Heat transfer

Thermodynamics is the branch of physics that studies thermal energy and is based on the so-called laws of thermodynamics that describe its properties and how energy is transferred and converted into other forms.

The laws of thermodynamics are as follows:

  • First Law of Thermodynamics (Principle of Conservation of Energy): The total energy of a physical system is constant, and the difference between the initial and final energy is equal to the amount of energy that is transferred or which becomes work.
  • Second Law of Thermodynamics (Principle of Entropy): In a thermodynamic transaction, the total amount of entropy of a system and its surroundings always increases or remains constant.
  • Third Law of Thermodynamics (Zero Entropy Principle): At absolute zero temperature, the entropy of a perfect pure system is zero.

Some of the main factors we have to keep in mind when we talk about thermodynamics are temperature, pressure, amount of substance and entropy.

In this section we talk about thermal equilibrium, temperature, the kinetic theory of gases, heat capacity, sensible heat, latent heat, phase changes, the heat capacity of a solid, the heat capacity of a gas, adiabatic systems, types of heat exchange, convection, conduction, radiation, psychrometric diagram, etc.

How to select an industrial heat exchanger

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How to select an industrial heat exchanger: the 7 technical criteria | BOIXAC Technical Guide › Industrial Heat Exchangers How to select an industrialheat exchanger: the 7 criteria Selecting a heat exchanger is not a catalogue choice. It depends on seven interdependent technical criteria —and many other variables that no guide can fully capture. Field experience and deep knowledge of real equipment behaviour are just as decisive as any formula. BOIXAC Technical Office 21 May 2026 Reading: ~8 min Indicative technical content — please read before continuing This guide describes some of the criteria involved in selecting an industrial heat exchanger. It is not a complete guide, nor can it be: there are process variables, installation conditions and accumulated experience factors that cannot be captured in any document. Any technical decision regarding real equipment requires a specific analysis of the particular process conditions. When someone asks “which heat exchanger do I need?”, the correct answer is never a catalogue model. Nor is it a list of seven criteria. Behind every industrial process there are variables that appear on no datasheet: the actual behaviour of a fluid under variable process conditions, the experience accumulated from similar applications, the subtleties that determine whether a solution will work well in the long run. This guide describes the documentable criteria. The rest comes from deep sector knowledge. Contents of this guide Criterion 1 — Characterise the process fluid Criterion 2 — Temperature conditions Criterion 3 — Required thermal power Criterion 4 — Allowable pressure drop Criterion 5 — Construction material Criterion 6 — Cleaning and maintenance Criterion 7 — Applicable PED regulations Indicative power calculator (Criterion 3) The 7 selection criteria 01 Characterise the process fluid The starting point is the precise characterisation of the two fluids that will flow through the equipment —the hot fluid and the cold fluid— under real operating conditions, not standard or laboratory conditions. For each fluid it is necessary to determine: type (gas, liquid, saturated steam, two-phase fluid), full chemical composition, pH, suspended or fibrous solids content, dynamic viscosity and thermophysical properties —density, specific heat and thermal conductivity— at the actual working temperature. When the fluid is a mixture, the mixture properties do not always coincide with those of any of its components. Corrosive, viscous or particle-laden fluids directly condition the admissible construction types and materials. The compatibility of a fluid with a given material depends on the exact composition, temperature and concentration: what is suitable in one environment may be completely unsuitable in a superficially similar one. A viscous fluid affects the flow regime and therefore the achievable heat transfer coefficient. Why it is not trivial: the thermophysical properties of a fluid change significantly with temperature. Air at 200°C has a density of 0.746 kg/m3 compared to 1.20 kg/m3 at room temperature. Using 20°C properties for a high-temperature process introduces significant deviations in basic calculations —greater the larger the temperature difference. Document: fluid technical datasheet and safety data sheet Common error: 20°C properties used for high-temperature processes 02 Define the temperature conditions The inlet and outlet temperatures of each fluid (T1 and T2) must be established precisely. From these, the log mean temperature difference (LMTD) is derived, which is the driving force for heat transfer and the basis of the design equation Q = U · A · LMTD. Checking the limits is as important as the central value. Maximum temperatures must be compatible with the structural material and fluid conditions; minimum temperatures, with the risk of unwanted condensation or acid dewpoint in combustion gases. The temperature at which combustion gases can condense acids in the exchanger varies depending on the fuel, air excess and other process conditions —and is one of the parameters that must be evaluated case by case. It should be noted that working with condensing gases —including gases from the combustion of natural gas or other fuels such as diesel or fuel oil— is perfectly viable technically when the equipment is designed for this condition. In these cases, the gas outlet temperature may be below the dewpoint, and the heat exchanger must be designed to handle this. Why the order of criteria matters: temperatures define the fluid properties used in all subsequent calculations. Defining the temperature first and then looking up properties at that temperature is the only rigorous order. Key data: inlet T / outlet T for each fluid Combustion gases: assess acid condensation risk (depends on fuel and conditions) Thermal oil degradation T: always consult the datasheet of the specific fluid 03 Determine the required thermal power The thermal power Q (kW) is the central sizing parameter. It is obtained by applying the thermodynamic formulas corresponding to the fluid type, using properties interpolated at the actual working temperature — not at ambient temperature. Sensible fluid (liquids, gases) Q = ṁ · cp(Tm) · ΔT ṁ Mass flow rate [kg/s]. If the flow is volumetric: ṁ = ρ(T1) · Q̇ — where ρ is evaluated at T1, not at T_m cp(Tm) Specific heat at mean temperature Tm = (T1+T2)/2 [kJ/(kg·K)] ΔT |T1 − T2| [K] Saturated steam (full condensation) Q = ṁ · hfg(Tsat) hfg Latent heat of vaporisation [kJ/kg], from IAPWS-IF97 tables. At 1 bar: 2,257 kJ/kg. At 4 bar: 2,134 kJ/kg. At 8 bar: 2,048 kJ/kg. Humid air (sensible + latent heat) Q = ṁas · |h1 − h2| ṁas Dry air flow rate = ṁmixture/(1+W1), where W1 is the inlet specific humidity h = 1.006·T + W·(2501 + 1.86·T) [kJ/kgda] — mixture enthalpy The calculated Q value is a starting point for the technical discussion. In practice, equipment selection takes into account the progressive degradation of heat transfer over time due to fouling. How much margin is appropriate in each case depends on the fluid, operating conditions, expected maintenance frequency and knowledge of the specific application. Why the formula is not enough: Q determines the order of magnitude of the required exchange surface, but the overall heat transfer coefficient U —on which the actual surface depends— varies enormously … Read more

