Economiser sizing for industrial boiler OEM manufacturers
Technical criteria for thermal sizing, mechanical integration and regulatory documentation for boiler manufacturers incorporating economisers as an integral component of their equipment.
For an industrial boiler OEM manufacturer, the economiser is not an optional accessory: it is a critical component that defines the overall efficiency of the assembly, conditions the structural design of the boiler and, to a large extent, determines the regulatory category of the final equipment. Integrating it correctly demands going well beyond a simple calculation of the heat transfer surface area.
1. Function and positioning of the economiser in the boiler assembly
An economiser is a gas-to-liquid heat exchanger located in the final section of the flue gas circuit, typically between the last boiler pass and the stack. Its function is to recover the enthalpy contained in the outlet gases — which in conventional natural gas boilers ranges between 150 and 280 °C — to preheat feedwater before it enters the steam generator, or to heat a secondary service fluid.
The thermal gain is directly proportional to the temperature drop of the flue gases across the economiser. As an indicative reference, every 20 °C drop in flue gas temperature in a natural gas boiler represents an approximate 1 % improvement in the overall plant efficiency. In boilers burning diesel, heavy fuel oil or biomass the margins may be greater, but the risk of acid condensation on the tubes demands careful analysis of the acid dew point, particularly when the gases contain SO₂.
In flue gases containing sulphur dioxide (SO₂), present in sulphur-bearing fuels such as heavy fuel oil or certain biogases, the acid dew point occurs at temperatures significantly higher than the water dew point. Operating below this point causes condensation of sulphurous and sulphuric acid on tube surfaces, severely accelerating corrosion. Economiser sizing must ensure that tube wall temperatures always remain above this critical threshold, the determination of which depends on the sulphur content of the fuel and the excess air used.
2. Thermal sizing variables
Thermal sizing of an economiser is based on forced convection heat transfer between the flue gases and the fluid to be preheated, separated by the tube wall. The variables the OEM engineer must define to initiate the sizing process are as follows:
| Variable | Description and OEM considerations |
|---|---|
| Flue gas mass flow rate (ṁg) | Expressed in kg/h or Nm³/h. Must correspond to the rated boiler output and, if required, to partial load conditions (50 %, 75 %). Flow variation affects the external convection coefficient on the tubes. |
| Gas inlet temperature (Tg,in) | Temperature of the gases at the economiser inlet, i.e. at the outlet of the last boiler pass. May vary with load conditions. |
| Gas outlet temperature (Tg,out) | Target gas temperature at the economiser outlet. Constrained by the minimum allowable temperature to avoid condensation (acid dew point or water dew point). |
| Fluid flow rate and inlet temperature | Feedwater or service fluid flow rate and its inlet temperature. In steam boilers, feedwater typically arrives between 60 and 105 °C from the deaerator. |
| Fluid outlet temperature (Tf,out) | Target fluid temperature at the outlet. Must maintain an adequate margin below the saturation temperature at working pressure to avoid local boiling in the tubes. |
| Gas composition | Content of CO₂, H₂O, SO₂, NOₓ, ash and particulates. Determines the corrosion risk, the fouling factor and the tube material selection. |
| Allowable pressure drop (ΔP) | Pressure drop limitation on both the gas and fluid sides, imposed by the overall boiler design and available fan capacity. |
Q = U · A · ΔTlm
Where Q is the thermal duty (W), U is the overall heat transfer coefficient (W/m²·K), A is the heat transfer area (m²) and ΔTlm is the log mean temperature difference between the two fluid streams. The value of U results from the detailed calculation of the interior and exterior convective coefficients, the wall resistance and the fouling factors on each side, and is highly dependent on the specific geometry of the economiser.
3. Economiser construction types for OEM integration
| Type | Characteristics for OEM integration | Preferred application |
|---|---|---|
| Helically finned tubes | Maximum surface density per unit volume. High U coefficient with clean gases. Prone to progressive fouling if gases contain fine particles or ash. | Natural gas or LPG boilers. Clean gases without particulates. |
| Continuous (plate) fins | High heat transfer area. Compact design. Air-blowing or sootblower cleaning integrable. | Diesel-fired boilers. Gases with moderate particulate content. |
| Bare tubes (no fins) | Lower surface density but maximum robustness against gases with high abrasive particle content, fly ash or corrosive condensates. Easy mechanical cleaning. | Biomass boilers, heavy fuel oil, process gases with particulates. Gases with elevated SO₂. |
| Condensing economiser | Operates below the water dew point, recovering the latent heat of condensation. Requires corrosion-resistant materials (316L stainless steel) and management of the generated condensate. | High-efficiency natural gas boilers. Projects targeting efficiency ≥ 107 % (LHV basis). |
4. Mechanical integration into the boiler assembly
4.1. Differential thermal expansion
Economiser tubes and the casing undergo thermal expansions of different magnitudes and rates during boiler start-up and shutdown cycles. Inadequate management of thermal stresses can cause fatigue at welded joints or irreversible deformation of the headers. Common solutions include floating header designs, expansion compensators in the connecting pipework and defined maximum heating rates (heat-up rates) in the operating procedures.
4.2. Fluid connections
Water circuit connections must be compatible with the boiler working pressure, which in industrial steam generators may exceed 10, 20 or even 30 bar. Flanges, sealing materials and wall thicknesses must be sized in accordance with the pressure-temperature tables of the applicable reference standards.
