Reheater

A reheater is a boiler heat-transfer section that heats steam after it has expanded through the high-pressure turbine and before it enters the intermediate-pressure or low-pressure turbine stages. Reheat improves cycle efficiency and reduces moisture in later turbine stages.

Reheaters are usually arranged in the convective pass, pendant section, horizontal pass, or backpass depending on boiler design. They operate at high steam temperature but lower pressure than the primary superheater. Because reheat steam flow is tied to turbine operation, reheater protection during start-up, shutdown, bypass, and low-load operation is important.

Operating role

The reheater must deliver target outlet temperature without overheating tubes. Control methods include burner tilt, gas recirculation, flue-gas dampers, spray attemperation in some designs, and sootblowing strategy. Fouling changes heat absorption and can shift temperature control burden to other parts of the boiler.

Ash deposits on reheater tubes reduce heat transfer, increase gas-side pressure drop, disturb gas distribution, and may create local tube metal temperature problems. Deposits can also trap corrosive salts, especially in units firing biomass, waste-derived fuels, or high-chlorine coals.

Failure modes

Common problems include ash fouling, slag bridging, tube erosion from sootblowers or fly ash, creep damage, oxide growth, thermal fatigue, corrosion under deposits, and overheating during low steam flow. A reheater leak is serious because it can force boiler shutdown and can damage neighbouring tubes.

Operators monitor reheat outlet temperature, spray flows where fitted, tube metal temperatures, draft loss, sootblower effectiveness, and gas temperature leaving upstream sections. A rising need for temperature control can indicate heat-transfer surfaces are moving away from design cleanliness.

Acoustic cleaning context

Sonic horns can support reheater cleanliness when deposits are dry and friable, especially in convective passes where frequent low-energy cleaning prevents ash from sintering. They do not replace sootblowers where slag or hard bonded deposits form. A good design considers horn frequency, line of sight, tube spacing, casing noise, access, and the path for dislodged ash to leave the section.

Operating and maintenance context

Reheater performance is tracked through hot reheat temperature, spray or burner bias, gas-side pressure drop, tube metal temperature, and unit load. In coal and biomass boilers, reheaters sit in ash-laden gas and can experience both fouling and high-temperature corrosion. In waste-fired units, chlorine and alkali compounds make deposit chemistry especially important. A clean reheater transfers heat predictably; a fouled one can shift heat absorption to other banks and force derating.

Maintenance checks include tube alignment, spacer condition, oxide scale, creep indications, sootblower coverage, erosion at bends, support damage, and deposit texture. Acoustic cleaning can help where deposits are dry and friable, especially on upper surfaces and between pendant elements. It is less useful for molten slag or bonded corrosion products. Cleaning intensity should be balanced against tube vibration, released ash load, and downstream hopper capacity.

Measurement context

Reheater fouling is visible in hot reheat temperature control, gas-side pressure drop, spray flow if fitted elsewhere in the cycle, and tube metal temperature margins. A deposit that insulates one bank can push heat absorption downstream and create unexpected temperature patterns. Operators therefore compare reheater trends with superheater, economiser, and air-heater behaviour.

Acoustic cleaning should be checked for tube vibration, released ash path, and whether deposits return more slowly after installation. The best outcome is stable heat absorption with less aggressive sootblowing, not simply a visibly cleaner tube after a short test.

Related terms

• Boiler
• Superheater
• Convective pass / backpass
• Sonic horn