Hot blast
as of 0.9.8
Hot blast is the furnace's own flue gas, soaked into a brick core and given back to the air on its way to the tuyeres. A cowper stove is that core: it charges on exhaust, then discharges into air. Nothing about it is required to make iron, and everything about it is worth building anyway, because it roughly triples the melt rate.
The builds are on the Cowper stove and Smoke stack pages.
What it buys
A furnace on plain blower air tops out at a 1540 C hearth, climbs at 4 C/s and melts at 0.61x. Blast at 1240 C lifts that to a 1740 C hearth, an 8 C/s climb and 2.0x, which is 20.4 units of iron per second against 6.2. There is no threshold to cross: the ceiling and the climb both interpolate from the cold figures to the hot ones across the blast temperature, so a part-charged stove is worth part of the boost. 1240 C is where the gain stops, and it is also the hottest a stove core gets.
The four connections
A stove is one block tall where you stand and four gas connections in two pairs. Taking the Cowper Stove Intake block as the front, at its own level:
| connection | where | carries |
|---|---|---|
| the intake block itself | front, low | furnace exhaust in |
| outlet | front, one block up | hot blast out |
| passthrough | back, one block up | air in from the blowers |
| outlet | back, low | spent exhaust on to the stack or the next stove |
The front pair is the furnace's side, exhaust in at the bottom and hot blast out at the top; the back pair is the blowers' side, air in at the top and spent gas out at the bottom. Each stream crosses the core on its way through. The four Heat Sinks stand in the column between them and show the core temperature when you look at one. A pipe merely routed past the intake face is not plumbed in: the stove only draws from a run whose pipe presents a connector back at it.
Charging
While exhaust is arriving, the core takes a share of the gap between the gas temperature and its own, every second, and the spent gas leaves the back outlet at 40 per cent of the temperature it came in with. The stove draws 24 L/s. Furnace exhaust arrives at 80 per cent of the hearth temperature, so about 1230 C from a furnace on cold blast.
| coal pile below the intake | share per second | time to close half the gap |
|---|---|---|
| none | 0.0012 | 9 min 40 s |
| any coal but anthracite | 0.0048 | 2 min 25 s |
| anthracite | 0.0064 | 1 min 50 s |
The core stops at 1240 C whatever it is fed. The coal pile goes in the cell directly below the intake block, which the build leaves open for it.
Discharging
While air is passing, the air leaves at the core's temperature and the core loses heat in proportion to the air actually drawn against its rated 24 L/s. At a full draw it sheds half of its margin over the incoming air in about 9 min 40 s. A stove doing nothing at all still bleeds toward ambient with a half-life of about 19 minutes, so a charged stove cannot be banked for later.
That is why stoves come in pairs: one charges on exhaust while the other blows, and they change places before the blowing one runs down.
Running a pair
- Build both stoves, with their heat sinks inside and the overflow line to the Smoke Stack already valved and open. Build that line before the first swap, not after it.
- Merge the hot-blast side of the network so both tuyeres draw from the same pipes. A split output drops the blast temperature on every swap.
- Run the furnace on cold blast first. A stove has nothing to charge from until the furnace has been alight for a while.
- Charge stove A: open its exhaust valve, leave its air valve shut. It reads "Heating Up!" and the heat sinks climb.
- Swap: shut A's exhaust valve, open A's air valve. It reads "Heating Air" and the furnace picks up the boost.
- Charge B the same way while A blows, and change them over when A's heat sinks drop toward the point where the boost is no longer worth it. Nothing in the code fixes that point; the in-game handbook suggests swapping below 800 C.
- One valve per stove at a time, always. Both open is the fault below.
The smoke stack
The stack is the safety valve of the whole gas side. It draws 96 L/s off the run and vents it, which is more than one furnace makes at full melt rate. Without it, or without a path to it, the exhaust run fills, the furnace chokes and the melt stops.
Route the surplus to the stack through a pressure-relief valve teed off the exhaust main. The furnace pushes its exhaust to 2.0 atm, and a freshly placed relief valve gates at 1.0 atm, so the default setting opens under load. A valve set as low as 0.5 atm also works, though the 0.8 atm choke point it was originally sized against predates the 0.9.5 release that raised the furnace's exhaust ceiling to 2.0 atm.
What goes wrong
"Cannot heat up, exhaust mixes with the air!" Both valves of one stove are open. Exhaust and air are arriving at the same stove, and it will not charge. Shut the air valve: the stove vents the gas stranded in its passthrough on the next tick and charges from the one after, so there is nothing to clear by hand. A stove that keeps the message has its air valve still open somewhere.
The furnace goes out every time you swap. The exhaust has nowhere to go for the moment the valve is shut. Build the overflow to the smoke stack, and merge the hot side so both tuyeres keep drawing during the swap. Cycling the valves faster does not help: no blast at 1.5 atm for 30 seconds is what puts the furnace out, and a second disruption at the same time puts it out at once.
The blast temperature falls below 1500 C on every swap. The output networks are still separate, so the tuyeres lose their supply while the stoves change places.
A cold stove reads Idle and does nothing. With no exhaust to charge from and no air to give heat to it has nothing to do, and that is not a fault. A charged stove is a different matter: it bleeds toward ambient whatever it is doing, so it cannot be banked. Charge one when it is about to blow.
Exhaust arrives at a tuyere. The furnace counts that as a disruption and starts its extinguish clock. Keep the exhaust and air sides apart the whole way; a stove hands on whatever medium its air passthrough carries.
The coal fire. The in-game handbook warns that burning low grades of coal makes the stove give off exhaust from its own main output. The code in 0.9.8 does not do that: the pile only picks the charging rate above, and a stove hands on whatever medium arrives at its passthrough. Putting the fire out before a stove blows still costs nothing, and a stove is meant to blow after it is charged, not while it is.
Whether the stoves work
The stove code in 0.9.8 does charge and discharge as described here, and the stoves are not required to smelt: cold blast clears iron's melting point on its own. A furnace that will not take the boost is usually one of the faults above, and the cold-blast route is always available as a fallback: it is slower, not broken.