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.

ATEX: Explosive atmospheres in industrial installations

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ATEX: Equipment Selection in Explosive Atmospheres for Chemical, Pharmaceutical and Food Industries | BOIXAC Technical guide › Industrial regulation ATEX: Zone Classification, Equipment Categories and Marking for Explosive Atmospheres in Industrial Installations Technical reference guide on ATEX directives 2014/34/EU (equipment) and 1999/92/EC (worker safety): Ex zones, equipment categories, gas groups, temperature classes and implications for manufacturers and operators of industrial installations with explosion risk. BOIXAC Tech SLUpdated: 2026Reading time: ~9 min Safety warning and limitation of liability — Mandatory reading This page is intended for general information and reference purposes only. ATEX regulations directly affect the safety of people and installations. No content in this guide constitutes technical, safety engineering or legal advice. Zone classification, equipment selection and the preparation of the Explosion Protection Document (EPD) require the involvement of a qualified technical professional with accredited experience in explosive atmosphere safety. BOIXAC Tech SL assumes no liability arising from the use of this information. For any real installation, consult an accredited notified body or an engineer specialised in ATEX. Explosive atmospheres represent one of the industrial hazards with the most potentially severe consequences. For manufacturers and operators of installations in sectors such as chemicals, pharmaceuticals, food, oil and gas or waste treatment, understanding the ATEX framework is not optional: it is a legal requirement and an unavoidable responsibility. 1. The two ATEX directives: manufacturers and operators ATEX 2014/34/EU — Equipment directive (manufacturers) Applies to manufacturers of equipment, protective systems, control devices and components intended for use in potentially explosive atmospheres. Establishes design, manufacturing, conformity assessment and CE marking requirements for Ex equipment. Replaced Directive 94/9/EC from 20 April 2016. ATEX 1999/92/EC — Workplace directive (operators) Applies to operators of installations where explosive atmospheres may be present. Establishes the obligation to classify Ex zones, prepare the Explosion Protection Document (EPD), select equipment appropriate for each zone and ensure worker training. Intersection with PED 2014/68/EU and Machinery Directive 2006/42/EC When a pressure equipment item is installed in an ATEX zone, PED (pressure risk), Machinery Directive (if part of an actuated assembly) and ATEX directives (ignition risk) apply simultaneously. When in doubt, the precautionary principle requires applying the most restrictive requirement. 2. Zone classification: the starting point Gas / Vapour / Mist (ATEX 1999/92)Zone 0Permanent hazardExplosive atmosphere present continuously, for long periods or frequently. Requires Category 1G equipment. Gas / Vapour / Mist (ATEX 1999/92)Zone 1Occasional hazardExplosive atmosphere that may occasionally form during normal operation. Requires Category 1G or 2G equipment. Gas / Vapour / Mist (ATEX 1999/92)Zone 2Unlikely hazardExplosive atmosphere not normally present and, if it does occur, only for a brief period. Requires Category 1G, 2G or 3G equipment. Combustible dust (ATEX 1999/92)Zone 20Permanent hazardCombustible dust cloud present continuously or frequently. Requires Category 1D equipment. Combustible dust (ATEX 1999/92)Zone 21Occasional hazardCombustible dust cloud that may occasionally form during normal operation. Requires Category 1D or 2D equipment. Combustible dust (ATEX 1999/92)Zone 22Unlikely hazardCombustible dust cloud not normally present or, if it occurs, only for a brief period. Requires Category 1D, 2D or 3D equipment. Frequent critical error — Zone classification is not optional A common shortcoming in existing installations is the absence of formal zone classification or its inadequate updating when production processes change. In the event of an accident, lack of classification and an up-to-date EPD results in direct criminal and civil liability for those responsible for the installation, regardless of whether the installed equipment was ATEX-certified. 3. Equipment categories, groups and temperature classes Category Group Suitable zones Max. permitted zone Main industrial applications Cat. 1G I / II Zone 0, 1, 2 Gas/vapour · Zone 0 Refineries, chemical plants, solvent storage. EPL level Ga/Da — very high protection. Cat. 2G II Zone 1, 2 Gas/vapour · Zone 1 Chemical and pharmaceutical plants, flammable liquid loading/unloading areas. EPL level Gb/Db. Cat. 3G II Zone 2 Gas/vapour · Zone 2 Food industry, chemical plant perimeter areas, flammable product warehouses. EPL level Gc/Dc. Cat. 1D I / II Zone 20, 21, 22 Dust · Zone 20 Flour, sugar and high-combustibility metal dust processing facilities. EPL level Da. Cat. 2D II Zone 21, 22 Dust · Zone 21 Food industry (spray areas), pharmaceutical, biomass processing. EPL level Db. Cat. 3D II Zone 22 Dust · Zone 22 Perimeter areas of combustible dust installations, silos, warehouses. EPL level Dc. Gas groups and subgroups: IIA, IIB, IIC Group II (surface) equipment is subdivided according to the minimum ignition energy of the gas or vapour present: IIA (propane, butane — high minimum ignition energy), IIB (ethylene — intermediate energy) and IIC (hydrogen, acetylene — very low minimum ignition energy, maximum risk). IIB-certified equipment is suitable for IIA and IIB gases, but not for IIC. Incorrect subgroup selection is one of the most common causes of non-conformity in ATEX audits. Maximum surface temperature classes (T1–T6) The maximum surface temperature of the equipment must be below the ignition temperature of the gas or vapour present, with a safety margin. Classes range from T1 (450°C max.) to T6 (85°C max.). For example, a T3 device (200°C max.) is suitable for gases with an ignition temperature above 200°C (acetone: 465°C ✓ / hydrogen sulphide: 270°C ✓ / diethyl ether: 160°C ✗). 