1. Introduction

In the aerospace and energy sectors, optimizing energy efficiency represents a critical issue. Industrial waste heat, often dissipated into the environment, constitutes a substantial but underexploited energy source. Its recovery not only reduces operational costs and greenhouse gas emissions, but also improves the overall reliability of installations. This document examines the principles, technologies and economic analysis methodologies associated with waste heat recovery, focusing on organic Rankine cycles (ORC) and heat exchangers, in accordance with NF and EN standards.

2. Fundamental Principles

2.1. Thermodynamics of Heat Recovery

Waste heat recovery is rooted in the principles of thermodynamics. In accordance with Carnot's second law, part of the thermal energy of a system is inevitably degraded. However, a significant fraction of this energy, especially at low and medium temperatures (80°C to 350°C), can be converted into a useful form. Heat transfer occurs by conduction, convection and radiation. The objective is to maximize the overall heat transfer coefficient (U) while minimizing losses.

2.2. Organic Rankine Cycles (ORC)

ORC systems use a heat source to evaporate an organic fluid (hydrocarbons, siloxanes, or refrigerants such as R245fa or R1233zd(E)) at a lower boiling point and pressure than water. This fluid, in its gaseous state, drives a turbine to produce electricity before being condensed and pumped back to the evaporator, thus closing the cycle. ORCs are particularly suited to low and medium temperature heat sources, typical of many industrial processes.

The thermal efficiency of an ORC cycle is influenced by the nature of the working fluid, the hot and cold source temperatures, and the performance of the components. Fluids like cyclopentane are used for hot spring temperatures around 200-250°C, with an electrical conversion efficiency of up to 15-20% for a sufficient temperature delta.

2.3. Heat Exchangers

Heat exchangers are the central components of any heat recovery system. They facilitate heat transfer between two fluids at different temperatures without direct contact. Common types include:

Plate exchangers: Compact, high efficiency, low pressure losses. Ideal for clean fluids.
Tubular exchangers (shell and tubes): Robust, suitable for clogging or high pressure/temperature fluids. In accordance with the EN 13445 standard (Pressure devices not subject to flame).
Finned exchangers: Used to improve heat transfer with gases, often on the gas side face.

The choice of exchanger depends on the properties of the fluids, temperatures, operating pressures and clogging constraints.

3. Technical Specifications and Standards

3.1. Applicable Standards and Certifications

The design and operation of heat recovery systems must meet strict standards:

Pressure Equipment Directive (PED) 2014/68/EU: Mandatory for exchangers and other pressure components.
Standard EN 13445: Specifies requirements for pressure vessels not subject to flame.
Standard EN 14276: Refers to pressure equipment for refrigeration systems and heat pumps.
Standard ISO 281: Applicable to bearings used in ORC turbines.
Standard IEC 60947-2: Concerns circuit breakers for the protection of electrical circuits generated by ORCs.
ATEX 2014/34/EU Directive: Essential if flammable organic fluids are used in potentially explosive environments (classified areas).
CE certifications: Essential for the free movement of products within the EU.

3.2. Component Specifications

ORC fluids: Critical point, saturation vapor pressures, material compatibility, GWP (Global Warming Potential), ODP (Ozone Depletion Potential). For example, R245fa has a GWP of 1030, encouraging the use of low GWP fluids like R1233zd(E) (GWP < 1).
Exchangers:
- Materials: Stainless steel (EN 1.4404 or EN 1.4571) for corrosion resistance, Titanium for aggressive fluids.
- Design pressure: Typically from 10 to 40 bar for exchangers, and up to 80 bar for certain ORC evaporators.
- Design temperature: Range from -20°C to 300°C, or even higher for specific applications.
- Exchange surface: Dependent on the thermal power to be recovered, calculated in m².

4. Selection and Sizing Guide

Selecting a waste heat recovery system is a rigorous engineering process.

4.1. Heat Source Analysis

Characterization of residual heat: temperature, flow rate, composition (if combustion gas), pressure, nature of the fluid (gas, liquid, two-phase), seasonal or operational variations. A typical source in industry may be an off-gas at 250°C with a flow rate of 10,000 Nm³/h.

4.2. Heat Exchanger Selection Criteria

The following table provides a decision matrix for the choice of exchanger technology:

Criterion	Plate Exchanger	Tubular Exchanger	Finned Exchanger
Thermal Efficiency	High (low temperature approach, < 3 K)	Medium to High (depends on design)	Medium (optimized for gases)
Max. Working Pressure	Up to 30 bar	Up to 100 bar and more	Generally low (< 10 bar)
Max. Service Temperature	Jusqu'à 200 °C	Up to 600°C and more	Up to 400°C
Fluid Clogging	Low (clean fluids)	High (clogging fluids, particles)	Medium (depends on fin geometry)
Compactness	Very compact	Less compact	Compact (gas side)
Initial Cost	Average	High	Average

4.3. Calculation of Return on Investment (ROI)

ROI is a determining factor in the investment decision. The Payback Period formula is commonly used:

Payback Time (years) = Initial Investment Cost / (Annual Savings - Annual Operational Costs)

Annual savings come from the reduction in primary energy consumption (natural gas, electricity) and possible carbon credits. Operational costs include maintenance, electricity consumption of auxiliaries (pumps), and insurance. A heat recovery project can typically have a payback time of between 2 and 5 years, with annual savings of several hundred thousand euros for large installations.

