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June 2006

Filtration and Vacuum Specialists since 1976

www.vacuumpumpusa.com




A Scientific Review of Dust Collection - Part 3

Electrostatic Collectors

Reference material by: Scientific Dust Collectors

Because our newsletters are a service to our valued customers we have decided to share some important and educational information on Scientific Dust Collection. Over the next few months we will be focusing on the use of Dust Collectors. We felt that the extensive information and it's importance in the industry would be very useful in helping our customers make an informed decision on their needs for dust collectors in their businesses. Because the information is extensive we will be spreading it over several months.

Electrostatic collectors operate by the forces generated by electrostatic charges which draw the dust particles to the collection plates. These particles lose their charges and agglomerate when they reach the grounded plates.

In general, the main advantages of an electrostatic precipitator are:

  • The efficiency can exceed 99 percent in some applications.
  • The size of the particles collected can be very small.
  • The precipitator can function at temperatures of 700 F and with special designs as high as 1300 F.
  • The pressure and temperature changes through the collector are small, usually less than 0.5 inches water column.
  • The collected dust is dry, an advantage for the recovery of loss product.
  • Large flow rates are possible.
  • Difficult acid and tars can be collected.
  • Collectors can tolerate extremely corrosive materials.
  • The electrical power requirement is low to clean the dirty gas.

As there are advantages to using electrostatic precipitation, there are also disadvantages:
  • The initial cost is generally more costly than other approaches to solve the pollution problem.
  • Some materials are extremely difficult to collect in an electrical precipitator due to very high or low resistivity.
  • Variable condition of airflow causes the precipitator to become very inefficient. Automatic voltage control improves the collector efficiency somewhat.
  • Space requirements for the equipment can be greater than those for other approaches such as bag houses and/or cartridge units.
  • Electrical precipitation is not applicable for the removal of materials in the gaseous phase.
  • A cyclonic precleaner may be needed to reduce the dust load before the precipitator.

Single Stage Precipitator

Figure 3-1 is a typical plate type precipitator. It 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. High voltage discharge electrodes are suspended vertically between the plates from an insulated mounting frame. The distance between the grounded plates are in the 4 to 6 inch range, and the voltage on the electrodes is between 40,000 and 60,000 volts. This voltage causes the gases to ionize and when this occurs the dust particles becomes negatively charged. The strength of this charge is a function of the dielectric characteristics of the dust. Some dusts will have a high charge and the forces to attract it to the grounded collecting plates will be high. 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 (or liquid droplets) have higher forces that attract them to the collection plates at a greater efficiency rate than others. Other factors include the other gases in the process stream. For instance, some sulfur compounds in boiler gas will increase collection efficiency.


Figure 3-1
The velocity of the gas passing through the plates will also affect the efficiency of collection. For instance, at a 50 fpm gas velocity, only half of the particles will reach the collecting plates with an associated collection efficiency of 50%. At 25 fpm, the efficiency might be 95% and at 12 fpm, it might be 99%. The pressure drop across the precipitator collection section will usually stay in the range of 0.2 to 0.5 inches of water.

From the above analysis, it is important to have a very even velocity distribution through the precipitation from side to side and from top to bottom in the collection compartment. If the velocity varies, the efficiency will be lower across the sections with higher velocity (and higher flow), and the collection efficiency might be much lower than might be predicted based on the average velocity. In designing these electrostatic dust collectors of the single state high voltage design, it is necessary to design the distribution baffles very carefully. This is accomplished with computer programs followed by modeling in a test laboratory.

In some precipitators, the high voltage electrodes are in the form of hanging wires with weights on the bottom of the wires to keep them straight. This is an economical approach, but many of the premium designs have fixed frames. The charging wires and/or electrodes can be viewed as "lightening rods", rods that drain charges from buildings. The closer the electrodes are placed to the grounding plates, the more effective the charging force becomes. With smaller electrode to plate distances, the voltage becomes lower and smoother to ionize the gas stream. Under the circumstances, smaller diameter wires are more effective. The more costly framed electrodes are built with points sticking out from the electrode frames.

Dust Removal from Plates

The collecting plates are cleared by rapping with an air powered anvil. The power supply is shut off during the rapping and the dust falls into the collection hopper.

Once the particles get a charge, they will migrate to any grounded (or uncharged surface), even a surface at a lower potential. The collection surface may include the high voltage insulators. If dust collects on the insulators, a path for the high voltage to ground is formed. Eventually, this will cause failure of the high voltage power supply in order to reduce or eliminate this effect, the insulators are pressurized with a blower and a flow of outside air is maintained in the collection compartment. Then the charged particles will not have enough attraction to rest on the insulator surfaces.



The Tubular Precipitator

consists of pipes with the electrodes in the center of the pipes. These designs have much more rigidity and are often employed with wet electrostatic precipitators. These designs either keep the walls continuously wet or use a washing system to clean the grounded electrodes. The construction and a schematic of the insulator supports are shown Figure 3-2. The pipe collection electrodes provide unusually effective gas distribution within the precipitator.

These types of precipitators are able to adjust to the expansion and contraction of parts as they are heated and are widely applied to higher temperature gas streams, especially boiler exhausts in power plants. They are sometimes subject to corrosive gases, and the life of the collectors and the frequency of maintenance depends on the thickness and ruggedness of the electrodes and the grounded collecting plates.



