Two Stage

The two-stage precipitator differs from the single-stage precipitator in both design and amount of voltage applied. The two-stage ESP has separate particle charging and collection stages (Figure). The ionizing stage consists of a series of small, positively charged wires equally spaced 2.5 to 5.1 cm (1 to 2 in.) from parallel grounded tubes or rods. A corona discharge between each wire and a corresponding tube charges the particles suspended in the air flow as they pass through the ionizer. The direct-current potential applied to the wires is approximately 12 to 13 kV.

The second stage consists of parallel metal plates less than 2.5 cm (1 in.) apart. The particles receive a positive charge in the ionizer stage and are collected at the negative plates in the second stage. Collected smoke or liquids drain by gravity to a pan located below the plates, or are sprayed with water mists or solvents that remove the particles and cause them to fall into the bottom pan. Two-stage precipitators were originally designed for air purification in conjunction with air conditioning systems. (They are also referred to as electronic air filters). Two stage

ESPs are used primarily for the control of finely divided liquid particles. Controlling solid or sticky materials is usually difficult, and the collector becomes ineffective for dust loadings greater than 7.35 x 10-3g/m3 (0.4 gr/dscf). Therefore, two-stage precipitators have limited use for particulate-emission control. They are used almost exclusively to collect liquid aerosols discharged from sources such as meat smokehouses, pipe-coating machines, asphalt paper saturators, high speed grinding machines, welding machines, and metal-coating operations.

            Representation of gas flow in a two-stage precipitator