Thermal & Combustion

Natural Draft Calculator

Determine the natural draft generated by a chimney's thermal buoyancy and compare it against the kinetic and frictional energy required to move flue gases through the duct.

Chimney Draft Flue Gas Combustion Engineering Furnace Design
Draft Balance Real-time
Available Draft
-
mmH2O
Total Requirement
-
mmH2O
Margin
-
mmH2O
- Enter valid parameters to see the draft balance status.

Available Draft vs. Total Requirement

Flow & Properties
Gas Velocity
-
m/s
Flow Rate
-
m³/h
Gas Density
-
kg/m³
Kinetic Head
-
mmH2O
Resistive Losses
-
mmH2O
Hydraulic Diameter
-
m
Physical Interpretation
- Enter valid parameters to see the engineering diagnosis.
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About This Simulator

Industrial furnaces, boilers, and process heaters rely on a chimney or stack to remove flue gases without the help of a fan. This natural, or thermal, draft is created by the density difference between the hot gases inside the stack and the cooler ambient air outside it – the same principle that drives air up a fireplace flue. The available draft is given by

ΔP = H · ρ0 · a · (tg - ta) / [(1 + a·ta)·(1 + a·tg)]

where H is the stack height, ?0 is the gas density at 0°C, a is the gas expansion coefficient, and tg and ta are the flue gas and ambient temperatures. This calculator compares that available draft against the total energy the flowing gas actually needs: the kinetic head to accelerate it to duct velocity, plus the resistive losses from friction and fittings along the duct run.

The result is a draft balance – a quick way to check whether a stack design is adequate before committing to a physical build, or to diagnose why an existing chimney is underperforming.

How to Use

  1. Chimney and thermal conditions. Enter the stack height and the flue gas and ambient temperatures. The gas temperature must exceed the ambient temperature for any draft to exist.
  2. Furnace and gas properties. Enter the furnace gauge pressure (negative for suction) and the base gas density at 0°C – 1.293 kg/m³ is a reasonable default for typical flue gas.
  3. Duct geometry. Choose a circular or rectangular section and enter its dimensions. This sets the cross-sectional area used to compute velocity and flow rate.
  4. Flow conditions. Either enter the gas velocity directly, or switch to dynamic pressure mode if that's what you measured – the calculator derives velocity from it using the real gas density. Enter the loss coefficient J for the duct run.
  5. Read the results. Compare Available Draft against Total Requirement. A positive margin means the stack can sustain the flow; a negative one means induced (fan-assisted) draft is needed.

Understanding the Results

Available Draft

The natural draft the chimney can generate from thermal buoyancy alone, expressed in mmH2O. It grows with stack height and with the temperature difference between flue gas and ambient air.

Total Requirement

The sum of the kinetic head (energy to accelerate the gas to duct velocity) and the resistive losses (friction and local losses along the path). This is the energy the system must supply for the gas to flow at the specified velocity.

Margin

Available Draft minus Total Requirement. A margin below roughly 10% of the requirement indicates the system is highly sensitive to small changes in temperature or roughness; below 25% is considered tight but workable; a healthy design typically keeps a comfortable reserve above that.

Physical Interpretation

A short engineering diagnosis that flags the most relevant issue in the current configuration – for example, insufficient thermal driving force, excessive velocity, an inefficient flattened duct shape, or a healthy stable operating point.

Frequently Asked Questions

What is natural draft?
Natural draft is the flow of flue gases through a chimney or stack driven purely by the density difference between the hot gases inside and the cooler air outside – no fan required. It depends on stack height and the gas-to-ambient temperature difference.
What happens if the margin is negative?
A negative margin means the available natural draft cannot overcome the kinetic and frictional losses at the specified flow rate. In practice this means the flow will not sustain itself, and induced or forced draft (a fan) is required.
Why does furnace pressure matter?
A slightly negative (suction) pressure at the furnace outlet helps keep combustion stable and prevents hot gases from leaking out through openings in the furnace casing. Strongly positive pressure can push flue gases into the boiler room.
How do I choose the loss coefficient J?
J is the sum of the friction coefficient (based on duct roughness and length-to-diameter ratio) and the local loss coefficients of every fitting, damper, and bend in the duct run. Values typically range from about 1 to 5 for a moderately complex duct system; consult applicable duct design references for your specific layout.
What are the limitations of this simulator?
This model assumes steady-state, single-phase gas flow with uniform temperature along the duct, and a simplified ideal-gas density correction. It does not account for wind effects on the stack, transient thermal behavior, or moisture condensation. For critical designs, validate results against your facility's engineering standards.