When metalworking professionals and hobbyists need intense heat for tasks like welding, cutting, or heating, they use oxy-fuel combustion, combining a fuel gas with pure oxygen. The choice of fuel gas dictates the maximum achievable temperature and how that heat is delivered to the workpiece. Acetylene burns significantly hotter than propane, a distinction that determines which applications each gas is best suited for.
Direct Comparison of Maximum Flame Temperatures
The maximum temperature a gas can reach is achieved only when mixed with pure oxygen, creating an oxy-fuel flame. When combined with oxygen, acetylene consistently produces the highest flame temperature of all commonly used industrial fuel gases. Oxy-acetylene flames typically reach temperatures between 5,600°F and 6,300°F (3,100°C to 3,480°C). This extreme heat makes acetylene the standard for certain high-temperature metal applications.
Propane, often used as liquefied petroleum gas (LPG), generates a very hot flame when mixed with oxygen but falls substantially short of acetylene’s peak. The maximum temperature for an oxy-propane flame generally ranges from 4,500°F to 5,100°F (2,480°C to 2,820°C). Oxy-acetylene can generate a flame that is approximately 600 to 1,200 degrees Fahrenheit hotter than oxy-propane. The large temperature differential is the primary reason the two fuels are used for different types of work, despite propane having a higher total heat content (BTU) per cubic foot.
The Chemical Mechanism of Heat Generation
The difference in maximum flame temperature is due to how quickly and where the energy is released during combustion, not the total heat energy released by the gas. The combustion process in an oxy-fuel torch is divided into two main zones: the inner primary cone and the outer secondary plume. The inner cone is the hottest and most concentrated part of the flame, performing the majority of the direct work on the metal.
Acetylene’s superior heat generation is rooted in its highly energetic chemical structure (\(\text{C}_2\text{H}_2\)), which features an unstable carbon-carbon triple bond. This triple bond releases a massive amount of energy in a highly concentrated area during the first stage of combustion. Approximately 40% of acetylene’s total heat output is released in the inner primary cone, creating an intensely hot and focused heat source.
Propane, a saturated hydrocarbon (\(\text{C}_3\text{H}_8\)), has single bonds between its carbon atoms that are much more stable than acetylene’s triple bond. Consequently, propane’s heat release is far more spread out between the two combustion zones. Propane releases less than 10% of its total energy in the inner cone; most of the heat is dispersed into the much larger, less-focused outer plume. This difference in energy distribution is the core reason propane has a lower maximum flame temperature, even though it has a higher overall heat value (BTU) than acetylene.
Choosing the Right Fuel for the Job
The distinct combustion characteristics of each gas dictate their suitability for different industrial and fabrication tasks. Acetylene’s extremely hot and concentrated inner cone makes it the only practical choice for gas welding steel, providing the necessary focused heat to melt the base metal. It is also preferred for applications requiring fast piercing or cutting of thinner materials due to its quick pre-heating capability. However, acetylene has safety limitations, as it cannot be used at pressures above 15 pounds per square inch (PSI) because it becomes unstable.
Propane is widely used for heating, brazing, soldering, and many cutting operations, especially for thicker materials. While its lower maximum temperature makes it unsuitable for gas welding, its high total BTU content and distributed heat are highly effective for general heating and pre-heating large workpieces. Propane is also significantly more stable, allowing it to be stored at higher pressures, and is typically a much more affordable and readily available fuel source than acetylene. The selection depends on whether the task requires the highest possible focused temperature (acetylene) or a large, high-volume heat output (propane).