Solid cakes are removed by rapping the plates at regular time intervals with a mechanical or electromagnetic rapper that strikes a vertical or horizontal blow on the edge of the plate. Through science,art,and experience designers have learned to make rappers that cause most of the collected cake to fall into hoppers below the plates. Some of the cake is al ways re-entrained thereby lowering the efficiency of the system. If the collected particles are liquid, e.g, sulfuric acid mist, they run down the plate and drip off. For liquid droplets the plate is often replaced by a circular pipe with the wire down its center. Some ESPs(mostly the circular pipe variety) have a film of water flowing down the collecting surface, to carmy the collected particles to the bottom without rapping There are many types of ESPs Support frarnes for Fig. 9.3 shows one of the Rapper systems discharge clectrodes Transfurmcr-rectifier sets most common in current u in the United States. Gas flo High-voltage insulators is from right to left. The gas enters at the right through 30s which the flow spreads from the much narrower duct distribution plate that across the entrance face of the converging nozzle on the left side(not shown) ma a uniform flow at the outlet and then reduce the that of the outlet duct the whole interior of the structure elect Particle collecting plates, the cutaway show Discharge electrodes only one set of plates and discharge electrodes. The ig.9-3 Cutaway view of a large, modern ESP showing the various parts. of rigid frames with many In this design the wire discharge electrodes have been Replaced by rigid frames with many short, pointed stubs short, pointed stubs, which serve the same function as the in Fig9.2. TH collecting surfaces are made of sheet metal sections with vertical joints that tend to trap the particles. Each pair of plates, along with the discharge electrode between them, acts like the single channel in the simplified version of an ESP shown in Fig 9.2. The rappers strike the supports for the discharge electrodes and the collecting plates at regular time intervals to dislodge the cake of collected particles. The multiple power supply transformer-rectifier sets supply DC current at m -40,000 V to the discharge electrodes. The collected particles, dislodged from the plates by tl rappers, fall into the particle collecting hoppers, from which they are automatically removed to storage. The drawing shows some of the structural steel frame and enclosure of the esp and the handrail on its top, but not the internal seals that hinder the gas from flowing around the area of the collecting plates Each point in space has some electrical potential V. If the electrical potential changes from place to place, then there is an electrical field, E= V/x, in that space. If we connect two such points with a conductor, then a current will flow. This v is the voltage we are all familiar with, and e is its gradient in any direction; the units of E are V/m In a typical wire-and-plate precipitator, the distance from the wire to the plate is about 4 to 6 in or 0. 1 to 0.15 m. With a voltage difference of 40 kV and 4-in. spacing, one would assume a field strength of 40 kv/0. 1 m=400 k V/m. This is indeed the field strength near the plate. However, all of the electrical flow that reaches the plate comes from the wires, and the surface area of the wires9-3 Solid cakes are removed by rapping the plates at regular time intervals with a mechanical or electromagnetic rapper that strikes a vertical or horizontal blow on the edge of the plate. Through science, art, and experience designers have learned to make rappers that cause most of the collected cake to fall into hoppers below the plates. Some of the cake is always re-entrained, thereby lowering the efficiency of the system. If the collected particles are liquid, e.g., sulfuric acid mist, they run down the plate and drip off. For liquid droplets the plate is often replaced by a circular pipe with the wire down its center. Some ESPs (mostly the circular pipe variety) have a film of water flowing down the collecting surface, to carry the collected particles to the bottom without rapping. There are many types of ESPs; Fig. 9.3 shows one of the most common in current use in the United States. Gas flow is from right to left. The gas enters at the right through an inlet diffuser (not shown) in which the flow spreads out from the much narrower duct to the perforated gas distribution plate that distributes the gas evenly across the entrance face of the precipitator. A similar plate and converging nozzle on the left side (not shown) maintain a uniform flow at the outlet and then reduce the cross-sectional flow area to that of the outlet duct. The whole interior of the structure is filled with discharge electrodes and collecting plates; the cutaway shows only one set of plates and discharge electrodes. The discharge electrodes consist of rigid frames with many short, pointed stubs, which serve the same function as the wires in Fig.9.2. The collecting surfaces are made of sheet metal sections with vertical joints that tend to trap the particles. Each pair of plates, along with the discharge electrode between them, acts like the single channel in the simplified version of an ESP shown in Fig. 9.2. The rappers strike the supports for the discharge electrodes and the collecting plates at regular time intervals to dislodge the cake of collected particles. The multiple power supply transformer-rectifier sets supply DC current at m -40,000 V to the discharge electrodes. The collected particles, dislodged from the plates by the rappers, fall into the particle collecting hoppers, from which they are automatically removed to storage. The drawing shows some of the structural steel frame and enclosure of the ESP and the handrail on its top, but not the internal seals that hinder the gas from flowing around the area of the collecting plates. Each point in space has some electrical potential V. If the electrical potential changes from place to place, then there is an electrical field, E = V/x, in that space. If we connect two such points with a conductor, then a current will flow. This V is the voltage we are all familiar with, and E is its gradient in any direction; the units of E are V/m. In a typical wire-and-plate precipitator, the distance from the wire to the plate is about 4 to 6 in., or 0.1 to 0.15 m. With a voltage difference of 40 kV and 4-in. spacing, one would assume a field strength of 40 kV/0.1 m = 400 kV/m. This is indeed the field strength near the plate. However, all of the electrical flow that reaches the plate comes from the wires, and the surface area of the wires Fig. 9-3 Cutaway view of a large, modern ESP showing the various parts. In this design the wire discharge electrodes have been Replaced by rigid frames with many short, pointed stubs