Heat exchangers for corrosive gases: degradation mechanisms, materials and applicable standards

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Heat exchangers for corrosive gases: materials, degradation mechanisms and standards | BOIXAC Technical guide › Materials › Corrosive process gases Heat exchangers for corrosive gases: degradation mechanisms, materials and applicable standards The selection of materials for finned-tube heat exchangers and gas-gas recuperators in the presence of corrosive industrial gases —H₂S, chlorine, HCl, SO₂, ammonia or HF— is one of the technical decisions with the greatest impact on equipment reliability and service life. BOIXAC Tech SLUpdated: May 2026Reading time: ~9 min Technical notice and limitation of liability This article is intended exclusively for informational purposes. The compatibility of materials with corrosive gases depends on multiple variables —concentration, temperature, pressure, presence of moisture, fluid velocity— that cannot be assessed generically. The material indications contained in this article are general guidance based on public technical literature and do not constitute engineering recommendations for specific applications. The definitive selection of materials for a real piece of equipment requires the assessment of a qualified materials or corrosion engineer. BOIXAC Tech SL assumes no liability arising from the use of this information for technical decisions on real equipment. In the chemical, petrochemical, gas treatment and fertiliser production industries, heat exchangers frequently operate in contact with gaseous streams containing substances that are aggressive towards conventional metallic materials. An error in the selection of the material for the tubes, fins or headers can manifest itself months or years after commissioning, with consequences ranging from loss of performance to structural failure of the equipment. Understanding the degradation mechanisms specific to each gas is the starting point of any rigorous selection process. 1. Degradation mechanisms: the necessary technical vocabulary The degradation mechanisms of metallic materials in the presence of corrosive gases are not limited to general corrosion through loss of thickness. In many industrial cases, the dominant mechanism is localised or mechanical-chemical in nature, with kinetics that can be difficult to detect before the damage is significant. SSC Sulfide Stress Cracking Cracking under stress in high-strength steels induced by atomic hydrogen in the presence of H₂S. Occurs without visible general corrosion. Especially severe in steels with hardness >22 HRC. NACE MR0175 / ISO 15156 HIC Hydrogen Induced Cracking Internal cracking in carbon steels caused by hydrogen pressure at material defects (MnS inclusions). Visible in cross-section as parallel laminations. NACE MR0175 / ISO 15156; API 571 SCC Stress Corrosion Cracking Cracking under stress in the presence of a specific corrosive environment. In austenitic stainless steels: chlorides at elevated temperatures. In brasses and Cu-Ni: ammonia with moisture. ASTM G36; ISO 7539; API 571 HTHA High Temp. Hydrogen Attack Atomic hydrogen diffuses into the steel at high temperature and reacts with the carbon, forming methane. Causes loss of strength and intergranular cracking. Specific to H₂ at elevated T. API 941 (Nelson curves) Pitting Pitting corrosion Localised corrosion that generates cavities or pits on the material surface. Characteristic of austenitic stainless steels in the presence of chlorides or halogens. Often initiates at surface inclusions. ASTM G48; EN ISO 11463 Galvanic Galvanic corrosion Acceleration of corrosion of the less noble metal in an electrochemical pair in the presence of an electrolyte. Critical at tube-fin joints with dissimilar materials (e.g. SS + Al) in humid environments. ASTM G71; ISO 7441 2. Most common corrosive gases in industrial process Hydrogen sulphide H₂S Industries: Petroleum refining, natural gas treatment, sulphuric acid production, wastewater Mechanisms: SSC, HIC, SOHIC, uniform corrosion in the presence of water NACE risk threshold: presence of H₂S with moisture; NACE MR0175 defines specific sour service conditions Indicative materials: SS 316L, Duplex 2205, Inconel 625, Titanium Gr.2. Hardness restrictions for carbon and low-alloy steels. Key standard: NACE MR0175 / ISO 15156; NACE MR0103 (refineries); API 571 Chlorine and hydrogen chloride Cl₂ / HCl Industries: Chlorine chemistry, PVC production, organic synthesis, metal pickling Mechanisms: Severe uniform corrosion in standard austenitic stainless steels; pitting and SCC in the presence of moisture; accelerated galvanic corrosion if in contact with aluminium Materials for tubes and fins: Titanium Gr.2 for wet Cl₂ and dilute HCl; high corrosion-resistance alloys for concentrated HCl. Standard austenitic stainless steels are not suitable for HCl service. Fins: Aluminium is incompatible with HCl environments. Alternative: stainless steel or titanium fins depending on concentration. Consult the Technical Office for services with high concentrations of HCl or Cl₂. Sulphur dioxide and trioxide SO₂ / SO₃ Industries: Combustion gases (fuel oil, diesel, coal), sulphuric acid production, metal sulphide smelting Mechanisms: Acid dew point corrosion (condensation of H₂SO₄); uniform corrosion at temperatures above the dew point is generally manageable Acid dew point: Variable depending on SO₃ and water vapour concentration; critical in the cold zone of combustion gas recuperators and economisers Indicative materials: SS 316L for moderate zones; SS 310S or specific alloys for high-corrosivity zones; avoid carbon steel in the possible condensation zone Ammonia NH₃ Industries: Fertiliser production (Haber-Bosch synthesis), industrial refrigeration, flue gas treatment (SCR) Mechanisms: Attack on copper and copper alloys (formation of soluble amino-cuprates); SCC in carbon and low-alloy steels in the presence of NH₃ and moisture Indicative materials: Austenitic stainless steels (316L, 304L); carbon steel for dry NH₃ at moderate ambient temperature. Avoid brasses, bronzes and Monel in the presence of NH₃ with moisture. Note: In NH₃ refrigeration systems, gaskets and seals are critical tightness points. Hydrofluoric acid HF Industries: Refinery alkylation (HF process), fluoropolymer production, stainless steel pickling Mechanisms: Severe corrosion in most metals; carbon steel forms a relatively protective fluoride layer in anhydrous or concentrated HF; titanium reacts violently with HF (not suitable) The NACE MR0175 / ISO 15156 standard: the reference standard for sour service The NACE MR0175 / ISO 15156 standard —Petroleum and natural gas industries — Materials for use in H₂S-containing environments in oil and gas production— is the reference technical document for material selection in H₂S environments. It defines the concept of «sour service», establishes the H₂S partial pressure thresholds that activate its requirements, and specifies the chemical composition, heat treatment and qualification testing conditions that materials must meet. Its application is not limited to oil and gas production: … Read more

