Industrial economiser: operation, applications and selection criteria | BOIXAC Technical guide › Energy recovery Industrial economiser: operating principle, applications and selection criteria The economiser is the component that converts the residual heat from boiler exhaust gases into a measurable reduction in fuel consumption. This guide analyses its operation, constructive types, main industrial applications and the technical parameters that determine its selection. BOIXAC Tech SLGuia tècnica industrialLectura: ~9 min Table of contents 1. Definition and function of the industrial economiser 2. Operating principle in an industrial boiler 2.1 Energy flow and positioning 2.2 Heated fluids: water, steam and thermal oil 3. Constructive types of economisers 4. Quantified energy and economic benefits 5. Main industrial applications 6. Selection and design parameters In an industrial boiler, between 10% and 20% of the fuel energy burned is lost as sensible heat in the exhaust gases discharged to atmosphere. The economiser is the device that recovers this energy and transfers it to the boiler feedwater, reducing fuel consumption without modifying the main process. 1. Definition and function of the industrial economiser An industrial economiser is a gas-liquid heat exchanger installed at the outlet of combustion gases from an industrial boiler or furnace. Its function is to transfer the residual enthalpy of these gases to the boiler feedwater, preheating it before it enters the boiler body. The term economiser derives directly from its function: to save fuel. By preheating the feedwater, the energy the boiler must supply to reach the vaporisation or working temperature is reduced, translating directly into lower natural gas, diesel or biomass consumption. 10–20%Energy lost in flue gases without economiser3–8%Typical fuel consumption reduction~1%Saving per 6 °C feedwater temperature rise1–3 yearsTypical payback period 2. Operating principle in an industrial boiler 2.1 Energy flow and positioning In a conventional industrial boiler, gases leave the boiler at temperatures typically between 200 °C and 450 °C. The economiser is installed precisely at this point — at the boiler gas outlet and before the stack — to extract residual enthalpy from these gases and transfer it to the feedwater. Gas inlet200–450 °C→EconomiserGas → liquid heat transfer→Gas outlet120–200 °C↕Water inlet40–80 °C→Preheated water130–220 °C to boiler Lower limit: acid dew point temperature The gas temperature at economiser outlet cannot be reduced indefinitely. In sulphur-containing fuels (diesel, heavy fuel oil, some industrial gases), the minimum temperature is set by the acid dew point temperature (typically 120–150 °C), below which condensed sulphurous acid attacks the metal surfaces of the economiser. For clean natural gas, this limit falls to approximately 55–65 °C. 2.2 Heated fluids: water, steam and thermal oil Although the classic function of the economiser is feedwater preheating, in industrial environments the recovered heat can be transferred to other process fluids: Boiler feedwaterClassic application. Water is preheated from the 40–80 °C typical at deaerator outlet to 130–220 °C, reducing the energy the boiler must supply to generate steam.High-pressure superheated waterIn high-temperature circuits for industrial heating processes, the economiser preheats the high-pressure circuit return water.Thermal oilIn thermal fluid boilers (Therminol, Dowtherm, Marlotherm), the economiser preheats the circuit return oil, reducing consumption by 5–12%.Combustion air (APH)In air pre-heater configuration, exhaust gases heat the combustion air before the burner, improving combustion efficiency and reducing NOₓ emissions. Industrial boiler economiser. Gas-liquid heat exchanger with helical finned tubes, designed to operate in combustion flue gases with inlet temperatures of 250–420 °C. 3. Constructive types of economisers The internal construction of the economiser determines its behaviour against combustion gases and its suitability for each application. Main typeHelical finned tubes Each tube carries a sheet metal fin wound helically. The helical geometry provides greater mechanical robustness and resistance to vibrations induced by combustion gas pulsations. The pitch between turns can be adjusted to accommodate particle-laden gases (fly ash, soot). Preferred application: natural gas, diesel, heavy fuel oil, biomass and industrial waste boilers. Environments with suspended particles in the gases. Compact alternativeContinuous finned tubes Flat perforated sheets through which tubes pass perpendicularly. Allow a higher surface density per unit volume, resulting in more compact equipment for the same recovery duty. Require gases without significant particle content to prevent inter-fin blockage. Preferred application: natural gas boilers in clean environments or with prior gas filtration. Installations where dimensional constraints are critical. BOIXAC gas-liquid heat exchangersCustom-designed and manufactured heat recuperators and economisers for industrial boilers, furnaces and combustion processes. View heat recuperators → 4. Quantified energy and economic benefits Installing a correctly sized economiser in an industrial boiler produces measurable and verifiable improvements in the overall performance of the installation. ⚡Reduction in fuel consumption The standard industry rule of thumb states that for every 6 °C rise in feedwater temperature, boiler fuel consumption decreases by approximately 1%. An economiser that raises the temperature by 60 °C can represent savings of 8–10% of fuel costs. 🌿Reduction in CO₂ emissions Lower fuel consumption translates directly into fewer CO₂ and NOₓ emissions per unit of useful energy produced. In facilities subject to emissions trading (EU ETS), the economiser is one of the interventions with the best investment ratio per tonne of CO₂ saved. 🔩Reduced thermal stress on the boiler Preheated feedwater reduces thermal shock at the boiler inlet, decreasing temperature gradients across the shell and tubes. Contributes to extending boiler service life and reducing preventive maintenance frequency. 💶Typical payback of 1 to 3 years In continuously operated industrial boiler installations (>4,000 h/year), return on investment is typically achieved within 12 to 36 months, depending on fuel price, boiler output and the recoverable temperature differential. 5. Main industrial applications The industrial economiser finds application in any process where a boiler or furnace generates residual combustion gases at a temperature sufficient to make heat recovery economically viable. Food and beverage industrySteam boilers for cooking, sterilisation, pasteurisation and drying processes. The economiser preheats the boiler feedwater, reducing energy consumption in the production process.Chemical and pharmaceutical industryThermal fluid boilers for reactors, distillers and dryers. The economiser preheats the circuit return oil, improving cycle efficiency and reducing natural gas consumption.Paper and textile industryLarge steam boilers for continuous drying processes. … Read more
