8. ELECTROSTATIC PRECIPITATOR
Electrostatic precipitator generally used to separate particulate matter that is easily ionized from a gas stream by using electrostatic charges. The principal actions are the charging of dust particles and forcing them to the collector plates. According to e.m.f. gradient the charge particle migrates and is attracted to collecting electrodes. Negatively charged particle are attracted towards the positive electrode and positively charged particles to the negative electrodes.
There are two types of electrostatic precipitators
| 1. Single stage unit |
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Single stage electrostatic precipitator is very common. In this unit ionization and collection are combined. In two stage unit ionization is followed by collection. Each operation occurs in different part of the apparatus.
8.1 Single stage unit
Single stage unit precipitator consists of a rectangular shell or casing in which a number of grounded plates are suspended parallel to each other and has equal spacing between plates to form channels through which the gas flows (figure 5.8). High voltage discharge electrodes are suspended vertically between the plates from an insulated mounting frame. The distance between the grounded plates is in the 4 to 6 inch range. The voltage on the electrodes is between 40,000 to 60,000 volts. This voltage causes the gases to ionize and when this occurs the dust particle becomes negatively charged, the strength of this charge is a function of the dielectric characteristics of the dust. Some of the dust particles will have a high charge and the forces attract it to the grounded collecting plates. The time interval is determined by the distance the dust particle has traveled to the grounded collector plate and the magnitude of the charged dust particle. Some dust particles have higher forces that attract them to the collection plates at a greater efficiency rate than others. This may include other gases present in the process stream. For instance, some sulfur compounds in boiler gas increases collection efficiency. Apart from that the velocity of the gas passing through the plates also affects the efficiency of collection. For example, at 17 m/min gas velocity, only fifty percent of the particles may reach the collecting plates with an associated collection efficiency of 50%. Say at 9 m/min, the efficiency might be 95% and at 4 m/min it might be 99%. The pressure drop across the precipitator collection section usually in the range of 0.2 to 0.5 inches of water.
If the velocity varies, the efficiency will be lower across the sections with higher velocity, and the collection efficiency might be much lower than the predicted efficiency based on the average velocity. While designing electrostatic collector for single stage high voltage design, it is necessary to design the distribution baffles very carefully.