Ventilation Airflow Research Articles

Effect of metro train on the critical driving force for preventing smoke back‐layering in tunnel fires

SummaryPreventing smoke back‐layering in longitudinal‐ventilated tunnels is an important topic in handling with fires safety issues. Previous studies focused on the critical velocity, while only very few of them paid attention to how to realize this parameter, that is, in a critical state, to overcome the resistance of the tunnel and the fire source to the airflow, the required driving force provided by the ventilation equipment installed in the tunnel. This study addressed this problem in the situation where the coupling effect of blockage and throttling effect was highlighted by conducting theoretical analysis and numerical simulations. The results showed that the presence of metro train blockage reduces the effective cross‐sectional area of the tunnel near the fire source and decreases the critical velocity. On the other hand, the blockage of trains also increases the resistance to the ventilation airflow and exacerbates the throttling effect, leading to the increase of the critical driving force. In such conditions, a stronger driving force to approach the critical velocity was needed. Besides, the length of train blockage was verified to exert limited influence on the value of critical velocity while it significantly affected the critical driving force. Furthermore, the calculated formula for the critical driving force was proposed with the prevention of smoke back‐layering considering the blockage effect to be emphasized.

“Demand Control” an Innovative Way of Reducing the HVAC System’s Energy Consumption

According to EIA, the Heating Ventilation and Air Conditioning (HAVC) systems account for about 25% of the U.S.’s total commercial building’s energy use. Therefore, advanced modeling and optimization methods of the system components and operation offer great ways to reduce energy consumption in all types of buildings and mainly commercial buildings. This research introduced an innovative integrated two-level optimization technique for the HVAC system to reduce the total energy consumption while improving the indoor thermal comfort level. The process uses actual system performance data collected for the building automation systems (BAS) to create accurate component modeling and optimization process as the first level of optimization (MLO). Artificial neural networks were chosen to be the tool used to serve the process of modeling. The second optimization level utilizes the whole system-level optimization technique (SLO) using a genetic algorithm (G.A.). The proposed two-levels optimization technique will optimize the system setpoints, the supply air temperature, duct static pressure, minimum zone air flowrates, and minimum outdoor air ventilation rate. The proposed technique has contributed to the field of modeling and optimization of HVAC systems through several new contributions. (1) Implementing the demand control methodology with the optimization process to modify the electricity consumption power profile when the demand signal is received. (2) Implement the occupancy schedule inputs into the optimization process to adjust the ventilation airflow rates accordingly. (3) Implement the real-time zone occupancy sensor readings and adjust the zone’s ventilation flowrates and minimum flowrates. (4) Lastly, implementing the method of zone minimum air flowrates setpoint rests to reduce reheat requirements. The proposed optimization process was tested and validated, resulting in savings in the total energy consumed by the chilled water VAV system by 13.4%, 22.4 %, followed by 31% for July, February, and October, respectively.

Improvement of the Chimney Effect in Stack Ventilation

The article is focused on the airflow in a ventilation system in a building. The work examines the methods which enhance the chimney effect. In this paper, three cases with different chimneys were analyzed for the full-scale experiment. These cases were characterized by different geometrical and material parameters, leading to differences in the intensity of the ventilation airflow. The common denominator of the cases was the room with the air inlet and outlet to the ventilation system. The differences between the experimental cases concerned the chimney canal itself, and more precisely its part protruding above the roof slope. The first experimental case concerned a ventilation canal made in a traditional way, from solid ceramic brick. The second experimental case concerned the part that led out above the roof slope with a transparent barrier, called a solar chimney. In the third experimental case, a rotary type of chimney cap was installed on the chimney to improve the efficiency of stack ventilation. All these cases were used to determine the performance of natural ventilation—Air Change per Hour (CH). Additionally, the paper presents a technical and economic comparison of the solutions used.

A review of airflow rate estimation techniques for natural ventilation in buildings

As natural ventilation involves local and global interactions, the estimation of these interactions can be performed by many approaches. Such approaches, rather more experimental and numerical than analytical, often require a great deal of instrumentation and equipment, which results in higher demands on project budget and funding. The present work is devoted to comprehending the natural ventilation concept, and to assess the existing experimental techniques already implemented for past researchers in the estimation of the ventilation airflow rate due to the wind and thermal buoyancy effects. A brief review of modeling techniques is also presented. This will provide a strong theoretical grasp of the natural ventilation process as part of the main elements in the thermal behavior of buildings. Ultimately, these bases are intended to help choose the most suitable techniques to estimate the natural ventilation airflow rate. The adequate benefit-to-budget technique appears to be the airtightness tests (blower door tests), since empirical Equations relating the airflow directly to the pressure difference in the building for both cases: infiltrations (openings closed) and openings opened, can be obtained. Also, the location of the leakages can be identified without complications, and this technique has the potential to estimate in situ the airflow capacity and friction characteristics of the openings.