Heat recovery in hydrogen production: heat exchangers in SMR, electrolysis and green H₂

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Heat recovery in hydrogen production: heat exchangers and condensing economisers | BOIXAC Technical guide › Energy › Industrial hydrogen Heat recovery in hydrogen production: heat exchangers, recuperators and condensing economisers Finned-tube heat exchangers and gas-gas recuperators are key equipment in the energy balance of hydrogen production plants, both in reforming processes and electrolysis installations. BOIXAC Tech SLUpdated: May 2026Reading time: ~9 min Technical notice and limitation of liability This article is intended exclusively for informational purposes. The temperature, pressure and efficiency ranges given are reference values from public technical literature; the actual conditions of each installation may differ. Normative references are based on the texts in force at the date of writing. BOIXAC does not act as a regulatory certification entity. Engineering technical decisions are the responsibility of the engineer in charge of the project. Hydrogen production —both by steam methane reforming and by electrolysis using renewable energy— generates high-temperature heat streams that represent real energy recovery opportunities. Finned-tube heat exchangers, gas-gas recuperators and condensing economisers are the reference technical solutions for harnessing these streams under the process and regulatory conditions specific to the sector. 1. Heat recovery opportunities in hydrogen plants In a hydrogen production plant, the heat streams available for recovery appear at several points in the process. Identifying and harnessing these streams —by means of finned-tube heat exchangers or gas-gas recuperators conceived for the specific conditions of each point— is one of the main vectors for improving the plant’s overall energy performance. Steam reforming (SMR / ATR) Flue gases: high-temperature furnace combustion gases. The principal opportunity for conventional recuperators and condensing economisers. Process gas cooling: process gases in the shift and purification stages. Moderate temperature; finned-tube heat exchangers. Acid dew point: decisive for the recovery strategy in the cold zone of the equipment. PEM and alkaline electrolysis (BOP) Stack cooling: the electrolyser generates heat that must be evacuated. Finned-tube heat exchangers in the cooling circuit. Drying of the produced H₂: the gas leaves saturated with vapour; a condenser or heat exchanger lowers the temperature to remove the water. Inter-stage compression cooling: H₂ compression generates heat between stages. Finned-tube intercoolers. Compression and conditioning Intercoolers: between H₂ compression stages up to storage or distribution pressure. Pressurised H₂ service; PED Group 1 regulatory requirements. Aftercoolers: final cooling of the compressed H₂ before storage. Drying and purification Gas drying: condensation of the water vapour in the produced H₂. Moderate temperature; materials for H₂ service. PSA feed cooler: cooling of the H₂ before the pressure swing adsorption purification unit. 2. The flue gas recuperator: the equipment with the greatest efficiency impact In reforming installations, the recuperator or economiser that cools the furnace combustion gases —preheating the combustion air or process water, or generating steam— is usually the heat transfer equipment with the greatest impact on the plant’s overall energy performance. The way this equipment is conceived with respect to the acid dew point of the combustion gas determines how much energy can be recovered. Conventional recuperator vs condensing economiser: the key design decision A conventional recuperator operates with the wall temperature above the acid dew point, recovering only the sensible heat of the gases. A condensing economiser operates deliberately below the dew point, also recovering the latent heat of the water vapour —which in natural gas combustion gases represents a significant fraction of the total available energy. The result is a lower gas outlet temperature and a higher overall thermal efficiency. BOIXAC can supply both solutions; the choice between them depends on the combustion gas composition, the temperature of the available cooling fluid and the project’s efficiency objectives. 3. Materials for heat exchangers in hydrogen service Hydrogen presents attack mechanisms on metallic materials that do not exist with other conventional fluids. Its high diffusivity in metals activates specific phenomena that must be considered when conceiving heat exchangers for this service. HTHA (High Temperature Hydrogen Attack): at elevated temperatures and H₂ partial pressures, atomic hydrogen diffuses into the steel and reacts with the carbon in the material to form methane, causing loss of strength and intergranular cracking. The reference standard is API 941, which defines the so-called Nelson curves: for each type of steel, they establish the maximum allowable combination of temperature and H₂ partial pressure in continuous service. Low-alloy Cr-Mo steels withstand more severe conditions than carbon steels. Hydrogen embrittlement (HE): at ambient or low temperatures, absorbed hydrogen can reduce the ductility of certain high-strength steels, increasing the risk of fracture under stress. Particularly relevant in high-pressure H₂ equipment. It is controlled through the selection of materials with controlled hardness. PED Group 1 classification: hydrogen is flammable and is classified as a Group 1 fluid under the PED. Heat exchangers with pressurised H₂ typically fall into high PED categories with involvement of a Notified Body. The requirements for non-destructive examination of welds are also stricter than in conventional services. Nelson curves (API 941): a non-negotiable limit in high-temperature H₂ service The API 941 standard establishes, for each type of steel, the maximum combination of service temperature and H₂ partial pressure above which the material is exposed to the risk of HTHA. Operating above these limits is one of the documented causes of catastrophic failures in process installations. In heat exchangers in high-temperature H₂ service, verification against the Nelson curves is a non-negotiable design requirement, and demands knowledge of the equipment’s maximum wall temperature —not only the mean fluid temperature— under the most unfavourable operating conditions. 4. Typical equipment configurations and materials Application Equipment configuration Tube material Key regulatory consideration Flue gas recuperator (conventional) Finned tubes, gas-gas or gas-liquid 316L / 321 stainless steel in the cold zone; carbon steel in the hot zone outside the condensation risk area Minimum wall temperature above the acid dew point. PED according to pressure and fluid. Condensing economiser Finned tubes, gas-liquid 316L stainless steel throughout the condensation zone Materials for contact with acid condensate. Drainage geometry. PED applicable. Process gas cooling (shift, purification) Finned tubes, gas-liquid 316L stainless steel; Duplex 2205 if H₂S present Verify Nelson … Read more

