The nameplate says 6,000 kg/h. The steam meter says 5,600 and falling. Before anyone phones the boilermaker, read this lesson — because in nine cases out of ten nobody lied, and no tube is fouled. The nameplate and the meter are answering two different questions.
What "F&A 100 °C" actually promises
Boiler capacity in India (and most of the world) is quoted "from and at 100 °C": the steam the boiler would raise from feedwater already at 100 °C, evaporated at 100 °C (atmospheric pressure). Under that condition every kilogram needs exactly the latent heat at atmospheric — 539 kcal — and nothing else. No feedwater heating, no pressure penalty.
It is a deliberately artificial condition, and that is its virtue: it is the same artificial condition for every make, so a 6 TPH boiler from any manufacturer promises the same thing — a furnace and heating surface able to transfer 6,000 × 539 ≈ 3.23 million kcal/h into water. F&A is not a flow promise. It is a heat promise, written in flow units.
What your boiler actually does
Your boiler works harder than the reference condition twice over: it lifts feedwater from well below 100 °C, and it evaporates at working pressure, where the total heat of steam is higher. Each real kilogram therefore costs more than 539 kcal — so the same 3.23 million kcal/h buys fewer kilograms:
The meter reading 5,600 at full fire is a healthy boiler. And if the deaerator trips and feed arrives at 30 °C instead of 90: 6,000 × 539 ÷ 634 ≈ 5,100 kg/h — the boiler "loses" half a tonne an hour without a single thing being wrong in the boiler house.
Where this bites, and where it pays
- Buying a boiler. If the process genuinely needs 5.8 TPH at pressure, a "6 TPH" nameplate is already too small on cold feed. Specify the duty — kg/h at your pressure and feed temperature — and let the vendor translate to F&A. The symptom of an undersized boiler is pressure sagging at peak load, then everyone blaming the burner.
- Comparing offers. F&A puts all makes on one scale — that is what it is for. Just never paste the F&A figure into a process heat balance.
- Feedwater temperature is capacity, free. The same curve read backwards: every 6 °C of feedwater heating buys back about 1% of evaporation (and 1% of fuel). It is the standing argument for condensate recovery and deaerators — their faculty-mates pick it up.
- Write your boiler's actual maximum (at your pressure and feed temperature) on the panel, next to the nameplate. Operators chase the wrong number otherwise.
- Trend feedwater temperature on the log sheet. A drifting deaerator quietly derates the boiler — and shows up here first.
- When a "capacity problem" is reported, check feed temperature and steam pressure before checking the boiler.
- F&A 100 °C = heat promise in flow clothing: rating × 539 kcal/h of transfer.
- Actual kg/h = F&A × 539 ÷ (hg − feed heat). At 10.5 kg/cm²g with 90 °C feed: ~94% of nameplate.
- Cold feed is the silent derater: 30 °C feed costs ~15% of capacity.
- Every 6 °C of feedwater ≈ 1% fuel and capacity.
"From and at" is Victorian test-bench language — it standardised the boiler trials of the 1800s, when every maker quoted capacity under whatever conditions flattered their design. The phrase has outlived imperial units, the boilers it was written for, and the empire itself: your 2026 IBR paperwork still speaks a sentence of nineteenth-century English.