Assessment age of air and thermal comfort in a classroom by adopting displacement ventilation with chilled ceiling system

This study aims to investigate the effect of the cooling load ratio covered by the chilled ceiling on the age of air and comfort level in a classroom in a hot and dry climate in Iraq-Hilla city. Air age, air exchange efficiency, and concentrations of pollutants in a classroom are investigated numerically by used AIRPAK software under displacement ventilation combined with a chilled ceiling system. Four cases are studied at different values of the cooling load covered by the chilled ceiling (0%, 25%, 50%, 80%) with respect to total classroom cooling load. Cooling load removes by chilled ceiling varied from (0 to 84.5 W/m2) based on the classroom area, and its temperature varied between (17.5-22.5oC). The displacement ventilation airflow rate was kept at 0.3m3/s, and the air temperature supply varied between (19.5-24.5oC) depend on the amount of cooling load covered by displacement ventilation. The results showed that the mean local air age increasing with height. The room mean air age increase and air exchange efficiency reduce with increasing load portion, which treated by the chilled ceiling. Increasing the portion of the load treated by chilled ceiling tends to improve comfort levels.

Application of an occupant voting system for continuous occupant feedback on thermal and indoor air quality – Case studies in office spaces

Smartphone applications or electronic devices in the form of occupant voting systems (OVS) have demonstrated to be feasible for acquiring feedback from occupants to support the building operation manager to identify indoor environmental quality (IEQ) problems and inappropriate control settings of heating, ventilation and air-conditioning (HVAC). The present paper aims to contribute to the growing research on OVS as a tool to support building operation management. The paper presents a study about a tangible OVS, denoted TiAQ, that occupants could use to vote on their here-and-now experiences regarding the thermal environment and indoor air quality. The main objectives were to identify whether there was an alignment between the votes collected with TiAQ and the monitored IEQ variables as well as to demonstrate the use of occupant votes to identify problematic IEQ conditions and appropriate strategies for HVAC control. The study demonstrated that thermal votes collected with TiAQ could be related to the variations in the indoor temperature and ventilation airflow. Additionally, collected votes could identify i.a. poorly set indoor temperature setpoint and appropriate control strategies that could reduce energy use by 46% and increase thermal comfort and cold complaints by 6% if current airflow and indoor temperature setpoint were lowered.

The application value of operating room ventilation with laminar airflow for surgical site infection: A protocol for a systematic review and meta-analysis.

Background:The presence of biological particles in the air inside operating theatres has the potential to cause severe surgical site infections. Recently, laminar airflow systems have been regarded as a means to reducing surgical site infections using airborne microbes. Still, other publications have argued the benefits of laminar airflow systems, stating the likelihood of adverse effects. Therefore, we will conduct this systematic study to evaluate the applicational value of adopting laminar airflow systems in operating theatres to minimize surgical site infections.Methods:Reporting of this study adheres to the guidelines of Preferred Reporting Items for Systematic Review and Meta-analysis Protocols. The authors will perform a systematic search on MEDLINE, Web of Science, EMBASE, the China national knowledge infrastructure, and the Cochrane Library from their commencement until June 2021. The search will identify relevant randomized and non-randomized controlled trials that evaluates the applicational value of using laminar airflow ventilation in surgical theatres to minimize surgical site infections. There are no restrictions on language. Two authors will independently screen the identified studies, perform data extraction, and use an appropriate method to evaluate the bias risk in the included studies.Results:The work done in the present study will enhance the existing literature on the applicational value of laminar airflow ventilation in surgical theatre to reduce surgical site infections.Conclusion:The outcomes are a reference for healthcare practitioners and patients when making informed decisions regarding care during surgeries.