Heat exchangers in refineries and petrochemicals: ASME BPVC Section VIII and PED 2014/68/EU

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Heat exchangers in refineries and petrochemicals: PED, ASME and API standards | BOIXAC Technical guide › Standards › Refinery and petrochemicals Heat exchangers in refineries and petrochemicals: PED, ASME BPVC and API standards The regulatory framework applicable to finned-tube heat exchangers, flue gas recuperators and condensing economisers in refinery and petrochemical installations. BOIXAC Tech SL Updated: May 2026 Reading time: ~9 min Technical notice and limitation of liability This article is intended exclusively for informational purposes. Normative references are based on texts published and in force at the date of writing and may have been subsequently amended. Determining the applicable code for a specific piece of equipment and the certification process are the responsibility of the engineer in charge of the project and, where applicable, the relevant inspection body. BOIXAC does not act as a notified body or regulatory certification entity. In refineries, petrochemical plants and other process installations, finned-tube heat exchangers, gas recuperators and economisers operate under demanding conditions and are subject to a specific regulatory framework. Understanding how the European Pressure Equipment Directive, the ASME code and sector specifications interact allows the correct conception of each piece of equipment to be established from the basic engineering phase. 1. Equipment operating in this environment In refining and petrochemical applications, finned-tube heat exchangers and gas recuperators fulfil essential functions in the thermal balance management of installations. The most common applications are heat recovery from combustion gases —where hot gases from the furnace or reformer transfer heat to combustion air or process water— and the cooling or heating of gaseous process streams. In these services, the conception of the equipment with respect to the acid dew point of the gas is one of the most impactful technical decisions. Operating above the dew point limits recovery to sensible heat; conceiving the unit as a condensing economiser —designed to operate deliberately below the dew point— also recovers the latent heat of the water vapour present in the gases, achieving a higher overall thermal efficiency. Both strategies are technically valid and applicable in process installations. Heat recovery in process installations: BOIXAC’s segment BOIXAC works in the conception and supply of finned-tube heat exchangers, gas-gas recuperators and economisers —including condensing economisers— for industrial installations in sectors such as refining, petrochemicals, hydrogen production and other high-temperature processes. For each project, the BOIXAC technical team works with the actual process conditions, fluids, temperatures and regulatory requirements to identify the appropriate solution. 2. PED 2014/68/EU: the mandatory framework in Europe For all pressure equipment placed on the market in the European Union, the Pressure Equipment Directive 2014/68/EU (PED) establishes the essential safety requirements that equipment must comply with before being put into service. Its application is mandatory regardless of whether the project also references international standards such as ASME or sector specifications such as API. Scope: the PED applies to pressure equipment with a maximum allowable pressure above 0.5 bar. Finned-tube heat exchangers and gas recuperators in industrial installations typically fall within its scope when they exceed the pressure and volume thresholds set out in Annex II. Fluid classification: the PED distinguishes between Group 1 fluids (flammable, toxic, oxidising or explosive under CLP) and Group 2 fluids (all others). In petrochemical installations, process gases containing hydrocarbons or H₂S are Group 1, activating the more demanding categorisation tables and potentially requiring the involvement of a Notified Body. CE marking: all equipment subject to the PED must bear the CE marking accompanied by the EU Declaration of Conformity before being put into service in Europe. Reference to other standards in a technical specification does not exempt equipment from this requirement. Technical documentation: the equipment’s technical file must demonstrate compliance with the PED’s essential safety requirements, including pressure calculations, material certificates and inspection records corresponding to the applicable conformity assessment module. PED category and conformity assessment module: determined from basic engineering The PED category of a piece of equipment —from I to IV— determines the applicable conformity assessment module and, with it, whether or not a Notified Body must be involved. The category results from the intersection of the fluid Group and the PS×V product (vessels) or PS×DN (piping). In petrochemical installations with Group 1 fluids at elevated pressures, categories III or IV are frequently reached. Defining the category in the basic engineering phase allows the timescales and resources of the certification process to be planned correctly. 3. ASME BPVC: international calculation reference The ASME Boiler and Pressure Vessel Code (BPVC), published by the American Society of Mechanical Engineers, is the reference code for the calculation and certification of pressure equipment in projects within the North American sphere and in numerous international projects where the process licensor or plant owner contractually requires it. Its knowledge is relevant for export projects and for installations where the client specifies ASME requirements. PED 2014/68/EU Scope: European Union market (mandatory) Marking: CE marking + EU Declaration of Conformity Calculation reference: EN 13445 (vessels), EN 13480 (piping) Inspection: Notified Body for categories III–IV Materials: EN 10028, EN 10216, EN 10217 and equivalents Documentation: EU Declaration of Conformity + technical file ASME BPVC Section VIII Scope: USA and international projects by contract Marking: U/U2/U3 stamp + dataplate (requires Certificate of Authorization) Calculation reference: ASME VIII Div.1 (prescriptive), Div.2 (analysis) Inspection: Authorized Inspector (AI) of an accredited AIA Materials: SA/SB designations (ASME Section II) Documentation: Manufacturer’s Data Report (Form U-1) When a European project simultaneously requires CE marking (PED) and ASME documentation, the reconciliation of both frameworks must be planned from the basic engineering phase: materials must be qualified under both systems, calculations must demonstrate compliance with both codes, and the inspection process must coordinate the requirements of the Notified Body and the Authorized Inspection Agency. Precisely defining the scope of each code from the outset prevents incompatibilities that could jeopardise the delivery schedule. Material equivalences: case-by-case verification The material designation systems of the PED (EN standards) and ASME (SA/SB designations) are not directly interchangeable. An SA-516 Gr.70 material and a P265GH (EN 10028-2) have similar compositions but … Read more

