How to Generate Process Air and Save Energy Through Heat Recovery

In industry, efficiency and sustainability are indispensable. They are undoubtedly amongst the most important challenges facing operators and system manufacturers today. However, the production of process air needs a relatively high amount of energy because a large amount of heat energy, known as process heat, is released in the process.

It is not just produced in the generated airflow, but also under the acoustic hood as a result of heat loss from the compressor, motor, and silencer. It frequently went unused in the past.

AERZEN, as a manufacturer of high-performance compressors, has long developed innovative solutions for using this heat energy practically. The company provides plant operators with cutting-edge compressed air technology and right-sized systems for heat recovery from a single source as a result.

The potential savings that could come from installing a heat recovery system are huge: up to 85 % of the heat energy can be utilized for other operations easily, such as drying processes, water heating, or preheating burner air. Energy losses are avoided, such as those caused by venting the heated air outside. Therefore, heat recovery means targeted, resource-saving energy conservation.

The investment costs for a heat recovery system are quickly compensated for by the energy savings and are relatively low. Additionally, older plants can be retrofitted and optimized without spending much time and money and AERZEN can help to implement tailored solutions. Heat recovery can be performed for a wide range of processes and uses:

  • Constructing thermal brakes
  • Warm air for processing tasks (e.g. drying processes in production)
  • Producing hot water for showers and washrooms
  • Supplying central heating systems
  • Sludge drying
  • Warm-air heating
  • Heating service water for cafeterias
  • Heating swimming pools

The goal of a plant designer is to minimize costs by making the most efficient utilization of the energy introduced into a process. Heat recovery is a simple way of meeting this challenge in the area of compressed air production. It is useful to look at the operating principle of an air compressor to understand why a working compressor produces heat energy and how that energy can be utilized.

Operating Principle of Compressors: Heat Sources and Options for Recovering Heat

The large quantity of waste heat that arises when compressed air is generated is nothing to do with the efficiency of the machine. It is a by-product which comes about necessarily when generating process air. Plant operators have long not been aware of the massive energy potential contained therein.

As a rule, the heat is produced in two different ways:

  1. By the waste heat (cooling air) from the motor, stage and silencer and the exhaust air from the oil cooler
  2. From the compression of the medium by the assembly

Compression

Machines like positive displacement blowers and screw compressors, compress the sucked-in surrounding air by using rotary pistons or screws. This form of air compression is a thermodynamic process that heats the sucked-in medium, such as gas or air.

From a physical perspective, the process converts electrical energy into heat energy. When compressors are employed, the process gas can hit temperatures of up to 280 °C. The logical manner to recovering heat from process air is to utilize heat exchangers; a medium such as water can flow through them and be heated to a preset temperature when they are integrated into the airflow, simultaneously removing heat from the airflow.

The heated water can be re-used as process or service water or fed into the central heating system. AERZEN offers precisely designed heat exchangers for this purpose, that convert the maximum amount of transferable heat with minimal pressure loss.

Cooling Air (Heat Radiation)

More heat energy is generated under the acoustic hood of the assembly by the radiation of the motor, rotary piston stage, compressor stage, oil cooler, piping and silencer. This strongly heated cooling air can also be put to practical use. The heat can be merged in the exhaust air ducts integrated in the assembly and transported as heating air to adjoining workshops or rooms.

So that a uniform ambient temperature is attained, a temperature-controlled outlet can regulate the airflow. If no heating is needed, for example in summer, the excess heat can be conducted outside.

Heat Recovery in Wastewater Treatment

One of Aerzener Maschinenfabrik’s core sectors is wastewater treatment. The family-owned company is a key player in this area and is continuously developing environmentally friendly and resource-saving complete solutions for customers around the globe. This includes not only the range of ‘custom-fit’, optimal machine technology, but also ideas for recovering heat.

The assemblies are usually employed to aerate the aeration tanks in wastewater treatment plants. Using this process, an airflow with a pressure of around 1 bar is produced by AERZEN positive displacement blowers, compressors and turbo blowers working together or as individual assemblies. The airflow is pumped into the aeration tanks (the ‘biology’ of the plant).

The oxygen which is contained in the air plays a key role in a crucial step in the process: biological sewage treatment. Airflow generation, measured in terms of the total energy costs for a wastewater treatment plant, naturally accounts for the largest part. Therefore, the potential for savings in this area is particularly high.

The possibilities are huge: just one compressor with a rating of 22 kW can fit the heating requirements of a family home. By administering heat recovery solutions and choosing the optimal machine technology for each application, Aerzener Maschinenfabrik’s many success stories show how the energy balance of a wastewater treatment plant can be massively improved.