4. The ATEX marking: how to read it ⟨Ex⟩ II 2G Ex d IIB T3 Gb ⟨Ex⟩ATEX markingIIEquipment group (surface)2GCategory / gas environmentEx dProtection type (flameproof enclosure)IIBGas subgroupT3Temperature class (200°C max)GbEPL level Most common protection types: Ex d (flameproof enclosure), Ex e (increased safety), Ex ia/ib (intrinsic safety), Ex p (pressurised enclosure), Ex n (non-sparking equipment, zone 2), Ex t (dust ignition protection by enclosure). The ATEX certificate is not permanent: any unauthorised modification to the equipment — including replacement of components with non-certified parts — invalidates the certificate and protection category. Maintenance in ATEX zones: maintenance operations must be carried out by trained and qualified personnel, with procedures appropriate for the classified zone. All interventions must be documented. 5. The Explosion Protection Document (EPD) Minimum EPD content: identification and classification of all Ex zones, inventory of installed equipment … Read more

Machine Directive 2006/42/CE industrial boiler manufacturer

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Machinery Directive 2006/42/EC for Boiler and Thermal Equipment Manufacturers | BOIXAC Technical guide › Industrial regulation Machinery Directive 2006/42/EC: Technical Guide for Boiler and Industrial Thermal Equipment Manufacturers Analysis of the essential health and safety requirements, conformity assessment and CE marking for OEM manufacturers integrating thermal components —economisers, heat exchangers, heat recovery units— into boilers and industrial machinery assemblies. BOIXAC Tech SLUpdated: 2026Reading time: ~8 min Note on the scope of this guide This page is intended for general information and reference purposes only. It does not constitute legal or engineering advice. The interpretation and application of Directive 2006/42/EC may vary depending on the specific product, the country of commercialisation and the individual circumstances of each manufacturer. BOIXAC Tech SL does not provide regulatory or legal advisory services and assumes no liability arising from the use of this information. For any conformity decision, consult an accredited notified body or a legal adviser specialised in product law. For OEM manufacturers of boilers, steam generators and industrial thermal equipment, the Machinery Directive 2006/42/EC is the legal framework that governs the conditions for placing products on the European market. Integrating third-party components —economisers, heat exchangers, heat recovery units— into a machinery assembly is not a minor technical detail: it determines the risk assessment scope, the technical documentation and the liability of the integrating manufacturer. 1. Scope: when does the Machinery Directive apply? Directive 2006/42/EC applies to machinery, defined as an assembly of linked parts or components, at least one of which moves, fitted with an appropriate actuating system. Industrial boilers with burners, automatic control systems and electrically or pneumatically actuated auxiliary components clearly fall within the scope of the directive. 🔥Industrial boilers with burnerAssemblies with automatic ignition, safety controls and actuated auxiliary components. ⚙️Industrial steam generatorsEquipment with automatic pressure, level and temperature regulation systems. 🏭Thermal machinery assembliesInstallations where several machines are assembled to perform a combined function. ⛔Passive components without moving partsHeat exchangers, economisers and recuperators without their own actuating system generally fall outside the direct scope. Intersection with PED Directive 2014/68/EU When a boiler integrates pressure-bearing components, two directives apply simultaneously: 2006/42/EC for mechanical and operational risks of the assembly, and PED 2014/68/EU for pressure-specific risks. The integrating manufacturer is responsible for managing both conformity frameworks. 2. Essential Health and Safety Requirements (EHSR) General safety principles (§1.1): Machinery must be designed so that, when used as intended, it does not endanger persons. Safety by design takes priority over protective devices and operating instructions. Materials and products (§1.3.2): Materials must be suitable for the working fluids, temperatures and pressures involved. The integrating manufacturer must verify that the materials of external components meet the requirements of the boiler’s working fluid. Surface temperature (§1.5.5): Accessible hot surfaces capable of causing burns must be insulated or guarded. Especially relevant for high-temperature economisers. Design pressure and temperature (§1.5.7): Machinery must withstand the anticipated loads with adequate safety margins, including maximum operating pressures of hydraulic and steam circuits. Control systems and emergency stop (§1.2): The boiler must be equipped with control systems enabling safe shutdown in the event of a failure, including integrated components. Instructions (§1.7.4): The instruction manual must include information on all integrated components, including maintenance instructions for third-party supplied components. 