Determining the critical insulation thickness of breathing wall: Analytical model, key parameters, and design

Breathing wall (BW) based on air-permeable porous medium offers an alternative solution for utilizing the conductive heat loss of building envelope to preheat the infiltration ventilation airflow within porous medium. However, current studies neglect the influence of pressure drop within porous medium on the energy performance of BW, which may lead to an overestimation or non-optimal design. In this study, we proposed a framework to determine the critical insulation thickness of BW for minimizing its convective heat loss and pressure drop related energy loss. An analytical model was developed and validated to calculate the heat loss of BW under third-type boundary condition. Darcy's law was applied to estimate the pressure drop of infiltration airflow and its associated energy loss. Case studies were conducted to identify the critical insulation thickness of BW. The critical thickness of BW under different scenarios was investigated. The results demonstrate the existence of critical thickness, which yields the lowest overall heat loss of BW. A larger infiltration airflow rate or lower air permeability of porous medium will result in a downward trend of this critical thickness. The outcomes of this study can provide a design guideline of BW for maximizing its energy saving potential.

Air distribution and thermal environment optimization on subway platform using an innovative attached ventilation mode

Subway systems are extensively used due to their safety and convenience. Scientific and reasonable air distribution is necessary for creating a comfortable environment on the subway platform. In the present study, three ventilation modes (four-side slot outlets of attached ventilation (FSOAV), both-side slot outlets of attached ventilation (BSOAV), and mixing ventilation (MV)) were investigated based on numerical methods. Visualization experiments were conducted to characterize the attached ventilation airflow. A series of evaluation indexes, including the air velocity, temperature, air diffusion performance index (ADPI), and relative warmth index (RWI), were utilized to assess the ventilation effects of the three modes. The average temperatures among the three modes were 29.09 °C, 29.85 °C, and 29.18 °C; the ADPI values were 81.28%, 84.78%, and 55.89%; and the RWI values were 0.236, 0.256, and 0.239, respectively. These results demonstrate that FSOAV outperforms BSOAV and MV in terms of air distribution and thermal environment. Thus, FSOAV is recommended for application on subway platforms. Considering the actual layout of the construction, the positional distribution and width of air outlets were discussed to optimize the ventilation performance of FSOAV. The thermal environment of attached ventilation with double-row air outlets is better than that with single-row air outlets, and the increase in velocity contributes to the uniform diffusion of airflow in attached ventilation. This research provides design references for ventilation design in subway stations.

Potential Mechanical Ventilation System For Emergency Care During Spaceflight Missions

Neurally Adjusted Ventilation Assist (NAVA) technology is an autonomous mechanical ventilator that is triggered by diaphragm activity (EAdi) to provide the necessary assisted breath required by the patient. This technology could be an asset toward medical autonomy during spaceflight missions. PURPOSE: To test if NAVA has potential for increasing medical autonomy during spaceflights and to test ventilatory assist with NAVA at rest and during steady state physical exercise. METHODS: EAdi, respiratory frequency (RF), tidal volume (VT) and pulmonary ventilation (VE) were measured on 5 participants (males, age: 24.8 ± 2.8 yrs, height: 1.74 ± 0.07 m, weight: 71.8 ± 11.9 kg, V02max: 51.42 ± 5.32 ml/kg/min) during 6 minutes of resting breathing (T0) and during 10 minutes at 70% of V02max on an ergocycle (T1: minutes 1-3 and T2: minutes 8-10). This protocol was performed without (WO) and with NAVA (W) 30 minutes later. Paired T-tests were performed between both conditions. RESULTS: A slightly lower EAdi was observed with NAVA at T0 (WO: 3.52 ± 1.04 μV, W: 2.67 ± 0.88 μV, p = 0.2761), T1 (WO: 15.74 ± 4.79 μV, W: 15.34 ± 6.16 μV, p = 0.9190) and T2 (WO:23.42 ± 4.4 μV, W:23.19 ± 2.83 μV, p = 0.9105). However, NAVA was activated at every inspiration meaning that the electrodes on the naso-gastric tube can capture EAdi at high RF; T0 (WO: 13 ± 3, W: 14 ± 6 breaths/min), T1 (WO: 21 ± 2, W: 18 ± 4) and T2 (WO: 27 ± 4 and W: 23 ± 4). VE was also slightly lower with NAVA at T2 (WO: 63.17 ± 7.72, W: 59.78 ± 7.52 L/min), but these results were not significant. VT was slightly higher with NAVA at T0 (WO:1.08 ± 0.3, W: 1.22 ± 0.75 L/breath), T1 (WO:2.11 ± 0.46, W:2.32 ± 0.8 L/breath) and T2 (WO:2.43 ± 0.47, W: 2.67 ± 0.76 L/breath), but these results were not statistically significant. Also, the mechanical ventilator portion triggered by NAVA was unable to offer sufficient positive pressure to maintain inspiratory flow during exercise (peaked during first minute of exercise). CONCLUSION: NAVA is light, portable, and auto adjustable, which are desirable characteristics for medical equipment for spaceflight missions. It provided assisted breathing at rest but was unable to offered limited mechanical ventilation airflow during exercise. Funding: Annie Martin received a scholarship from the Natural Sciences and Engineering Research Council of Canada (NSERC) for this project.