Example: Filderstadt-Bonlanden Wastewater Treatment Plant in Baden-Württemberg

To follow the strict environmental requirements at the Filderstadt-Bonlanden wastewater treatment plant, which was built in the German state of Baden-Württemberg in the 1960s, four Delta Hybrid rotary lobe compressors were supplied with a heat recovery system as part of a modernization and renovation project in cooperation with AERZEN.

The combination of assemblies supplies 100 % oil- and absorbent-free compressed air in a highly energy-efficient manner for the aeration of the aeration tanks. The oil- and absorbent-free operation means not only improved process reliability for the plant, but also savings on maintenance and service.

Once the modernization measures were finished, the system started circulating the machine-warmed ambient air using an extraction system and employing it to heat other engineering rooms.

The biggest savings were gained by installing a tube bundle heat exchanger into the main pipeline of the system. Using the heat exchanger, heat is taken from the process air flow and utilized to heat water. Water heating costs could be eliminated completely as a result of this.

Example: Essen-Kupferdreh Wastewater Treatment Plant

A further example of highly efficient heat recovery applied in cooperation with AERZEN can be observed at the Essen-Kupferdreh wastewater treatment plant. The plant uses four positive displacement blowers to send air to the aeration tanks.

The plant now makes efficient and practical use of the waste heat from the blowers thanks to the installation of a tube bundle heat exchanger. The converted energy from the component provides heat energy for the heating system and the buffer tank for warm water. The savings as a result of this are huge: heat recovery alone allows the plant in Essen-Kupferdreh to save over 30,000 euros a year.

Example: Wertach Wastewater Treatment Plant in Bavaria (Allgäu)

It was possible to avoid the installation of a new burner for heating boiler water at the Wertach wastewater treatment plant in Bavaria, thanks to the utilization of a heat exchanger. In Wertach, two Delta blowers with fixed speeds are employed for the basic workload.

When the needs are higher, during the season when skiers and hikers visit in large numbers for example, a third blower with a changeable speed is activated. The 68 °C warm air produced by the compression process is cooled to less than 30 °C by an air-water plate heat exchanger.

Thus, the resulting delta of nearly 40 °C can be utilized to store the boiler water of the heating system. The heat exchanger integrated in the system is equipped with flow-optimized profiles and causes almost no pressure loss in the process air flow. It is switched off easily if no heating is required, in summer for example.

A mechanical valve in the piping allows a portion or all of the air to be routed directly to the aeration tanks instead of through the heat exchanger, and so making demand-driven control and heat recovery a possibility at all times. In Wertach, heat recovery made it possible to save around 1,850 liters of fuel oil per year.

Heat Recovery in Pneumatic Conveying

Pneumatic conveying, with the help of air, involves the transport of bulk goods such as flour, cement, sand or powder. It is employed in a number of industries, like bulk goods, food processing, chemicals and materials. AERZEN compressors, blowers and rotary lobe compressors are often employed to create the required airflow.

Extremely high outlet air temperatures can be attained in the process. The ambient air, which normally has a temperature of approx. 20 °C, is sucked in and due to the compression process, can hit temperatures of up to 280 °C, in an application involving screw compressors for example. The process air often has to be cooled back down to stop the transported bulk goods from being damaged by the high temperatures.

With the help of a heat exchanger, the heat can be removed from the compressed air and utilized for heating, water heating or process heat. The pressure loss caused by the installation of a heat exchanger is between 1-3% in relation to the absolute intake pressure in the aftercooler, depending on the design. Compared to the amount of energy saved by the heat exchanger The resulting increase in the drive output of the motor is nominal.

When Does it Pay to Recover Heat?

Before deciding to procure a heat recovery solution a number of factors should be considered. To start with, the structural conditions must be checked, as it is crucial that the waste heat source and the heat sink are positioned close together. The plans should factor in higher costs for piping and energy losses due to transport if they are too far apart.

If a lot of effort is required to transport or store the heat, higher investment costs have must be considered. In addition, 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. Typically, it could be said that heat recovery is particularly advantageous when utilized with large assemblies and constantly running production processes.

In cases where the waste heat is lower but the workload higher a heat recovery system can also pay off. In principle, heat recovery is more profitable the more continuously the assemblies remain in operation.

Summary

The large amount of energy needed to produce compressed air can be recovered and employed in a number of ways. With AERZEN technologies and expertise, new projects in the areas of wastewater treatment and pneumatic conveying can be designed to make use of heat energy for heating rooms, halls or water (process or service water) or for steps in the production process.

Multiple different applications are possible. Existing plants can also be modernized to increase their efficiency using simple means. In this field, AERZEN offers its customers a wide range of services, for example, a 24-hour telephone information service, a machine diagnosis and the AERaudit, in which service employees create a plan for modernizing a plant.

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

This information has been sourced, reviewed and adapted from materials provided by Aerzener Maschinenfabrik GmbH.

For more information on this source, please visit Aerzener Maschinenfabrik GmbH.

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