3. Conformity assessment: applicable procedures Procedure Notified body Application for boilers Resulting documentation Annex VIIISelf-assessment Optional Machinery not listed in Annex IV. Standard boilers where the manufacturer applies harmonised standards (e.g. EN 12952, EN 12953). Internal technical file + CE Declaration of Conformity Annex IXEC type-examination Mandatory Annex IV machinery or where harmonised standards are not applied. High-power boilers or non-standard configurations. EC type-examination certificate + Technical file + CE Declaration Annex XFull quality assurance Mandatory Alternative to Annex IX for manufacturers with a quality system approved by a notified body. Suitable for series OEM manufacturers. Approved quality system + CE Declaration Harmonised standards: the safest route to conformity Applying harmonised standards published in the OJEU confers a presumption of conformity with the corresponding EHSR. For fire-tube boilers, the reference standard is EN 12953. For water-tube boilers, EN 12952. For general machinery risk assessment and reduction, EN ISO 12100 is the central reference. 4. Integrating manufacturer liability for third-party components Integrating manufacturer liability — critical point If a third-party component does not meet the technical requirements needed for safe integration, liability for the non-conformity of the assembly rests with the integrating manufacturer, not with the component supplier. Supplier qualification diligence is a conformity requirement, not merely a commercial best practice. PED Declaration of Conformity (where the component exceeds Article 4 thresholds of Directive 2014/68/EU), indicating risk category and conformity assessment module applied. Technical datasheet with design parameters: PS (maximum allowable pressure), TS (maximum design temperature), DN, materials of construction, design fluid and use limitations. Installation and maintenance instructions in the official language of the country of commercialisation. Material traceability for components in contact with pressurised or high-temperature fluids. 5. CE Marking and Declaration of Conformity The CE marking is not a quality mark or an external approval certificate: it is the manufacturer’s declaration that the product meets all applicable legal requirements. CE marking is mandatory for placing on the European market (EEA). Its absence constitutes a legal violation. The technical file must remain accessible to market surveillance authorities for a minimum of 10 years from the date of manufacture of the last unit. The CE Declaration of Conformity must accompany each unit and be available in the official language of the destination country. 6. New Machinery Regulation 2023/1230/EU: the upcoming change Regulation (EU) 2023/1230 will replace Directive 2006/42/EC from 20 January 2027. The shift from directive to regulation means direct application without national transposition. Key changes introduced by Regulation 2023/1230 The most significant changes include: requirements for control systems incorporating artificial intelligence, new cybersecurity requirements for connected machinery, extended scope to partially completed machinery, and reinforced sustainability and ecodesign requirements. OEM manufacturers should begin reviewing their technical files well ahead of the January 2027 application date. Thermal components for industrial boilers — BOIXAC BOIXAC … Read more

Pressure Equipment Directive PED 2014/68/UE

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Directive 2014/68/EU (PED): regulatory framework for pressure equipment | BOIXAC Technical blog › Regulations & certification Directive 2014/68/EU (PED):the regulatory framework for pressure equipment in the EU A technical guide to the scope of application, risk category classification and conformity assessment modules established by the Pressure Equipment Directive. BOIXAC Tech SL Updated: 2026 Reading time: ~8 min Note on the scope of this article This text is exclusively informational and educational in nature. It does not constitute legal, technical or engineering advice, and cannot under any circumstances replace the specific analysis carried out by a qualified professional on a particular piece of equipment. The correct application of Directive 2014/68/EU — including equipment classification, determination of the assessment module and obtaining CE marking — always requires the involvement of competent engineers and, for higher categories, a duly notified Notified Body. BOIXAC assumes no liability arising from decisions made based on the content of this article. Directive 2014/68/EU of the European Parliament and of the Council, of 15 May 2014, on the harmonisation of the laws of Member States relating to the making available on the market of pressure equipment — commonly known as the Pressure Equipment Directive or PED — is the European regulatory instrument governing the design, manufacture and conformity assessment of pressure equipment intended for the internal market. For any manufacturer or industrial user of pressure equipment — heat exchangers, vessels, boilers, process pipework and fittings — understanding the scope and logic of this Directive is a prerequisite for operating safely and in legal compliance within the European Economic Area. 1. Background and regulatory context Directive 2014/68/EU repealed and recast the previous Directive 97/23/EC, which ceased to apply on 19 July 2016. The recast did not substantially modify the essential safety requirements or the conformity assessment tables, but aligned the legislation with the New Legislative Framework (NLF) of the European Union — in particular Regulation (EU) No 765/2008 and Decision 768/2008 — introducing explicit obligations for all economic operators in the supply chain: manufacturers, authorised representatives, importers and distributors. Key regulatory reference Directive 2014/68/EU of the European Parliament and of the Council, of 15 May 2014 (OJ L 189, 27 June 2014, pp. 164–259). Full entry into force: 19 July 2016. 