A fuel gas burner brings fuel and oxidizer together and holds a flame where heat is needed. This calculator compares two orifice diameters and pressure drops using a low-pressure flow approximation. It estimates a relative firing rate for the same gas; it does not size a complete burner or establish a safe operating setting.
Fuel Gas Burner Interactive Calculator
Compare orifice diameter and pressure drop with a reference condition. Read the relative flow change, jet area and equivalent reference-orifice pressure.
Equation Used
For the same gas, density and discharge coefficient, the low-pressure approximation compares flow using orifice area and the square root of pressure drop across the orifice. Enter differential pressure, not supply pressure alone. The result is relative to the reference condition; it does not establish combustion quality.
- Same fuel gas, temperature, and discharge coefficient at both conditions.
- Low-pressure non-choked burner orifice flow.
- Firing rate is proportional to gas flow through the orifice.
Metering, mixing and flame holding
The illustrated atmospheric arrangement uses a gas jet to entrain air into a mixing passage. Other burner designs supply air separately or mix the streams nearer the outlet. The flow path in the drawing is one arrangement, not a universal burner layout.
Orifice flow depends on the pressure difference across the opening and on the gas and orifice properties. Gas flow alone does not determine mixture distribution, ignition, flame stability or emissions. Actual burner systems also need appropriate fuel and air controls and flame supervision.
The sectional animation follows gas through a jet into a flared air inlet, a narrow mixing throat and a capped burner pipe. Primary air enters around the jet, and the mixture leaves through the upper ports. Two enlarged jet openings below the pipe compare the reference and actual diameters at the same scale. Gas and air particles show direction only; flame color, size and motion do not diagnose combustion or predict temperature, flow velocity or mixture strength.
Using the comparison
Use the calculator to examine the sensitivity of a reference jet's estimated flow to diameter and differential pressure. It can help explain why a small dimensional change matters, provided the same gas and flow assumptions remain applicable.
Furnaces, ovens and process heaters require burner-specific selection and commissioning. This page supplies no verified settings for a named furnace, gas conversion, fuel train or flame safeguard. The numerical controls are comparison ranges rather than allowable operating limits.
The relative-flow calculation
For an idealized low-pressure orifice, Q ≈ C_d A√(2ΔP/ρ). Q is volumetric flow at the density ρ used in that equation. It is not automatically a standard-volume flow such as SCFH. C_d depends on the actual orifice; no universal drilled-jet value is assumed here.
When C_d and density are the same at both conditions, the ratio becomes R = (d_actual/d_spec)²√(ΔP_actual/ΔP_spec). Firing Rate displays 100R percent of the reference. This also represents the heat-input ratio only if the fuel heating value is unchanged. Change from Spec is 100(R − 1), so a negative result means lower estimated input.
Jet Area is πd_actual²/4 in mm². Equiv Pressure is ΔP_spec R²: the pressure drop that the reference diameter would need to produce the estimated actual flow under the same approximation. It is not an adjustment recommendation for the actual jet.
The relation excludes compressibility corrections and choked flow. It does not solve air entrainment, fuel composition changes, burner backpressure or combustion. The reference and actual pressure inputs must both describe the drop across their respective orifices.
Example: a small diameter difference
The default reference diameter is 1.40 mm and actual diameter is 1.45 mm, with a 7 in. w.c. pressure drop for both. R = (1.45/1.40)² = 1.072704. The estimated firing rate is 107.3% of the reference, and Change from Spec is +7.3%. Jet Area is about 1.65 mm².
The equivalent drop at the reference diameter is 7 × R² = 8.05 in. w.c., displayed as 8.1. This is a mathematical comparison. It does not instruct the reader to alter a regulator or enlarge a jet.
With equal diameters, changing only the compared pressure drop from 7 to 3 in. w.c. gives √(3/7) = 0.6547, or 65.5% of reference flow. Changing it from 7 to 14 gives 141.4%. These ratios do not establish flame stability or permissible turndown.
The selectable extremes can produce firing-rate ratios from about 3.3% to 3,456.4% and equivalent pressures far outside the entered pressure range. Such mathematical extrapolations do not validate the approximation or a physical burner at those conditions.
What this does not select
An atmospheric mixer, a forced-air burner and an oxygen-fired system have different fuel and oxidizer arrangements. Their permitted fuels, firing ranges and operating procedures come from the actual equipment documentation. A jet-flow ratio cannot choose between them.
Changing an orifice or pressure can affect fuel delivery without establishing an acceptable air-fuel mixture. Equipment adjustments and fuel conversions require the manufacturer's specified parts, procedures and combustion checks by qualified personnel. Flame appearance or a calculator percentage cannot replace those checks.
Frequently asked questions
Should I enter gas supply pressure?
Enter the pressure difference across the orifice. Supply or manifold pressure alone is equivalent only when the downstream reference makes that appropriate.
Does 107% mean the burner is unsafe?
It means the model estimates 7% more input than the stated reference under its assumptions. It does not evaluate the equipment’s permissible firing range or combustion.
What does a negative Change from Spec mean?
The estimated input is below the reference. For example, 65.5% of reference corresponds to a −34.5% change.
Does Equiv Pressure tell me how to adjust the regulator?
No. It is the theoretical pressure drop at the reference diameter that gives the calculated actual flow. It is not a setting for the burner.
Can this calculate an absolute BTU/hr rating?
No. It calculates a ratio and area. Absolute heat input needs valid flow data, gas properties and heating value at consistent reference conditions.
Can the tool convert natural gas equipment to propane?
No. The ratio assumes unchanged gas properties. Use the equipment manufacturer’s approved conversion provisions and qualified service.
Does a flame color in the animation diagnose combustion?
No. The color and size are illustrative and do not calculate temperature, carbon monoxide, mixture or flame stability.
Why might measured flow differ from the comparison?
Gas density, discharge coefficient, differential pressure and the approximation’s validity can differ. The tool cannot diagnose a burner fault or determine corrective settings.
Burner and flow references
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