By Bill Frantz, Tom Roaten, Wei Sun and Mindi Young, in, The NEBB Professional – Q4 2021 Edition
Pressure differences between spaces in a building are commonly used to control the flow of air and possible contaminants away from clean spaces and toward less clean spaces by way of cascading pressurization. Wei Sun [1] states that “room pressurization or depressurization is normally used to achieve desired airflow directions and to minimize airborne particle, biological, gas and/or chemical contamination from a less-clean or contaminated room into a cleaner or protected room. In this approach, air pressure differentials are created mechanically between rooms to introduce the intended air movement through room leakage openings.”
Traditional Applications and Emerging Applications
James Coogan [2] notes that this method of contamination control is used in areas such as hospital isolation rooms, hospital pharmacies, and clean manufacturing applications. In addition to these “traditional” applications, he also notes some “non-traditional” usages such as smoke control in office towers during a fire event, separating restrooms from other parts of a building, managing kitchen odors and smoke in restaurants, and any building where it’s important to keep unconditioned outside air out of occupied spaces.
Pressure differences and controlled leakage are also important in Hot Aisle Containment (HAC) style data halls (energy savings) and manufacturing control rooms (dust and fume control). Andy Persily [3] points out that mail rooms, loading docks, and public lobbies are also areas where controlled leakage is beneficial, particularly regarding Chemical Biological Radiological and Nuclear (CBRN) threats.
New Considerations, Pandemics, Extreme Climate Events
More recently, Wei Sun [1] stated that “during the COVID-19 pandemic, it has become urgently important to design, operate and retrofit buildings correctly to minimize or prevent airborne pathogen transmissions between rooms. Recent guidance from ASHRAE [4] indicates that health suites in K-12 schools should be configured to operate as cascading, pressure-controlled spaces with a flow direction from corridors to the health suite lobby, and into individual sick rooms to be then exhausted to the exterior.
Tim Roaten explains in early 2020, resident rooms in both independent and assisted living facilities were being retrofitted to create negative pressure spaces [5]. The creation of negative pressure spaces was designed to both isolate residents who had been diagnosed with COVID-19 and to aid in reducing transmission of the virus throughout the facilities, many of which are connected by shared or common spaces. Negative pressure spaces were created by using portable HEPA filter fan units.
While in most instances, the capacity of the portable HEPA filter fan units should have been sufficient to create the desired pressure differential, the greatest challenge was the integrity, or lack thereof, of the room and building envelope. Senior Living facilities are constructed for aesthetics and resident comfort, not pressure control. In many cases, to achieve the desired differential pressure relationships, temporary walls were added to corridors and the existing acoustical ceiling tiles were caulked in place in the ceiling grid system [7].
The need to consider pressurization in non-traditional applications is driven not only by urgent health crises like pandemics but also by extreme climatic events such as smoke from wildfires, decaying detritus from hurricanes, and poor AQI days in our urban areas. Mindi Young points out that, during historic West Coast wildfires, proper cascading pressure control and controlled leakage between interior spaces was critical to maintaining indoor air quality in an advanced manufacturing facility. These same extreme events also impact non-manufacturing facilities. It’s likely that design strategies and techniques will carry over to these non-traditional applications.
Enclosure Tightness and Suspended Ceiling Systems
Many experts acknowledge the important role that the air leakage characteristics of an enclosure plays in determining net airflows for pressurization. Andy Persily [3] states, “Success requires that the envelope is sufficiently tight and that the net airflow into the building is large enough to overcome the pressures created by outdoor weather conditions. The amount of airflow required is directly related to the building envelope leakage—the leakier the envelope, the more airflow is needed.”
Wei Sun [1] points to this: “To reach the same pressure differential, it is more cost-effective to make a room tighter (by better sealing) which requires a smaller offset between incoming (supply) and leaving (return or exhaust) airflows, than a looser room which requires a larger offset… and makes the task of HVAC design and air balance more complicated.” Air leakage between spaces in buildings is gaining more attention.
When the technology is implemented in more “non-traditional” applications, it will be critical to do it right and do it in an energy-efficient way. The industry is going to need more knowledge about leakage characteristics of interior enclosures, including partition walls, doors, lights, fire protection systems, and suspended ceiling systems.
