Superheated Steam: Generation, Properties and Applications

Superheated steam is steam whose temperature has been raised above the saturation temperature corresponding to its pressure. The difference between actual steam temperature and saturation temperature is known as the degree of superheat.

This condition is especially important when steam is used to produce mechanical work, such as in steam turbines. Superheating, however, does not automatically improve every steam application. Saturated and superheated steam behave differently in heat-transfer processes.

How superheated steam is produced

While saturated steam remains in contact with liquid water, additional heat evaporates more water rather than increasing steam temperature above saturation. To produce superheated steam, evaporation must first be completed and dry steam must then pass through an additional heating stage.

This is normally achieved in a heat exchanger called a superheater. It may be incorporated as an additional boiler heat-transfer stage or installed as a separate unit. The heat source may be boiler flue gas or a separately fired source.

If saturation temperature is 170°C and steam is heated to 220°C:

Degree of superheat = 220 - 170 = 50°C

Unlike saturated steam, superheated steam does not have a unique pressure-temperature relationship. At one pressure, it may exist over a range of temperatures.

Why superheated steam is used in turbines

In a steam turbine, steam expands through nozzles and transfers energy to the rotor blades. As mechanical work is produced, steam energy decreases.

If steam entered the turbine exactly at saturation, the energy reduction would cause part of the steam to condense. Liquid droplets moving at turbine velocities are undesirable because they may contribute to water hammer and severe erosion of turbine internals.

Supplying superheated steam provides an energy margin before the steam reaches the saturation line.

Thermodynamic efficiency and superheat

Two theoretical approaches used to evaluate heat engines are the Carnot cycle and the Rankine cycle.

ηC = (Ti - Te) / Ti

For a turbine supplied at 90 bar(a) and 450°C, exhausting at 0.06 bar(a) with a 10% wet condition, the exhaust saturation temperature is 36.2°C.

• Ti = 723.0 K
• Te = 309.2 K

ηC = (723.0 - 309.2) / 723.0 = 57.2%

For the same example, using an inlet enthalpy of 3,256 kJ/kg, an exhaust mixture enthalpy of 2,188.7 kJ/kg and condensate enthalpy of 152 kJ/kg, calculated Rankine thermodynamic efficiency is approximately 34.4%.

These values represent thermodynamic rather than mechanical efficiency.

Superheated steam properties and tables

Superheated steam tables provide specific volume, internal energy, specific enthalpy and specific entropy for defined combinations of pressure and temperature. Both pressure and temperature must be known.

The table values apply at 1.013 bar(a).

How much additional energy is in superheat?

At 1.013 bar(a), dry saturated steam at 100°C has a specific enthalpy of approximately 2,676 kJ/kg. At the same pressure, superheated steam at 400°C has a specific enthalpy of 3,278 kJ/kg.

Superheat enthalpy = 3,278 - 2,676 = 602 kJ/kg

The mean specific heat capacity over this interval is:

602 / (400 - 100) ≈ 2.0 kJ/(kg·°C)

Specific heat capacity is not constant. It varies with pressure and temperature. Representative values include approximately 2.0 kJ/(kg·°C) at 125°C and 1.013 bar(a), and 3.5 kJ/(kg·°C) at 400°C and 120 bar(a).

Why superheated steam is not always suitable for heating

Superheated steam must first cool to saturation temperature before condensation begins. The heat released during desuperheating is relatively small compared with the enthalpy of evaporation.

• relatively little heat is released before saturation is reached,
• temperature gradients may develop over the heat-transfer surface,
• heat-transfer rates are lower while steam remains superheated,
• a larger heat-transfer area may be required.

When steam is part of the process material

In some duties, condensation supplies moisture as well as heat. Conditioning animal feed before pelletising is one example. Steam with significant superheat may initially provide insufficient moisture and can over-dry the meal.

Can pressure reduction create superheated steam?

During throttling through a pressure reducing valve, steam specific enthalpy is treated as essentially constant while pressure falls.

If retained enthalpy at the lower pressure is below dry saturated steam enthalpy, the steam remains wet. If it is higher, remaining moisture evaporates and the excess energy appears as superheat.

Example: reducing 6 bar(g) steam to 1 bar(g)

Steam enters at 6 bar(g) with dryness fraction x = 0.95.

