Closed-loop heat recovery systems – A sustainable solution for heat recovery?

With rising energy costs and increasing sustainability requirements, the efficient use of waste heat is becoming an increasingly important priority for industrial companies. A proven approach to reducing energy consumption in ventilation systems is the use of a closed-loop heat recovery system. This technology provides an efficient, economical and technically reliable method of heat recovery, even when handling heavily contaminated or corrosive exhaust air streams – without any direct contact between the exhaust and supply air.

But what exactly is a closed-loop heat recovery system? How does it work, and where is it used? In this article, we explain the operating principle of a closed-loop heat recovery system and demonstrate why it is a sustainable solution, particularly for demanding industrial environments such as surface finishing, the chemical industry and research facilities. In HVAC applications, this type of system is also referred to as a run-around coil system. In industrial applications, however, the term closed-loop heat recovery system is more commonly used, particularly for systems handling corrosive exhaust air and using corrosion-resistant plastic heat exchangers.

If you are planning or optimising an industrial ventilation system, this article provides a comprehensive overview of heat recovery using a closed-loop heat recovery system. It also explains how Calorplast plastic heat exchangers help maximise efficiency, corrosion resistance and operational reliability within the system.

Key facts at a glance

A closed-loop heat recovery system is a closed heat recovery solution specifically designed for industrial ventilation systems.

It transfers thermal energy from the exhaust air to the supply air without any direct air contact.

It is ideal for applications with hygienically sensitive or corrosive exhaust air, such as surface finishing, the chemical industry and laboratories.

A typical closed-loop heat recovery system consists of two separate heat exchangers connected by a closed liquid circuit.

Heat is transferred by means of a heat transfer fluid circulated by a pump.

Closed-loop heat recovery systems are low-maintenance, reliable and highly energy-efficient.

They provide a sustainable way to reduce heating demand and lower operating costs in industrial ventilation systems.

How does a closed-coop heat recovery system work?

A closed-loop heat recovery system uses two separate heat exchangers connected by a closed liquid circuit. Heat from the exhaust air is absorbed, transported and transferred to the incoming supply air efficiently, with minimal maintenance requirements and without any mixing of the air streams.

What is the purpose of a closed-loop heat recovery system?

A closed-loop heat recovery system is more than just a component of an air handling system. It performs essential functions that improve energy efficiency and process reliability in industrial ventilation systems. Its primary tasks include recovering waste heat, conditioning exhaust and supply air, and providing temperature-controlled energy transfer for sensitive production areas. Especially in industries handling aggressive or contaminated exhaust air, a closed-loop heat recovery system offers a safe, reliable and cost-effective solution for reducing energy consumption and improving environmental performance.

Treatment of supply and exhaust air

Many industrial processes generate contaminated or chemically aggressive exhaust air that must not come into direct contact with fresh air. A closed-loop heat recovery system ensures complete separation between the exhaust and supply air streams. Using two separate heat exchangers, thermal energy is transferred from the warm exhaust air to a heat transfer fluid and then released to the supply air. As a result, the supply air is efficiently preheated without any risk of cross-contamination—an essential requirement for controlled environments such as chemical production facilities and cleanrooms.

Supply air temperature control

One of the key advantages of a closed-loop heat recovery system is its ability to precisely control the temperature of the supply air. Instead of relying on additional heating or cooling systems, the system uses the thermal energy contained in the exhaust air to precondition the incoming fresh air. This not only reduces heating costs but also helps maintain stable temperature conditions in temperature-sensitive production areas and laboratory environments. As a result, the system contributes significantly to an energy-efficient and reliable ventilation process.

Design of a gas-to-water heat exchanger and air circulation in a closed-loop heat recovery system

Efficient control of air circulation is essential for both process quality and indoor climate in industrial facilities. In many closed-loop heat recovery systems, a gas-to-water heat exchanger is the key component responsible for transferring thermal energy between the exhaust and supply air streams.Die Steuerung der Luftzirkulation innerhalb einer Anlage ist für die Prozessqualität und das Raumklima von zentraler Bedeutung. In vielen Kreislaufverbundsystemen ist ein Gas-Wasser-Wärmetauscher zentrales Element, um die Luftzirkulation zu regulieren.

