What is Boiler Feed Water Enthalpy?
Feed water enthalpy is the total energy content of water entering the boiler, expressed per unit mass, usually in kJ/kg.
In simple terms, enthalpy tells us how much energy the feed water already contains before it enters the boiler. This energy mainly depends on the temperature and pressure of the feed water.
For example, if the feed water enters the boiler at a higher temperature, its enthalpy will also be higher. This means the boiler needs to provide less additional heat to convert the water into steam.
How Does Feed Water Enthalpy Affect Boiler Efficiency?
Feed water enthalpy has a direct effect on the amount of heat required inside the boiler.
When feed water enthalpy increases, the difference between steam enthalpy and feed water enthalpy decreases. Therefore, the boiler requires less heat from fuel to produce the required steam.
This can improve the overall boiler efficiency and fuel economy.
Heat required = Steam enthalpy − Feed water enthalpy
Role of Feed Water Temperature
Feed water temperature is one of the main factors affecting its enthalpy. Equipment such as feed water heaters, deaerators and economizers increases the temperature of feed water before it enters the boiler.
Therefore, maintaining the feed water at the designed temperature helps reduce fuel consumption and improves boiler performance.
For example
Why Is It Important in a Thermal Power Plant?
In a thermal power plant, improving feed water enthalpy is an important part of the regenerative feed heating system.
Steam extracted from the turbine is used to heat the feed water through feed water heaters. The economizer then further recovers heat from the flue gas and transfers it to the feed water.
This increases the energy already available in the feed water before combustion heat is added in the boiler.
Key Benefits of Higher Feed Water Enthalpy
How to Calculate Boiler Feed Water Enthalpy
Boiler feed water enthalpy is the specific enthalpy of water entering the boiler, normally expressed in kJ/kg. There are two common ways to calculate it.
Accurate method
Using Steam Tables
h ≈ hf at the same temperature
Use feed water temperature and find the corresponding saturated-liquid enthalpy from steam tables.
For compressed liquid feed water, enthalpy is approximated as the saturated-liquid value at the same temperature.
Quick estimation
Using Specific Heat
h ≈ Cp × (T − Tref)
For a simple estimation, multiply specific heat of water by the temperature rise above a reference point.
The constant cp method is useful for quick checks but becomes less accurate at higher temperatures.
Feed Water Enthalpy from Steam Tables
For actual boiler calculations, use the feed water temperature and pressure and find the corresponding water enthalpy from steam tables or a reliable steam-property calculator. For compressed liquid, a common approximation is h ≈ hf at the same temperature.
| Feedwater Temperature (°C) | Saturation Pressure (bar) | Feedwater Enthalpy hf (kJ/kg) |
|---|---|---|
| 20 | 0.023 | 83.9 |
| 30 | 0.042 | 125.7 |
| 40 | 0.074 | 167.5 |
| 50 | 0.123 | 209.3 |
| 60 | 0.199 | 251.1 |
| 70 | 0.312 | 293 |
| 80 | 0.474 | 334.9 |
| 90 | 0.702 | 376.9 |
| 100 | 1.013 | 419 |
| 110 | 1.433 | 462 |
| 120 | 1.985 | 503.7 |
| 130 | 2.701 | 546.5 |
| 140 | 3.614 | 589.3 |
| 150 | 4.758 | 632.2 |
| 160 | 6.116 | 675.5 |
| 170 | 7.919 | 719 |
| 180 | 10.02 | 763.2 |
| 190 | 12.55 | 807.5 |
| 200 | 15.55 | 852.4 |
| 210 | 19.05 | 897.6 |
| 220 | 23.2 | 943.2 |
| 230 | 27.95 | 990.1 |
| 240 | 33.48 | 1037.8 |
| 250 | 39.73 | 1085.4 |
| 260 | 47.15 | 1133.5 |
| 270 | 55.18 | 1181.5 |
| 280 | 64.16 | 1230 |
| 290 | 73.96 | 1278 |
| 300 | 85.88 | 1325.6 |
For example, at feed water temperature = 150°C, From steam tables, the saturated-liquid enthalpy at 150°C is approximately hf ≈ 632 kJ/kg.
Feed Water Enthalpy Estimation Formula
Feed water enthalpy is the amount of energy contained in the feed water before it enters the boiler. It is generally expressed in kJ/kg.
h ≈ Cp × (T − Tref)
h ≈ 4.18 × T (when Tref = 0°C)
Where
The small difference between the steam-table value and the constant-cp estimate occurs because the specific heat of water is not exactly constant over the temperature range.
Feed Water Enthalpy Sample Calculation
Given
- • Feed water temperature = 150°C
- • Specific heat of water = 4.18 kJ/kg·K
- • Reference temperature = 0°C
Apply the estimation formula
h ≈ Cp × (T − Tref)
h ≈ 4.18 × (150 − 0)
h ≈ 627 kJ/kg
Compare with steam-table value
Steam-table hf at 150°C ≈ 632 kJ/kg
Difference ≈ 5 kJ/kg due to varying specific heat
Result
Estimated Feed Water Enthalpy ≈ 627 kJ/kg
The value 627 kJ/kg means that, under this simplified reference convention, each kilogram of feed water contains approximately 627 kJ of sensible energy relative to 0°C. If the feed water temperature is increased, its enthalpy also increases and the boiler needs to supply less additional heat to convert the water into steam.
Frequently Asked Questions
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