Laboratory Testing of Suspended Ceilings and Components
Motivation
Air within an interior space can leave that space by many different parallel paths. In addition to planned paths such as return air and transfer grills, doors, electrical outlets, wall penetrations, light switches, light fixtures, fire protection sprinklers, and ceiling-mounted sensors all represent potential fugitive leak paths. Suspended ceilings frequently form the “top of the box” in a room and are often overlooked as leak points or wrongly dismissed as impractical to make tight.
Armstrong first became interested in quantifying leakage rates across perforated metal ceiling planes in 2001. At that time, the context was related to fire sprinkler performance and how flow through perforated panels could influence the reaction times for sprinklers. Soon after in 2002, Armstrong leveraged what we learned to explore a novel “ceiling supply plenum” approach to air distribution.
Later, in 2012, the company extended low leakage suspended ceiling systems to control leak points and save operating energy in Hot Aisle Containment (HAC) data halls. In early 2020, at the onset of the COVID-19 pandemic, Armstrong anticipated that pressurization could be an important way to manage potential contaminant flow. We saw that retrofit of existing spaces into pressure-controlled spaces could be a critical need. We re-started work on low leakage rate suspended ceiling systems.
Small-scale Test Apparatus
Supporting all this work is a small-scale test apparatus based on ASTM E-283 “Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, And Doors Under Specified Pressure Differences Across the Specimen” [6]. Although not specifically written for suspended ceilings, we saw it as a practical and fundamentally sound approach for our testing.
The test apparatus consists of a 48 x 48 [in] chamber that is 12 [in] deep. The chamber is well-sealed on five sides against air leakage. Samples are mounted in removable frames that clamp and seal tightly like a lid onto the top of the box. Air is introduced into the test box via compressed air or by metering pressure blower. Flowrate is measured via rotameter or calibrated orifice plate.
The ceiling assembly is sealed in a test frame so that air must flow through the sample from one side to the other. The pressure difference across the sample is measured via a micromanometer. Airflow through the sample is measured, and resulting pressure differences across the test sample are recorded. Typical “power law” equations are used to fit the data and characterize the flow resistance.
If the sample is a grid and tile assembly, then airflow is normalized by the area of the assembly. If the sample is a single item, such as a fire sprinkler, lay-in light fixture, high hat light fixture or other penetration, the sample tray is sealed so that all air must pass through/around the item under test. Leakage rate [cfm/ft2] is determined across pressure differences ranging from 0.000 to 0.090 [in WC].
Leakage Rates in Suspended Ceilings
Measured leakage rates will vary depending on pressure difference across the ceiling plane, direction of the pressure difference (positive or negative), type of ceiling panel (permeability), weight of tile, type of edge, and the use of hold-down clips. The plot shows a typical test result for an AirAssure mineral fiber ceiling tile with special soft gasketing on the perimeter of the tile.
Pressurization and Depressurization of Test Rooms
Suspended ceilings frequently form the “top of the box” in a room. To pressurize a room for contaminant control, one must supply excess air (for positive pressure) or exhaust excess air (for negative pressure). The net airflow required depends on leaks into or out of the space. When leaks are reduced, the enclosure’s airtightness is better, and the amount of air required to either pressurize or depressurize is also reduced. This results in reduced energy to move the air and better control of resulting pressure differences.
Factors Modifying Performance
Spring clips or “hold-down clips” can be used to press the gasketed tile more firmly onto the grid. The use of clips improves the seal significantly. In the case of positive pressure rooms, clips reduce the tendency for tiles to lift off the grid. Clips are recommended for positive pressure rooms when the pressure difference across the ceiling system is expected to approach about 0.090 [in WC]. Pressures above this level start to equal the weight of the tile and can cause the tile to lift from the grid.
Suspended ceiling tiles are frequently cut to fit the perimeter of a room. When the best possible seal is required, it is recommended to apply gasketed to the tile at the cut edges and seal the gap between the wall and the perimeter molding of the ceiling.
Pressurization and De-pressurization Room Tests
To demonstrate the impact of low leakage rate ceilings on pressure-controlled spaces, a laboratory test was conducted. A test room of 10 x 12 [ft] in size had air supplied by a pressure blower and exhausted by a reverse flow fan filter unit. The net airflow in-out could be controlled. Resulting net flow rates and room pressures were measured. The test was repeated for a standard ceiling system and an AirAssure low leakage rate ceiling system.
