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Heat recovery

Generate process air and save energy in a single step

Heat recovery concepts from AERZEN

Sustainability and efficiency are indispensable in the industry. Today, these goals undoubtedly rank among the most critical challenges facing system manufacturers and operators. However, generating process air requires a relatively high amount of energy by its very nature.

A large quantity of heat energy, known as process heat, is released in this process. It arises in the generated airflow and under the acoustic hood due to heat loss from the motor, silencer, and compressor. In the past, it frequently went unused.

As a manufacturer of high-performance compressors, AERZEN has long developed innovative solutions for putting this heat energy to practical use. As a result, the company offers plant operators cutting-edge compressed air technology and a right-sized sewage heat recovery system from a single source, all of which prioritize renewable energy efficiency.

Installing a sewer heat recovery system can save up to 85% of the heat energy for other operations. This means targeted, resource-saving thermal energy conservation at high efficiency.

How Renewable Energy From Sewer Heat Works

A heat transfer system can turn the hot water from sewer pipes into an eco-friendly district energy source for nearby buildings. This process helps combat climate change, all with the help of heat pumps. These systems extract heat from sewage water.

A heat pump can recover heat from sewage water and sewer pipes from existing sewers and other international wastewater systems. The heat pump then transfers that sewage water to clean water distribution pipes. Since the pipes and heat pump lie on a closed-loop system, the dirty water never touches the sewage water.

Instead, heat pumps absorb the sewer heat and use that temperature for district energy. The heat pump sends the sewage water back into the existing sewers after the heat transfer, while the warm, clean water is used for heating and cooling nearby buildings and providing warm water for swimming pools.

Using Sewage Heat Recovery for Heating and Cooling Systems

The investment cost for a sewage heat recovery system is relatively low and quickly compensated for by energy savings. Older plants can also be retrofitted and optimized without spending time and money. AERZEN is happy to help you implement individual solutions. Industrial settings can use heat recovery for a broad range of processes and uses:

  • Supplying the central heating system for individual buildings
  • Warm-air heating for cool buildings
  • Sludge drying
  • Warm air for processing tasks (e.g., drying methods in production)
  • Producing a hot water temperature for showers and washrooms
  • Heating service water for cafeterias
  • Constructing thermal brakes
  • Heating water for swimming pools and washing machines

The goal of every plant designer is to reduce costs by making the most efficient use possible of the energy introduced into a process. In compressed air production, heat recovery offers a simple way to meet this challenge. It is helpful to look at the operating principle of an air compressor to understand why a working compressor generates heat energy and how people can use that energy.

Operating principle of compressors: Heat sources and options for recovering heat

The large quantity of waste heat produced when compressed air generates says nothing about the machine's efficiency. It is a by-product that arises when making process air. Plant operators have long been unaware of the massive district energy potential contained within these systems.

As a rule, the heat gets produced in two different ways: on the one hand, from compression of the medium by the assembly, and on the other, by the waste heat (cooling air) from the motor, stage, and silencer and the exhaust air from the oil cooler.


Machines such as screw compressors and positive displacement blowers compress the sucked-in surrounding air using screws or rotary pistons. This form of air compression acts as a thermodynamic process that heats the sucked-in medium, for example, air or gas. From a physical standpoint, the process converts electrical energy into heat energy. When businesses use their compressors, the process gas may reach temperatures as high as 280°C.

The logical approach to recovering heat from the process air is to use heat exchangers; when heat exchangers work into the airflow, a medium such as water can flow through them and be heated to a preset temperature, simultaneously removing heat from the airflow. The hot water can be reused as process or service water or fed into a central or district heating system. AERZEN offers precisely designed heat exchangers for this purpose, to convert the maximum amount of transferable heat with minimal pressure loss.

Cooling air (heat radiation)

Additional heat energy is created under the acoustic hood of the assembly by the radiation of the motor, oil cooler, compressor stage, rotary piston stage, piping, and silencer. In addition, this strongly heated cooling air offers a practical solution for building cooling systems.

The heat can be consolidated in the exhaust air ducts integrated into the assembly and transported as heating air to adjoining rooms or workshops throughout the central plant. A temperature-controlled outlet can regulate the airflow to achieve a fairly constant temperature. For example, if you do not need heating in summer, you can conduct the excess heat outside the building.

Heat recovery in wastewater treatment

Wastewater treatment represents a core sector of Aerzener Maschinenfabrik. The family-owned company is a global player in this area and continuously develops resource-saving and environmentally friendly all-in-one technology for clean energy and reducing greenhouse gas emissions. That includes the selection of "custom-fit," optimal machine technology and concepts for recovering heat.

The assemblies serve the role of aerating the aeration tanks in wastewater treatment plants. In the process, AERZEN positive displacement blowers, turbo blowers, and compressors working jointly or as individual assemblies generate an airflow with a pressure of approximately one bar.

The airflow pumps into the aeration tanks (the "biology" of the plant), and the oxygen in the air plays a decisive role in the biological sewage treatment process. Airflow generation naturally accounts for the most significant percentage of the total energy costs for wastewater treatment buildings. The potential for savings in this area is, therefore, exceptionally high. Moreover, the possibilities are vast: just one compressor with a rating of 22 kW can meet the heating needs of a single-family home.

