Industrial utilization of waste heat: ORC systems, heat exchangers and calculation of return on investment

1. Introduction

Industrial enterprises of Ukraine daily face the challenge of optimizing energy consumption and ensuring uninterrupted operation of equipment. A significant part of the consumed energy is transformed into waste heat, which is often released into the atmosphere, which leads to economic losses and a negative impact on the environment. Effective utilization of waste heat (WHT) is a critically important engineering task for increasing energy efficiency, reducing production costs, and strengthening the country's energy security.

This article is a technical reference covering key aspects of CHP, including Organic Rankine Cycle (ORC) systems, modern heat exchangers and ROI methodology. The implementation of such systems provides not only economic benefit, but also increases the reliability of technological processes, contributing to long-term stability of production.

2. Fundamental principles

Classification of waste heat

Waste heat is classified by temperature range, which determines potential technologies for its utilization:

• High-temperature (>400 °C): Output gases of furnaces, dryers, metallurgical processes.
• Medium-temperature (100–400 °C): Waste heat from internal combustion engines, steam boilers, process fluids.
• Low-temperature (<100 °C): Cooling water for compressors, diesel generators, technological effluents.

Principles of heat exchange

Heat exchangers are the main components of UHT systems. The principle of their operation is based on the transfer of thermal energy from a hot working body to a cold one through a separating wall. The main mechanisms of heat exchange are:

• Thermal conductivity (conduction): Heat transfer through the wall material.
• Convection: Transfer of heat by moving streams of liquid or gas.
• Emission (radiation): Transfer of heat by electromagnetic waves (less significant for most industrial heat exchangers except for high temperature applications).

The key parameters are the overall heat transfer coefficient (U), the heat transfer area (A) and the logarithmic mean temperature difference (LMTD).

Heat exchange equation: Q = U × A × LMTD, where Q is the transferred thermal power (kW).

Organic Rankine Cycle (ORC) Systems

The ORC system converts thermal energy into mechanical and then electrical energy using an organic working agent that has a lower boiling point and higher saturated vapor pressure compared to water. This allows efficient use of low- and medium-temperature waste heat (from 70 °C to 350 °C).

The main components of the ORC system:

1. Evaporator: Heat from the spent source is transferred to the working agent that is vaporized.
2. Expander (turbine): The superheated steam of the working agent expands, which rotates the turbine and generates mechanical energy. The efficiency of the turbine is usually 80-85%.
3. Generator: The mechanical energy of the turbine is converted into electrical energy.
4. Condenser: The steam of the working agent condenses, giving off heat to the cooling medium (air, water).
5. Pump: The liquid working agent is fed back to the evaporator, closing the cycle.

Common organic working agents include R245fa, n-pentane, isobutane, toluene. The choice of agent depends on the temperature range of the heat source and safety requirements (flammability classification according to EN 378).

Typical electrical efficiency of ORC systems for low-temperature sources is 10-25%.

3. Technical characteristics and standards

Heat exchangers

The design and manufacture of heat exchangers is regulated by a number of international and national standards to ensure safety and reliability:

• EN 13445: "Leaky pressure vessels" is a European standard covering the design, manufacture, inspection and testing of pressure vessels.
• ASME Boiler and Pressure Vessel Code, Section VIII: "Rules for the construction of pressure vessels" is an American standard, often used internationally.
• Directive 2014/68/EU (PED): "Pressure Equipment" - requires CE marking for heat exchangers operated in the European Union.
• DSTU EN ISO 15614-1: "Technical requirements and qualification of welding procedures of metallic materials" - to ensure the quality of welds.

Materials are chosen according to the temperature, pressure and aggressiveness of the working environment. Common materials: stainless steel (AISI 304, 316), duplex alloys, nickel alloys. Design pressure can reach 10-40 bar, temperature - up to 450 °C.

Quality control includes hydrostatic testing (according to EN 13445-5) and non-destructive testing (ultrasound, radiography).

The UkrSEPRO certification confirms the compliance of the equipment with the national standards of Ukraine.

ORC systems

Standardization of ORC systems covers their components and integration into electrical networks:

• EN 378: "Refrigerating systems and heat pumps. Safety and environmental requirements" - refers to the selection of working agents and the safety of systems that use them.
• IEC 61508: "Functional safety of safety-related electrical/electronic/programmable electronic systems" - for ORC control and safety systems.
• DSTU EN 50549: "Requirements for generating units connected in parallel to distribution networks" is a mandatory standard for connecting ORC generators to the Ukrainian power system.
• DSTU IEC 60034-1: "Electric rotary machines. Part 1. Nominal modes of operation and operating characteristics" - for the standardization of generators used in ORC systems.

