Weld Preheat Energy and Fuel
Heat needed, fuel energy after efficiency, and propane in pounds and gallons to bring a steel mass to a preheat temperature. The preheat temperature comes from carbon-equivalent or the WPS; hot-work hazards govern.
Formula and source
Heat needed = mass x specific heat x (preheat temp - start temp); fuel = heat needed / efficiency; propane lb = fuel / 21,600 Btu/lb; propane gal = fuel / 91,500 Btu/gal.
Specific heat of carbon steel (about 0.11 Btu/lb-degF) and the heating value of propane (about 21,600 Btu/lb, 91,500 Btu/gal), by name; sensible-heat first principles.
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