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.
FAQ
My 6 TPH boiler never delivers 6 TPH. Is it undersized or faulty?
Probably neither. The nameplate is F&A 100 °C — an artificial reference condition — while your meter reads real kilograms at working pressure from real feedwater. At 10.5 kg/cm²g with 90 °C feed, a healthy "6 TPH" boiler tops out near 5,630 kg/h. Write that actual maximum on the panel; a meter reading 5,600 at full fire is a boiler doing exactly what it promised.
Why do manufacturers still quote F&A instead of real output?
Because it is the one condition that is identical for every make: feedwater already at 100 °C, evaporation at 100 °C, exactly 539 kcal per kilogram. That puts a 6 TPH boiler from any factory on the same scale — a heat promise of about 3.23 million kcal/h. The system fails only when someone pastes the F&A figure into a process heat balance as if it were deliverable flow.
How much capacity do I lose when the deaerator trips and feed runs cold?
At the practice plant, feed falling from 90 °C to 30 °C drops the maximum from about 5,630 to about 5,100 kg/h — half a tonne an hour gone with nothing broken. The working rule: every 6 °C of feedwater is worth about 1% of capacity and 1% of fuel, which is the standing argument for condensate recovery (D6) and a healthy deaerator (B5).
How should I specify a new boiler so this doesn't bite me?
State the duty the process actually needs — kilograms per hour at your working pressure and your real feedwater temperature — and let the vendor translate that into an F&A rating. Specify only "6 TPH" and you may buy a boiler whose honest maximum at your conditions is 5.6, then spend a year blaming the burner for sagging pressure at peak load.
Can a boiler ever beat its F&A nameplate?
Not at working pressure with normal feedwater — the derate curve never quite reaches 100%, because every real kilogram costs more than the reference 539 kcal. The nameplate is only achievable under the artificial test condition itself. If your meter reads above nameplate, suspect the meter or the fuel accounting before congratulating the boiler.
Check yourself
Five quick questions on this lesson. No marks, no records — the score is for you.