How heat exchangers contribute to EU 2030 targets and the Energy Efficiency Directive 2023/1791

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How heat exchangers contribute to EU 2030 targets and the Energy Efficiency Directive 2023/1791 | BOIXAC Technical blog › Sustainability and energy efficiency How heat exchangers contribute to EU 2030 targets and the Energy Efficiency Directive 2023/1791 The EED 2023/1791 and the Fit for 55 package have turned industrial energy efficiency into a legal obligation. We analyse the regulatory framework and the role of heat recovery systems as a verifiable efficiency measure. BOIXAC Tech SLDirective (EU) 2023/1791 · Fit for 55 · EU 2030Technical read — 8 min Important notice — informational content onlyThe contents of this article, including references to dates, thresholds and regulatory obligations, are strictly informational and for general guidance only. European regulations and their national transposition are subject to changes. BOIXAC Tech SL accepts no liability arising from decisions taken based on this article. Always consult a qualified legal or energy adviser. Table of contents The context: energy efficiency as a legal obligation Corporate obligations under the EED 2023/1791 The Energy Efficiency First principle The Fit for 55 package and the EU Taxonomy Heat recovery as a verifiable efficiency measure Industrial waste heat: the available potential The energy audit as a starting point The convergence of the EED 2023/1791, the Fit for 55 package and the EU 2030 climate target creates a framework in which recovering waste heat from industrial processes is no longer an optional improvement but a priority measure that mandatory energy audits will systematically place on the agenda. 55%EU GHG emission reduction by 2030 (vs 1990) 11.7%EU final energy consumption reduction by 2030 1.9%Mandatory annual energy savings 2028–2030 10 TJConsumption threshold for mandatory energy audit The context: energy efficiency as a legal obligation For decades, energy efficiency in industry was a voluntary decision. The adoption of the Fit for 55 package in 2021 and the entry into force of Directive (EU) 2023/1791 of 13 September 2023 — the new Energy Efficiency Directive (EED), recast — have turned energy efficiency into a legal obligation for a significant number of European industrial companies. The central objective is clear: to reduce the EU’s final energy consumption by at least 11.7% by 2030 compared to reference projections, as an essential contribution to the climate target of reducing emissions by 55% compared to 1990 levels (Regulation (EU) 2021/1119). Corporate obligations under the EED 2023/1791 The main novelty of the EED 2023/1791 is that obligations no longer depend on company size but on actual energy consumption. Key deadlines and thresholds of the EED 2023/1791 11 October 2025: deadline for transposition of the Directive into national legislation of EU Member States. 11 October 2026: first mandatory energy audit for companies with average annual consumption exceeding 10 TJ (≈ 2.78 GWh) over the three preceding years. Minimum frequency: every four years. 11 October 2027: mandatory implementation of a certified Energy Management System (EnMS) (ISO 50001) for companies with consumption exceeding 85 TJ (≈ 23.6 GWh). ℹ️ Dates are from the Directive text published in the OJ EU. National transposition may introduce variations. Always consult the applicable national regulations. The ‘Energy Efficiency First’ principle The EED 2023/1791 elevates for the first time to legal status the Energy Efficiency First principle. Recovering waste heat from one’s own processes must be the first option to evaluate before installing new heat generation. Practical implication for industry An industrial process with flows of hot gases, cooling water or thermal effluents is, under the EED 2023/1791 framework, an internal energy resource that must be systematically evaluated. Failing to recover it is a missed opportunity that the mandatory energy audit will bring to light. The Fit for 55 package and the EU Taxonomy The EED 2023/1791 is part of the Fit for 55 package, which includes the revision of the EU ETS, the Renewable Energy Directive (RED III), the EU Taxonomy Regulation and the EED itself. Financing opportunity: EU Taxonomy and green finance Investments in industrial heat recovery may qualify as EU Taxonomy-aligned activities. This qualification facilitates access to green financing, sustainable bonds and European programmes such as InvestEU or Next Generation EU funds — particularly relevant for companies operating in or exporting to the EU. Heat recovery as a verifiable efficiency measure Technically measurable and verifiableSavings are obtained using Q = ṁ · cp · ΔT, where all variables can be continuously measured and independently verified. Compatible with the M&V protocols required by the EED to certify savings. Eligible for support mechanismsIn France (CEE), Spain (CAE) and other EU countries, industrial heat recovery installations have standardised operation sheets enabling financial incentives based on kWh saved over equipment lifetime. Directly reduces CO₂ emissionsBy recovering heat that would otherwise require burning fuel, direct CO₂ emissions are reduced (Scope 1, GHG Protocol / ISO 14064). Compatible with EU ETS and CSRD 2022/2464/EU reporting requirements. Industrial waste heat: the available potential According to estimates from various European energy agencies, the total potential of industrial waste heat in the EU stands at around 300–400 TWh/year. Nearly half corresponds to temperatures above 100 °C. Where recoverable waste heat can be found Combustion gases (furnaces, boilers, turbines): typical temperature 200–600 °C. Process steam and condensate: temperature 100–200 °C. Compressor and machinery cooling water: temperature 30–90 °C. Hot process effluents: variable. The energy audit as a starting point Inventory of available waste heat flows: flow rate, temperature, gas composition, intermittency. Estimate of recoverable thermal power and associated annual energy. Study of potential uses for recovered heat. Techno-economic analysis with estimated investment, annual fuel savings and ROI. Identification of applicable support schemes and available grants in the country of operation. Difference between an indicative estimate and a formal audit A simplified estimate is useful as an initial screening. For EED 2023/1791 obligations, a formal energy audit by a qualified independent expert is required, compliant with EN ISO 50002 or EN 16247-1. Legal notice, limitation of liability and regulatory referencesStrictly informational and general guidance article. Regulatory references have been compiled from official published sources (OJ EU, EUR-Lex). BOIXAC Tech SL accepts no liability arising from the … Read more

