD fan supplies combustion air; the ID fan removes flue gas. Operators (or automatic draft controls) adjust fan speeds/dampers so furnace pressure stays near the desired setpoint — often slightly negative. Balanced draft is the industrial standard for large boilers with high gas-side resistance.

Dampers

Dampers are adjustable plates or vanes that throttle air or flue-gas flow. Locations include:

• Burner air registers / inlet dampers (control combustion air)
• FD fan inlet or outlet dampers
• ID fan inlet dampers or variable-speed control
• Stack or breeching dampers (isolation or draft control on some plants)

Damper position is part of the air–fuel ratio. A stuck closed inlet damper starves the fire; a stuck open damper can overcool the furnace with excess air. On automatic package burners, damper motors and linkages are calibrated with the fuel valve (parallel positioning or fully metered systems). Never “fix” a linkage without understanding the combustion curve — you can create a smoky or oxidizing fire that damages tubes and wastes fuel.

Breeching and Chimney

Keep breeching gas-tight where designed, supported, and free of heavy soot accumulation. Leaks on a negative system pull cold air into the gas path and hurt draft/efficiency; leaks on a positive system can spill flue gas into the boiler room — a carbon monoxide hazard.

Draft and the Combustion Triangle

Draft equipment is how the plant delivers the oxygen leg and removes products of combustion:

• Too little draft/air → incomplete combustion, smoke, soot, CO (Section 6.1)
• Too much draft/air → excess air heat loss (Section 6.4)
• Unstable draft → unstable flame, nuisance flame-safeguard trips, hard light-offs

When Arkansas sample material asks you to name draft types or define forced vs induced, answer with fan location and push vs pull, not with brand names. When you walk a real Arkansas boiler room, find the FD fan, the ID fan (if any), the breeching, and the stack dampers — that map is your airflow story for every light-off and every smoky stack complaint.