The analysis should also include the Net Present Value (NPV) and the Internal Rate of Return (IRR) for a complete financial assessment over the life of the project (generally 15-20 years).

5. Good Installation and Commissioning Practices

Rigorous installation and commissioning ensures system performance and durability.

Piping network: Design to minimize pressure losses (EN 13480 standard). Materials compatible with fluids and temperatures.
Thermal Insulation: Essential to maintain efficiency. Complies with the NF EN ISO 12241. standard
Support and Vibrations: Prevention of mechanical stress and vibrations which can damage exchangers and pipes.
Purge and Leak Test: Before commissioning, purge the systems to remove air and impurities. Carry out pressure leak tests in accordance with the PED.
Instrumentation and Control: Installation of temperature (NF EN 13398), pressure (NF EN 837), flow sensors for system supervision and regulation.

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6. Failure Modes and Root Cause Analysis

Understanding failure modes is crucial for preventive and corrective maintenance.

6.1. Heat Exchangers

Fouling: Accumulation of deposits (mineral, organic, corrosion) on the exchange surfaces, reducing heat transfer and increasing pressure losses. Visual indicators:Decrease in thermal performance, increase in temperature on the cold fluid side, increase in differential pressure.
Corrosion: Degradation of the material by chemical reaction. Visual indicators: Stains, perforations, leaks, thickening of deposits.
Erosion: Wear due to high fluid velocity or the presence of abrasive particles. Visual indicators: Thinning of the walls, grooves.
Leaks: Breakage of joints, cracks in tubes or plates. Visual indicators: Loss of fluid, mixing of fluids, drop in pressure.
Thermal Stress: Due to rapid temperature variations, which can lead to deformation or cracking.

6.2. ORC Systems

Pump failure: Cavitation, bearing wear (ISO 281), motor failure. Visual indicators: Abnormal noise, vibrations, drop in flow.
Turbine problems: Blade wear, imbalance, bearing failure. Visual indicators:Excessive vibrations, loss of electrical power.
Degradation of Working Fluid: Thermal decomposition of organic fluid, leading to the formation of corrosive by-products and reduced performance. Visual indicators: Change in fluid color, increase in pressure/temperature, chemical analyses.
Control System Malfunction: Incorrect regulation of flow rates, temperatures or pressures.

7. Predictive Maintenance and Condition Monitoring

These techniques are essential for anticipating failures and optimizing the lifespan of equipment.

Temperature and Pressure Monitoring: Use of PT100 sensors (NF EN 60751) and pressure transmitters to monitor hot/cold spots and pressure differentials across the exchangers.
Vibration Analysis: Early detection of bearing problems, imbalance or misalignment in pumps and turbines. Complies with ISO 10816. standard
Fluid Analysis: Regular monitoring of ORC fluid quality (acid number, moisture content, decomposition products) to prevent degradation.
Infrared Thermography: Identification of areas of fouling or defective insulation on exchangers and pipes.
Acoustic Monitoring: Detection of internal leaks or cavitation.

This data, combined with machine learning algorithms, helps predict failures and plan maintenance proactively, reducing unplanned downtime and repair costs.

8. Heat Recovery Technologies Comparison Matrix

The selection of waste heat recovery technology depends on the source characteristics and energy needs. Here is a comparison of common solutions:

Technology	Hot Spring Temperature Range	Main Application	Conversion Efficiency	System Complexity	Relative Initial Cost
ORC (Organic Rankine Cycle)	80°C - 350°C	Electricity production from low/medium temperature heat	10% - 25% (electric)	Average	Medium to High
Recovery Steam Generator (HRSG)	> 250°C	Steam production for processes or electricity (water/steam Rankine cycle)	Up to 90% (thermal)	High	High
Economizer (Feedwater Preheater)	150°C - 400°C	Boiler feed water preheating	Up to 90% (thermal)	Low	Low
Heat Pump (Absorption/Compression)	50°C - 100°C	Temperature rise for heating or hot water production	typical COP 3-7	Average	Average
Steam Turbine (small format)	> 300°C	Electricity production from high pressure steam	20% - 35% (electric)	High	High

9. Conclusion

Waste heat recovery is an essential strategy for industries keen to optimize their energy consumption and reduce their environmental footprint. ORC systems and heat exchangers are proven technologies, offering solutions suitable for various waste heat sources. Rigorous design, based on current standards (PED, EN 13445, ATEX), compliant installation and predictive maintenance are the pillars of the success of these projects. The economic analysis, including the calculation of the ROI, makes it possible to justify these transformative investments.

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10. References

NF EN 13445-1:2021 – Pressure vessels not subject to flame – Part 1: General.
Directive 2014/68/EU of the European Parliament and of the Council of May 15, 2014 on the harmonization of the laws of the Member States relating to the making available on the market of pressure equipment.
NF EN ISO 10816-1:2009 – Measurement and evaluation of mechanical vibrations of non-rotating machines – Part 1: General guidelines.
International Institute of Refrigeration (IIR) – Technical Guides on Organic Rankine Cycle.
US Department of Energy – Waste Heat Recovery Technology & Applications.