Boiler sizing
Boiler Capacity Calculator
Tells you what capacity of steam boiler a process needs, in the rating convention Indian boilers are actually sold in — kg/h "from and at" 100 °C. It also tells you what that boiler will really evaporate at your working pressure and feedwater temperature, which is a different and usually smaller number.
Size my boiler
Step 1 of 6
How the calculation works
Process steam demand is not boiler capacity. Five factors sit between them, and each one is a judgement call rather than a constant, so all five are shown and can be changed.
- 1Process demandEither the steam demand you already know, or a heat load worked out from mass, specific heat and temperature rise. For a process that returns condensate, the useful heat per kilogram of steam is the latent heat — using total enthalpy instead inflates demand by roughly a quarter.
- 2DiversityNot every steam user runs at once. Continuous plants sit near 1.0; batch plants with staggered cycles are lower.
- 3Distribution and trap lossesRadiation from the header and an allowance for traps and leaks. Long runs and poor lagging push this up.
- 4Peak and expansion marginBatch startup draws far more than the running average. Expansion margin is yours to choose — it is often the difference between two standard boiler sizes.
- 5Conversion to F&A 100 °CThe result is multiplied by the evaporation factor, (h_g at working pressure − h_f at feedwater temperature) ÷ 2256.9, to get the rating a boiler is quoted against.
Worked example
A plant with 1500 kg/h of process steam demand at 10.54 kg/cm²(g), feedwater returning at 80 °C, on a 10% expansion margin:
| Process steam demand | 1,500 kg/h |
|---|---|
| After diversity (0.90) | 1,350 kg/h |
| After distribution and trap losses (6%) | 1,431 kg/h |
| After peak margin (20%) | 1,717 kg/h |
| After expansion margin (10%) | 1,889 kg/h — actual evaporation required |
| Continuous blowdown (200 → 3000 ppm TDS) | 135 kg/h |
| Feedwater requirement | 2,024 kg/h |
| Evaporation factor at 10.54 kg/cm²(g), 80 °C feed | 1.0838 |
| Required rating | 2,047 kg/h F&A 100 °C |
That falls between a 2000 and a 2500 kg/h standard size. Both are shown, because the honest answer is that dropping the 10% expansion margin fits the smaller boiler — and a 22% oversized boiler runs at part load, where efficiency is worse.
Common questions
What does "kg/h F&A 100 °C" mean on a boiler nameplate?
"From and at 100 °C" is a rating datum, not an operating condition. It is the evaporation the boiler would achieve if feedwater entered at 100 °C and left as steam at 100 °C at atmospheric pressure, where each kilogram absorbs 2256.9 kJ. Because real feedwater is colder than 100 °C and real steam is hotter, every kilogram absorbs more than that, so the actual output is always lower than the nameplate figure.
How much less than the nameplate will my boiler actually produce?
Divide the nameplate rating by the evaporation factor. A boiler rated 2000 kg/h F&A 100 °C, run at 10.54 kg/cm²(g) with 30 °C feedwater, has an evaporation factor of 1.176 and so delivers about 1700 kg/h. With feedwater preheated to 100 °C the same boiler gives about 1910 kg/h. This is the most common reason a new boiler turns out to be undersized.
Does boiler efficiency change the capacity I need?
No. A plant that needs 2000 kg/h of steam needs 2000 kg/h at any efficiency — the F&A rating is an output rating, with the design efficiency already built into it. Efficiency determines how much fuel is burnt to deliver that steam, and therefore the running cost, not the size of the boiler.
What feedwater temperature should I assume?
Use your real figure if you have one, because it is the single biggest lever in the whole calculation — moving from 30 °C to 100 °C changes the required rating by about 12%. If condensate is returned and a deaerator is fitted, 80–105 °C is typical. If the plant runs on cold make-up water, 30 °C is closer. Where the temperature is uncertain, size on the colder figure.
Why does the calculator add a blowdown allowance?
Dissolved solids entering with the feedwater concentrate in the boiler as steam leaves, so a fraction of the boiler water is continuously blown down to hold total dissolved solids within the design limit. The rate is feedwater TDS divided by the difference between the boiler limit and the feedwater TDS. With good feedwater it is under 1%; with poor feedwater it can exceed 10%, which is both a heat loss and a reason to look at treatment or heat recovery.
Do I need superheated steam?
For indirect process heating, almost never. The useful energy is the latent heat released as steam condenses, and superheat has to be removed before condensation starts, which lowers the heat transfer coefficient while adding cost. Superheat is needed for turbine drive, or where steam is the drying medium in direct contact with the product.
Is my boiler covered by the Boilers Act, 2025?
The Boilers Act, 2025 (Act No. 12 of 2025) came into force on 1 May 2025, replacing the Boilers Act 1923; IBR 1950 technical regulations continue under the savings clause. A vessel is excluded from the definition of "boiler" only if its capacity is below 25 litres, or its design and working gauge pressures are both below 1 kg/cm², or the water is heated below 100 °C. Any real industrial process boiler is covered, which means state registration and periodic inspection are mandatory.
How is boiler capacity calculation actually done?
The boiler capacity formula that matters in India is the evaporation factor: divide the heat your process needs by (h_g at working pressure minus h_f at feedwater temperature), then by 2256.9 kJ/kg to express it F&A 100 °C. This tool does that from IF97 properties rather than a printed table.
Why is boiler capacity kg/hr rather than a power rating?
Because Indian procurement quotes steam output, not heat input. Boiler capacity kg/hr — specifically kg/h F&A 100 °C — is what appears on the nameplate and in the tender, so a figure in kW has to be converted before it can be compared with anything.