Energy saving and CO₂ reduction calculator for industrial heat recovery

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    Technical blog › Energy efficiency Energy saving and CO₂ reduction calculator for industrial heat recovery Indicative tool to estimate recoverable thermal power, fuel savings and approximate CO₂ emission reduction from installing a heat recovery heat exchanger. Enter your process data and get an estimate in seconds. BOIXAC Tech SL GHG Protocol reference factors · User-editable values Indicative tool — estimative results with no regulatory value Important notice — please read before using this tool This tool is strictly indicative. Results are simplified estimates based on the thermal balance Q = ṁ · cp · ΔT · η and reference emission factors. They have no technical, legal or regulatory validity whatsoever. BOIXAC Tech SL accepts no liability arising from the use of this tool or its results for any purpose. 🌍 1 · Select territory — Select a territory —International (generic) 📊 Reference emission factors — user-editable ↺ Restore The values shown are GHG Protocol generic references. You may edit them to match the actual conditions of your process or energy supply contract. Click “Restore” to revert to the original values. Fuel Emission factor Reference source These are generic GHG Protocol reference values. Apply the official emission factors published by the competent authority in your country for any formal purpose. 2 · Process data Hot fluid or gas flow rate kg/hm³/h (gas)kg/s Mass flow rate of the hot stream available for heat recovery. Typical values: industrial furnaces 2,000–50,000 kg/h; steam boilers 1,000–20,000 kg/h; cogeneration engines 500–5,000 kg/h. Inlet temperature°CTemperature at the process outlet, before the heat exchanger. Target outlet temperature°CMinimum outlet temperature of the hot fluid. For combustion gases, never go below the acid dew point (typically 120–150 °C for natural gas, 140–160 °C for diesel). Specific heatkJ/(kg·K)Dry air ≈ 1.006 · Combustion gases ≈ 1.05–1.15 · Steam ≈ 2.0 · Water ≈ 4.18 kJ/(kg·K) Annual operating hoursh/yearContinuous operation: 8,760 h/year. 2-shift, 5 days: ≈ 4,000 h/year. Estimated heat exchanger efficiency%Typical industrial heat recovery: 65–85%. Default conservative value: 75%. 3 · Fuel Fuel replaced — Select the fuel. The emission factor is taken from the table above. Fuel price€/kWhAdapt the price to your actual contract. Boiler / heat generator efficiency%Conventional boiler: 85–90%. Condensing: 95–105%. Steam: 80–88%. Reference CO₂ price (optional)€/t CO₂Indicative carbon market price. Set to 0 to ignore this factor. 4 · Investment (optional — for ROI) Estimated equipment and installation cost€Includes equipment, installation and commissioning. Leave blank to skip ROI. Note: ROI may appear very short (months) for high flow-rate, high ΔT processes — always verify against a real quotation and actual process conditions. Additional annual maintenance cost€/yearCleaning, inspection, spare parts. Typically 0.5–2% of equipment cost per year. Calculate estimate ↺ Reset Indicative estimate Calculation detail (estimative) Parameter Estimative value Limitation of results These results are purely estimative. They have been obtained using the simplified thermal balance Q = ṁ · cp · ΔT · η, without considering radiation or pipe conduction losses, seasonal load variations or the acid dew point. They do not represent the actual behaviour of any specific equipment or installation. For a rigorous technical estimate, please contact the BOIXAC technical office. Legal notice and limitation of liability Strictly informative and indicative tool. Results have no technical, legal or regulatory validity and may not be used for any official, contractual or regulatory purpose. The emission factors shown are indicative reference values. BOIXAC Tech SL accepts no liability for decisions taken based on the results of this tool. Do you need a real technical estimate for your process? The BOIXAC technical office analyses the actual conditions of your process and proposes the optimal heat recovery solution with a detailed thermal balance. Contact our technical office

Heat exchanger parameter glossary and unit converter

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Heat exchanger parameter glossary and unit converter | BOIXAC Technical tools › Heat exchangers Heat exchanger parameter glossary and unit converter Select any parameter from a heat exchanger calculation datasheet to view its definition and convert the value between the most common industrial units. Parameter: — Select a parameter —Thermal capacityHeat transfer surface areaOverall heat transfer coefficient (U)Log Mean Temperature Difference (LMTD) Volumetric air flow rateMass air flow rateFace velocity on the coilInlet air densityInlet air temperatureOutlet air temperatureInlet relative humidityOutlet relative humiditySpecific humidityInlet air enthalpyOutlet air enthalpyPressure drop — air sideFouling factor — air sidePartial heat transfer coefficient — air side Volumetric fluid flow rateMass fluid flow rateFluid velocityInlet fluid temperatureOutlet fluid temperatureTotal pressure drop — fluid sidePartial heat transfer coefficient — fluid sideFouling factor — fluid side Number of rowsNumber of tubes per rowCoil lengthFin pitchNumber of circuitsTube outer diameterTube inner diameterFin thicknessCoil internal volumeAtmospheric pressure / Altitude 🔍 Select a parameter from the dropdown to view its definition and the unit converter. Note on conversions Converted values are obtained by applying international standard conversion factors. Temperature conversions (°C, °F, K) include the origin offset where applicable. Results have up to 4 significant figures. This tool is for guidance only; for engineering calculations, always verify against applicable reference standards. Do you need a detailed calculation for your process? The BOIXAC technical office analyses the actual conditions of your process and proposes the optimal heat exchanger solution. Contact our technical office

