Vapor Depletion in Processing Membrane Filters: The Effects of Chamber Parameters

Abstract

The vapor field around an ice crystal growing on a membrane filter in a static diffusion chamber is modeled and the associated losses in deposition nuclei due to crystal-crystal vapor depletion calculated. The expression which is derived relates the depletion losses to the aerosol activity spectrum and the processing characteristics of the chamber, such as time constant, supersaturation and plate separation. In particular, it is suggested that depletion losses are affected most by changes in the time taken to reach the required supersaturation: with crystal-crystal depletion, a slow rise to the processing level increases the losses. The model, along with some of its underlying assumptions, is compared with experimental results using an AgI aerosol, and found to be in reasonable agreement. Some applications of the model to other types of filter processing chambers are also discussed, and a similar but untested model for depletion losses due to the presence of hygroscopic particles is given as an appendix.