cross flow recuperator

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: 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