2. Scope of application The Directive applies to the design, manufacture and conformity assessment of pressure equipment and assemblies with a maximum allowable pressure (PS) greater than 0.5 bar gauge. Element Description under the Directive Vessels Housings designed and built to contain fluids under pressure, including shell-and-tube heat exchangers. Piping Piping components intended for the transport of fluids, including pipes, piping systems, fittings, expansion joints and hoses. Safety accessories Devices protecting against the exceedance of allowable limits: safety valves, pressure relief devices, automatic control systems, etc. Pressure accessories Devices with an operational function subject to pressure: non-return valves, regulators, steam traps, filters, etc. Assemblies Several items of pressure equipment assembled by a manufacturer to constitute an integrated functional installation. Notable exclusions The Directive expressly excludes from its scope, among others: simple pressure vessels covered by Directive 2014/29/EU; aerosol generators; equipment intended for vehicle operation; certain water distribution networks; nuclear equipment; and well-control equipment for the extractive industry. 3. Fluid classification and its significance One of the Directive’s central pillars is the classification of the fluids contained in the equipment, which determines — together with the pressure and volume or nominal diameter parameters — the applicable risk category. Directive 2014/68/EU updated the classification relative to the previous legislation, aligning it with Regulation (EC) No 1272/2008 (CLP) on classification, labelling and packaging of substances and mixtures. Group Fluids included (simplified criterion) Group 1 Fluids considered hazardous: explosive, extremely or highly flammable, toxic, very toxic, oxidising and corrosive under the CLP Regulation, as well as any fluid at a maximum allowable temperature (TS) above its flash point. Group 2 All fluids not included in Group 1, commonly referred to as “non-hazardous fluids”. Key technical consideration The Directive establishes that thermal oils are classified as Group 1 when the maximum allowable temperature of the equipment exceeds the flash point of the oil in question, regardless of its CLP classification. This specific criterion, set out in Commission Guideline B-41, has direct implications for the resulting category of the equipment. 4. Risk categories The Directive establishes four risk categories (I to IV) for pressure equipment. Category assignment is determined using the tables in Annex II, which cross-reference fluid parameters with equipment parameters (PS, volume V or nominal diameter DN). Category IMinimal risk Low-pressure or small-volume equipment. The manufacturer may self-certify via Module A (internal production control). Category IILow risk Requires involvement of a Notified Body in the production phase. Available modules: A2, D1, E1. Category IIIModerate risk Notified Body involvement in design and/or production. Modules: B+D, B+F, B+E, B1+D, G, H. Category IVHigh risk Maximum requirements. Notified Body required at all stages. Permitted modules: B+D, B+F, G, H1. 5. Conformity assessment modules Module Name NB required Summary A Internal production control No Manufacturer’s self-declaration. Applicable to Category I only. A2 Internal production control with supervised checks Yes The NB performs random inspections of the finished product. B EU-type examination (production type) Yes The NB examines a representative specimen. Must be combined with a production-phase module (D, E or F). B1 EU-type examination (design type) Yes The NB examines the design technical documentation without a physical prototype. D / D1 Production quality assurance Yes The NB approves and supervises the manufacturer’s quality system in the production phase. E / E1 Product quality assurance Yes The NB approves and supervises the quality system for final inspections and testing. F Product verification Yes The NB verifies each produced unit by examination and testing. G Unit verification Yes Individual assessment of each item by the NB, both in design and production. Common for one-off equipment. H / H1 Full quality assurance Yes The NB approves and supervises the complete quality management system. H1 adds design examination and special surveillance. 6. Economic operator obligations Manufacturer Ensure the equipment … Read more

Water quality requirements industrial boiler EN12953-10

