PSI - Issue 77

Alexander Čaja et al. / Procedia Structural Integrity 77 (2026) 177 – 182 Author name / Structural Integrity Procedia 00 (2026) 000 – 000

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4. Analysis of measurement results Fig. 3 compares the PPD indices for variants A0_1 to A0_5 at different height levels – H1 (ankles), H2 (body centre of gravity) and H3 (head). The graph also shows two reference limits: the thermal comfort limit (10% PPD) and the acceptable environment limit (20% PPD). The results show a clear trend – with increasing air flow speed, thermal comfort deteriorates. At the lowest speed (A0_1, 0.15 m·s⁻¹) the PPD was approximately 8%, which represents almost optimal conditions. This variant was the only one that met the acceptability limit at all height levels. With a slight increase in speed to 0.175 m·s⁻¹ (A0_2), the PPD reached 10%, which is still below the 20% threshold. At a speed of 0.20 m·s⁻¹ (A0_3), dissatisfaction increased slightly to 12%. From a value of 0.225 m·s⁻¹ (A0_4), dissatisfaction increased more significantly – PPD was approximately 18% and at maximu m speed (A0_5, 0.25 m·s⁻¹) it reached up to 25%. A more detailed breakdown by height levels shows that while at A0_1 PPD was around 10–15% at all levels, at A0_2 it already exceeded 20% at the torso level (H2). Variants A0_3 and A0_4 brought further deterioration, with PPD reaching 30– 38% at H2 and H3. The most unfavorable variant was A0_5, where the PPD reached almost 70% at the torso level and more than 60% at the head level, which confirms that excessive air flow leads to a strong feeling of draft and significant dissatisfaction. Regarding the measurement of particle concentration in the space, the sequence of measurements (from A0_5 to A0_1) could have been influenced by the operator's activity, which disturbed the cleanliness of the environment. Nevertheless, the results confirm that the higher the outflow velocity from the laminar field, the lower the particle concentration under the operating table. At lower outflow velocities, a significant increase in particle concentration was observed, which could be caused by a reduced volumetric flow rate and a deteriorated ability of the flow to remove particles. Conversely, higher flow velocities removed particles more effectively, but at the same time caused the movement of particles from the floor back to the ceiling and their recirculation in the peripheral areas of the room. Overall, the results show a trade-off between particle removal efficiency and thermal comfort. The optimal balance is achieved at lower airflow rates, especially for variants A0_1 to A0_2 (0.15– 0.175 m·s⁻¹), where both cleanliness and comfort requirements are met. 5. Conclusion The thermal comfort assessment shows high dissatisfaction with the environment and the thermal-humid microclimate at air flow rates from the laminar field starting from 0.2 m·s⁻¹. This fact supports the design of a prototype laminar field that works with lower outlet speeds and smaller volume flows while simultaneously reducing pollution in the clean room. For particle concentrations in reference volumes, the influence of the volume flow rate of the HVAC system and the rate of fluctuation of the turbulent component of the air flow rate was confirmed - the lower the fluctuations, the lower the intensity of turbulence and the lower the level of environmental pollution. Higher fluctuations can only be compensated by high air flows and a higher intensity of air exchange. Another important factor is the clogging of HEPA filters, which capture up to 99.97% of particles with a size of 0.3 μm. The gradual deposition of dust particles, microorganisms, pollen or smog leads to a decrease in air flow, an increase in pressure loss in the air conditioning system and, consequently, to an increase in energy consumption. Therefore, regular inspection and replacement of HEPA filters is crucial not only for maintaining high air quality, but also for the efficient and economical operation of the system. Research has also shown that reducing the volumetric air flow by up to 40% has brought about energy savings of fans by more than 30%. This proves that optimizing ventilation has significant economic benefits. However, clean rooms also need to be viewed from the perspective of the service life of their structure. Significant temperature gradients can cause material fatigue and cracking of seals. Improperly controlled humidity leads either to electrostatic discharges at low humidity or to condensation and corrosion at high humidity. Uncontrolled pressure differences can damage doors, walls or cause leaks. Therefore, the right choice of materials is important and modular systems are increasingly being used, which are more durable and flexible.