Particle Deposition in Ventilation Ducts: Connectors, Bends and Developing Turbulent Flow

Abstract

In ventilation ducts the turbulent flow profile is commonly disturbed or not fully developed and these conditions are likely to influence particle deposition to duct surfaces.Particle deposition rates at eight S-connectors, in two 90° duct bends and in two ducts where the turbulent flow profile was not fully developed were measured in a laboratory duct system with both bare steel and internally insulated ducts with hydraulic diameters of 15.2 cm. In the bare steel duct system, experiments with nominal particle diameters of 1, 3, 5, 9 and 16 µm were conducted at each of three nominal air speeds: 2.2, 5.3 and 9.0 m/s. In the insulated duct system, deposition of particles with nominal diameters of 1, 3, 5, 8 and 13 µm was measured at nominal air speeds of 2.2, 5.3 and 8.8 m/s. Fluorescent techniques were used to directly measure the deposition velocities of monodisperse fluorescent particles to duct surfaces.Deposition at S-connectors, in bends and in straight ducts with developing turbulence was often greater than deposition in straight ducts with fully developed turbulence for equal * Address correspondence to Prof. William W Nazaroff, Department of Civil and Environmental Engineering, 661 Davis Hall, University of California, Berkeley, CA 94720-1710 USA. Tel.: +1-510-642-1040. Fax: +1-510-642-7483. E-mail: nazaroff@ce.berkeley.edu. 1 particle sizes, air speeds and duct surface orientations. Deposition rates at all locations were found to increase with an increase in particle size or air speed. High deposition rates at Sconnectors resulted from impaction and these rates were nearly independent of the orientation of the S-connector. Deposition rates in the two 90° bends differed by more than an order of magnitude in some cases, probably because of the difference in turbulence conditions at the bend inlets. In straight sections of bare steel ducts where the turbulent flow profile was developing, the deposition enhancement relative to fully developed turbulence generally increased with air speed and decreased with downstream distance from the duct inlet. This enhancement was greater at the duct ceiling and wall than at the duct floor. In insulated ducts, deposition enhancement was less pronounced overall than in bare steel ducts. Trends that were observed in bare steel ducts were present, but weaker, in insulated ducts.

Particle loading rates for HVAC filters, heat exchangers, and ducts

“…Using the defined base cases, parametric influence was also determined with the use of a loading rate ratio (LRR), for the filter and coil loading rates only. The ducts were excluded from all succeeding discussion because they load so little relative to the other components (duct loading is distributed over the entire duct length and filter and coil loading are normalized by area normal to the volumetric flow) and duct deposition has been studied extensively elsewhere (Sippola and Nazaroff, 2003, 2004, 2005; Zhao and Wu, 2006a,b). The LRR is equal to the adjusted loading rate divided by the Rural or Urban base case loading rate, and the adjusted loading rate is the result of holding all parameters in the base case constant except for the varied parameter.…”

“…Airborne particulate matter continually deposits on components of operating heating, ventilation, and air‐conditioning (HVAC) systems. Particle deposition is a strong function of particle size and researchers have characterized size‐resolved particle deposition to HVAC filters (Hanley et al., 1994; Hinds, 1999), heat exchangers (Siegel, 2002; Siegel and Nazaroff, 2003), and ducts (Sippola and Nazaroff, 2003, 2004, 2005; Zhao and Wu, 2006a,b). The particle mass loading rate to an HVAC system component is the product of the particle mass concentration immediately upstream of that component, the deposition fraction onto the component, and the volumetric flow rate through the HVAC system.…”

Experimental Study of Solid Particle Deposition in 90° Ventilated Bends of Rectangular Cross Section with Turbulent Flow

“…Since the particle deposition is influenced by numerous factors on straight duct surfaces (Lin et al 2003;Zhao and Wu 2007;Zhang and Chen 2009), the aerosol deposition in bends becomes more complex for different flow conditions and potential inertial impaction (Sippola and Nazaroff 2005). Up to date, limited experiments have been conducted to study aerosol deposition in a single bend.…”

“…Aerosol deposition in typical ventilation system of commercial or public buildings has been seldom examined. Sippola and Nazaroff (2005) investigated two types of bends in this kind of ventilation system during their experiments. The bend deposition was found to be greater than that in straight ducts with fully developed turbulent flow.…”

Evaluation of sampling methods for Bacillus spore-contaminated HVAC filters

“…Once inside, airborne contaminants are easily distributed throughout the building by the heating, ventilation, and air conditioning (HVAC) system (Thatcher and Layton, 1995; Sippola and Nazaroff, 2005; Krauter and Biermann, 2007; Krauter et al, 2005; Kowalski et al, 2003). Recently, it was proposed that sampling HVAC filters from buildings within the suspected contamination zone can help responders determine the affected areas rapidly (Ackelsberg et al, 2011; Van Cuyk et al, 2012; Hong and Gurian, 2012; Stanley et al, 2008).…”