Railcraft Boilers Math

Note: this is accurate as of GTNH 2.1.2.3qf

The two Railcraft Boilers (Solid Fueled Boiler Firebox and Liquid Fueled Boiler Firebox) function essentially the same, just taking different kinds of fuels. The steam production and water consumption of the boilers is straightforward: each low-pressure tank produces 10L steam/tick at maximum heat and the high-pressure produces 40L steam/tick; and each 160L of steam produced consumes 1L of water. The fuel consumption uses a complicated formula, scaling non-linearly with the number and type of tanks and the current heat of the boiler. Below is a summary of the fuel consumption per tick for max-heat boilers:

So, for example, one piece of charcoal (1600 burn time) in a solid-fueled firebox with 3x3x3 low-pressure boiler tanks lasts $$1600/13.66875 \approx 117$$ ticks, and a bucket of creosote (4800 burn time ) in a liquid-fueled firebox with 3x4x3 high-pressure tanks lasts $$4800/41.4 \approx 116$$ ticks.

Note that this math refers to the consumption at maximum heat. The fuel consumption increases with heat, but not linearly. The fuel efficiency (amount of steam generated for each unit of fuel consumed) significantly increases with heat. For example, a max-heat boiler with 3x4x3 high-pressure tanks produces twice as much steam as a similar boiler at half-heat, but it consumes only 5% more fuel. This ratio depends on the type and amount of steam tanks.

GTNH changes
The GTNH version of Railcraft modifies the high-pressure tanks to produce 40L steam/tick when fully heated, as opposed to the 20L steam/tick in "vanilla" Railcraft. Water consumption is doubled as well.

The fuel consumption curve is slightly altered as well, reaching its maximum value at 500C, rather than at 1000C. Between 20C and 500C, the fuel consumption curve of high-pressure tanks is similar to the curve for low-pressure tanks, and between 500C and 1000C the consumption stays constant. Note that a max-heat boiler consumes fuel at the same rate as in "vanilla" Railcraft.