Development of a five-stage, round jet, virtual cascade impactor

Conventional cascade impactors are of limited usefullness as source samplers because momentarily high particle concentrations in stacks can cause rapid sample buildup leading to particle re-entrainment and subsequent size analysis errors. To prevent excessive buildup of deposited aerosols, sample times must be so short that samples are not representative of aerosols present in stacks over extended times. In addition, sticky substrate collection surfaces must usually be avoided because of elevated temperatures within stacks. This leads to increased particle bounce within the impactor and additional size analysis errors. Virtual cascade impactors eliminate problems of particle bounce and re-entrainment. A prototype five-stage, round jet, virtual cascade impactor operating in the range of 15 to 25 standard liters per minute has been built. The device will pass through standard sampling ports. The sharpness of cutoff for each stage is limited by the requirement for positive flow of a fraction of sampled aerosol into each collector. Sample sizes are limited by buildup which causes excessive pressure drops across collector filters and altered fractional flow through collectors; however, the range of acceptable pressure drops is broad