Design Calculators - Mechanical

This calculator determines the Heat Duty (heat absorbed by the process fluid) and Thermal Efficiency (%) of an industrial furnace. It considers the heat input from fuel combustion and accounts for radiant heat losses. This tool is designed for practical assessment and optimization of industrial fired heaters and furnaces.

Key Inputs:

• Unit System: Select Metric (kg, kJ, °C) or Imperial (lb, BTU, °F) for consistency.
• Fuel Mass Flow Rate (M_fuel): The mass flow rate of fuel supplied to the furnace.
• Fuel Higher Heating Value (HHV): The energy content per unit mass of the fuel. Use measured HHV from reliable fuel analysis (e.g., ASTM standards). Typical HHV ranges (approx.): Natural Gas ~48-55 MJ/kg (20,700-23,700 BTU/lb), Fuel Oil ~40-44 MJ/kg (17,200-18,900 BTU/lb), Coal ~15-30 MJ/kg (6,450-12,900 BTU/lb).
• Process Fluid Mass Flow Rate (M_fluid): The mass flow rate of the fluid being heated inside the furnace coils.
• Process Fluid Data Input Type: Choose whether to specify the energy change using enthalpies or temperatures and specific heat.
  • Enthalpies (h_in, h_out): Use for fluids undergoing phase change (e.g., steam generation, vaporization) or when precise enthalpy values are available from thermodynamic tables (e.g., steam tables, process simulation software).
  • Temperatures (T_in, T_out) & Specific Heat (C_p): Use for sensible heating of fluids (no phase change) where specific heat is relatively constant over the temperature range.
• Radiant Heat Loss (%): The estimated percentage of the total heat input lost to the surroundings through radiation from the furnace casing. This value can typically range from 0.5% to 5% or more depending on insulation, age, and design.

Calculated Outputs:

• Total Heat Input (Q_input): The total thermal energy released by the combustion of the fuel, presented in both selected units and **MW**.
• Heat Absorbed by Process Fluid (Q_output / Heat Duty): The actual heat transferred to the process fluid, presented in both selected units and **MW**. This is the primary useful output of the furnace.
• Radiant Heat Loss (Q_rad): The estimated energy lost from the furnace through radiation, presented in both selected units and **MW**.
• Furnace Thermal Efficiency (\(\eta\)): The percentage of the fuel's energy that is successfully absorbed by the process fluid.

Important Note: This calculator provides a direct method energy balance. For comprehensive analysis, especially for troubleshooting or detailed design, a complete heat loss (indirect) method, considering all forms of losses (flue gas, convection, radiation, unburnt fuel, etc.), is recommended following international standards like API 560 (Fired Heaters for General Refinery Service) or ISO 13705.