To ensure complete separation between exhaust air and supply air, Calorplast uses two gas-to-water heat exchangers within the closed-loop heat recovery system. The warm exhaust air from the production area passes through the exhaust air heat exchanger before being discharged outdoors, while fresh outside air flows through the supply air heat exchanger before entering the building. The two heat exchangers are connected by two pipes forming a closed hydraulic circuit. This hydraulic system is controlled to regulate the flow rate of the heat transfer fluid, ensuring that the required temperatures are achieved. The more precisely the hydraulic circuit is controlled, the more efficiently heat can be recovered and transferred from the exhaust air to the supply air.Um die Abluft und Zuluft in Industriebetrieben kompromisslos voneinander zu trennen, setzt Calorplast zwei Gas-Wasser-Wärmetauscher im Kreislaufverbundsystem ein. Die Abluft aus der Produktionshalle strömt durch den Abluftwärmetauscher nach außen und die frische Zuluft strömt durch den Zuluftwärmetauscher von außen in die Produktionshalle. Die beiden Wärmetauscher sind dabei über zwei Röhren, die das hydraulische System bilden, miteinander verbunden. Dieses hydraulische System nutzt entsprechende Steuerungen, um den Wärmetauschern zu signalisieren, welche Wassermengen durch die Wärmetauscher zirkulieren müssen, um die gewünschten Temperaturen zu erreichen. Je präziser die hydraulische Steuerung ist, desto effektiver kann die Wärme aus den Wärmetauschern herausgezogen werden. So wird die thermische Energie zwischen Zu- und Abluft übertragen.

How a closed-loop heat recovery system works in detail

A closed-loop heat recovery system is a proven solution for heat recovery in industrial ventilation systems, particularly where exhaust air and supply air must remain physically separated due to plant layout or safety requirements. The system operates on the principle of indirect heat transfer using a liquid heat transfer fluid. Two heat exchangers are connected via a closed hydraulic circuit—one located in the exhaust air stream and the other in the supply air stream. Thermal energy is absorbed from the exhaust air, transported through the closed circuit and transferred to the supply air. This significantly reduces the energy required to heat or cool the supply air without any direct mixing of the air streams. Indirect heat recovery is particularly beneficial in systems handling contaminated or corrosive exhaust air, as it combines high operational safety with excellent energy efficiency.

The role of the heat exchanger in a closed-loop heat recovery system

The heat exchanger is the core component of every closed-loop heat recovery system. It is responsible for transferring thermal energy between the air stream and the circulating heat transfer fluid. A typical closed-loop heat recovery system consists of two identical heat exchangers: one on the exhaust air side, where thermal energy is absorbed from the warm exhaust air, and one on the supply air side, where this energy is released. The heat transfer fluid—typically water or a water-glycol mixture—circulates continuously between both heat exchangers and transports the thermal energy.

The choice of heat exchanger depends on the specific requirements of the application. In industrial environments exposed to chemically aggressive media, Calorplast gas-to-water heat exchangers are an ideal solution. Their all-plastic design provides outstanding corrosion resistance, hygienic operation and a long service life. Thanks to their modular design, these heat exchangers can be adapted to different air volumes and installation conditions, making them particularly suitable for customised industrial ventilation concepts.

Combined with intelligent flow control and precise system regulation, the heat exchanger ensures that the closed-loop heat recovery system operates efficiently, reliably and with minimal maintenance over the long term. This transforms waste heat into a valuable energy resource while contributing to sustainable industrial energy use.

Closed-loop heat recovery systems in practice: typical applications

A closed-loop heat recovery system is the preferred solution wherever high standards of hygiene, safety or process purity are required, and where exhaust and supply air must not be mixed due to technical or regulatory requirements. By completely separating the air streams while efficiently recovering heat, the system provides a reliable solution for demanding industrial and building services applications. Especially where contaminated or chemically aggressive exhaust air is present, a closed-loop heat recovery system not only improves energy efficiency but also maximises operational safety.

