Formula
\(F = \frac{h_s - h_{fw}}{h_{fg,100}} \qquad \dot m_e = F\,\dot m_s\)
\(\text{actual evaporation ratio} = \frac{\dot m_s}{\dot m_f} \qquad \text{equivalent evaporation ratio} = \frac{\dot m_e}{\dot m_f}\)
Variables
| Symbol | Meaning | SI unit |
|---|---|---|
| \(\dot m_s\) | steam generated | kg/s |
| \(p\) | steam absolute pressure at the boiler outlet | Pa |
| \(\text{state}\) | dry saturated, or superheated at \(T_s\) | – |
| \(T_s\) | steam temperature when superheated | K |
| \(T_{fw}\) | feedwater temperature | K |
| \(\dot m_f\) | fuel burnt (optional) | kg/s |
| \(\dot m_e\) | equivalent evaporation from and at 100 °C | kg/s |
| \(F\) | factor of evaporation | – |
| \(\dot m_s / \dot m_f\) | actual evaporation ratio | kg/kg |
| \(\dot m_e / \dot m_f\) | equivalent evaporation ratio | kg/kg |
| \(h_s\) | steam enthalpy | J/kg |
| \(h_{fw}\) | feedwater enthalpy | J/kg |
| \(h_{fg,100}\) | latent heat of vaporisation at 100 °C | J/kg |
Method and standards
The "from and at 100 °C" rating convention used for shell boilers (BS 845-1 and boiler-maker practice). All enthalpies come from FireCAD's IAPWS-IF97 implementation; \(h_{fg,100}\) is evaluated on the saturation line at 373.15 K.
Assumptions and limits
- Feedwater enthalpy is taken at the steam pressure (the difference to the feed pump outlet pressure is negligible here).
- Wet steam and blowdown are not included; for boiler efficiency use the PTC 4 calculators.
References
- IAPWS R7-97(2012) IF97
- BS 845-1
Questions and answers
What does "from and at 100 °C" mean?
It is the mass of water at 100 °C that the same heat would turn into dry saturated steam at 100 °C. It lets boilers with different steam conditions and feedwater temperatures be compared on one basis.
Why is the factor of evaporation above 1?
Raising feedwater at 105 °C to dry saturated steam at 10.5 bar(a) needs 2 338 kJ/kg, more than the 2 256 kJ/kg latent heat at 100 °C, so the factor is 1.036.
Which latent heat at 100 °C is used?
2 256.47 kJ/kg from IAPWS-IF97 at 100 °C. Older references round it to 2 257 kJ/kg (539 kcal/kg); the difference is 0.02 %.