## 1 Introduction  
Cleanrooms are defined as areas with a low concentration of airborne particles. It is necessary to ensure their proper function for healthcare or precision manufacturing. A high concentration of airborne particles can affect product quality and functionality. These particles transport viruses, bacteria, and increase the risk of contamination during surgery in the operating room. The key principle of keeping a low concentration of particles is to supply a large amount of filtered air and thus reduce the particle concentration in the ventilated room to the required level [1]. Forced ventilation systems have high-performance requirements, as they control not only the transportation and filtration of air into a ventilated room, but also adjust the temperature and humidity of this air, resulting in a high amount of energy consumed for their operation [2, 3, 4, 5]. Depending on the cleanroom classification, a clean zone's entire air volume must be replaced from once every 3 min (ISO EN 8) up to once every 10 s (ISO EN 4-1) [6]. According to studies [7, 8], clean rooms consume 50-70% of the electricity supplied for HVAC (Heating, Ventilation, and Air Conditioning) systems in order to function properly. A survey conducted in Taiwan demonstrates the use of high-tech HVAC systems, which only consumes approximately 35% of electricity by using clean air recirculation. This value represents about 7.4% of total energy consumption [9]. Our objective is to reduce the operating cost of these energy-intensive systems while maintaining the required quality of the cleanroom's internal environment (low concentration of particles in the air). As the industry develops, enforcing progressively stricter requirements for environmental cleanliness in the operation of medical and pharmaceutical facilities, the fiscal advantages that come with the appropriate modification of HVAC systems grow ever larger [5]. This approach also contributes to sustainable business. Since the 20 th century, many studies and scientific articles have focused intensively on reducing the energy requirements of clean rooms. The air change rates (ACH) value, the location of the inlet/outlet, the source of contaminants, as well as the influence of external factors in airflow distribution and air quality were monitored [10; 11; 12; 13; 14; 15]. Several research studies have identified various barriers to energy efficiency in cleanrooms and have suggested options to improve energy efficiency [16; 17; 18]. One of these options is to optimize airflow and its regulation by improving the energy efficiency of the ventilation system [19]. For example, reducing the energy cost of fan power by 33% would contribute to a 66% reduction in energy consumption [20]. Previous studies have shown that demand-controlled filtration can lead to annual energy savings ranging from 28% to 72% [21; 22; 23; 24]. Another way to reduce the energy consumption of a cleanroom is to move the air supply and exhaust distribution elements closer to the source of the pollution and the clean zone of the ventilated space. This can reduce energy consumption by up to 20% while maintaining the required environmental quality. Using ventilation system circulation outside of the occupied space will reduce the energy consumption of fans by up to 70% [19]. However, this solution is not appropriate in several countries from a legislative point of view. A cleanroom environment has many potential sources of contamination. These may vary depending on the number of people in the room, the purpose of the room, etc. However, the main sources of contamination are people, the material of their clothing, the room's structural elements, and the ventilation system itself, whose filters work with specified efficiency [25]. The actual rate of particle generation is often an unknown parameter when designing a cleanroom. As a result, HVAC systems are usually oversized to ensure compliance with the classification requirements [2; 3; 7; 26].