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EN 12953-10: Water Quality Requirements in Industrial Shell Boilers | BOIXAC Technical blog › Standards & operations EN 12953-10: Water Quality Requirements in Industrial Shell Boilers Technical analysis of the parameters the standard defines for feedwater and boiler water, and their significance for the integrity and safety of steam generation systems. BOIXAC Tech SL Updated: 2026 Reading time: ~10 min Note on the scope of this article This text is intended solely for informational and educational purposes. It does not constitute technical, engineering or water treatment advice, and cannot under any circumstances replace the specific analysis carried out by a qualified specialist on a given installation. The values and parameters mentioned are drawn from EN 12953-10 and the specialist technical literature; they must always be interpreted in the context of the current version of the standard, the boiler manufacturer’s instructions and the requirements of the authorised inspection body. BOIXAC assumes no liability for decisions taken on the basis of the content of this article. Water quality is, alongside design and manufacturing conditions, the single factor that most influences the long-term integrity of a shell boiler. The European standard EN 12953-10 establishes minimum water quality requirements for feedwater and boiler water in this type of equipment, with the fundamental aim of minimising risk to personnel and surrounding installations. For process engineers, maintenance managers and plant operators running steam generation systems, understanding the framework this standard defines — which parameters it controls, why, and according to what criteria — is an essential element of plant technical management. 1. Normative framework and scope The standard EN 12953-10:2003 forms part of the EN 12953 series, which collectively regulates the design, manufacture, documentation and operation of shell boilers (also referred to as firetube boilers). Part 10 deals specifically with the quality requirements for feedwater and boiler water. Its scope covers all shell boilers heated by combustion of one or more fuels or by hot gases, intended for the generation of steam and/or hot water. The standard applies to components between the feedwater inlet and the steam outlet of the generator. The quality of the steam produced is expressly excluded from scope; where specific steam purity requirements apply, additional normative documents are required. Relationship with Spanish operating regulations Spanish Royal Decree 2060/2008 of 12 December, approving the Pressure Equipment Regulations, requires operators of steam or hot water boilers to maintain water within the specifications of UNE-EN 12953-10 (shell boilers) or UNE-EN 12952-12 (water-tube boilers). Compliance is therefore a legal obligation for the installation operator. 2. Technical purpose of the standard: damage mechanisms to be prevented Scale and deposits The precipitation of calcium, magnesium and silicate salts onto heat transfer surfaces creates layers of low thermal conductivity. A deposit as thin as 1 mm can increase fuel consumption by around 5–8 % and locally raise the metal wall temperature to values that compromise its integrity. Corrosion Dissolved oxygen and free carbon dioxide are the primary corrosive agents. Oxygen corrosion produces localised pitting that can progress until the tube wall is perforated. Inadequate pH promotes various forms of chemical attack on carbon steel. Foaming and carry-over The presence of total dissolved solids (TDS) at elevated concentrations, or of certain organic substances, can cause foam formation at the water surface. This leads to carry-over of boiler water droplets into the steam (priming), contaminating the steam with salts. Sludge and blockages Suspended impurities and precipitates not removed by blowdown can accumulate as sludge in low-velocity water zones, impeding circulation and heat transfer, and promoting under-deposit corrosion. 3. Fundamental distinction: feedwater and boiler water The standard precisely distinguishes two types of water that have different requirements and are controlled independently. Feedwater is the water entering the boiler to replace the evaporated volume. It is typically a mixture of recovered condensate and make-up water, having undergone the necessary external pre-treatment processes. Boiler water is the water present inside the boiler drum during operation. Because feedwater is a continuous source of impurities, boiler water undergoes progressive concentration of these substances. Its admissible parameters are managed through system blowdown. 4. Quality parameters: technical description pHat 25 °C Determines the acidic or alkaline character of the water. Moderately alkaline feedwater pH inhibits oxygen corrosion; in boiler water, alkalinity is required to maintain steel passivation. Total hardnessCa + Mg, mmol/l Expresses the concentration of calcium and magnesium ions, the main scale-forming species. The standard requires extremely low levels in feedwater, which in practice necessitate softening or demineralisation treatment. Dissolved oxygenO₂, mg/l Primary corrosive agent. Must be eliminated by combining thermal deaeration with oxygen scavenger dosing. The standard distinguishes limits according to the design pressure of the boiler. Direct conductivityµS/cm at 25 °C Indirect indicator of total dissolved solids (TDS) concentration. The standard classifies the operating regime according to whether feedwater direct conductivity is above or below 30 µS/cm. Acid conductivityµS/cm, after cation exchanger Determined by passing the sample through a strongly acidic cation exchanger. Particularly sensitive to CO₂, chlorides and sulphates, providing a more reliable measure of aggressive anions. Total ironFe, mg/l Originates primarily from corrosion of steel pipework in the condensate circuit. Forms deposits on heating surfaces that degrade heat transfer performance. Total copperCu, mg/l Originates from corrosion of copper-alloy equipment and pipework in the circuit. Its deposition on steel surfaces can accelerate galvanic corrosion. SilicaSiO₂, mg/l Forms calcium and magnesium silicate scale with very low thermal conductivity and high mechanical hardness, difficult to remove without chemical cleaning. Its limit in boiler water varies with operating pressure. Oils and greasesmg/l Their presence causes intense foaming and water carry-over with steam. Can promote corrosion by forming films on metal surfaces that alter heat transfer conditions. Total organic carbon (TOC)mg/l C Organic substances can thermally decompose under boiler operating conditions, generating carbonic acid and other acidic products that raise acid conductivity and cause corrosion. 