Ask a plant what its steam costs and you will usually get the fuel bill divided by the steam meter: "about ₹600 a tonne". That number is not wrong — it is incomplete, and the gap decides real money: projects approved against the wrong steam cost, products priced on it, savings claims measured with it. This lesson builds the honest number, layer by layer, on the practice plant — and then shows you which version of it to use for which decision.
Layer by layer
Fuel — the base of the stack, from lesson A1's arithmetic: 574 kcal into every kilogram, at 75% boiler efficiency, on husk at ₹2,500/t: ≈ ₹600/tonne of steam.
Water and treatment — every tonne of steam consumes water: the un-returned condensate fraction plus blowdown. At the practice plant's 60% return, each tonne of steam drags roughly half a tonne of treated make-up behind it — water, softening, chemicals: ≈ ₹45/tonne. (Improve the return and this layer shrinks — the condensate lesson's project attacks exactly this line.)
Electricity — the boiler house runs on motors: FD/ID fans, feed pump, fuel handling. For a husk-fired shell boiler, typically 4–6 kWh per tonne of steam: ≈ ₹40/tonne.
People — operators round the clock, ash handling, water testing. A three-shift boiler house crew against 3,600 t/month: ≈ ₹70/tonne, varying widely with plant size (this layer is why big plants make cheaper steam).
Maintenance and consumables — annual overhaul, refractory, gaskets, gauge glasses, the trap register: ≈ ₹25/tonne as a fair mid-band.
Capital — the boiler, its chimney, water plant and piping were bought with money that has a cost. Spread over a 15-year life at Indian financing rates: ≈ ₹30–50/tonne depending on what was paid and when.
Two costs, two jobs — don't swap them
The stack gives you two different numbers, and using the wrong one is the classic steam-economics error:
- The full (average) cost — ≈ ₹820/t. Use it for costing production, pricing products, comparing steam against other utilities, and deciding whether to replace the boiler house wholesale. It answers: what does steam cost this business?
- The marginal cost — ≈ ₹650–700/t. Fuel, water, treatment and a little electricity — the layers that actually shrink when you save a tonne. Operators' salaries and the boiler's EMI do not fall because a leak was fixed. Use the marginal cost to evaluate savings projects. It answers: what does the next tonne cost — or the avoided tonne save?
The marginal figure survives the finance department's questions. The other two do not — one undersells your project, the other gets caught. Savings claimed at full cost are the fastest way to lose an MD's trust in energy projects generally, which costs more than any single project.
The number is a habit, not a report
The steam cost is not calculated once. Fuel prices move seasonally (husk swings with the paddy calendar), return fractions drift, efficiency decays between overhauls. A plant that recomputes its ₹/tonne quarterly — one line per layer, measured not assumed — owns the single number that ranks every lesson in this faculty: what a leak costs, what a failed trap costs, what the flash plume costs, what recompression earns. It is the exchange rate between engineering and money.
- Build your own stack this month: six layers, one page. Fuel from weighbridge records, not purchase orders; steam from the meter, not the nameplate.
- No steam meter? The stack is your argument for one — every layer divides by that reading (the metering lesson covers honest measurement).
- Print both numbers — full and marginal — on the energy notice board, dated. Watch project conversations change.
- Recompute quarterly and when fuel is re-contracted. A steam cost older than six months is a rumour.
- Six layers: fuel, water+treatment, electricity, people, maintenance, capital. ≈ ₹820/t at the practice plant vs ₹600 fuel-only.
- Full cost prices production; marginal cost (≈ ₹650–700) prices savings. Never swap them.
- Steam cost is a quarterly habit with measured inputs — the exchange rate for every efficiency decision.
- Every layer is attackable, and each has its own lesson in this academy.
The kilocalorie on this site's masthead is the same unit printed on your biscuit wrapper: one kcal heats one kilogram of water one degree. A working adult runs on about 2,500 kcal a day; the practice plant's boiler eats 3.4 million kcal an hour — the metabolism of a town of thirty thousand people, burning in one furnace, which is why a few percent of it is worth an engineer's whole month.