4.3. Access for inspection and cleaning
Directive 2014/68/EU and the periodic inspection standards for pressure equipment require that pressurised components be accessible for visual inspection, ultrasonic testing or hydraulic proof testing. The design must include inspection ports, vent flanges and adequate drain points. In economisers located in hot gas passes, access for gas-side cleaning (removal of ash or deposits from fins) is equally critical for maintaining thermal performance over the service life.
4.4. Structural support
The in-service weight of the economiser — including self-weight, internal fluid and, in some designs, accumulated ash deposits — must be carried by the boiler structure or independent supports anchored to the frame. The OEM manufacturer must validate that the load-bearing structure resists both static and dynamic loads (flow-induced vibration, seismic loading if applicable) without transferring them to the pressure circuit headers.
5. Material selection for industrial flue gases
| Material | Temperature range | Limitations and considerations |
|---|---|---|
| Carbon steel S235/P235GH | Up to ~450 °C wall temperature | Standard material for clean gases and moderate temperatures. Low cost. Not suitable for gases with significant SO₂ at temperatures near the acid dew point. |
| Low-alloy steel 16Mo3 | Up to ~530 °C wall temperature | For high-pressure steam boilers where wall temperature exceeds the capacity of conventional carbon steel. Improved creep resistance. |
| Stainless steel AISI 316L | Up to ~550 °C wall temperature | Resistance to corrosion by acid condensates (acid dew point). Indispensable in condensing economisers and in gases with SO₂. Higher cost; thermal and economic justification required. |
| Stainless steel AISI 310S / Alloy 800 | Up to ~1 050 °C wall temperature | For process gases at very high temperatures. Reserved for specific process heat recovery applications (furnaces, gas turbines). |
| Copper / Brass | Up to ~200 °C wall temperature | High thermal conductivity. Limited to non-corrosive gases and fluids. Common in low-pressure heating boilers. |
6. Regulatory requirements — economiser as part of a PED assembly
When the economiser is integrated into an industrial boiler as a component of the final assembly — i.e. when the OEM manufacturer places the boiler on the market as a complete unit including the economiser — the equipment is subject to Directive 2014/68/EU (Pressure Equipment Directive, PED) as an assembly within the meaning of Article 2.5 of the Directive.
This means the OEM manufacturer is responsible for:
- Determining the risk category of the assembly by applying the tables in Annex II of the Directive to the parameters of both the economiser (PS, volume, fluid group) and the boiler, retaining the highest resulting category.
- Ensuring that the economiser supplied by a third party has its own EU Declaration of Conformity and CE marking, or incorporating the economiser's sizing and certification within its own technical file for the assembly.
- Verifying that the junction between the economiser and the boiler body meets the essential safety requirements of Annex I of the Directive, particularly with regard to structural welds and required non-destructive testing.
- Including in the operational documentation (instruction manual) the safe operating specifications for the economiser, recommended inspection intervals and criteria for decommissioning.
- If the assembly category requires the involvement of a Notified Body (NB), coordinating its participation in the design and/or production phases corresponding to the selected conformity assessment module.
If the OEM manufacturer procures the economiser from an external supplier and integrates it into the boiler assembly, it assumes responsibility for verifying that the component complies with the Directive's requirements and is suitable for the service conditions of the final assembly. The presence of CE marking on the economiser does not exempt the OEM manufacturer from its responsibility as manufacturer of the assembly. Any modification to the original economiser design to adapt it to the boiler may invalidate the component's prior conformity.
7. Technical documentation to be provided by the economiser supplier to the OEM
- Thermal and hydraulic design data sheet: thermal duty, flow rates, inlet and outlet temperatures, pressure drops on both sides, overall U coefficient and heat transfer area.
- Material specification: standardised designations for tube, header, shell and gasket materials; inspection certificates 3.1 or 3.2 in accordance with EN 10204 for pressure-bearing materials.
- Welding documentation: welding procedure qualifications EN ISO 15614 and welder qualifications EN ISO 9606 for pressure joints.
- Non-destructive examination (NDE) results performed during manufacture: visual inspection, radiography or ultrasonic examination of welds, hydrostatic pressure test.
- Mechanical strength calculation: justification of wall thicknesses, joints and headers in accordance with the applicable harmonised standard.
- EU Declaration of Conformity and CE marking if the economiser is placed on the market as an independent item of equipment.
- Installation, operation and maintenance manual, including the allowable operating limit conditions (maximum PS, minimum and maximum TS, minimum flow rate to prevent local boiling).
8. In-service performance indicators and replacement criteria
A correctly sized and installed economiser undergoes progressive performance degradation over its service life due to the build-up of calcareous scale inside the tubes (fluid side) and ash or soot deposits on the outside (gas side). The plant maintenance engineer — or the OEM manufacturer's service technician — can monitor economiser performance through the flue gas outlet temperature: a progressive increase in this temperature relative to the design value, under comparable steady-state conditions, indicates a reduction in heat transfer capacity and the need for intervention.
As an indicative criterion, a sustained increase in flue gas outlet temperature exceeding 20–30 °C above the original design value justifies a detailed inspection of the heat transfer surfaces. The decision to clean, repair or replace the economiser is a technical decision that must be taken by a specialist, taking into account the actual condition of the equipment, spare parts availability and the opportunity cost of the outage.