Saturation Pressure and Temperature: Why Pressure Sets the Steam Temperature
Water boils at 100 °C only at atmospheric pressure. Raise the pressure and it boils hotter; lower it and it boils cooler. For saturated steam, pressure and temperature are locked together, so the pressure gauge on a steam line is also a thermometer.
You'll learn
- Explain why boiling point depends on pressure
- Read saturation temperature from pressure, and the reverse
- Choose a steam pressure for a process temperature
- Know where vacuum steam and altitude change the answer
Before this: Latent heat
Boiling point is not fixed
Water boils when its molecules have enough energy to push back against the pressure above them. Press harder and they need more energy, which means a higher temperature. That is how a pressure cooker cooks faster: its lid holds the pressure up, so the water inside boils well above 100 °C.
A boiler is a very large pressure cooker. At the pressures Indian process boilers run at, the water boils somewhere between about 150 °C and 220 °C, and the steam leaves at that same temperature. Water and steam sitting together at their boiling point are called saturated, and the temperature is the saturation temperature.
The useful consequence: for saturated steam, knowing the pressure means knowing the temperature. Operators set process temperatures by setting steam pressure.
The saturation curve
| Pressure, kg/cm²(g) | Saturation temperature, °C |
|---|---|
| 0 | 100.0 |
| 1 | 120.1 |
| 2 | 133.2 |
| 3 | 143.2 |
| 5 | 158.3 |
| 7 | 169.8 |
| 10 | 183.3 |
| 15 | 200.5 |
| 20 | 213.9 |
| 30 | 234.7 |
| 40 | 250.7 |
Computed from IAPWS-IF97, atmospheric pressure taken as 1.0332 kg/cm².
- A jacketed vessel must bring its product to 140 °C.
- Heat only flows with a temperature difference across the jacket wall; allow 15 °C.
- The steam must condense at 140 + 15 = 155 °C.
- Find the pressure at which water saturates at 155 °C, from a steam table or the curve: 4.51 kg/cm²(g).
Sensitivity, vacuum and altitude
The curve is steep at low pressure and flat at high pressure. Around 1 kg/cm²(g), one kg/cm² of pressure changes saturation temperature by about 15.8 °C; around 10 kg/cm²(g), by only 4.0 °C. So a small pressure swing in a low-pressure process heater is a large temperature swing, and a low-pressure gauge needs to be accurate.
Below atmospheric pressure the same rule holds. Steam at half an atmosphere of vacuum, −0.5 kg/cm²(g), condenses at 82.4 °C. Vacuum steam is used to heat products that would be damaged at 100 °C, such as some food and pharmaceutical processes, while keeping the even, constant-temperature heating that condensing steam gives.
Altitude matters for anything vented to atmosphere. At about 1,000 m, where atmospheric pressure is roughly 0.9 bar absolute, water boils at 96.7 °C. Boilers work on gauge pressure, which is measured from the local atmosphere, so a plant on a plateau sees slightly lower saturation temperatures at the same gauge reading than one at sea level.
Key takeaways
- For saturated steam, pressure and temperature are locked together.
- Pick steam pressure from the product temperature plus a heating-surface temperature difference.
- The curve is steep at low pressure: small pressure changes there are large temperature changes.
- Vacuum steam heats below 100 °C; gauge readings depend on local atmospheric pressure.