Closed-loop heat recovery systems in hospitals

Hygienic separation between exhaust air from operating theatres, isolation wards and infectious areas and the supply air for other parts of the building is essential.
Closed-loop heat recovery systems enable efficient heat recovery without mixing the air streams, preventing the transfer of germs and airborne contaminants.
They play an important role in meeting hygiene standards such as VDI 6022, particularly in intensive care units, isolation rooms and laboratory areas.

Closed-loop heat recovery systems in surface finishing

Surface finishing processes generate highly corrosive exhaust air containing acids, metal ions and aerosols that must never come into contact with the supply air.
A closed-loop heat recovery system equipped with Calorplast plastic gas-to-water heat exchangers ensures safe thermal separation of the air streams.
The system not only improves energy efficiency but also protects the ventilation system against chemical corrosion.

Take a look at our Surface Finishing Case Study to learn more about closed-loop heat recovery systems in this industry.

Closed-loop heat recovery systems in laboratories

Research facilities, universities and pharmaceutical production plants require complete separation between contaminated exhaust air and clean supply air.
Closed-loop heat recovery systems provide reliable heat recovery while maintaining process safety, even under changing operating conditions and demanding air quality requirements.
In buildings with multiple independent laboratory areas, they are often a cost-effective alternative to completely redesigning the ventilation system.

Advantages and disadvantages of closed-loop heat recovery systems

Using a closed-loop heat recovery system in industrial ventilation applications offers numerous technical and economic benefits. The system has proven particularly effective in demanding environments with corrosive exhaust air or strict hygiene requirements. As with any technology, certain limitations should also be considered. Below is an overview of the main advantages of this heat recovery solution.

Advantages

High heat recovery efficiency

Up to 70% of the thermal energy contained in the exhaust air can be recovered through the indirect heat transfer process.

Compact Installation

As no direct air connection is required, the exhaust and supply air units can be installed in separate locations.

Easy Retrofit

Closed-loop heat recovery systems are particularly well suited for retrofitting existing ventilation systems, often without major structural modifications.

Excellent Insulation Properties

Plastics have low thermal and electrical conductivity, making them excellent insulating materials for electrical applications and for the thermal separation of industrial systems. Many modern electrical devices would not be possible without them. At the same time, plastics are also highly suitable for heat exchangers. Thanks to specialised heat exchanger designs, efficient heat transfer can be achieved while offering outstanding corrosion resistance, excellent chemical resistance and a significantly lower weight than many conventional materials.

Integrated capacity control

The circulation pump within the closed hydraulic circuit allows the system capacity to be adjusted precisely to changing operating conditions.

Efficient energy use

Recovering waste heat from industrial processes reduces heating demand and improves the overall energy efficiency of the ventilation system.

Reduced risk of contamination

Because the exhaust and supply air streams remain completely separated, the risk of cross-contamination is effectively eliminated.

Wide material compatibility

In combination with Calorplast corrosion-resistant plastic heat exchangers, the system is ideally suited for chemically aggressive environments.

Nachteile

Higher investment costs

Compared to conventional air handling systems, a closed-loop heat recovery system requires a higher initial investment due to the additional hydraulic circuit, including pumps, piping and control components.

Risk of frost at low outdoor temperatures

Without appropriate frost protection measures or advanced control strategies, freezing may occur within the heat exchanger during cold winter conditions.

Additional maintenance for the pumping system

The circulation pump and heat transfer fluid require regular inspection and maintenance to ensure reliable and efficient long-term operation.

Thermal losses within the hydraulic circuit

Depending on the piping layout and insulation quality, heat losses may occur within the hydraulic loop, reducing the overall heat recovery efficiency if appropriate design measures are not implemented.

Greater Efficiency for Your Ventilation System

Closed-loop heat recovery systems provide a proven and sustainable solution for heat recovery in industrial ventilation systems, especially where strict hygiene requirements, corrosive exhaust air or physically separated air streams make direct heat recovery impossible. Using two separate heat exchangers connected by a closed liquid circuit, they ensure efficient, safe and contamination-free heat transfer. Combined with Calorplast corrosion-resistant plastic heat exchangers, these systems deliver high energy savings, maximum operational reliability and a significant reduction in both emissions and operating costs, making them a long-term solution for demanding industrial applications.