Displacement ventilation in hospitals

Engineered Systems, July, 2007 by Robert Gulick

Two North American consulting firms set out to investigate the safety and economic possibilities of using displacement in hospitals instead of traditional ventilation strategies. Here, one firm reports on testing in an ER waiting room setting (which itself can be an infection transmission problem waiting to happen). The research also raised a couple of unexpected questions--when does overhead ventilation "imitate" displacement, and are air changes always all they're cracked up to be?

Can sustainable ventilation approaches be as successful in health care environments as they have been in other buildings? This is the question Mazzetti and Associates asked itself in early 2006 in response to a challenge from a sustainable-minded client--Kaiser Permanente.

The short answer is yes, but the more detailed questions of where and how well have led to a year-and-a-half-long journey that continues.

The obvious concern that differentiates health care facilities from other building types is infection control. With a guiding principle of saving energy while maintaining or improving patient and staff safety, the journey began. In a day-long workshop, a matrix matched health care spaces with the most promising sustainable strategies. Invited to the workshop to critique the proceedings were Andrew Streifel, a recognized infection control expert from the University of Minnesota and officials from California's Office of Statewide Health and Planning Development (OSHPD). The applications with the greatest possibilities from the workshop were displacement and natural ventilation in patient rooms and public areas. Because natural ventilation is less precise and more difficult to analyze, displacement ventilation became the initial focus. However, parallel research is proceeding on natural ventilation applications.

Traditional overhead ventilation mixes room air from an overhead supply diffuser, ideally creating a homogenous environment throughout the entire room volume. Displacement ventilation supplies cool air low at a low velocity, allowing it to pool across the floor. As the air picks up heat, it rises due to convection in a vertical temperature gradient, ideally in a laminar piston effect.

The goal of displacement ventilation is to condition only the occupied volume and carry contaminants directly up out of the occupied zone without mixing them. Low sidewall displacement ventilation was chosen over raised floor displacement for health care facilities, due to cleanliness concerns. The presumed benefit of displacement ventilation was equal or better environmental comfort and ventilation effectiveness at lower air exchange rates.

When Streifel considered the displacement mechanism for contaminant removal, he was optimistic that it might also have benefits for the control of airborne infectious particles. The formulas for mixing dilution via air exchange rates--applied to overhead mixing ventilation--do not directly apply to displacement ventilation. A hypothesis was created that low sidewall displacement ventilation at lower air exchange rates would be equal to or better than overhead mixing ventilation for environmental comfort, ventilation effectiveness, and airborne particle control. If the hypothesis proved to be true, the energy savings and economic benefits would be dramatic.

THE STUDY--PHASE I

As Kaiser authorized a research study to test the hypothesis, Mazzetti & Associates became aware that Stantec, an A/E firm in Vancouver, BC, Canada, was planning similar research. The two firms agreed to collaborate on the research and share their findings publicly. Stantec took a holistic look at an emerging best practices hospital, and focused on an outpatient exam room. This space was representative of outpatient diagnostic areas where undiagnosed patients may be present. They evaluated overhead, displacement, and natural ventilation supplemented by radiant heating and cooling.

Mazzetti focused on in-patient rooms and emergency waiting rooms. Inpatient rooms were selected, because patients are generally diagnosed and collectively, they represent a significant portion of hospital space. Negative isolation rooms and positive environment rooms were excluded at this time. The emergency waiting room was chosen, as it is one of the areas of highest concern for airborne infection transmission from undiagnosed contagious patients. This article discusses the Mazzetti & Associates portion of the research.

STUDY SCOPE

The study included the following elements:

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* Literature research--to assess prior research to avoid repetition and inform our research

* Mock-up testing--conducted at the test labs of EH Price Company in Winnipeg, MB, Canada

* Field testing--to test both displacement and natural ventilation at the Kaiser Modesto Hospital in California, a new facility available prior to occupancy

* CFD modeling--to corroborate both mock-up and field testing

* Energy modeling--to quantify the energy savings of the sustainable strategies

* Life-cycle costing--to confirm the business case