Boiler Ratings: TPH, F&A 100 °C and the Evaporation Factor
A boiler’s nameplate says how much steam it makes, but under standard conditions, not yours. In India the standard is “from and at 100 °C”. Converting that rating to real output at your pressure and feedwater temperature is the single most useful calculation when buying or checking a boiler.
You'll learn
- Explain TPH and the F&A 100 °C rating basis
- Calculate the evaporation factor
- Convert a nameplate rating to actual output, and a demand to a rating
- See how feedwater temperature changes real capacity
Before this: Enthalpy, Latent heat
What “5 TPH, F&A 100 °C” means
TPH is tonnes of steam per hour; 5 TPH is 5,000 kg/h. But the same boiler makes different amounts of steam depending on how hot its feedwater is and what pressure it runs at, so a rating needs a reference condition. Indian boilers are rated “from and at 100 °C” (F&A 100 °C): the steam the boiler would make from water already at 100 °C, into steam at 100 °C and atmospheric pressure.
Real plants are never at that condition. Feedwater is usually cooler than 100 °C and steam is at a higher pressure, so each kilogram of real steam needs more heat than a kilogram of F&A steam. The result: a boiler delivers less actual steam than its F&A rating.
The evaporation factor
| Symbol | Meaning | Unit |
|---|---|---|
| Enthalpy of steam at working pressure | kJ/kg | |
| Enthalpy of the feedwater | kJ/kg | |
| 2256.9 | Latent heat at 100 °C: the F&A reference | kJ/kg |
- Working pressure 10 kg/cm²(g); cold feedwater at 30 °C.
- Steam enthalpy 2,780.1 kJ/kg; feedwater 126.7 kJ/kg.
- Evaporation factor:
- Actual output:
Sizing and checking with the factor
Sizing runs the other way. A process needing 4,000 kg/h of actual steam at 10 kg/cm²(g) from 30 °C feedwater needs a boiler rated 4,703 kg/h F&A before any margin for distribution losses, blowdown or future load. Buying a “4 TPH” boiler for a 4,000 kg/h demand undersizes it by 18% on day one.
Feedwater temperature is the lever a plant controls. Returning condensate, a deaerator or an economizer raises it, and every degree reduces the evaporation factor and raises real output. That is why a capacity complaint is sometimes solved in the feed tank rather than by buying a bigger boiler.
The reference latent heat, 2,256.9 kJ/kg (539 kcal/kg), is a convention, held fixed so that ratings stay comparable. Feedwater enthalpy is taken at boiler pressure, not at its own saturation pressure; the difference is under 1 kJ/kg but it is what makes published evaporation factor tables reproduce exactly.
Key takeaways
- Indian boilers are rated F&A 100 °C: steam from 100 °C water into 100 °C steam.
- Evaporation factor = .
- Actual output = rating ÷ factor: a 5 TPH boiler at 10 kg/cm²(g) from 30 °C feed makes about 4,253 kg/h.
- Hotter feedwater raises real output; compare quotations on the same basis.