Pillow plate for fermentation and thermal control in wineries and breweries

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Pillow plate heat exchangers in breweries and wineries: fermentation cooling and tank temperature control | BOIXAC Technical blog · Food industry › Breweries and wineries Pillow plate heat exchangers in breweries and wineries: fermentation cooling and tank thermal control Why dimple plate technology outperforms conventional jackets for fermentation tank cooling: heat transfer coefficient analysis, CIP cleaning and design criteria for beer and wine production. BOIXAC · Technical OfficeUpdated: 2026Reading: ~11 min Note on the scope of this article This article is of a general technical and informational nature. Heat transfer coefficient values, temperature ranges and design criteria given are indicative; the definitive sizing of a pillow plate heat exchanger for a specific application requires analysis of the actual process conditions by qualified engineers. BOIXAC assumes no liability for decisions taken on the basis of this article. Temperature control during fermentation is one of the technical parameters with the greatest influence on the organoleptic profile of the final product in breweries and wineries. The difference between a fermentation proceeding at 12 °C and one that peaks at 18 °C may be the difference between a clean product and one with undesirable ester and fusel alcohol profiles. Pillow plate technology — also known as dimple plate — has progressively replaced half-pipe jackets and conventional annular jackets on next-generation stainless steel fermentation vessels, thanks to thermal, hygienic and constructional advantages that become especially evident in tank volumes exceeding 5,000 litres. 1. Operating principle of the pillow plate (dimple plate) A pillow plate is a heat exchanger formed by two stainless steel sheets joined at their perimeter and by a regular matrix of resistance spot welds, creating an internal labyrinthine cavity with a very narrow cross-section. When a refrigerant fluid (typically aqueous glycol) circulates through this cavity, the dimple geometry induces local turbulent flow — even at low volumetric flow rates — maximising the internal convection coefficient. The outer sheet of the pillow plate is welded directly to the surface of the fermentation vessel, so that the tank wall simultaneously serves as the load-bearing structure and the heat transfer surface. The embossed geometry of the dimples distributes the refrigerant pressure uniformly across the entire plate surface, allowing operation at relatively high internal pressures (up to 10–15 bar depending on design and sheet thickness) with minimal material thickness. 2. Technical comparison: pillow plate vs. conventional jackets Parameter Pillow plate (dimple plate) Half-pipe jacket Conventional annular jacket Internal convective coefficient (hi) High: dimple geometry induces local turbulence. Typical values: 3,000–8,000 W/m²·K. Moderate-high: tubular flow. 2,000–5,000 W/m²·K. Low-moderate: wide annular flow, often laminar. 500–2,000 W/m²·K. Cooling distribution across tank surface Excellent: continuous, uniform coverage of all covered surface. Good along the pipe length; zones between pipes lack direct contact. Variable: risk of dead zones in large-section annular circuit. Refrigerant fluid volume in circuit Very low: narrow flow passage (typically 3–6 mm). Reduced glycol volume in circuit. Moderate. High: large annular cross-section. Thermal response time Very fast: low fluid volume, reduced thermal inertia. Rapid control system response. Fast-moderate. Slow: large fluid volume, high thermal inertia. Slow response to setpoint changes. Cleanability — product side Excellent: smooth external surface in contact with product, suitable for CIP cleaning. Good. Good. 3. Specific applications in breweries and wineries 3.1. Fermentation vessel cooling in brewing In bottom-fermentation (lager) beer production, temperature control is especially critical because the yeast working window (typically 8–14 °C for standard lager strains) is narrow and the heat generated by alcoholic fermentation is significant: approximately 2.3 kJ are released per gram of fermented sugar. In a 50-hl fermenter with 12 °P wort, the cooling duty required at peak fermentation activity can be between 3 and 8 kW depending on the fermentation rate. Pillow plates welded to the cylindrical tank wall (and, in some designs, to the cone) allow this heat extraction to be distributed homogeneously, avoiding radial temperature gradients that could create localised sub-cooling zones where yeast activity is inhibited or premature precipitation occurs. The fast response of the system — due to the low refrigerant circuit volume — facilitates the use of PID control systems that maintain temperature setpoints within ±0.5 °C, difficult to achieve with high-inertia conventional jackets. 3.2. Must temperature control in wine fermentation In white and rosé winemaking, fermentation temperature control (typically between 12 and 18 °C) is critical for preserving volatile varietal aromas that are lost through volatilisation if temperatures are exceeded. Pillow plates on AISI 304 or 316L stainless steel tanks allow low fermentation temperatures to be reached and maintained with modest refrigeration systems. The ability to reach temperatures close to 0 °C uniformly and in a controlled manner — the so-called cold tartrate stabilisation — is an application that highlights the thermal performance of pillow plate technology over less efficient alternatives. 3.3. Craft breweries and microbreweries In craft breweries with smaller fermenters (100–2,000 litres), pillow plate technology offers additional advantages due to its compatibility with relatively small glycol systems and the ease of integration on cylindroconical stainless steel tanks. The typical configuration consists of one or two independent pillow plate zones (cylindrical and conical sections) connected to a glycol circuit with independent zone control valves, allowing programmable temperature profiles throughout fermentation. 4. Pillow plate sizing criteria for fermentation vessels Peak fermentation thermal duty (Qmax): estimated from fermentation rate, wort concentration (°P or °Brix) and tank volume. In beer production, indicative reference values range from 50 to 150 W per hl of fermenter capacity at peak activity, depending on the recipe and yeast used. Available temperature differential (ΔT): difference between fermenting product temperature and refrigerant inlet temperature to the plate. Minimum refrigerant temperature: in aqueous glycol circuits, glycol temperatures of -2 to -5 °C are generally sufficient for most standard fermentation applications; lower temperatures are used for tartrate stabilisation. Tank surface coverage: the proportion of the total tank surface covered with pillow plate (typically 40–70 % of the lateral surface) must be sufficient to ensure cooling uniformity and avoid vertical temperature gradients in the product. Refrigerant circuit working pressure: pillow plate … Read more

