Cement, Energy and Environment

' -; ' ., • ) } • > ., Step 4. Ifmeasured collection efficiency is significantly lower than designed performance, determine cause of poor performance If the measured collection efficiency, with good mechanical conditions, is significantly less than the designed value, then the precipitator is performing poorly. Any one or a combination of several factors, each of which needs investigation, may cause poor performance. These factors include electrical operating conditions, non-uniform gas velocity distribution, gas sneakage past the electrified fields, particle re-entrainment without rapping , and rapping re– entrainment. The following steps describe procedures to pinpoint other problem areas. Step 4.1. Determine if the operating currents are completely useful in the precipitation process. At this point, the electrical operating conditions should be examined to determine whether or not the operating currents are completely useful in charging and collecting particles. If excessive sparking or back corona is occurring in the ESP, then the measured currents will not be completely useful in the precipitation process. On the contrary, the currents may actually be detrimental to the performance. Excessive Sparking Excessive sparking can lead to increased particle re-entrainment. Excessive sparking disrupts the surface of the collected dust layer and reduces the electrical holding force over large regions of the layer because of decreased currents to those regions. If sparking is occurring , then the extent of the sparking should be measured with spark rate meters. Acceptable levels of sparking are in the range of 1 0-30 sparks per minute. If excessive sparking is occurring, then the operating voltage should be lowered; the power supply controllers should be adjusted or upgraded. Although the operating voltages and currents will be lower, the ESP performance may improve (unless the problem is caused by severe ESP mechanical problems) and the use of the lower operat ing electrical cond itions in the ESP wiil give better agreement between designed and measured collection efficiencies. Back Corona In a condition of severe back corona, the measured current can be very much greater than the current that is actually useful in the precipitation process. If severe back corona is occurring, the operating voltage should be lowered to operate with a lower current density that will not lead to the formation of back corona. The reduced voltages and currents will usually result in improved performance, and the use of the lower electrical operating points in the ESP will give better agreement between designed and measured collection efficiencies. Electrode Misalignment If the spacing between plates or between discharge electrodes and plates is out of alignment by over 1/2 inch (1/4" preferred), then corrective measures should be taken. Significant misalignment of the electrodes is detectable from secondary voltage-current curves obtained under air load conditions. The air load V-1 curves should be the same for different electrical sections with similar geometry, up to the current limits of the T– R sets. If these curves differ significantly, then misalignment of some of the electrodes is expected. Distorted plates can result in very low secondary voltages with high values of localized currents. The electrode alignment should always be checked to avoid reaching wrong conclusions. For example, low secondary voltages may be attributed wrongly to high dust resistivity, when mechanical deformation of the electrodes may be responsible. Step 4.2. Determine if performance is limited by non-uniform inlet gas temperature or dust loading Excessive temperature variation across the face of the ESP or mass loading at the inlet of the precipitator can be detrimental to the ESP performance. The uniformity should be checked by sampling the gas temperature and mass loading over the inlet duct of the precipitator, under actual operating conditions. A temperature variation 50°C would cause an order-of– magnitude change in the electrical resistivity of most dust. Even large variations in the ambient temperature can be measurable in the ESP performance. Large increases in the particulate mass loading or perhaps the condensation of 19