Weld Preheat Energy and Fuel
Heat needed, fuel energy after efficiency.
Example
You enter
- Steel mass to preheat (lb) 200
- Start temperature (°F) 70
- Preheat temperature (°F) 300
- Torch-to-part efficiency (%) 25
- C steel 0.11
- Propane (Btu/lb) 21600
You get
- Heat needed in the steel 5060 Btu
- Fuel energy after efficiency 20240 Btu
- Propane (lb) 0.94
- Propane (gal) 0.22
Checked against published worked examples from: Carbon-steel specific heat / propane heating value.
Details, formula, and sources
ScopeHeat 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 or tableHeat 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.
Source and editionSpecific 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.
Free accessThe sensible-heat relation and the property values are public.
What governsMath aid for personal verification. Stop work and consult the qualified person on site if any number does not match the field condition.
API field namessteel_lb, start_temp_F, preheat_temp_F, efficiency_pct, c_steel, propane_btu_lb, heat_needed_btu, fuel_btu, propane_lb, propane_gal
- Steel specific heat 0.11 Btu/lb-degF default (compute input c_steel; not on the page)carbon-steel property tables
- Propane heating value 21,600 Btu/lb (compute input propane_btu_lb; not on the page) and a fixed 91,500 Btu/gal for the gallon figurepropane property tables
- Open-torch efficiency 25% default (roughly 15 to 30% for an open torch on a plate); an enclosed heat, blanket, or induction is far higher; hold the preheat through the weld and verify the interpass temperaturewelding preheat practice