Fluids are non-solid items, such as water and oil. They can normally only exist inside entities for fluid handling (like pipes), and buildings that have fluids as input ingredients or products (like an oil refinery).
Liquids can be destroyed by removing buildings or pipes in which they are contained. Only one type of fluid can occupy a given pipe segment or tank at a time; no two fluids will ever mix, but will instead block each other from flowing. They cannot be carried by the player, moved using inserters, dropped on the ground, or stored in chests, unless the fluids are stored in barrels. They cannot be spilled or even dumped in a lake, and are counted in continuous fractions, rather than discrete integers.
In the game, fluid is held in entities that behave as vessels (fluid boxes) of a defined size (volume). The vessels automatically connect to each other if their inputs/outputs are adjacent (pipes connect to all directions) and allow fluids to flow between them.
The volume of fluid contained in a fluid box is a value between 0 and the maximum volume. For instance, the pipe can hold 100 units of fluid, therefore the value in the pipe can be a number between 0 and 100. The level of fluid in a given entity is manifested by a percentage of the entity's maximum volume that is being occupied by a fluid. It can be observed in pipes and tanks; they have windows through which the fluid is seen at a certain level, or perhaps even as just a small trickle.
All connected tanks and pipes are treated as a single vessel in that the level of fluid must be equal in all parts, to even out pressure exacted by a higher fluid level on smaller ones. This is why level is also often referred to as pressure, even though pressure is actually caused by a difference in level between two entities. All flow of fluid that happens between pipes is to achieve this balance (pumps practically ignore it and buildings disrupt it; more on that further below). The flow rate between pipes is dependent on pressure (the difference in level between the adjacent entities), it becomes slower as pipes even their levels out.
Coming back to how the 'level' is defined, this also means that all connected pipes and tanks attempt to even out to the same percentage of their respective volumes. For example, if 12 550 units of fluid are left to flow into a storage tank of 25 000-unit capacity with one pipe of 100-unit capacity connected, there will be 12 500 units in the storage tank and 50 units in the pipe, both being filled to the same percentage (50%) of their capacities, even though the amounts themselves are obviously unequal.
Machines that produce fluids put them in their output slots, which are related to a specifically labeled output pipe socket somewhere on the machine (pressing Alt reveals the labels). The slot will attempt to empty itself into the entity connected to the machine's socket, unless it is full, or contains a non-matching fluid. Machines that consume fluids also have an accordingly labeled pipe input socket. If an entity containing the correct fluid is connected to it, the machine will start behaving like a pipe that can never be filled, meaning the fluid from connected pipes and tanks drains into the machine at a fixed rate, until the machine's input slot is full. There may be machines that have pipe sockets for both input and output (like a drill placed over uranium ore). They then drain the fluid for themselves first, and once full, behave as a regular pipe that attempts to even out its level with adjacent entitites. If there are multiple output/input sockets for one fluid on a machine, their activity is distributed to them equally unless some of them are blocked/full.
Temperature is currently only relevant in heating water as a medium for power generation. Even though all fluids in the game have a temperature value, it is generally the default 15°C.
Energy, whether harnessed from fuel in boilers, or from nuclear power through heat exchangers, can be used to turn water to steam, being a liquid form of work. Steam holds energy at a ratio of 0.2 kJ per °C per unit. In other words: 0.2 kJ of work is necessary to heat a unit of steam by one °C. Since steam/water is set to have a maximum temperature of 1000°C and minimum of 15°C, the most work that can be done on one unit is 197 kJ.
In practice, this is barely utilized in a great variety: Boilers only output steam of 165°C temperature, and heat exchangers only output 500°C hot steam, never hotter, never colder; if insufficient energy is supplied, the heaters do not output steam altogether. The steam also does not grow colder over time. Using the 165°C steam in a steam engine has the same effect as using it in a steam turbine, although it is impractical, since turbines are made to consume 500°C (superheated) steam, generating proportionally more power. All of this makes for no need of exact calculations.
Fluids can be transported through pipelines, barrels, or railway. It is generally practical to use piping for short-distance distribution to machines (or barrelling, if there is need to use belts), and railway transportation for longer distances.
Pipes are the most basic way to channel fluids from A to B. They automatically connect to any adjacent pipe and can do so to all four cardinal directions simultaneously. Underground pipes only work in two opposite directions, linking to another underground pipe on one side, and to another entity on the other. If a pipe section becomes too long without using pumps, all fluid inside it will be "spread thin", resulting in very slow flow and preventing machines to use its contents effectively. Tanks behave the same as pipes, except their volume is much greater, which can cause this inconvenience over a much smaller distance if multiple tanks are used. Underground pipes can help alleviate this issue; although they can connect a distance of up to 10 tiles, their volume is always equivalent to two pipes.
Pumps use electrical power to transfer fluids in one direction very quickly. They also block any back-flow, which means they can pressurize a section of piping, filling it as much as possible. This is very useful to counteract the "thin spread" outlined above, among other things. They can also be disabled using the circuit network which stops fluid flow through the pump.
The table below shows how fast will fluid flow in a pipeline with a certain frequency of pumps. If a higher flow rate is desired, pumps should be placed more frequently. Because underground pipes only count as 2 regular pipes in terms of volume, a full-length section only counts as two pipes in this table, if a pump is placed between each underground section. Placing a filled storage tank before a pump ensures maximum possible flow rate and is therefore a suitable start of any pipeline.
| Number of pipes
between two pumps
| Maximum flow|
Barrels are used by Assembling machines to effectively "bottle" fluids into an item that can be handled like any other item; carried in an inventory, placed in chests and handled by Inserters. This allows the player to transport fluids via the belt transport system. Assembling machines are also used to empty the barrels, depositing their contents to pipes and leaving an empty barrel for another use.
Railway is another method of transporting fluids, and can be conducted in two ways: Either the fluids are directly pumped into a fluid wagon, or they are poured into barrels and loaded into cargo wagons. Both methods have their distinct differences: The cargo wagon can hold different types of fluid barrels, however the fluid wagon can hold more fluid (25k versus 20k) and can be emptied and filled in mere seconds, at speeds inserters with barrels require an inadequate expenditure of resources to match; while Stack inserters can transfer barrels quickly, machines for barreling fluids are slow. On the other hand, the fluids can be barreled/unbarreled while trains are en route.