Dust Resistivity Lab
Dust resistivity has long been recognized as the primary factor in determining the performance of electrostatic precipitators (ESPs).
Changes in dust resistivity from optimum values to extreme values can change the collection efficiency of ESPs dramatically. Therefore ESP sizing and performance evaluation must always include resistivity as a primary factor. A typical dust resistivity analysis is shown on Figure 1. Resistivity plots of this type are used as a standard engineering design “tool” in the ESP industry.
Since resistivity is such an important factor in ESP design and performance evaluation, KC-Cottrell operates our own laboratory facility to measure resistivity. This laboratory equipment and test personnel have been in service for more than 30 years. Our equipment can handle one to four dust samples simultaneously, while varying gas temperature and moisture content. This gives us the capability to process a large number of dust samples in a relatively short period of time, as opposed to laboratory equipment that might test only one sample at a time.
Dust Resistivity Lab – Definition
Laboratory resistivity (OHM-CM) of a dust is the ratio of the applied electric potential across the dust layer to the induced current density.
The value of the resistivity for a dust sample depends upon a number of variables, including dust chemistry, dust porosity, dust temperature, composition of gaseous environment (i.e. gas moisture), magnitude of applied electric field strength, and test procedure.
In working with electrostatic precipitators (ESP), resistivities are encountered in the range from about 1E4 to 1E14 OHM-CM. The optimum value for resistivity is generally considered to be in the range of 1E8 to 1E11 OHM-CM. In this range the dust is conductive enough that charge does not build-up in the collected dust layer and insulate the collecting plates.
Additionally the dust does not hold too much charge and is adequately cleaned from the collecting plates by normal rapping. If resistivity is in the range 1E12 to 1E14 OHM-CM, it is considered to be high resistivity dust. This dust is tightly held to the collecting plates, because the dust particles do not easily conduct their charge to ground. This insulates the collecting plates and high ESP sparking levels result (also poor ESP collection efficiencies). Conversely if the dust is low resistivity, 1E4 to 1E7 OHM-CM, the dust easily conducts its charge to the grounded collecting plates. Then there is not residual charge on the dust particles to hold them on the plates. Thus these particles are easily dislodged and re-entrain back into the gas stream. ESP gas velocities are generally designed in the 2.5-3.5 FT/S range, if high carbon particles are to be collected.
Dust resistivity lab – Procedure
The test procedure is in accordance with IEEE-548, Standard Criteria for the Laboratory Measurement of Fly Ash Resistivity.
The apparatus used for the testing is a custom built arrangement utilizing a high temperature oven, a controlled temperature water bath for gas humidity adjustment, a DC power source, and an electrometer for current flow measurement. Resistivity values are calculated from
= (V/I). (A/L) where
= resistivity, OHM-CM
V = applied voltage, Volts
I = measured current, Amperes
L = Ash thickness, cm
A = current measuring electrode face area, cm²
The resistivity testing was conducted in ascending temperature order.
Resistivity Laboratory Request
Dust sample sizes should be 3 to 5cc. Do not send large sample amounts to the resistivity laboratory, as this creates disposal costs for us.
The samples can be in plastic bottles or double bagged in plastic bags. In this time of terrorism concerns, each sample should have a Material Safety Data Sheet (MSDS) with it (i.e. an explanation of what chemicals are in the dust).
The following data should be included in the cover letter for the samples;
Date of Sample______________________________________
Process Sampling Point____________________________
ESP Operating Temperature__________________________ C
Laboratory Resistivity Cell Gas Moisture___________ %(Vol.) *
(Note: Each moisture level tested constitutes a billable resistivity test. So for example, one dust sample tested at three different moistures is actually considered three separate/billable resistivity tests)
Unless otherwise stated, the resistivity testing will be performed in ascending order, which is standard for our evaluations. If for some reason, the customer also wants resistivity measured in descending order, this should be requested.
* If gas moisture is unknown, the resistivity lab will base moisture content upon typical values observed in the past for the process type.