5. Boiler water parameters: the role of blowdown Because boiler water concentrates progressively, quality management requires an active strategy for impurity removal. The fundamental tool for this is blowdown. The standard provides for the dosing of chemical conditioning agents into boiler water … Read more

HRSG

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Heat Recovery Steam Generator (HRSG): the role of economisers and heat exchangers | BOIXAC Technical guide › Energy recovery › HRSG Heat Recovery Steam Generator (HRSG): the role of economisers and heat exchangers Heat Recovery Steam Generator (HRSG) systems depend on the quality of their heat transfer components. This guide analyses the role of economisers and heat exchangers in optimising these systems, the determining design parameters and the selection criteria for demanding industrial applications. BOIXAC Tech SL Guia tècnica industrial Lectura: ~10 min Table of contents HRSG system fundamentals Definition and application context Thermal architecture and main components The economiser in an HRSG system Function and thermal positioning Key design parameters Heat exchangers: types and integration Quantifiable benefits of thermal integration Component selection criteria In an industrial context where energy efficiency is a determining factor for competitiveness and regulatory compliance, recovering residual heat from exhaust gases represents one of the best cost-benefit interventions available. HRSG (Heat Recovery Steam Generator) systems are the reference solution for this application, and their overall efficiency depends largely on the quality and design of their heat transfer components: in particular, economisers and auxiliary heat exchangers. 1. HRSG system fundamentals 1.1 Definition and application context An HRSG is a thermal recovery system that harnesses the enthalpy of hot exhaust gases from a gas turbine, internal combustion engine or industrial furnace to generate pressurised steam. This steam can be used for electricity generation in combined cycles, industrial heat processes or centralised district heating systems. The main applications of HRSGs include combined cycle gas-steam power plants (CCGT), industrial cogeneration facilities, petrochemical plants and refineries, and processes in the paper, cement and steel industries. 1.2 Thermal architecture and main components A conventional HRSG operates with exhaust gases flowing in counter-current or cross-flow against the water-steam circuit. Energy is transferred successively through several thermal sections, each optimised for a specific temperature range: Gas inlet Hot exhaust gases 400–650 °C at gas turbine outlet. Up to 900 °C in industrial furnaces. Section 1 Superheater Raises saturated steam temperature above the saturation point, preventing condensation in turbines. Section 2 Evaporator Converts liquid water into saturated steam at constant pressure. Phase change zone. Section 3 Economiser Preheats feedwater to near saturation point, extracting residual energy from already-cooled gases. Gas outlet Cooled gases 90–180 °C under optimal conditions. The economiser is key to minimising this value. Note on acid dew point temperature In applications with sulphur-containing fuels, the gas temperature at HRSG outlet cannot be reduced below the acid dew point temperature (typically 120–150 °C for gases containing SO₂), to prevent sulphurous acid condensation on economiser surfaces. This parameter is a critical design limit that directly constrains the maximum achievable energy recovery. 2. The economiser in an HRSG system 2.1 Function and thermal positioning The economiser is a gas-liquid heat exchanger positioned in the low-temperature zone of the HRSG, where exhaust gases have already transferred most of their energy to the evaporator and superheater. Its function is to extract residual enthalpy from these gases to preheat the boiler feedwater. The energy gain is directly proportional to the temperature difference between the water entering and leaving the economiser. A well-designed economiser can raise feedwater temperature from the typical 40–80 °C at deaerator outlet to 180–240 °C, drastically reducing the energy the evaporator must supply to achieve phase change. Industrial boiler economiser. Gas-liquid heat exchanger with helical finned tubes, designed to operate in combustion flue gas streams with inlet temperatures of 250–450 °C. 2.2 Key design parameters Designing an economiser for an HRSG requires the simultaneous analysis of multiple thermal, mechanical and process parameters. The main determining factors are: Parameter Typical range Design impact Gas inlet temperature 200–650 °C Determines material selection and potential corrosion regime Gas outlet temperature 90–200 °C Limited by acid dew point; constrains maximum recovery Water pressure 10–180 bar Defines tube wall thickness and PED requirements Water inlet temperature 40–120 °C Risk of condensation in humid gases; may require recirculation Pinch point temperature 8–20 °C Difference between saturation temperature and gas temperature at same section Gas mass flow rate Process-specific Determines pressure drop on gas side and ID fan power Particle content 0–50 g/Nm³ Determines free passage between fins and cleaning method required 3. Heat exchangers: types and integration Beyond the economiser itself, an HRSG system may incorporate various types of heat exchangers depending on the thermal needs of the associated process. 