CO2 dynamics and heterogeneity in a cave atmosphere: role of ventilation patterns and airflow pathways

Understanding the dynamics and distribution of CO2 in the subsurface atmosphere of carbonate karst massifs provides important insights into dissolution and precipitation processes, the role of karst systems in the global carbon cycle, and the use of speleothems for paleoclimate reconstructions. We discuss long-term microclimatic observations in a passage of Postojna Cave, Slovenia, focusing on high spatial and temporal variations of pCO2. We show (1) that the airflow through the massif is determined by the combined action of the chimney effect and external winds and (2) that the relationship between the direction of the airflow, the geometry of the airflow pathways, and the position of the observation point explains the observed variations of pCO2. Namely, in the terminal chamber of the passage, the pCO2 is low and uniform during updraft, when outside air flows to the site through a system of large open galleries. When the airflow reverses direction to downdraft, the chamber is fed by inlets with diverse flow rates and pCO2, which enter via small conduits and fractures embedded in a CO2-rich vadose zone. If the spatial distribution of inlets and outlets produces minimal mixing between low and high pCO2 inflows, high and persistent gradients in pCO2 are formed. Such is the case in the chamber, where vertical gradients of up to 1000 ppm/m are observed during downdraft. The results presented in this work provide new insights into the dynamics and composition of the subsurface atmosphere and demonstrate the importance of long-term and spatially distributed observations.

Flow Field Analysis of Adult High-Flow Nasal Cannula Oxygen Therapy

The mechanical ventilation of human body is a complex human-computer interaction process. High flow nasal cannula oxygen therapy (HFNC) is a new type of ventilation, which is often measured by lung pressure, respiratory work, and other parameters. The purpose of this paper is to analyse the pressure, flow, and strain rate of upper respiratory tract with different flow and oxygen concentration by using finite element simulation, to guide professionals to adjust the appropriate flow and oxygen concentration parameters of HFNC machine. This paper studies the complex human-computer interaction environment of human respiratory tract and ventilation airflow. The 3D model of respiratory tract established by the conversion of image scanning data was taken as the research object. The flow state of the gases in the respiratory tract was judged by Reynolds equation. After that, RNG K-ε model was applied to the research object, and the simulation diagram of airway pressure, flow rate, strain rate, and trace diagram of flowing particles were obtained under the finite element method. The results explain some clinical phenomena in HFNC and guide people to make better use of mathematical tools to study human-computer complex environment.

COVID-19 spread in a classroom equipped with partition – A CFD approach

In this study, the motion and distribution of droplets containing coronaviruses emitted by coughing of an infected person in front of a classroom (e.g., a teacher) were investigated using CFD. A 3D turbulence model was used to simulate the airflow in the classroom, and a Lagrangian particle trajectory analysis method was used to track the droplets. The numerical model was validated and was used to study the effects of ventilation airflow speeds of 3, 5, and 7 m/s on the dispersion of droplets of different sizes. In particular, the effect of installing transparent barriers in front of the seats on reducing the average droplet concentration was examined. The results showed that using the seat partitions for individuals can prevent the infection to a certain extent. An increase in the ventilation air velocity increased the droplets’ velocities in the airflow direction, simultaneously reducing the trapping time of the droplets by solid barriers. As expected, in the absence of partitions, the closest seats to the infected person had the highest average droplet concentration (3.80 × 10−8 kg/m3 for the case of 3 m/s).

