A gas crucible furnace heats metal inside a separate crucible using combustion gases around its outside. This calculator estimates average fuel heat input for a simplified charge, temperature rise, melt time and assumed efficiency. The diagram illustrates the gas path; it does not predict a temperature distribution or specify a burner installation.
Gas Crucible Furnace Interactive Calculator
Compare charge mass, temperature rise, melt time and assumed efficiency using a fixed-property melting estimate.
Equation Used
The result is average fuel heat input for an illustrative charge, with fixed heat capacity and latent heat. Efficiency is applied once. Alloy-specific enthalpy, furnace warm-up and the actual firing schedule are not solved separately.
- Illustrative fixed properties: cp = 0.38 kJ/kg-K and Lf = 200 kJ/kg. These are assumptions, not certified data for every brass alloy.
- Efficiency is the assumed fraction of fuel energy reaching the charge over the stated interval; the individual losses are not calculated.
- Propane use is estimated using 46.4 MJ/kg lower heating value.
- The temperature rise is entered independently. The model assumes the chosen interval spans complete melting; it does not check an alloy liquidus or predict pour quality.
How heat reaches the metal
The burner supplies hot combustion gas to a refractory-lined chamber. Heat reaches the outside of the crucible by radiation and convection, then passes through the crucible wall to the charge. Tangential burner entries are used in some furnaces to direct gas around the vessel. The actual distribution depends on the burner, chamber, exhaust and crucible geometry.
The shell, lining, crucible, support and exhaust each have a different role. The lining limits heat loss; the crucible contains the charge; the support positions it in the chamber. These parts must be selected together for the furnace and alloy. There is no universal clearance, support height or number of heats that applies to every crucible.
The section shows hot gas rising outside the crucible and leaving through the lid opening. The plan shows a tangential burner and circulation around the vessel. Charge mass changes the fill symbol, not a dimensioned vessel size. Gas markers indicate direction only; their speed is not a measured gas velocity. The calculator contains no flame, chemistry or heat-transfer-field simulation.
Stationary and tilting furnaces
MIFCO offers both stationary crucible furnaces and center-pivot tilting melters. Its product specifications include examples with tangential burners and a crucible rest block. In a stationary arrangement, compatible handling equipment moves the crucible; in a tilting arrangement, the furnace assembly rotates for pouring.
A furnace specification identifies which alloys, crucibles and operating conditions it supports. Suitability depends on the exact alloy and equipment, especially for reactive metals.
Average heat-input estimate
The modeled energy delivered to the charge is E = m(c_pΔT + L_f). Here m is charge mass in kg, c_p is fixed at 0.38 kJ/(kg·K), ΔT is the entered temperature rise in K and L_f is fixed at 200 kJ/kg. The result is kJ.
Average fuel heat input is P = E/(η × 60t), where η is efficiency as a fraction and t is time in minutes. Since kJ/s equals kW, the result is kW. Multiplying by 3412.142 gives BTU/hr. The two heat-input outputs describe the same power in different units.
Estimated propane flow is 3600P/46400 kg/hr, using the assumed lower heating value of 46.4 MJ/kg. Average charge throughput is 60m/t kg/hr. Efficiency has already been applied in P; dividing by it again would double-count the allowance.
This constant-property approximation adds a full latent-heat term for every selected temperature rise. It is meaningful only when the interval includes complete melting. A detailed alloy calculation would use its enthalpy change, including the solid and liquid temperature ranges. Warm-up energy, holding time and losses are represented only insofar as the entered overall efficiency accounts for them.
Example using the calculator defaults
For 25 kg, a 920 K rise, 25 minutes and 8.93% assumed efficiency, the charge energy is 25 × (0.38 × 920 + 200) = 13,740 kJ. Dividing by 0.0893 × 1500 s gives 102.58 kW of average fuel input, approximately 350,000 BTU/hr.
The estimated propane flow is 7.96 kg/hr, equivalent to about 3.32 kg over the stated 25-minute interval. Average charge throughput is 60 kg/hr. These values apply to the stated assumptions and require an equipment-specific assessment before selecting a burner.
With the same charge and temperature rise, doubling the time halves the required average power and hourly fuel flow. Fuel per charge is unchanged in this simple model because efficiency is held fixed. Real longer cycles can have different losses; this calculator does not infer that change.
Across the input limits, average fuel power ranges from about 0.317 to 2186.67 kW and charge throughput from 0.667 to 360 kg/hr. The selectable numerical range is not an equipment rating. A burner needs an appropriate control range and a compatible combustion system as well as an average heat input.
What the comparison can establish
Gas firing, electric resistance heating and induction supply heat in different ways. An energy balance helps compare assumptions, but it does not establish purchase cost, melt cleanliness, lining life or throughput for equipment that has not been specified.
Changing the efficiency input is a sensitivity comparison. It is not a recommendation to alter an air gate, gas pressure, flue opening or burner arrangement. Those changes affect combustion and require the furnace manufacturer’s procedure and qualified commissioning.
Crucible condition, compatible handling equipment, measured metal temperature and the prescribed refractory drying process remain separate operating matters. None can be assessed from this average-power output.
Frequently asked questions
Is Heat Input the heat absorbed by the charge?
No. It is the estimated fuel input after division by the assumed efficiency. The charge absorbs the modeled energy m(cpΔT + Lf).
Should I divide the output by efficiency again?
No. Efficiency has already been applied once in the calculation.
Can I enter an aluminium or bronze charge?
The fixed properties do not change with alloy. Use the result only as an illustrative calculation unless those properties adequately represent the intended enthalpy change.
Why does doubling melt time halve power but not fuel per charge?
The model holds efficiency constant, so the same energy is delivered more slowly. Actual heat losses may change with the duration.
Does Fuel Input select a commercial burner?
No. It reports the estimated average fuel power in BTU/hr. Maximum demand, modulation, gas supply, flame supervision and furnace compatibility need equipment-specific assessment.
Does the animation show actual flow and temperature?
No. It shows the intended circulation around the crucible. Gas speed, turbulence and the temperature field are not solved.
Can this specify a gas conversion or a refractory repair?
No. Fuel conversion, combustion settings and lining repair must follow the applicable manufacturer’s instructions; the energy estimate does not determine them.
Is a fixed crucible lifespan implied?
No. Alloy, crucible grade, service conditions and handling affect life. This calculator has no wear or thermal-shock model.
Manufacturer references
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