This simple test clearly shows the benefit of controlling leakage through the ceiling plane. De-pressurizing to -0.020 [in WC] required an exhaust flow of 150 [cfm] with a conventional system. Achieving the same pressure result with a low leakage ceiling system required an exhaust flow of only 90 [cfm]. This 40% reduction in airflow translates to a 78% reduction in fan power.
Practical Applications
The effect of low leakage suspended ceilings was recognized in two practical installations.
Improved Pressure Control in an Airborne Infection Isolation Room
Encompass Health Rehabilitation Hospital, located in Middletown, Delaware, is a medical inpatient rehabilitation facility designed to help patients recover from stroke and other complex neurological and orthopedic conditions. The facility includes one Airborne Infection Isolation Room (AIIR). The Airborne Infection Isolation Room is designed/required to operate at a pressure of -0.020 [in WC] with respect to the adjacent corridor.
Over the course of the past several years, the room has experienced frequent low-pressure nuisance alarms, which could distract medical staff from other duties. The dedicated exhaust fan system serving this room was operating at its maximum capacity, and the room could only achieve and sustain a differential pressure of -0.0189 [in WC] in relationship to the corridor.
Rather than replace the existing exhaust fan, the conventional suspended ceiling was replaced with a low leakage rate ceiling. The introduction of the low leakage ceiling tightened the enclosure enough to change the actual room differential pressure to -0.0368 [in WC] without increasing the exhaust airflow rate and eliminated the nuisance alarms.
Pressurization to Improve Air Quality in a Manufacturing Break Room
Armstrong World Industries operates a mineral fiber ceiling tile manufacturing plant in Macon, GA. As part of a facilities upgrade, there was a desire to change the 890 [ft2] central break room into a positive pressure space. This was expected to keep nuisance dust out of the break room and provide a clean, comfortable place to eat and relax between shifts.
Prior to the renovation, the break room operated at -0.015 [in WC] pressure with a net flow inward from the manufacturing plant into the room. The conventional suspended ceiling was replaced, and sufficient excess supply air was supplied to the space to create a +0.021 [in WC] pressure relative to the manufacturing plant. This created an outward flow of air and significantly reduced the particulate in the break room. The low leakage ceiling system only required 650 [cfm] excess supply air to achieve the target pressure.
If a conventional ceiling was used, it is estimated that 1250 [cfm] excess supply air would have been necessary. Room conditions were monitored with Awair Omni IEQ sensors before and after the changes were implemented. Before the changes, the room operated at a negative pressure of -0.015 [in WC], and airflow was clearly from the manufacturing plant into the break room. After the changes, the room operated at positive +0.021 [in WC], and excess airflow was outward from the (clean) breakroom to the (less clean) manufacturing plant. The reduction in particulate concentrations was remarkable.
Conclusions
Pressurization and de-pressurization of spaces is a well-known means of controlling airflow and the possible transfer of contaminants. It’s commonly applied in critical environments such as hospital isolation rooms, hospital pharmacies, and clean manufacturing applications.
The industry may be at a point where “typical buildings” may need to adopt some of the design techniques to respond to urgent health crises such as pandemics and extreme climatic events. Greater attention to enclosure leakage and surfaces like suspended ceilings may be necessary to achieve design goals. The ability to economically retrofit existing spaces into pressurized spaces may become important. Furthermore, it may become more common to require a space to easily “toggle” between a normal mode of operation and a critical event mode of operation.
Sources
- “Room Pressure, Flow Offset, Airtightness and Pressurization Strategies”, Wei Sun, ASHRAE Journal, Dec 2020.
- “Space Pressurization: Concept and Practice”, ASHRAE Distinguished Lecture Series, Jim Coogan, Siemens Building Technologies, Boston Chapter, February 2017.
- “Building Ventilation and Pressurization as a Security Tool”, Andy Persily, ASHRAE Journal, Sep 2004.
- “ASHRAE Epidemic Task Force, Core Recommendations, Schools and Universities”, updated 5-14-2021.
- “Implementing a negative pressure isolation space within a skilled nursing facility to control SARS-CoV-2 transmission”, Shelly L. Miller PhD, Debanjan Mukherjee PhD, Joseph Wilson BS, Nicholas Clements PhD, Cedric Steiner MBA, American Journal of Infection Control, 2021.
- “ASTM E283–04 (Reapproved 2012) Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen”.
- “Airborne Infectious Disease Management Methods for Temporary Negative Pressure Isolation”, Minnesota Department of Health, Office of Emergency Preparedness Healthcare Systems Preparedness Program, Feb 2007.
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