This invention relates to apparatus for removing particles from a gas stream and more particularly to apparatus for charging and collecting submicron particles entrained in a gas stream.
Gas streams, particularly in industrial settings, often contain particulates which must be removed therefrom for environmental or other reasons. Large particles, i.e. above 1-3 microns in size, are relatively easy to separate from the gas stream and conventional apparatus can remove them with high efficiency. Submicron particles, on the other hand, are more difficult to remove and the collection efficiencies of conventional apparatus with respect to them are lower.
Various types of apparatus are used to collect submicron particles, some with relatively high efficiency, but they do have disadvantages. These apparatus typically use an ionizer to charge the particles and then provide a large surface area at a different potential to collect them. However, high charges on submicron particles are difficult to achieve in conventional ionizers. The voltage gradient and current densities of these ionizers are not generally sufficient to quickly and highly charge submicron particles. In many cases this charging can be increased only at the expense of undesirably increased power consumption. Consequently, these apparatus either have a relatively long transit time (e.g., seconds) for particles in the ionizer, which is obtained by flowing the gas stream through the apparatus at a low velocity, or they have a large amount of collection area to collect the less highly charged particles, or both. These alternatives are all undesirable since they require a larger apparatus to handle a given amount of gas than would be required if the particles were more highly and rapidly charged (e.g., in milliseconds).
Some apparatus have electrodes for generating a precipitating field downstream of the ionizer to increase the rate at which charged particles move toward the collecting surface. But these electrodes create another problem, viz., arcing and sparking between the electrodes and the collecting surfaces. During arcing the precipitating fields decrease and particles go uncollected.
High efficiency collection of submicron particles is achieved in some apparatus at the expense of large pressure drops along the gas stream. For example, fiber beds do a credible job of removing submicron particles, but the pressure drop across the bed is undesirably high.