Steam and boilers

A boiler burns coal under a water vessel and forces the steam into a pipe network. One litre of water becomes 16 L of steam. Steam temperature follows the line's pressure:

steam temperature = 100 C * (pressure + 1) ^ 0.25

which is 100 C on an unpressurised line, about 150 C at 4 atm and about 157 C at the 5 atm a Cornish Boiler holds.

The chain is always the same: water feeds a boiler, the boiler drives an engine, the engine turns one sub-machine. This page is the boiler and the water; see Steam engines and sub-machines for the other half and Pipe networks and pressure for the plumbing.

The Cornish Boiler

Quantity	Value
vessel capacity	1600 L
water needed before it will boil	300 L
water it will hold and boil	up to 1000 L
steam output at full rate	64 L/s
choke pressure	5.0 atm
exhaust vented while burning	16 L/s
exhaust back-pressure that puts the fire out	0.8 atm
heating time before it boils	180 s
explosion radius	3 blocks

It is raised by right-click construction: place the block and right-click it with the materials in the hotbar to take it through its stages. It brings its own brickwork, grate and hatches; there is nothing to wall in around it. The whole build is 22 metal plates, 8 rods, 32 rivets and 44 fire bricks, and it reserves a volume 3 wide, 6 long and 3 high, less the corners of the top course.

Rivets, not nails. A riveted seam is the one joint that is both strong and tight, and no stage of a pressure vessel accepts anything else.

Firing it

The boiler answers on two hatches.

• The main hatch is the firing door at the near end. Hold right-click with an empty hand to swing it, right-click coal into the open door to charge the grate, and hold right-click on a full grate to light it.
• The man hatch is the cover on top of the barrel. Hold right-click to swing it, then pour water in from a bucket or bail it back out.

The grate holds four courses of four, so 16 units of a single fuel, and the fill is readable on the block as it burns down. Anything whose burn temperature is 1000 C or more is admitted. What separates fuels is mostly how long a charge lasts, not how hot it burns: the boiler is limited by its heating surface, so any fuel at 1200 C or above raises steam at the vessel's full rate. A cooler fuel runs at

rate = (flame temperature - steam temperature) / (1200 C - steam temperature)

which also stretches the 180-second heating time in the same proportion.

Water

The vessel is primed by hand through the man hatch. Filling from a stack of buckets fills one bucket's worth and wastes the rest, so pour one bucket per slot, or pump.

The automatic intake on the boiler's south face draws from a feed network at up to 20 L/s and stops at half the vessel's capacity, which is 800 L. Hand pouring is not capped by that and can reach the 1000 L boiling ceiling.

Pressurised feedwater is dangerous. Every litre admitted above 1 atm flashes an extra litre of steam per atm of feed pressure, which spikes a boiling vessel toward a burst. Gate or throttle any pumped feed.

Water reaches a boiler through one of two pumps. The Manual Fluid Pump is hand-cranked, needs no engine, and moves 2 L/s at 1 atm as long as a working Fluid Intake feeds its input line; the crank support in its model marks the input side. The engine-driven Engine Fluid Pump is a sub-machine and moves far more; see Steam engines and sub-machines.

Venting and bursting

A burning boiler makes 16 L/s of exhaust and has to put it somewhere. An open pipe end passes only about 8 L/s, so the exhaust run needs a Pipe Outlet capped with a chimney, which vents 16 L/s, or a route to a stack. If back-pressure on the exhaust port reaches 0.8 atm the fire goes out after 10 seconds.

On the steam side, a boiler that keeps firing with nowhere to send its steam sits at its choke pressure and explodes after 30 seconds there. The emergency release is the man hatch, which vents about 200 L/s. An unpiped steam outlet only leaks at 16 L/s, which is nowhere near enough to save an over-pressured vessel.

A burst shatters every block within 3 blocks whose resistance is under 20, which takes pipes, ports, coal piles and soft terrain and leaves other machines standing, and scatters 0.4 of the boiler's construction materials. Mining a boiler intact returns 0.8 of them instead.

A Cornish Boiler's 5 atm ceiling sits above the Watt Beam Engine's 4 atm breaking point. A pressure valve between the two, with an overflow, is not optional.

Step by step

1. Place a Fluid Intake on a pond, on a full cube of water at least 3 blocks deep, with no other intake within 6 blocks.
2. Raise a Cornish Boiler and pipe its exhaust port, at the far corner, out through a Pipe Outlet capped with a chimney.
3. Pipe the intake to a Manual Fluid Pump and the pump's output into the boiler's south face, or pour by hand, one bucket per slot.
4. Get at least 300 L into the vessel.
5. Open the main hatch, charge the grate with one fuel, and hold right-click to light it. Shut both hatches.
6. Wait about three minutes for it to start boiling.
7. Pipe the steam outlet, on top of the barrel, to whatever is consuming steam, through a Piping (Pressure Valve) gated below the consumer's breaking pressure.

What goes wrong

The fire keeps going out. The exhaust is backed up. Vent it through a chimney rather than an open pipe end.

The boiler will not start. It holds less than 300 L, or the man hatch is open and the steam is going straight out of it.

It boils slowly. The fuel burns below 1200 C. Lignite, the only common one, burns at 1100 C and so runs at about 0.90 of the rate, which stretches the 180-second heat-up to about 200 seconds. It is a 10 per cent penalty, not a different order of machine.

It exploded with a safety valve fitted. Check that the valve's output has somewhere to go. A pressure valve with nothing piped past it vents, which works; a pressure valve whose output run is already at the same pressure moves nothing at all, because gas only goes downhill.

It exploded straight after a relog or a server restart. This one is carried over from the mod line this code came from, where several players saw a boiler at critical pressure immediately after a chunk reload with safety valves fitted and the hatch open. No fix is recorded and it has not been re-tested here. Until it has been, vent the boiler and stop the run before a restart.

Filling from a stack of buckets lost most of the water. Pour one bucket per slot, or use a pump.