©2023 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 65, no. 11, November 2023.
About the Author
David Schurk is director of Healthcare and Applied Engineering Markets for Innovative Air Technologies, in Covington, Ga.
Anyone who has experienced the wet inside an HVAC air-handling unit (AHU) serving critical care environments knows this situation can create a breeding ground for mold and microbial growth, an unacceptable condition particularly when serving hospital patient care areas or other such sensitive settings. Engineering controls outlined in this article are intended to positively impact both the performance and hygiene of air-handling systems by improving a building’s indoor air quality, helping protect exposed occupants from airborne and surface source contaminants and improving building sustainability.
Mold, fungi and bacteria growing on HVAC system components can be aerosolized and distributed by ductwork to various spaces within the building, resulting in considerable concern regarding occupant well-being.1 One issue is wet and moldy air filters, which create unsustainable operating conditions for air handling in any HVAC application. In addition to health consequences, this moisture may reduce filter life or efficiency and add additional airside pressure drop, which increases fan energy consumption and operational costs.2
Filters become wet for various reasons, most of which can be eliminated with proper AHU design, selection and control. Ensuring cooling coils are kept clean and air velocities through coils are kept low enough to prevent moisture carryover (blow-off) into the airstream is critical. If humidification devices have been inadvertently installed upstream of filters, their humidistat, sensors and valves must be properly installed and functioning correctly. But even with the most judicious AHU design, final filters may still become wet for what appears to be no good reason. Besides physical wetting by condensate carryover, another relatively simple explanation exists as to how this can happen.
HVAC System Dynamics
Cold air leaving a refrigerant-based direct-expansion (DX) or chilled water cooling coil will typically approach saturation (the air is near 100% relative humidity)3 (Figure 1). When this airstream enters a final filter (Figure 2), or any component that increases its velocity, the pressure and temperature of the air will drop as the air accelerates. Even a slight increase in air velocity accompanied by a small drop in temperature may be enough to allow the airstream to cool to a point where moisture is condensed. In cooling systems that operate for many hours a day, considerable amounts of moisture may collect on filters or other internal surfaces within the AHU. If there is not sufficient downtime that allows for this moisture to evaporate, filters will remain wet and microbial growth can proliferate.4
This wetting situation may be more evident with blow-through fan configurations and when filters are positioned immediately downstream of cooling coils in what is considered the final-filter position.
When designing a critical-care AHU that includes final filters, one may consider using a draw-through (cooling coil located before the fan) versus blow-through (cooling coil located after fan) configuration.
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