Evaluating the cooling potential of a geothermal-assisted ventilation system for multi-family dwellings in the Scandinavian climate

In recent years, the increasing occurrence of heatwaves raises the cooling need of residential buildings in Scandinavian countries, which are traditionally not equipped with active cooling systems. Indoor overheating caused by such heatwaves leads to severe consequences for occupants, especially kids and seniors. Efficient and economical cooling solutions are urgently needed to cope with frequent heatwaves. The present study investigated the novel usage of the geothermal-assisted mechanical ventilation with heat recovery (GEO-MVHR) system for cooling purposes in typical Swedish multi-family dwellings. The cooling potential of the system and its contributions to thermal comfort were evaluated. Dynamic simulations were conducted to assess the system's cooling performance under two climate scenarios: the climate of 2018 representing an extreme year with excessively hot summer and the climate of a typical meteorological year. The GEO-MVHR system shows great potential in mitigating indoor overheating with improved thermal comfort. A ventilation airflow rate of 0.50–0.70 l/s/m2 is suggested for multi-family dwellings to maximize the cooling potential of the GEO-MVHR system. The indoor operative temperature could be reduced by up to 3 °C with the GEO-MVHR system operating for cooling. Modulating the supply air temperature of the GEO-MVHR system based on indoor thermal conditions is recommended, as it shows the advantage of avoiding unnecessary overcooling and energy saving.

Laminar airflow versus turbulent airflow in simulated total hip arthroplasty: measurements of colony-forming units, particles, and energy consumption

The optimal type of ventilation in operating theatres for joint arthroplasty has been debated for decades. Recently, the World Health Organization changed its recommendations based on articles that have since been criticized. The economic and environmental impact of ventilation is also currently an important research topic but has not been well investigated. To compare how large, high-volume, laminar airflow (LAF) and turbulent airflow (TAF) ventilation systems perform during standardized simulated total hip arthroplasty (THA), as they pertain to colony-forming units (cfu), particle counts, and energy consumption. Two identical operating theatres were used to perform simulated THA. The only difference was that one was equipped with LAF and the other with TAF. Cfu and particles were collected from key points in the operating theatre, and energy was measured for each simulation. Thirty-two simulations were done in total. LAF had significantly reduced cfu and particle count when compared with TAF, at both 100% and 50% air influx. Furthermore, it was shown that lowering the air influx by 50% in LAF did not significantly affect cfu or particles, although reducing the fresh air influx from 100% to 50% significantly lowered the energy consumption. Most simulations in TAF did not meet the cleanroom requirements. Cfu were significantly lower in LAF at both 100% and 50% air influx. It is possible to reduce fresh air influx in LAF operating theatres by 50%, significantly reducing energy consumption, while still maintaining cfu and particle counts below the ISO classification threshold required for THA surgery.

Empirical validation of a multizone building model coupled with an air flow network under complex realistic situations

Building energy simulation (BES) assesses the energetic and thermal behaviour of buildings and can be used for the evaluation of energy conservation measures (ECMs). However, building modelling under realistic conditions can be challenging for simulation programs. The aim of this work is to test whether a multizone building model developed with a simulation tool can handle the complex situations of an in-use building. Monitoring data from the Twin Houses empirical validation experiment of the International Energy Agency, Energy in Buildings and Communities (IEA EBC) Annex 71 is used for that purpose. The multizone building model is developed with the energy simulation software TRNSYS coupled with an air flow network in TRNFLOW. The innovative aspect of this work lies in the validation of a combined multizone building model under stochastic occupancy gains, taking into account room-wise heating profiles, ventilation, and interior air flow currents due to window and door operation. The results of a coheating test showed that the errors in the simulated heating power for the whole house are a coefficient of variation of the root mean square error (CV(RMSE)) of 12.72% and a normalised mean bias error (NMBE) of −8.35%. Nevertheless, more difficulties were found in separately predicting the heating power of each individual room. However, the combined multizone air flow model was able to predict indoor temperatures in all rooms with root mean square error (RMSE) and mean absolute error (MAE) values of<1 °C.

HAZARD ANALYSIS DUE TO THE INFLUENCE OF THE STRONG FIRES IN THE ROAD TUNNELS

In this article presented an analysis of the hazards caused by the influence of strong fires in the road tunnels. Have been considered the main factors that affect the reliability of the results of definition of the period of time during which there is the possibility of evacuation of people. The dependence of the period of time marked by the characteristic parameters of fire: from the power of spatial localization and duration of the ascending phase of its development. The paper noted that the duration of the evacuation period, will be less than the duration of the ascending phase of development of a fire and also noted that this period tends to narrowing due to the influence of turbulent diffusion fluxes. In this paper it is assumed that the turbulent diffusion fluxes occur between the zones in which the ruling are toxic carbon gases of fire and fresh ventilation air flow.