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

Heat exchanger for lime and calcium carbonate industrial minerals plant

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Heat exchangers in calcination plants: lime, calcium carbonate and industrial minerals | BOIXAC Technical blog · Minerals industry › Calcination and industrial minerals Heat exchangers in calcination plants: lime, calcium carbonate and industrial process minerals Technical criteria for heat recovery from rotary kiln flue gases with abrasive dust, high temperatures and CO₂ content: typology selection, materials and cleaning strategies. BOIXAC · Technical OfficeUpdated: 2026Reading: ~11 min Note on the scope of this article This article is of a general technical and informational nature. The temperature values, gas compositions and material ranges indicated are orientative and are based on process references from the industrial minerals industry. Definitive sizing and equipment selection for a specific application requires a detailed analysis of the actual plant conditions by qualified engineers. BOIXAC assumes no liability for decisions taken on the basis of this article. Plants producing quicklime, hydrated lime, precipitated calcium carbonate and other industrial process minerals operate with rotary kilns that generate considerable volumes of combustion gases at temperatures typically between 300 and 600 °C at the preheater outlet. Recovering this residual thermal energy represents one of the best cost-benefit energy efficiency improvements available in the sector, but the nature of the gases — with high concentrations of abrasive dust, significant CO₂ content and, occasionally, sulphur compounds — demands very specific selection and technical design. 1. Production context: rotary kiln and calcination gases The calcination of calcite (CaCO₃) to produce quicklime (CaO) is an endothermic reaction requiring process temperatures between 900 and 1 100 °C inside the kiln. The resulting combustion gases — enriched with CO₂ released by the mineral’s decarbonation — leave the kiln at temperatures that depend on the kiln type and preheating system used: Kiln type / process Typical flue gas temperature at outlet Particularities for the heat exchanger Long rotary kiln without preheater 350–600 °C High fine lime dust load (CaO/CaCO₃). High abrasivity. Large gas flow rate. Rotary kiln with cyclone preheater 200–350 °C Dust partially separated in cyclones. More moderate temperature. Risk of condensation if excessively cooled. Shaft kiln 150–280 °C Gases with very high CO₂ (up to 30–40 % v/v). Moderate dust. High CO₂ concentration may influence the selection of the receiving fluid. Rotary kiln for dolomite / magnesite 400–700 °C Dust containing MgO and CaO components. Very high abrasivity. High gas temperature. 2. Degradation mechanisms specific to this industry 2.1. Mechanical abrasion by particle impact CaO, CaCO₃ or dolomite particles in kiln gases present a Mohs hardness of 3 to 5 and a particle size distribution that, despite passage through pre-collection cyclones, includes fractions up to 200–500 µm. When they impact tube surfaces at typical gas velocities (8–15 m/s), they cause erosive wear that is particularly severe at fin edges in finned tubes and at bends in gas direction-change zones. The erosion rate is proportional to particle concentration, particle hardness, the third or fourth power of the impact velocity and the cosine of the impact angle. Minimising it requires design action: reducing gas velocities in the exchanger ducts (typically below 10 m/s in heavily abrasive applications), avoiding geometries that generate direct impingement on surfaces and selecting high erosion-resistant materials for maximum exposure points. 2.2. Fouling and blockage by dust deposition CaO particles progressively depositing on tube and fin surfaces form an insulating layer that reduces the overall heat transfer coefficient (U) proportionally to its thickness. Under high dust-loading conditions without active cleaning, accumulation can reduce the economiser’s thermal performance by 30–50 % within weeks or months. Dry CaO or CaCO₃ dust deposits are generally relatively soft and friable, and can be removed by mechanical vibration, steam blowing (sootblowing) or rapping, provided the equipment design includes adequate access and cleaning systems. Specific risk: hydration of quicklime in the presence of moisture Under conditions of elevated moisture in the gases or during start-up and shutdown cycles with partially cooled gases, CaO (quicklime) particles can hydrate by reacting with moisture in the gases to form Ca(OH)₂. This exothermic reaction can generate hard and expansive deposits on tube surfaces, significantly more difficult to remove than dry dust deposits. Exchanger design and management of minimum wall temperatures during start-ups and shutdowns must account for this risk, especially in kilns processing quicklime without an efficient dust pre-collection system. 3. Heat exchanger typology selection Typology Advantages for calcination gases Limitations and risks Recommended application Bare tubes (no fins) Maximum abrasion resistance. No preferential wear point due to fin geometry. Direct mechanical cleaning. Lower tendency for dust retention. Lower surface density per unit volume than finned tubes. Gases with high dust loading (>5 g/Nm³) and elevated abrasivity. Helically finned tubes High surface density. Good U coefficient. Compactness. Dust accumulation in inter-fin channels. Difficult mechanical cleaning. Risk of irreversible blockage. Clean gases (<1–2 g/Nm³). Not recommended for calcination gases without efficient post-collection. Continuous (plate) fins Better cleaning access than helical fins. Dust accumulation in channels. Moderate dust loading (1–5 g/Nm³). Post-collection cyclones of good efficiency. 4. Material selection for abrasive-corrosive environments Material Abrasion resistance Wall temperature limit Remarks Carbon steel S235/P235GH Moderate ~450 °C Suitable for moderate temperature zones with reasonably clean gases after pre-collection. Susceptible to SO₂ near the acid dew point. Cr-Mo low-alloy steel (13CrMo4-5, P91) Good ~550 °C Improved high-temperature oxidation and erosion resistance compared to carbon steel. Stainless steel AISI 310S Good–very good ~1 050 °C Excellent high-temperature oxidation resistance. For the first tube rows exposed to the hottest gases (>500 °C). High wear-resistant cast iron (Ni-Hard, white iron) Excellent ~400 °C Maximum impact abrasion resistance. Used for deflectors and shell liners. Brittleness limits use; not suitable for pressure-bearing tubes. Nickel-base alloy (Inconel 625, Alloy 800H) Very good ~1 000 °C For extreme conditions: very high temperature combined with corrosive gas. High cost; application justified on a case-by-case basis. Common strategy: material zoning In heat exchangers for calcination gases, it is common practice to apply a material zoning strategy: the first tube rows, exposed to the hottest gases and highest particle velocities, are built with more resistant materials (310S stainless steel … Read more