Environmental protection and economic efficiency
By Lena Arenz, KÃ¶nigswinter
Magnesium is now considered the building material of the 21st century. Lightweight metal is characterized by its lightness, excellent casting properties and ease of machining. In addition, magnesium can be recycled almost indefinitely compared to other materials. The STIHL company had already established a magnesium die-casting plant in Weinsheim in 1971. With a casting capacity of approx. 6000 tonnes per year, the plant is today considered one of the largest and most modern magnesium die-casting plants in Europe. More than 20 hot and cold chamber technology die-casting machines with clamping forces of up to 1000 tons are used in the fully automated production. 790 employees create more than 26 million components per year for the parent company’s chainsaws and other power tools, but also for external customers.
Focus on sustainability
The company is committed to environmental protection and energy efficiency at a high level and to their continuous improvement, both in business processes and in products. In accordance with these principles, emphasis was placed on eco-efficiency when selecting a new exhaust air filtration system.
In addition, climatic conditions have been taken into account in the design of the exhaust air technology. In Weinsheim, Rhineland-Palatinate, the thermometer hovers around zero degrees during the winter months. Therefore, when awarding the project, STIHL attached importance not only to reliable purification of the exhaust air, but also to highly efficient heat recovery. In 2015, KMA Umwelttechnik GmbH installed a unique heat recovery system (Fig. 1) which heats the entire die-casting foundry to a constant temperature of 18 Â° C during the winter months without introducing conventional energy sources such as electricity or gas. The multi-stage heat recovery system is based on two hydraulic circuits in which 45.6 mÂ³ and 22.8 mÂ³ of ethylene glycol flow as carrier fluid every hour.
Multi-step recovery of valuable process waste heat
Twelve exhaust air filter systems with a total capacity of 236,000 mÂ³ / h are installed in a weatherproof housing on the roof of the hall of the die-casting foundry. In addition to filter elements (Fig. 2), the systems are fitted with extract air heat exchangers in order to recover the heat contained in the extract air. The extract air from production, contaminated by oil fumes, is drawn under the hall ceiling via fans and first flows through the extract air filters. Downstream are the heat exchanger units. Since the exhaust air has a temperature of at least 29 Â° C even in winter, it has valuable thermal energy which is extracted in the heat exchanger by means of a hydraulic carrier fluid during the cold season. . Each heat exchanger has a capacity of max. 112.5 kW / h.
The heat transfer fluid circuit leads to the basement under the foundry hall. The central air supply system for supplying fresh air to the foundry is located here. The fans draw in fresh air from the outside. Via a first heat exchanger (Fig. 3), the energy recovered from the exhaust air is transferred to the supply air by means of a compound cycle system. In this way, the blown air can be heated to at least 11 Â° C even on cold winter days. In order to bring the supply air to the required inlet temperature of 18 Â° C, the system has a second stage heat exchanger (Fig. 4).Here, the energy supply is ensured by integration into the cooling water circuit of the casting machines. Cooling water is normally discharged via a cooling tower. With a temperature of approx. 30 Â° C, it is an ideal carrier fluid for the second stage of the heat exchanger during the cold season and thus heats the air in the hall to 18 Â° C. Every hour, 236,000 mÂ³ of heated fresh air is introduced into the hall through ventilation slots in the hall floor.
Heating system independent of production
At the same time, another autonomous system heats an additional 120,000 mÂ³ of fresh air. Four of the twelve rooftop filtration systems are not connected to the large hydraulic circuit, but form their own in order to protect the hall from freezing even when production is stopped, such as during holiday periods. Here too, the incoming air from outside is first heated to approx. 11 Â° C. But the second heat exchanger differs from that of the larger system. It does not work exclusively with heated cooling water, but can also work with conventionally heated water.
Economy and ecology: no contradiction
The foundry operates five working days a week with a daily operating time of 24 hours. The heating period during the winter months is on average 12 weeks, so the calculation is based on 1440 hours. Due to the high smoke load, there is an air change of 13 times per hour in the foundry. This corresponds to an air flow of 360,000 mÂ³. If a conventional gas-fired hall heating system is used, the heating costs for these 12 weeks would be around â¬ 36,000. In contrast, the operating costs of the KMA ULTRAVENT filter and heat recovery system are only around â¬ 5,900. They consist of the consumption of electrical energy for the fans, pumps and the cleaning system. The KMA system thus saves almost 84% of annual heating costs in comparison. At the same time, the environment benefits from clean air as oily smoke and aerosol substances are filtered out.
But even more impressive is the eco-effect of the heat recovery system used at STIHL. While a conventional heating system would result in an annual CO2 impact of around 363 tonnes under the mentioned operating conditions, the carbon footprint with a KMA filter system is only 55 tonnes, which means 85% d less CO2 emissions. And the introduction of a CO2 tax will further widen this gap, since 25 euros per tonne of CO2 have been due since 2021, increasing successively to reach 55 euros by 2025.
The sustainable treatment of production exhaust air is a major issue for all players in the die-casting industry. The wide range of customer-specific parameters and local conditions lead to customized solutions that enable foundries all over the world to achieve a high environmental standard. KMA offers its customers an energy-optimized filtration system, tailored to the individual needs of the foundry and local conditions. The ULTRAVENT system developed by KMA includes all components for exhaust air purification and heat recovery. It combines environmental and climate protection with tangible benefits in terms of operating costs for the foundry. The example of the plant operated at STIHL shows that the main die-casting manufacturers have taken steps to ensure sustainable purification of the exhaust air at all their production sites.