🔧 Helical finned tubes Preferred type for economisers in combustion gas streams with particulate content. Individual helical fins per tube provide greater mechanical robustness and vibration resistance. Fin pitch can be configured to minimise fouling in loaded gas streams. 📐 Continuous finned tubes Compact alternative for clean or filtered gases. Higher surface density per unit volume than helical fins, but requires particle-free gases to prevent inter-fin blockage. Common in combined-cycle gas turbine applications. ⚙️ Bare multi-tube For applications where the internal fluid is high-pressure steam or water and the external fluid is a gas with high particle loading or corrosive compounds. The absence of fins simplifies external cleaning and reduces blockage risk. 🌡️ Air pre-heaters (APH) In HRSG configurations coupled to burners, preheating combustion air with residual energy from exhaust gases improves burner efficiency and reduces fuel consumption. The gas-gas heat exchanger is the central component of this recovery. Helical finned tube heat exchanger. Standard type for economisers in HRSG systems with combustion gases containing suspended particulates. 4. Quantifiable benefits of thermal integration Incorporating correctly sized economisers and heat exchangers in an HRSG system produces measurable improvements in several operational and environmental indicators. ⚡ Improvement in overall energy efficiency A well-sized economiser can reduce gas outlet temperature by 80–150 °C, equivalent to recovering 3–8% of the total fuel energy burned. In combined-cycle plants, the economiser directly contributes to the overall electrical efficiency of the cycle. 💶 Reduction in fuel consumption Increasing feedwater temperature reduces the energy the evaporator must supply. For every 6 °C increase in feedwater temperature, boiler fuel consumption is reduced by approximately 1% under typical operating … Read more

Types of heat exchangers

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Types of Heat Exchangers: Classification by Construction and Operation | BOIXAC Technical guide › Heat transfer Types of Heat Exchangers: Classification by Construction and Operation Encyclopaedic guide to the main families of heat exchangers: from the distinction between direct and indirect contact to classification by fluid pair. Reference base for engineers, designers and technical managers. BOIXAC Tech SL Referència tècnica enciclopèdica Lectura: ~12 min Table of contents Classification by construction Direct contact Indirect contact Tube heat exchangers Plate heat exchangers Classification by operation Liquid–liquid heat exchangers Liquid–gas heat exchangers Gas–gas heat exchangers Bulk solid heat exchangers Selection criteria and design impact There are many types of heat exchangers and multiple ways to classify them. This article classifies them according to classification by construction and classification by operation, which considers the fluid pairs involved and their physical properties. 1. Classification by construction 1.1 Direct contact In direct contact heat exchangers, the two fluids are completely mixed. Cooling towers are the most representative example. Limitation of direct contact Fluid mixing can cause contaminant transfer between circuits. This makes direct contact unsuitable for most process cooling, energy recovery, gas treatment, food liquid and bulk solid systems where circuit separation is a technical or sanitary requirement. 1.2 Indirect contact In indirect contact heat exchangers, the two fluids remain permanently separated by a physical element — usually a metal plate or tube wall — acting as the heat transfer surface without allowing any mixing. Focusing on the two main families — tubes and plates — a comparison can be drawn as follows. Special case: rotary heat recuperators Rotary heat recuperators are a special case within indirect contact. The two fluids traverse the same space alternately. A slight cross-contamination is theoretically possible, but in industrial practice is considered negligible. Feature Tube heat exchangers Plate heat exchangers Compactness Less compact for the same duty High compactness: maximum surface in minimum volume Transfer coefficient Moderate, depending on tube and fin design High thanks to turbulence induced by corrugations Flow cross-section Wide; less susceptible to fouling Narrow channels; risk of blockage Viscous / loaded fluids Highly recommended. High tolerance to particles and viscosity Unsuitable for dirty, viscous or sticky fluids Maintenance and cleaning Simple. Rarely clog; low maintenance cost More susceptible to scaling; more frequent cleaning required Dusty / abrasive environments Excellent performance Not well suited Preferred application Gas-gas, gas-liquid, liquid-liquid in demanding conditions Liquid-liquid in clean, controlled circuits 1.3 Tube heat exchangers Tube heat exchangers consist of cylindrical, flat or oval tubes, the cross-section being selected according to the specific requirements of each system. 