ON AIR FLOW STRUCTURE IN STATION VENTILATION CONNECTIONS OF SUBWAYS

One of the important microclimatic criteria for the comfort and safety of passenger transportation in subways is dust concentration in passenger and staff area. It is known that fine dust concentration in subways is significantly exceeded relative to the permissible regulatory level. To reduce the dust concentration to the maximum allowable level, it is necessary to filter the tunnel air not only for the systems of local ventilation of staff rooms, but also for tunnel ventilation and general ventilation of passenger rooms at the stations. It is better to install filters in station ventilation connections, since significant circulating air flows pass through them. To determine the type and parameters of filters, it is required to know the magnitude and direction of an airflow rate, i.e. the structure of its velocity field. Due to the complex topology and presence of separation zones, the airflow structure, gradient of air velocities in the cross-section of ventilation connections and, accordingly, justification of filter installation locations in a ventilation connection requires a separate study. The paper substantiates a decomposition approach to mathematical modeling of aerodynamic processes by transition from a linear open-loop model of the subway line to a closed circular one; geometric parameters of the adopted design model; mathematical statement of the problem. Computational aerodynamics methods allowed determining the minimum, average and maximum projections of air velocity on the normal to the cross section of a ventilation connection and air flows through this cross section. This makes it possible to justify the requirements for the placement and operating parameters of the filtration equipment.

Simulation and experimental study of residential building with north side wind tower assisted by solar chimneys

Natural ventilation combined with passive cooling systems can reduce the cooling energy demand in residential buildings. However, the main difficulty in designing natural ventilation systems driven by buoyancy and wind is the simultaneous estimation of ventilation airflows and indoor temperatures. The purpose of this study was to overcome this difficulty by developing a tool that combines a ventilation model (envelope flow) with a thermal multi-zone model of a bioclimatic dwelling known as “La Casa de la Tierra”. This building, located in Murcia (Spain), incorporates a central wind tower, four solar chimneys , and construction elements with thermal inertia capacity. The ventilation and the thermal models were completely integrated and solved simultaneously. On-site collected data were used to validate the models. In the validation of the air flow envelope model, maximum and minimum root mean square error values of 10.9 m 3 /h and 7.25 m 3 /h, respectively, were obtained for the simulated and measured flows of the solar chimneys. The maximum value corresponded to chimney 2 where an average air flow rate of 58.2 m 3 /h was registered, while the minimum value corresponded to chimney 3 where an average air flow rate of 30 m 3 /h was measured. Different passive cooling strategies were simulated, combining natural ventilation induced by the wind tower and solar chimneys with passive geothermal and evaporative cooling systems. For these systems, the predicted percentage of dissatisfied people was less than 10%, and electrical energy savings of approximately 42% were achieved. These results contribute to increasing awareness of the role that passive strategies can play in achieving the objectives set by the EU for residential building energy efficiency .

Laminar airflow decreases microbial air contamination compared with turbulent ventilated operating theatres during live total joint arthroplasty: a nationwide survey

Preventing surgical site infections and prosthetic joint infections is crucial for patient safety after total joint arthroplasty. Microbial air contamination has been suggested as a risk factor. Therefore, the ventilation system that will reduce air contamination most effectively in operating theatres (OTs) has been discussed. To determine whether laminar airflow (LAF) ventilation is superior to turbulent airflow (TAF) ventilation by looking at the colony forming units (cfu) count during live total hip and knee arthroplasties. Furthermore, to explore whether the number of OT personnel, door and cabinet lock openings and technical parameters of the ventilation systems have an impact on the number of cfu. Active air sampling and passive sedimented bacterial load were performed in 17 OTs, equipped with either LAF or TAF ventilation, during 51 live surgeries while observations were noted. LAF OTs reduced cfu counts compared with TAF OTs during live surgery (P<0.001). All LAF OTs provided ultraclean air whereas TAF had nine procedures exceeding the threshold of 10 cfu/m3. Door and cabinet lock openings and number of personnel did not influence the cfu count, while it decreased with increasing volume and total air change per hour (P<0.05). All LAF OTs had cfu counts within recommendations and provided lower cfu counts compared with TAF OTs. The number of OT personnel and total openings did not have an influence on cfu counts. Increased volume of the OT and total air change per hour showed a decrease in active cfu counts.