Economizers and condensers

 

As economizers and condensers we understand the heat exchange system that allows to recover the surplus energy of a process to reuse it, achieving economic and environmental savings. That is, obtaining a more efficient and sustainable installation.

Within the economizers we find many different formats depending on the singularities of each project, if we want to recover the energy surplus from a flow of air, extraction gas, combustion fumes, water, superheated water, vapors or oils thermal among others. There are hundreds of options.

In this sense, the technical office of Boixac Tech studies in detail the characteristics of the project to select the optimal solution, both in terms of performance and durability. Providing solutions with diverse materials such as steel, stainless steel, titanium, copper, cupro-nickel, aluminum or manganese aluminum, among others. Thus responding to the most demanding conditions such as dusty currents, with high fouling coefficients or sticky, viscous or sediment-bearing fluids.

Whether it is plate or pillow-plate heat exchangers, cross-flow heat exchangers, tube and fin heat exchangers, finned heat exchangers, concentric tube heat exchangers or double tube heat exchangers, at Boixac we have the solution.

Economizer sizing for OEM industrial boilers

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Economiser sizing for industrial boiler OEM manufacturers | BOIXAC Technical blog · OEM integration › Industrial economisers 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. BOIXAC · Technical OfficeUpdated: 2026Reading: ~10 min Note on the scope of this article This text is exclusively technical and informational in nature. It does not replace the specific analysis of a concrete project by qualified engineers. The values and ranges given are indicative; the definitive sizing of any economiser requires a detailed study of the actual process conditions, the regulatory classification of the equipment and, where applicable, the intervention of a Notified Body. BOIXAC assumes no liability for decisions taken on the basis of this article. 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. Key term: acid dew point 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. Fundamental sizing equation 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 … Read more