Aerzener Maschinenfabrik’s many success stories show how implementing sewer heat recovery technology and choosing the optimal machine technology for each application can markedly improve the energy balance of a wastewater treatment plant. You can find several examples here:

Another example of highly efficient sewer heat recovery implemented in cooperation with AERZEN is the Essen-Kupferdreh wastewater treatment plant. The plant uses four positive displacement blowers to supply air to the aeration tanks.

Thanks to tube bundle heat exchangers, the plant now makes practical and efficient use of the wastewater heat from the blowers. The converted energy from the component now supplies thermal energy to the heating system and the buffer tank for warm water. The resulting savings range up to 30,000 euros each year.

At the Wertach wastewater treatment plant in Bavaria, it was possible to forgo the installation of a new burner for heating boiler water, thanks to the use of a heat exchanger. In Wertach, two Delta blowers with fixed speeds carry out the essential workload. However, when the requirements are higher, for example, during seasons where hikers and skiers visit in large numbers, a third blower with a variable speed is activated.

The warm, 68°C air generated by the compression process is cooled to under 30°C by an air-water plate heat exchanger. The resulting delta of almost 40°C can store the boiler water of the heating system. In addition, the heat exchanger integrated with the system is equipped with flow-optimized profiles and causes virtually no pressure loss in the process of airflow.

When additional heating is not necessary, it is easy to switch off. A mechanical valve in the piping enables all the air to be routed directly to the aeration tanks instead of through the heat exchangers, thus making demand-driven control and heat recovery possible whenever. In Wertach, sewer heat recovery made it possible to save approximately 1,850 liters of fuel oil per year. You can find more information on the subject in our customer magazine's article entitled ‘Efficient heating with lost heat’

The Filderstadt-Bonlanden wastewater treatment plant, built in the 1960s in the German state of Baden-Württemberg, needed to conform with strict environmental requirements. Therefore, mechanics constructed four Delta Hybrid rotary lobe compressors with a heat recovery system as part of a modernization and renovation project in cooperation with AERZEN.

The combination of assemblies provides 100% oil- and absorbent-free compressed air for the aeration of the aeration tanks in a highly energy-efficient manner. The oil and absorbent-free operation means savings on service and maintenance and improved process reliability for the plant. Once the modernization measures were complete, the system began circulating the machine-warmed ambient air via an extraction system and using it to heat other engineering rooms in the building.

But fitting a tube bundle heat exchanger into the main pipeline of the system achieved the most significant savings. In the heat exchanger, heat is removed from the process airflow and used to heat water. As a result, heat exchangers could eliminate the cost of wastewater heating altogether. In our customer magazine, you can learn more about heat recovery in Filderstadt-Bonlanden in an article entitled ‘Every kilowatt put to optimal use’.

Heat recovery in pneumatic conveying

Pneumatic conveying involves transporting bulk goods such as flour, powder, sand, or cement with the help of air. Many industries use pneumatic conveying, such as food processing, bulk goods, chemicals, and materials. AERZEN compressors, blowers, and rotary lobe compressors often create the required airflow.

Pneumatic conveying can also achieve very high outlet air temperatures. The ambient air, which usually has an average temperature of approx. 20°C, is sucked in and can reach up to 280°C due to the compression process, for example, in an application involving screw compressors. The process air often has to be cooled back down to prevent the transported bulk goods from being damaged by the high temperatures. The heat can be removed from the compressed air with the help of a heat exchanger and used for heating, wastewater heating, or process heat.

Depending on the design, the pressure loss caused by installing a heat exchanger is 1-3% in relation to the absolute intake pressure in the aftercooler. Therefore, the resulting increase in the drive output of the motor is nominal compared to the amount of energy saved by the heat exchanger.

When does it pay to recover heat?

Several factors should be considered before deciding to procure a heat recovery system.

To begin with, the structural conditions should be checked. It is vital that the sewer heat recovery source and the heat sink be positioned close together. If they are too far apart, the plans should factor in energy losses due to transport and higher costs for piping. If it takes a lot of effort to transport or store the heat, the higher investment cost must be reckoned with. Furthermore, there must be a Δ T of at least 5-10 k between the plant’s waste heat source and heat sink in order for a heat recovery installation to pay off.

In general, heat recovery technology proves especially beneficial when used with large assemblies and constantly running production processes. But a heat recovery system can also pay off in cases where the waste heat is lower but the workload is higher. Therefore, in principle, heat recovery systems become more profitable as the assemblies continuously remain in operation.


The large amount of energy required to generate compressed air can be recovered and used in various ways. With AERZEN technology and expertise, new projects in wastewater treatment and pneumatic conveying can make use of thermal energy for heating rooms, halls, water (process or service water), or for steps in the production process. Many different applications are possible.

Existing plants can also modernize using simple means that increase their efficiency. In this field, AERZEN offers its customers a broad portfolio of services. For example, a 24-hour telephone information service, a machine diagnosis and the AERaudit, in which service employees develop a plan for modernising a plant.

And with the help of AERZEN’s Room Ventilation Calculator, engineers and wastewater treatment plant owners can find a suitably designed heat exchanger for their plant themselves.