Shell

The shell structure encloses the electrodes and supports the precipitator components in a rigid frame to maintain proper electrode alignment and configuration (Figure). The support structure is especially critical for hot-side precipitators because precipitator components can expand and contract when the temperature differences between the ESP (400°C or 752°F) and the ambient atmosphere (20°C or 68°F) are large. Excessive temperature stresses can literally tear the shell and hopper joints and welds apart. The outer sheet or casing wall is usually made of low-carbon or mild-grade steel that is 0.5 to 0.6 cm (3/16 to 1/4 in.) thick.

Collection plates and discharge electrodes are normally attached to the frame at the top so that the elements hang vertically due to gravity. This allows the elements to expand or contract with temperature changes without binding or distorting.

Shells, hoppers, and connecting flues should be covered with insulation to conserve heat, and to prevent corrosion resulting from water vapor and acid condensation on internal precipitator components. If the ESP is installed on a coal-fired boiler, the flue gas temperature should be kept above 120°C (250°F) at all times to prevent any acid mists in the flue gas from condensing on ESP internal components. Insulation will also help minimize temperature- differential stresses, especially on hot-side precipitators. Ash hoppers should be insulated and heated because cold fly ash has a tendency to cake, making it extremely difficult to remove. Insulation material is usually 10 to 15 cm (4 to 6 in.) thick.