Steam

Steam is the in-game fluid used primarily for power generation and certain processing chains. Its energy content is proportional to temperature above an ambient baseline of 15°C: each unit of Steam stores exactly 200 joules per degree Celsius above 15°C. Steam produced by boilers is 165°C, while steam produced by heat exchangers (and by acid neutralisation in some setups) is 500°C; higher-temperature Steam therefore contains more energy per unit than lower-temperature Steam.

Steam does not lose heat while sitting in pipes or storage tanks. The energy invested to heat water into Steam is recoverable because Steam engines and steam turbines in-game operate at 100% efficiency. If Steam fed to a steam engine or turbine is cooler than the optimal operating range, output power is reduced but no energy is destroyed. If Steam exceeds the maximum temperature that a steam engine or turbine can handle, the device’s output is capped at its maximum and any extra thermal energy in the Steam is wasted.

A practical example: a storage tank holding 25,000 units of Steam at 165°C contains (200 J / unit / °C) × 25,000 units × (165 − 15)°C = 750,000,000 J = 750 MJ. The same tank at 500°C contains (200 J / unit / °C) × 25,000 × (500 − 15)°C = 2,425,000,000 J = 2.425 GJ, an amount comparable to hundreds of fully charged accumulators. Under the assumption of perfect efficiency of boilers and heat exchangers, one unit of Steam corresponds to approximately 47.9 g of water, so a full 25,000-unit tank contains roughly 1,195 kg of Steam by that equivalence.

• The fill gauge on Steam storage tanks visually fills from the top downward.
• Steam is the only fluid in Factorio that is not a true liquid in real-world terms and is the only fluid that cannot be stored in barrels.
• Steam can be supplied from any compatible source to either steam engines or steam turbines; choice of generator affects how temperature maps to usable power but both device types accept Steam from any source.
• Steam interacts with the fluid system and the electric system, and it is used in certain production chains related to Oil processing.