Boiler Auxiliaries
Core concepts
Auxiliaries include feedwater pumps, air preheaters, economizers, superheaters, sootblowers, draft fans, safety valves, and blowdown equipment. Understand the purpose and placement of each: economizers recover heat from flue gas to feedwater; air preheaters recover heat to combustion air; superheaters raise steam temperature above saturation. Know the difference between attemperation (desuperheating) and superheat control. Understand blowdown (bottom and surface/continuous) and its role in chemistry control.
Traps to watch
- Confusing economizer and air preheater functions.
- Assuming superheaters can operate dry (they need steam flow for cooling).
- Mixing bottom blowdown (removes sludge/solids) with surface blowdown (controls dissolved solids, TDS).
- Forgetting that sootblowers should not be operated during unstable firing.
- Overlooking the need for continuous blowdown to maintain TDS limits.
Worked example / rule of thumb
An economizer raising feedwater from 100 °C to 140 °C at 10 kg/s with cp = 4.18 kJ/(kg·°C) recovers Q = 10 × 4.18 × 40 = 1,672 kW. Rule of thumb: each 6 °C rise in feedwater temperature improves boiler efficiency by about 1%.