• h_f = 697.5 kJ/kg
• h_fg = 2,066.0 kJ/kg

h = 697.5 + (2,066.0 × 0.95) = 2,660.2 kJ/kg

At 1 bar(g), dry saturated steam enthalpy is 2,706.7 kJ/kg. Because retained enthalpy is lower, the steam remains wet with a final dryness fraction of approximately:

x = 0.979 or 97.9%

Example: reducing 10 bar(g) steam to 1 bar(g)

Steam enters at 10 bar(g) with x = 0.98.

• h_f = 781.6 kJ/kg
• h_fg = 2,000.1 kJ/kg

h = 781.6 + (2,000.1 × 0.98) = 2,741.7 kJ/kg

At 1 bar(g), dry saturated steam enthalpy is 2,706.7 kJ/kg, leaving:

35 kJ/kg

The final steam temperature is approximately 136°C compared with a saturation temperature of 120°C.

Degree of superheat = 16°C

The Mollier chart

The Mollier chart plots specific enthalpy against specific entropy and is therefore an h-s diagram. It includes constant enthalpy, entropy, pressure, temperature and dryness-fraction lines, together with the saturation line.

Above the saturation line is the superheated region. Below it is the wet-steam region.

Expansion and throttling on the Mollier chart

Ideal expansion

Ideal expansion through a steam turbine is treated as a constant-entropy process and is shown as a vertical movement on the h-s chart.

Ideal throttling

Throttling through a pressure reducing valve is treated as a constant-enthalpy process and is shown as a horizontal movement.

Example of isentropic turbine expansion

For an initial condition of 50 bar(a) and 300°C and a final pressure of 0.04 bar(a), entropy remains constant.

A Mollier-chart reading gives approximately:

• specific enthalpy: 1,890 kJ/kg,
• dryness fraction: 0.72.

Numerical evaluation gives the same dryness fraction of 0.72 and a specific enthalpy of approximately 1,873 kJ/kg. The small difference reflects graphical reading accuracy.

Engineering checks for superheated steam

• confirm whether pressure is absolute or gauge,
• determine actual temperature and degree of superheat,
• check specific enthalpy and entropy,
• determine final condition after expansion or throttling,
• check dryness fraction if the process enters the two-phase region,
• relate the steam condition to the actual process duty.

In pressure reducing stations, downstream steam condition cannot be determined from inlet and outlet pressure alone. Initial dryness and enthalpy may determine whether the outlet is wet, dry saturated or superheated.

Selection of pressure reducing valves, control valves and steam trapping and condensate equipment should therefore take the actual steam condition into account.

Conclusion

Superheated steam is steam heated above the saturation temperature corresponding to its pressure. The additional superheat enthalpy is particularly useful in power-generation applications because it allows steam to expand before entering the wet region.

In heating applications, superheated steam must cool to saturation before condensation begins and evaporation enthalpy is released. A large degree of superheat can therefore reduce heat-transfer rate and create temperature gradients.

A sufficiently large pressure reduction can also generate superheated steam under suitable inlet conditions without a separate superheater. Superheated steam tables and the Mollier chart are key tools for evaluating these changes.

Frequently Asked Questions

What is superheated steam?

Steam at a temperature higher than the saturation temperature corresponding to its pressure.

What is the degree of superheat?

The difference between the actual temperature of superheated steam and the saturation temperature at the same pressure.

Why is superheated steam used in turbines?

It provides an energy margin during expansion and delays entry into the wet-steam region, reducing liquid-droplet formation through turbine stages.

Is superheated steam preferable for heat exchangers?

Not necessarily. It must first cool to saturation temperature, and heat-transfer rates are lower during the superheated stage while temperature gradients may develop.

Can a pressure reducing valve create superheated steam?

Yes. If the enthalpy retained after pressure reduction exceeds the enthalpy of dry saturated steam at the new pressure, the excess appears as superheat.

What does a Mollier chart show?

It plots specific enthalpy against specific entropy and allows steam pressure, temperature, dryness fraction and thermodynamic processes to be evaluated graphically.

What is the difference between turbine expansion and throttling through a pressure reducing valve?

Ideal turbine expansion is treated as constant entropy, while ideal throttling is treated as constant enthalpy.

Does superheated steam have a constant specific heat capacity?

No. Its specific heat capacity varies with pressure and temperature.