AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Impact of Relative Humidity on HVAC Filters Loaded with Hygroscopic and Non-hygroscopic Particles
JAMES MONTGOMERY, Sheldon Green, Steven Rogak, University of British Columbia
Abstract Number: 554 Working Group: Control Technology
Abstract The key characteristics of an air filter – flow resistance and filtration efficiency – are strongly affected by captured particles. The impact of exposing loaded HVAC air filters to a relative humidity (RH) other than that experienced during loading is investigated to develop an understanding of the role of RH throughout filter operation. Flat sheets of commercial filter media were loaded with hygroscopic, non-hygroscopic, or a mixture of particles, in a laboratory apparatus. When a filter loaded with hygroscopic particles in dry air is exposed to an elevated relative humidity of 20% or 40% the flow resistance reduces by up to 10% and 45%, respectively, depending on the filter being tested. Investigation of filter efficiency against 130nm particles before and after changes in RH in the same samples shows reductions of 5 and 20 percentage points, respectively. Further increasing RH causes additional drops in flow resistance and efficiency whereas reverting back to a lower humidity does not change the filter characteristics. Exposing an unloaded filter medium to the same RH changes produces a negligible change in flow resistance and filtration efficiency. The irreversibility of the particle-loaded filter characteristics implies that the RH increases are associated with an irreversible change in the particle structure. The response to humidity is similar, but with a lower dependence on RH, when a 50:50 mixture of hygroscopic and non-hygroscopic particles is used. Exposure of filters loaded with only non-hygroscopic particles does not show the same dependence on RH. Imaging of agglomerates of hygroscopic particles captured on filter fibers shows changes in agglomerate structure when RH is increased within the range of the experimental conditions. This observation supports the hypothesis that small increases in size of individual particles due to changes in relative humidity well below deliquescence can result in large changes to the filter performance.