Figure3-2
The Two Stage Precipitator

A schematic is shown in Figure 3-3. The grounded plates are about an inch apart and have an intermediate plate that is also charged. Instead of the 40,000-60,000 volt D.C. supply, the two stage precipitator has a 13,000-15,000 volt supply with the intermediate supply at 7,500 volts.

This collector was developed for HVAC (heating and ventilating service). It provides very efficient dust collection and is designed with a self cleaning washing system. The dust load in this service is between 0.01 to 0.1 grains per 1,000 cubic feet. The washing system is a light duty unit designed for 250 cycles. Since the usual cleaning is only required monthly, this unit exceeds the life of other components of the HVAC systems.

The high voltage electrodes consist of very fine wire stretched across springs. At 15,000 volts, a finer wire is required for ionization. The plates have to be maintained at more precise distances and to manufacture these components requires very special tooling.

In this kind of service, the air distribution is usually very even since the dust collecting filtering device operates the same velocities as the heating and cooling coils.

Figure 3-3



Industrial Dust Venting with Two Stage Precipitators

In the early seventies, Two Stage units were supplied as general ventilation modules in industrial plants where welding, burning, and grinding operations were preformed. The units had integral fans and drew air from the plant at one end and blew it out of the opposite end. Because the load to these precipitators was 10 to 50 times as high, these units typically required cleaning 2 to 7 times a week.

Since the precipitators were designed for 250 cleaning cycles, major maintenance was required within months. The required maintenance consisted of removing the precipitator frames and manually cleaning them. These assemblies were delicate and often the electrode wires were broken and the collection efficiency suffered. The washing mechanisms would also require replacement or an overhaul.

Soon these two stage electrostatic filters were being applied to hooded and ducted automatic welding machines or to welding booths. In these applications the dust loading was increased to 30-50 grains per 1,000 cubic feet per minute.

Insulator Deterioration

As discussed above, the charged dust particle will be attracted to a grounded element, or any element at a lower potential than the charge carried by the particle. Some of these particles will be collected on the intermediate charged plate while others will be attracted to the insulators. However, an electrical charge inherently cannot be bled to ground so it adheres to the insulator. The particle sometimes can be washed off during the cleaning cycle, but some of it will paint the insulator. The particle sometimes can be washed off during the cleaning cycle, but some of it will paint the insulator. The particle sometimes can be washed off during the cleaning cycle, but some of it will paint the insulator. Soon a leakage path forms from the high voltage charging wires to the intermediate plate which results in not maintaining enough voltage to the power supply to perform the function of ionizing in the precipitator. The normal maintenance in this case would be to install new insulators. This requires some specialized abilities from the maintenance personnel and is presently performed by specialized maintenance organizations.

Pressurized Insulators

Single stage precipitators have the insulators installed in compartments through which air from outside the precipitator is drawn or blown into the insulator compartment. The charged particle must overcome the velocity vector of the air that is flowing towards the precipitator so that few, if any, particles will reach the insulators. This allows insulatators in very heavy dust load service to operate for many years.

The same approach was taken on two stage precipitators. This allowed their application to become more widespread and to be applied on industrial processes as severe as asphalt saturators.



Plating

Most of the two stage precipitator collectors were applied on processes like welding. It was especially effective since it could tolerate condensed hydrocarbons as well as the particulate fume. The cleaned gas was discharged into the room instead of outside. The electrostatic is very sensitive like all precipitators to even flow distribution. When applied to industrial hooded processes, it is difficult and expensive to get even flow across the collection plates.

With even distribution, a correctly selected lower velocity collector can achieve a collection efficiency of 99%. But if an improperly designed distribution component is installed in front of fhe collector, the efficiency may drop to 90% or lower.

Figure 3-4

The charged particles leaving a properly designed precipitator will quickly lose their static charge. Normally this will occur within a few inches of the discharge into the room. Under some atmospheric conditions, notably low humidity, this zone may extend to a couple of feet.

If velocity distribution is poor, the distance required to dissipate the charge may be several feet (Figure 3-5). Under certain conditions of low humidity, this distance may extend indefinitely, even up to or more than a hundred feet. In that occurrence, all the surfaces in the room become collecting plates. This includes the walls, machines and operator eyeglasses, etc.

This phenomenon is called plating.

Figure 3-5

Competitive pressures have led many suppliers to offer precipitators that operate at higher velocities. Many times, even on welding fume collectors, these units would only achieve efficiencies in the 90-95% range. From a design viewpoint, this seemed sufficient since it was quite effective in eliminating the haze in the work area. Unfortunately, they did not always consider the plating phenomenon. This gave the two stage precipitators a bad reputation and contributed to the rapid rise of pulse jet cartridge collectors for welding fume collection.

De-ionizing Sections

The designers came up with an effective remedy to remove the charges from the dust particles. They applied an alternating current to the high voltage power supply and this effectively removed the charge from the particles that were coming though the collector. This de-ionizing could be accomplished even at fairly high velocities.

We would like to acknowledge the contribution of Scientific Dust Collectors for their contributions to this newsletter. Excerpts taken from " A Scientific Review of Dust Collection" by Scientific Dust Collectors.

Look for "Filter Media" next month



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