1.3.1 Bare tubes When the internal and external exchange surfaces are similar — fluids with comparable specific heats — bare tubes are used: bare-tube multi-tube exchangers for gas-to-gas, and tubular, multi-tube, shell-and-tube, coaxial or double-pipe, and fire-tube configurations for liquids. Multi-tube heat exchanger. Common in liquid-liquid applications with clean or moderately viscous fluids. 1.3.2 Finned tubes When the two fluids have very different specific heats — a common situation when one fluid is a gas and the other a liquid or steam — the exchange surface must be compensated by adding fins on the side of the fluid with lower specific heat. Why are fins necessary? Quantitative example The specific heat of gas (dry air) is approximately 1.214 kJ/m³·K, while that of water is 4.186 kJ/m³·K. Water can give up or absorb almost 3.5 times more energy per unit volume than air. To compensate for this imbalance, the exchange surface on the gas side is enlarged using fins. Gas (dry air) — 1.214 kJ/m³·K1.214 kJ/m³·KSaturated steam — ~2.010 kJ/m³·K~2.010 kJ/m³·KThermal oil — ~2.000 kJ/m³·K~2.000 kJ/m³·KWater — 4.186 kJ/m³·K4.186 kJ/m³·K Finned tubes Continuous fins (transverse to the tubes) Continuous perforated sheets through which tubes pass perpendicularly. Uniform distribution of fin surface. Common in industrial HVAC and heat recuperators for exhaust gases with relatively clean air. Finned tubes Helical fins (wound around the tubes) Sheets wound helically around each individual tube. Greater mechanical robustness and vibration resistance. Used for combustion gases, industrial fumes and streams with some particle content. Heat recuperator (economiser) for industrial boiler. Gas-liquid application with helical finned tubes. 1.4 Plate heat exchangers Plate heat exchangers consist of flat or corrugated plates acting simultaneously as heat transfer surface and as structural element of the flow channel. Plates Pillow plate heat exchanger Emerging technology of great versatility. The cushion-shaped surface allows working with viscous, sticky and particle-laden fluids, and transferring energy to granular solids as an alternative to fluidised beds. Plates Cross-flow heat exchanger Plate system in perpendicular flow configuration, widely used in HVAC energy recovery. Achieves high efficiency values but requires advanced air filters due to the difficulty of internal cleaning. Welded plate heat exchanger Plates are joined by welding, forming a rigid assembly without gaskets. Internal cleaning is not possible; only applicable with completely clean fluids generating no scaling. Gasketed plate heat exchanger Gaskets allow individual plates to be dismantled, cleaned and replaced. More versatile than the welded type, but channels remain narrow and susceptible to blockage with viscous or particle-laden fluids. 2. Classification by operation Classification by operation considers the fluid pairs involved and their physical properties. Correct selection is essential to maximise efficiency and ensure long-term installation reliability. Liquid–LiquidPillow plate · Welded platesGasketed plates · Concentric tubesCoaxial · Shell-and-tube · Double pipeLiquid–GasBare tubesContinuous finned tubesHelical finned tubesHeat recuperatorsGas–GasMulti-tube · Bare tubesCross-flow · RotaryFlue gas recuperatorsBulk solidsPillow plate(alternative to fluidised beds) 2.1 Liquid–liquid heat exchangers In applications where both fluids are liquids, specific heats are usually similar. Selection depends mainly on fluid viscosity, suspended particle content and operating pressures. 2.2 Liquid–gas heat exchangers This is the situation where the difference in specific heats is most significant. Gas has a much lower specific heat than typical liquids, making it necessary to considerably increase the exchange surface on the gas side using fins. 2.3 Gas–gas heat exchangers When both fluids are gases, their specific heats are similar. However, the low convection coefficient of gas makes it necessary to increase the total surface to … Read more

Conduction, convection & radiation

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CONDUCTION, CONVECTION & RADIATION HEAT TRANSFER IN NATURE In nature, we find fascinating examples of heat transfer through conduction, convection, and radiation—three fundamental mechanisms in thermodynamics. For example, imagine a summer morning at the beach. Early in the morning, the air remains calm because there is a thermal equilibrium between the temperature of the air mass over the sea and the air mass over the land. As the Sun heats the Earth’s surface, the temperature of the air over the land rises more quickly than that of the air over the sea. This creates a thermal contrast: the warm air over the land rises, while the cooler air from the sea moves toward the land to take its place. This movement of air masses is a clear example of thermal convection, the same principle that allows hot air balloons to rise. The more the Sun heats the land, the stronger this temperature difference becomes, increasing the speed of the sea breeze. This rising warm air favors the formation of small cumulus clouds, and if the temperature difference is significant enough, cumulonimbus clouds may appear, which are responsible for sudden summer storms. Unlike radiation, which transfers energy without direct contact (such as the Sun’s rays heating the sand), convection depends on the movement of fluids like air or water. On the other hand, thermal conduction occurs when two objects at different temperatures come into contact—for example, when we walk barefoot on hot sand at noon and feel the heat transferring to our feet. So, the next time you’re at the beach and notice the sea breeze picking up at midday, think of BOIXAC, the specialists in thermal exchange for the industry.