Hot blast
as of 0.9.9
Hot blast is preheated air blown into the furnace tuyeres. A cowper stove is a regenerative heat exchanger: it soaks heat out of the furnace's own exhaust into a stack of iron heat sinks, and then gives that heat back to cold air passing through it. It is the only thing in the mod that raises the temperature of a blast, and the only reason the hot blast furnace can reach iron's melting point. A Smoke Stack Intake vents whatever exhaust the stoves do not take.
The four pipes of a stove
A stove has two lines running through it and no switch of its own. Which half-cycle it runs depends entirely on which line the player has left open.
| Connection | Where | Carries |
|---|---|---|
| Cowper Stove Intake | the front face of the intake block | furnace exhaust in |
| bottom pipe outlet | the far side of the stove, at intake level | spent exhaust out |
| pipe passthrough | the far side, one block up | cold air in |
| upper pipe outlet | directly above the intake | hot blast out |
Bottom pair is exhaust, upper pair is air. Two valves per stove, one on each line, is the working arrangement.
How it works
Charging on exhaust
While exhaust arrives at the intake the stove draws 24 L/s of it and moves heat into its core:
core temperature += (exhaust C - core C) x rate x seconds
| Fuel in the cell under the intake | Rate per second | Time constant |
|---|---|---|
| a burning anthracite coal pile | 0.0064 | about 156 s |
| any other burning coal pile | 0.0048 | about 208 s |
| nothing | 0.0012 | about 833 s |
The coal pile only multiplies the rate; the stove still charges without one, five times slower. The core is capped at 1240 C.
The exhaust leaves much cooler than it arrived, in two steps. The stove takes 0.3 of the gap between the exhaust and the core out of the exhaust each second, and the bottom outlet then carries the volume the stove drew at 0.4 of what is left, never below ambient. Exhaust arriving at 1000 C at a cold stove leaves at about 282 C, which is what the next stove in the chain or the stack gets.
The four Heat Sink blocks inside the stove are set to the core temperature every tick, so looking at one is how the charge is read.
Blowing hot air
With the exhaust line shut and the air line open, air passing through leaves at the core temperature, and the core falls at 0.0012 per second of the difference. With anthracite under it a stove charges about five times faster than it discharges, which is why a plant runs stoves in pairs or threes: one blowing, the others charging.
The stove draws 24 L/s out of the air run. What it pushes into the hot outlet is 24 L/s, or the whole volume the passthrough is holding when that is more than 24 L, so a well charged air main delivers more than 24 L/s of hot blast, bounded only by the receiving run's throughput. The stove adds no pressure of its own: the air leaves at the pressure it arrived with.
What the blast is worth
The furnace gains 0.35 C of process temperature per degree the blast is above ambient. A stove at its 1240 C cap is worth about 427 C, which is what takes a full furnace from about 1420 C on cold blast to about 1850 C on hot.
The blast itself
Air counts as blast at 2.5 atm, which is the threshold the converter gates on; the furnace's own demand is set by its burden and is 2.0 atm at a 20 % coke charge. The Air Blower is the source: driven by a Cornish Engine, it injects 144 L/s per unit of engine power into the pipe run on its left-hand side, at the engine's inlet steam pressure times 0.75.
| Cornish throttle | Engine power | Air injected |
|---|---|---|
| Low | 0.2 | 28.8 L/s |
| Normal | 0.4 | 57.6 L/s |
| High | 0.8 | 115.2 L/s |
Because the output pressure is three quarters of the inlet steam pressure, 3.4 atm of steam at the engine is the floor for a 2.5 atm blast. A Cornish Engine does not engage below 5 atm of steam anyway, so a running engine clears it with room; a sagging steam line is what loses it. A twin-tub blower cannot: it tops out at 2.2 atm, under the blast threshold.
The smoke stack
A Smoke Stack Intake draws 48 L/s off its network and vents it to the sky. That is one whole furnace's exhaust, and it is the relief the whole system depends on: when the exhaust run backs up, the furnace chokes and goes out. The stack refuses liquids and vents air, steam or exhaust; the plume is coloured by what it is venting.
Running the pair
- Build two stoves and one stack. Pipe the furnace's two outlets into the stoves' intakes, and put a relief line from the exhaust run to the stack behind a pressure valve set below 1 atm.
- Pipe the air main from the Air Blower to the stoves' upper passthroughs, and the stoves' upper outlets to the furnace tuyeres. Merge the two tuyere feeds into one network so both tuyeres see the same pressure.
- Put a valve on each of the four lines of each stove.
- Light the furnace first. A stove has no heat of its own; nothing happens until exhaust is flowing.
- Open the exhaust valves on stove A and the air valves on stove B, with the other two lines shut on each. Watch a heat sink in stove A climb.
- When the blowing stove has cooled, swap: shut its air line, open its exhaust line, and open the other stove's air line. The furnace keeps its blast from the stove that has been charging.
- Keep a coal pile burning in the cell under each intake to charge faster, and let it burn out before that stove is put back on blast.
What goes wrong
"Cannot heat up, exhaust mixes with the air!" Both lines of that stove are open. Run one at a time. When the air valve is shut the stove vents the air still stranded in the passthrough, so the message clears within a second or two; if it does not, the air valve is still feeding it.
The stove will not take air after a swap. Air left in the run blocks the stove, which cannot push into a side already at higher pressure. Drain or vent the line rather than cycling valves faster.
The blast collapses on every swap. The two tuyeres are on separate networks. Merge the output side into one run so both tuyeres share the pressure, and the temperature holds through the change.
The furnace goes out when the stoves are switched. The exhaust has nowhere to go. Build the relief line to the stack before the first switch, with the pressure valve set below 1 atm; the furnace chokes when its outlets cannot push.
A cold stove is not broken. With no flow on either line a stove neither heats nor cools. It is waiting, not stuck.
Anthracite is not required. It is the fastest charge, ordinary coal is nearly as fast, and no coal at all still works.
The stove will not craft. Its recipe needs a straight cast pipe, which has no recipe of its own. See below.