Typical ORC modules for industrial waste heat recovery range in electrical power from 50 kW to several megawatts. The mean time between failures (MTBF) of quality ORC turbines is 30,000 - 50,000 hours between overhauls.

4. Guide to selection and calculation

UHT technology selection matrix

The choice of the optimal UHT technology depends on many factors. The table below is a simplified decision matrix:

Heat exchanger calculation

For effective calculation of the heat exchanger, it is necessary to have the following initial data:

• Temperature at the inlet/outlet of the hot coolant (T_h,in, T_h,out)
• Temperature at the inlet/outlet of the cold coolant (T_c,in, T_c,out)
• Mass flow rates of both coolants (ṁ_h, ṁ_c)
• Specific heat capacity of heat carriers (c_p,h, c_p,c)

Thermal power (Q) is determined by the formula: Q = ṁ × c_p × ∆T.

Example: For a plate heat exchanger utilizing 200 kW of thermal power from flue gases, the choice of materials (e.g. AISI 316) and plate configuration will be critical to achieve an overall heat transfer coefficient of U = 300-500 W/(m²·K).

Calculation of return on investment (ROI)

Assessment of the economic efficiency of the UHT project is key. The main indicators are net present value (NPV), internal rate of return (IRR) and payback period (PBP).

The simple payback formula:

PBP = Initial Investment / Annual Savings

Annual savings consist of:

• Savings on electricity: Utilized electricity (MWh/year) × Cost of electricity (€/MWh)
• Fuel savings: Reduction in fuel consumption (m³/year or kg/year) × Fuel cost (€/m³ or €/kg)
• Savings on CO₂ emissions (if carbon quotas are available): Reduction of CO₂ emissions (t/year) × Quota cost (€/t)
• Minus operating and repair costs (O&M costs)

Example: The investment in the ORC system is €500,000. The system generates 1000 MWh/year of electricity. The cost of electricity is €150/MWh. Annual operating costs €20,000.
Annual savings = (1000 MWh/year × €150/MWh) - €20,000 = €150,000 - €20,000 = €130,000.
PBP = €500,000 / €130,000 ≈ 3.85 years. This indicator is attractive for industrial investments.

5. Best Practices for Installation and Commissioning

Correct installation and commissioning are crucial for the long-term and reliable operation of UHT systems.

Preparatory works

• Site analysis: Assessment of available space, availability of engineering communications (water, electricity).
• Designing foundations: Providing a stable foundation for heavy equipment such as ORC turbines and large heat exchangers.
• Piping: Optimization of length and diameters to minimize pressure losses and ensure optimal hydraulic characteristics.
• Electrical integration: Development of schemes for connecting ORC generators to the enterprise's internal network and external network (according to DSTU EN 50549).

Installation

• Heat exchangers: Installation taking into account thermal expansion compensation, correct orientation and strong mounting. Compliance with the tightening moments of the fastening elements for flange connections and gaskets.
• ORC modules: Provision of vibration isolation, exact centering of the turbine/expander and generator (collinearity tolerance up to 0.05 mm). Filling with a working agent after vacuuming the system (up to a residual pressure of no more than 10 Pa).
• Security: Application of lockout and tagout (LOTO) procedures during installation work to prevent unauthorized start-up.

Commissioning

• Leakage testing: Hydrostatic testing of pipelines and equipment according to EN 13445-5.
• System flushing: Removal of contamination from pipelines before filling with working fluids.
• Control system calibration: Setting temperature, pressure, and flow sensors. Checking the logic of the controllers (compliance IEC 61508).
• Performance testing: Checking the actual heat capacity of the heat exchangers and the electric power of the ORC system for compliance with the design parameters.
• Personnel training: Conducting detailed training for operators and maintenance personnel on safe and efficient operation of the system.

6. Failure modes and root cause analysis

Heat exchangers

• Fouling: Formation of deposits on heat exchange surfaces.

Indicators: A decrease in the overall heat transfer coefficient (U), an increase in the temperature difference between the inlet and outlet of the coolant, and an increase in hydraulic resistance.
Primary causes: Insufficient preparation of heat carriers (water, flue gases), incompatibility of working fluids, low flow rate.

• Corrosion: Destruction of the heat exchanger wall material.

Indicators: Leaks, thinning of the walls, change in the color of the coolant.
Root causes: Incorrect choice of material for an aggressive environment, chemical attack, insufficient corrosion inhibition.

Leaks: Seal failure due to damage to gaskets, welds or tubes.

Indicators: Mixing of coolants, pressure drop, detection of drops.
Root causes: Incorrect installation, vibration, thermal cycles, mechanical damage, aging of gaskets.

Erosion: Wear of material due to the influence of high-speed streams of abrasive particles.

