Ironmaking in the hot blast furnace
as of 0.9.9
The hot blast furnace is the steel tier's ironmaking furnace. It is the same machine as the cold blast furnace of Iron Industry Expanded, built with a sealed top, a charging pair instead of an open hopper, and two gas outlets that let its own waste heat be recovered and blown back in. It takes burden and fuel in alternating courses and pours molten pig iron and slag out of two taps.
What changes from the cold furnace
| Cold blast furnace (iiex) | Hot blast furnace (siex) | |
|---|---|---|
| Wall brick | refractory brick of any tier | tier 3 refractory brick only |
| Top | open stack, charged through one tall hopper | sealed, charged through Reinforced Hopper and Bell Hopper |
| Exhaust | none, the open top is the chimney | two outlets, 48 L/s |
| Blast | ambient air off a twin-tub blower | preheated air off a charged cowper stove |
| Anchor | a cold blast furnace core, one per tier | Blast Furnace Core, one tier-less block |
The two exhaust outlets are the whole structural difference. Run the same build on unheated air and it behaves exactly like a cold furnace: there is no hot-blast switch anywhere.
The tier gate is the wall, not the machine. A tier 1 or tier 2 wall does not complete this structure; all 99 wall cells must be tier 3 refractory bricks.
How it works
Heat
The furnace works out one process temperature every second and melts when it clears iron's melting point of 1482 C.
T_process = T_in - T_loss
T_in = 950 C + 900 C x coke factor x air factor + 0.35 x (blast C above ambient)
T_loss = 120 C + up to 310 C of cold charge + 1 C per degree the weather is below 20 C
The coke factor is 1.0 at a burden of 20 % coke, 1.175 at 30 % and 0.825 at 10 %, clamped to 0.35 and 1.25. The air factor is 0.5 with no blast at all and 1.0 with the full demanded air arriving.
At 20 % coke, a full shaft and full cold blast that is 950 + 900 - 430 = about 1420 C, which is short of the melt line. The preheat term is what closes the gap: a cowper stove at its 1240 C cap adds 0.35 x 1220 = about 427 C of T_in, which puts a full furnace near 1850 C. See Hot blast.
Air
Both the pressure the furnace demands and the air it draws come from the burden's coke fraction, not from the furnace:
| Burden coke | Pressure demanded | Air drawn per tuyere | Air for both tuyeres |
|---|---|---|---|
| 30 % | 1.25 atm | 21 L/s | 42 L/s |
| 20 % (the reference) | 2.0 atm | 14 L/s | 28 L/s |
| 10 % | 2.75 atm | 7 L/s | 14 L/s |
Pressure is clamped to 1.2 atm at the bottom and 6.0 atm at the top; the draw factor is clamped to 0.4x and 1.8x of 14 L/s. A coke-rich burden is easy to blow and thirsty; a lean one is frugal and needs steam-tier pressure to push through. A twin-tub blower tops out at 2.2 atm, so any burden leaner than about 17 % coke needs an Air Blower on a steam engine.
Blast that arrives below 10 % of the demanded volume counts as starvation, and sustained starvation puts the fire out.
Exhaust
Each of the two outlets is offered 24 L of exhaust per second at 0.8 of the furnace's own temperature, so the furnace vents 48 L/s. That is exactly one Smoke Stack Intake (48 L/s) or two cowper stoves (24 L/s each). When an outlet cannot push its litres into the run, the furnace reads as choked and stops burning, so the exhaust side needs at least as much sink as the furnace makes.
Charge
The shaft is 38 chargeable cells at 32 units a cell: 1216 units of charge. Charge is laid in courses, and the furnace refuses to stack fuel on fuel, so a load of coke onto a column that already shows coke at the top goes to another column instead. Each drop lands on the lowest column, which is what keeps the stockline level without any work from the player.
Burden is the ore-and-flux item made by the burdenmaker of Iron Industry Expanded. Fuel is coke or charcoal. The hoppers take either, one grade at a time.
Yield and tapping
Every charge unit of ore that melts yields 8.5 units of molten pig iron and about 1.42 units of slag. Both land in the same two-cell hearth and share one volume, so slag left undrained takes room the iron wants.
The iron tap sits in one wall at hearth level and the slag tap in the opposite wall one course higher; which compass wall each lands in follows the core's facing, and the build outline shows it. Neither pours into the air: molten metal does not back-flow, so a canal or a casting bed has to be under a tap before it is opened, or the metal solidifies at the spout.
Building and running one
- Lay the Blast Furnace Core in the hearth floor, then build the walls, the tuyeres, the taps and the two outlets around it. Hot blast furnace carries the figure and the full materials list.
- Plumb the two tuyeres to a blast main and the two outlets to cowper stoves and a smoke stack.
- Set the Reinforced Hopper on top with the Bell Hopper directly under it.
- Fill the tank with burden, let the bell drip it in, then fill with fuel, and keep alternating. Dropping is on by default; ctrl plus right-click on the tank stops and starts it.
- Start the blower and get the blast main above the pressure the burden demands before lighting.
- Open a tap and right-click it with a torch. The furnace lights only when every column has charge in front of the tuyeres and there is fuel at the bottom of each column.
- Put a canal or a bed under the iron tap and open it once the hearth has metal in it. Drain slag out of the upper tap as it builds.
What goes wrong
"It just says extinguishing." The blast is missing or too weak. Air has to be flowing at pressure before the torch goes in, not after. Check the blast main's pressure against the table above for the burden you actually charged, not against a number from a video. The blast pressure at the tuyere is not the safety-valve setting on the boiler, and it is not the hopper's fill readout.
It lights and goes out in seconds. Either a column is empty in front of a tuyere, or there is no fuel at the raceway. A furnace that has burned out keeps no fuel at the bottom, so it cannot relight off its own residue: charge a fresh fuel course first.
It burns but never melts. The blast is cold. Cold blast at the reference burden runs about 60 C under iron's melting point; a charged stove is not optional at this tier.
It says choked. The exhaust run is full. One furnace needs one smoke stack or two stoves taking gas; a run with a shut valve at both ends and no relief backs up within seconds.
The structure will not complete. The wall bricks must be tier 3. The taps and the tuyeres are pinned to a facing in the layout, so one installed the wrong way round leaves the count short.
It is built and vents nothing. The outlet cells accept any orientation of a pipe outlet, and the furnace never checks that what it pushes into is connected. An outlet turned the wrong way completes the structure and then joins nothing, and the furnace reports no fault while the stoves starve.
The bell drips nothing. The bell pulls from the Reinforced Hopper directly above it and drops into the furnace core it is anchored to. Neither link is validated by the structure; the only warning is a line on the tank saying there is no bell below.