A COMPARISON OF VENTILATION PERFORMANCEBETWEEN LAMINAR FLOW AND HIGH INDUCTIONDIFFUSERS IN A TUBERCULOSIS ISOLATION ROOM

INTRODUCTIONIn this technote, a study is presented of ventilation conditionsin a tuberculosis (TB) isolation room. The purpose of the CFDmodeling carried out was to assess the efficacy of theventilation in a typical room planned for construction at anexisting healthcare facility. Although thermal comfort in theroom was also considered, the primary goal of theassessment was to determine the level of protection affordedhealthcare workers by the ventilation system under varyingoperating conditions (heating and cooling modes) and systemconfigurations including supply diffuser types. The designobjective was to achieve a local ventilation rate of 12 orgreater in all parts of the room with a volumetric supply airchange rate of 15 ACH or less to the room. Given the mixingtype of ventilation system implemented, this requires that thestagnant zones be eliminated.

Protection from these particles by general dilution was knownto be unachievable at the outset of the project. However, itwas of interest to know what levels of dilution are likely andwhether the different ventilation configurations play a role inthis close proximity dilution. Of equal importance was theconcentration of particles in the vicinity of the door. If thedoor region were contaminated with particles, some may escape into the hallway when the door is opened, since no anteroomwas planned for the isolation rooms in this facility. In companion Technote #22, a discussion on the design requirements ofHUAC systems in patient isolation rooms is presented.

Room description, loads and ventilation system configuration

Two CFD simulations are presented here: they show the predicted conditions representing a laminar diffuser arrangement anda square 4-way inductive diffuser.

The room itself had a floor plan of approximately 16.26 m2 (175 ft2) and was approximately 46.72 m3 (1650 ft3 ) in volume. Theheat loads and boundary conditions applied to the room for the different HVAC configurations were the same and represent asummer design case.

The two HVAC arrangements are compared on the basis of the age-of-air and relative concentrations of contaminated aerosolparticles.

The extension from this work suggests that ventilation rate analysis alone can be misleading becauseconditions may be calculated to be adequate for fresh air supply, when in fact the air distributionsystem does not remove the contaminants effectively.

ISSUE NO. 23

TECHNOTES

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ASSESSMENT METHODOLOGY

The results of the CFD process lead to a prediction of the steady state flow field for each of the diffuser arrangements fromwhich age-of-air, age-of-contaminant and thermal comfort analyses were conducted.

The layout and air change rate contours of the TB isolation room are presented in Figures 1 & 2 for an HVAC configuration usinglaminar (CSA approach) and squarediffusers (ASHRAE approach)respectively. See Technote #22 for adiscussion of these two approaches.These plots show that the room withthe laminar flow diffuserarrangement (Figure 1) had a smallpocket of stale air at the ceilinglevel near the door. This pocket hasan age-of-air of 300 seconds orlarger. In other words, the zone isventilated at a local air change rateof less than 12 ACH. Figure 2 showsthe size of the stale air pocket forthe four-way diffuser HVACconfiguration. It turns out that forthis configuration, the age-of-air inall areas of the room is less than300 seconds. Which means that allcritical regions in the room arebeing ventilated at a rate of 12 ACHor better.

The method of assessing theventilation efficiency in the roomwas to model a cough. This wasachieved by implementing a releaseof contaminant from the mouth ofthe patient at 3.56m/s (700 fpm)which emulated a cloud of particles.First, a solved steady state flowfield was established for the room.Then, using a transient (time-varying) solver, the patient’s mouthwas turned into a source of airladen with particles. At 0.15seconds after the start of thecough, the flow from the mouth wasstopped resulting in a total releasevolume of 0.5 l (0.018 ft3). Theparticle concentrations were thenmonitored at various locations inthe room. The most importantlocations can be considered 1) thehealth care worker and 2) locationsnear the door.

The pulsed release of a tracerrepresenting a cloud ofcontaminated aerosol particlespermits one to assess thecontaminant removal efficiency ofthe different ventilationconfigurations.

Figure 1: TB Isolation Room Laminar Diffuser Configuration: Boundary Conditions andRegions with Age-of-Air >300 seconds (<12 ACH)

Figure 2: TB Isolation Room Square 4-way Inductive Diffuser: Boundary Conditions andRegions with Age-of-Air >270 seconds (<13.3 ACH)

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Figure 4: Particle concentration for square four-way diffuser configuration.

Figure 3: Particle concentration for laminar diffuser configuration.