Indicators: Thinning of the walls, formation of grooves, increased noise.
Primary causes: Excessive flow velocities, presence of solid impurities in coolants.

ORC systems

• Expander/turbine failure: Bearing wear, blade damage.

Indicators: Increased vibration (above normal ISO 10816), reduced power, unusual noise.
Root causes: Imbalance of the rotor, malfunction of the lubrication system, ingress of foreign objects.

• Degradation of working agent: Decomposition of organic liquid due to overheating or chemical reactions.

Indicators: Decrease in system productivity, change in the chemical composition of the agent (liquid analysis), formation of non-condensable gases.
Root causes: Excessive operating temperature, ingress of oxygen or moisture into the system.

• Pump cavitation: Formation and collapse of steam bubbles in the pump.

Indicators: Characteristic noise (cracking), vibration, decrease in pump performance.
Root causes: Insufficient cavitation margin (NPSH), incorrect pump selection, excessive suction liquid temperature.

• Control system failure: Incorrect operation of sensors, controllers, actuators.

Indicators: Unstable operation of the system, emergency stops, incorrect readings of parameters.
Root causes: Electrical interference, sensor drift, software errors, power failure.

7. Predictive maintenance and condition monitoring

Implementation of predictive maintenance (PTO) systems allows you to detect potential malfunctions before they develop, minimize downtime, and optimize repair schedules.

Heat exchangers

• Monitoring thermal efficiency: Regular calculation of total heat transfer coefficient (U) and LMTD. Deviation from the basic values ​​indicates contamination or deterioration of efficiency.
• Monitoring the pressure drop: An increase in the pressure drop (∆P) across the heat exchanger indicates contamination or blocked channels.
• Vibration analysis: Detection of flow-induced vibrations or component wear.
• Analysis of coolants: Regular control of the composition of cooling water and process fluids for the presence of corrosive agents, deposits, and mechanical impurities.
• Infrared Thermography: Detection of hot spots or uneven flow distribution, which may indicate contamination or internal damage.

ORC systems

• Vibration monitoring: Constant monitoring of the vibration of bearings, turbine/expander, pumps according to the ISO 10816. standard.Analysis of the vibration spectrum allows detection of imbalance, misalignment, wear of bearings.
• Analysis of the working agent: Periodic analysis of the physical and chemical properties of the organic agent (acid number, moisture content, non-condensable gases, decomposition products) to prevent degradation.
• Temperature and pressure monitoring: Continuous monitoring of parameters at key points of the cycle (evaporator inlet/outlet, condenser, expander) to track deviations from normal operating modes.
• Acoustic monitoring: Use of acoustic sensors to detect leaks of the working agent or cavitation of pumps.
• Monitoring of electrical parameters: Control of the output power of the generator (voltage, current, power factor) and the current of the electric motors of the pumps to assess their efficiency and condition.

8. UHT technology comparison matrix

The table below provides a comparative analysis of the three main approaches to waste heat recovery, enabling UNITEC-D engineers and their partners to make an informed decision.

9. Conclusion

Utilization of industrial waste heat is a strategic direction for increasing the energy efficiency and competitiveness of Ukrainian enterprises. ORC systems and high-efficiency heat exchangers are proven technologies that allow you to convert wasted energy into valuable resources — heat and electricity. A comprehensive engineering approach, including careful selection of equipment, professional installation and implementation of predictive maintenance systems, guarantees reliability and quick return on investment.

UNITEC-D GmbH, as a reliable supplier of high-tech industrial components, offers a wide range of equipment that complies with CE and UkrSEPRO standards for the implementation of the most complex waste heat recovery projects. From high-quality heat exchangers to precise sensors and components for ORC systems - we provide Ukrainian industry with reliable solutions.

For a detailed overview of the range of components for waste heat recovery systems, including high efficiency heat exchangers, valves, actuators and sensors, visit the UNITEC-D e-catalogue: https://www.unitecd.com/e-catalog/

10. List of references

1. EN 13445-1:2021. Unfired pressure vessels. Part 1: General. CEN, 2021.
2. ISO 10816-1:1995. Mechanical vibration - Evaluation of machine vibration by measurements on non-rotating parts - Part 1: General guidelines. ISO, 1995.
3. DSTU EN 50549-1:2020. Requirements for generating units connected in parallel to distribution electric networks. Part 1: Connection to low-voltage distribution networks. SE "UkrNDNC", 2020.
4. Directive 2014/68/EU of the European Parliament and of the Council of 15 May 2014 on the harmonization of the laws of the Member States relating to the making available on the market of pressure equipment (recast).
5. DSTU IEC 60034-1:2018. Electric rotary machines. Part 1. Rated operating modes and operating characteristics (IEC 60034-1:2017, IDT). SE "UkrNDNC", 2018.