Terms Used in Boiler House Efficiency

In industrial steam generation, 65% of a boiler’s lifecycle expense is attributed to fuel. Hence, it makes it essential for plants worldwide to focus on improving and sustaining the efficiency of their boilers. This glossary is designed to act as a quick guide for identifying and understanding the essential terminology and technical definitions related to the efficiency of industrial boiler systems.

Methods of calculating boiler efficiency

  1. Direct Efficiency:
    A calculation of boiler performance based on the ratio of heat output (steam mass multiplied by enthalpy) to heat input (fuel mass multiplied by Gross Calorific Value). It provides an average performance snapshot but cannot provide instantaneous analysis of specific losses.
  2. Indirect Efficiency:
    This method is also known as the heat loss method and calculated as 100 – (L1 + L2 + L3 + L4 + L5 + L6 + L7), where losses include dry flue gas (L1), water vapor from (L2), moisture in fuel (L3), moisture in air (L4), unburnt carbon (L5/L6), and radiation/unaccounted losses (L7). Following the BS F45 standard, this allows for real-time spot analysis of energy losses.

Terms used in boiler combustion

Stoichiometric (Theoretical) Air: The exact amount of air required for the complete combustion of a specific unit of fuel (e.g., ~14 kg of air for 1 kg of furnace oil).

Excess Air: Additional air supplied above the theoretical limit to ensure combustion stability. While necessary, high excess air (e.g., maintaining 8% O2 instead of 3.5%) draws more electrical power and carries away significant furnace heat.

Oxygen Trimming: An automated mechanism that senses oxygen at the furnace outlet and modulates the FD fan to maintain O2 within an efficient band i.e typically 1.5%–2% for gas and 3% for furnace oil.

The Three Ts of Combustion
The 3 core principles for complete combustion are:

  1. Temperature (attaining ignition),
  2. Time (sufficient residence time for stabilization)
  3. Turbulence (using secondary or tertiary air to mix fuel and oxygen).

Fluidised Bed Combustion (FBC) Terms

Fluidisation: The process of lifting fuel and sand particles using air pressure so the mixture behaves like a fluid. This facilitates a two-stage combustion where fixed carbon burns near the nozzles and volatile matter burns at the top of the bed.

Balanced Draft: A safety and efficiency state where the Forced Draft (FD) and Induced Draft (ID) fans are synchronized to maintain a near-zero or slightly negative pressure (ideally -0.5 mm WC) at the furnace null point.

Bed Height: A critical operating parameter for FBC boilers, typically maintained between 350 mm and 600 mm WC to sustain fluidization.

Terms used in heat loss and boiler water management

Stack Loss (Dry Flue Gas Loss): The loss of heat through hot gases exiting the chimney. It is directly proportional to the Δ T, the difference between stack temperature and ambient temperature.

TDS (Total Dissolved Solids): The concentration of minerals in boiler water. For saturated boilers, the TDS should be maintained at 3,500 PPM, manual control often causes wide variances (1500–3000 PPM), whereas Auto-Blowdown ensures a stable, higher TDS that minimizes heat and water waste.

Three-Element Drum Level Control: A control logic using three signals i.e. drum level, steam flow, and feedwater flow to provide precise water management and reduce the shrinking and swelling effects during boiler load fluctuations.

Terms used in boiler fuel benchmarking

Gross Calorific Value (GCV): The total heat released by fuel, considering the latent heat of moisture as a loss.

Net Calorific Value (NCV): The heat available excluding the energy required to vaporise moisture.

Steam-to-Fuel Ratio: A benchmark of how many kilograms of steam are produced per kilogram of fuel. At 100°C feedwater, Indonesian coal (5800 GCV) should ideally achieve a ratio of 7.2, while Indian coal (4600 GCV) averages 5.9–6.0.