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DILUTION OF COUGHFigures 3a and 4a present the time varying concentration ofparticles at various locations in the room for the laminar and4-way square diffusers respectively for the first 60 seconds.Measurement points in the room are shown in Figures 1 and 2as colored numbers corresponding to the lines on the charts.The initial reference concentration is 10,000 at the mouth ofthe patient. Thus, a concentration of 10 at any given locationin the room represents a dilution of 103 . A monitor wasestablished in the path of the cough, location 1,approximately 0.12 m (4.75 in.) from the patient’s mouth.Locations 5 and 6 near the door region are 0.61 m (2 ft) fromthe face of the door, 1.52 and 0.91 m (5 and 3 ft) highrespectively.

The plots show that the concentration peaks at the mouth ofthe healthcare worker at approximately 5 seconds andrapidly tails off for both ventilation strategies. The peakmagnitude of the cough does not change significantly forthe two strategies. Figures 3b & 4b show the concentrationsfor a 10-minute duration. In addition to the concentrations ateach location, the concentration for a hypotheticallyperfectly mixed room was also included on these plots.Figure 4b shows how the distribution of cough aerosols forthe 4-way diffuser room design approaches the perfectlymixed result at approximately 3 minutes for the locationsmonitored. However, Figure 3b highlights how the aerosol inthe laminar flow room requires up to 8 minutes to approachconditions that would reflect a fully-mixed room.

These plots also permit one to evaluate the time it takes foraerosol to reach the door region of the two rooms. For the 4-way square diffuser (Figure 4a), the cough aerosol reachesthe door in approximately 35 seconds (for a particleconcentration of 0.01). The results for the laminar diffuserconfiguration illustrate how the lack of induced mixingsignificantly increases the time it takes for the aerosolparticles to reach the doors to approximately 120 seconds.Comparison of Figures 3b and 4b shows that the peakconcentration at the door is about three times higher for thefour-way diffuser.

It is clear that there are consequences to the selection of theventilation strategy and diffuser type that are not evidentuntil one assesses the time varying particle concentrations.For example:

• The rapid mixing (ASHRAE approach) of the square 4-waydiffusers does not reduce the concentration of particles atthe health care worker’s head. However, the peakconcentration near the door jumps up by a factor of three.

• The peak concentration along the cough path is notinfluenced by the diffuser selections modeled here.

• Having laminar diffusers (CSA approach) in the near doorregion appears to help prevent the particle-laden air frompenetrating into the door region—the peak concentrationsare less than those for the perfectly mixed room. However,the region is poorly ventilated by fresh air as indicated byFigure 1.

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ConclusionsA standard ventilation rate analysis has notable limitations for patient isolation room ventilation. The prediction ofcontaminant concentration is helpful in comparing proposed ventilation strategies. The extension from this work suggests thatventilation rate analysis alone can be misleading because conditions may be calculated to be adequate for fresh air supplywhen, in fact, the air distribution system does not remove the contaminants effectively. The conclusions drawn from thespecific ventilation analyses presented are:

1. It is difficult to ensure that all regions of the room meet the 12 ACH requirement despite providing a supply rate of 15 ACHto the room, unless the room is vigorously mixed with supply diffusers. This can result in undesired mixing ofcontaminants into some regions.

2. The specification of a minimum air change rate does not ensure adequate ventilation in the room: a contaminant removalefficiency would appear to be more appropriate.

3. It is not possible with normal room ventilation to control the particles released during a cough to prevent them fromreaching the breathing zone of healthcare worker, thus this study supports the CDC recommendation that all healthcareworkers tending to potential TB patients wear respiratory protection.

4. A rapid mixing ventilation strategy (ASHRAE approach) has the effect of transporting large numbers of particles to thedoor which leads to the risk of a loss of containment should the door be opened.

5. A ventilation strategy in which supply air is introduced at the ceiling with a lower supply velocity (CSA approach) canresult in fewer particles arriving at the door.

The Role of CFD ModelingThe design of air distribution systems in patient isolation rooms can be greatly assisted by computersimulations based on Computational Fluid Dynamics (CFD) modeling.

CFD can predict air velocities, temperatures and contaminant concentrations throughout the room for arange of design challenges. This information can be interpreted in terms of indoor air quality indices that canbe compared against health criteria and also thermal comfort indices to assess patient comfort.

Supply diffuser locations, types, flow rates, exhaust air vent locations, distributions of heat loads in theroom, arrangements of furniture and other blockages to air movement can be assessed and comparisonsmade to judge the best design alternatives.

"Evidence from many studies leaves no doubt that hospital air quality andventilation play decisive roles in affecting air concentrations of pathogens...and,in this way, have major effects on infection rates"

Roger Ulrich, Xiaobo Quan, Center for Health Systems and Design, College of Architecture, Texas A & M University

Craig Zimring, Anjali Joseph, Ruchi Choudhary, College of Architecture, Georgia Institute of Technology

Report to The Center for Health Design for the "Designing the 21st Century Hospital Project". This project is funded by the Robert Wood Johnson Foundation. September 2004.

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