Superheaters: Heating Steam Beyond Its Boiling Point
A superheater takes dry saturated steam from the drum and heats it further, above its saturation temperature. Superheated steam is what turbines need, and producing it safely depends on one rule: the superheater tubes must always have steam flowing through them while hot gas flows outside.
You'll learn
- Explain what a superheater does and where it sits
- Calculate the heat a superheater adds
- Describe how steam temperature is controlled
- Understand why superheaters fail and how to protect them
Before this: Wet, dry and superheated steam, Shell, drums, furnace and tubes
What a superheater is for
Steam leaving a boiler drum is saturated: it is at the boiling temperature for its pressure, and the slightest cooling makes it condense. A superheater is a bank of tubes placed in the hot gas path, through which that steam passes after the drum. It comes out hotter than its saturation temperature, with a margin before any of it can condense.
Turbines need that margin. Wet steam erodes turbine blades, and superheat lets steam expand through a turbine and give up work without turning to water too early. Most process heating does not need superheat, which is why process boilers usually have none.
How much heat a superheater adds
- 20,000 kg/h of steam. Saturation at 40 kg/cm²(g) is 250.7 °C with = 2,800.8 kJ/kg.
- Enthalpy at 400 °C: 3,214.0 kJ/kg. Degree of superheat: 149 °C.
- Heat added:
Superheaters are described by where they sit. A convective superheater is in the gas path after the furnace and picks up heat mainly by contact with the gas; its outlet temperature rises with load. A radiant superheater sees the flame directly; its outlet temperature falls as load rises. Large boilers combine the two to flatten the curve.
Temperature control and protection
Steam temperature is usually held by an attemperator, also called a desuperheater: a spray of clean water into the steam between superheater stages. The spray evaporates, absorbing heat and lowering the temperature. The water must be very pure, because everything dissolved in it is deposited in the superheater or carried to the turbine.
A superheater tube is cooled only by the steam inside it, which carries heat away far less effectively than boiling water does. Starve it of steam flow while hot gas passes outside and the metal quickly runs past its limit. That happens most often at start-up, when the boiler is firing but no steam is yet being drawn. The standard protection is to open the superheater drains and start-up vent so that steam flows through the tubes from the moment it forms, and to keep firing rates low until the flow is established.
Superheater tubes also run in the hottest metal temperatures in the boiler, so they are often made of creep-resistant alloy steels, and their life is limited by creep: slow deformation under stress at high temperature. Running even modestly above the design steam temperature for long periods consumes that life much faster.
Key takeaways
- A superheater heats saturated steam above its boiling point, mainly for turbines.
- Superheater duty = steam flow × ().
- Attemperators spray pure water to control steam temperature.
- Superheater tubes are cooled only by steam flow: keep it flowing, especially at start-up.