question: How do moving objects, like ceiling fans or fans in general, collect so much dust? Wouldn't it moving just keep dust from collecting on it?
comment: Humidity in the air allows things to stick to surfaces the way a fan works is by pulling air from one side and pushing on another, through this action it pulls dust and moisture over itself which collects on the surface.
passages: Resistance affects electrical conditions in the dust layer by a potential electric field (voltage drop) being formed across the layer as negatively charged particles arrive at its surface and leak their electrical charges to the collection plate. At the metal surface of the electrically grounded collection plate, the voltage is zero, whereas at the outer surface of the dust layer, where new particles and ions are arriving, the electrostatic voltage caused by the gas ions can be quite high. The strength of this electric field depends on the resistance and thickness of the dust layer.
   The majority of electrostatic precipitators installed are the plate type. Particles are collected on flat, parallel surfaces that are 8 to 12 in. (20 to 30 cm) apart, with a series of discharge electrodes spaced along the centerline of two adjacent plates. The contaminated gases pass through the passage between the plates, and the particles become charged and adhere to the collection plates. Collected particles are usually removed by rapping the plates and deposited in bins or hoppers at the base of the precipitator.
  
  Section::::Electrostatic precipitators (ESP).:Types of precipitators.:Tubular precipitators.
   Fugitive dust acquires an electronic charge when dispersed in the air. Spraying an oppositely charged water fog can be used to effectively control dust in an industrial setting. If so charged, less water is needed to cause the particulate matter to drop from the air. This method has been tested with many materials including silica flour, sulfur dioxide, and fly ash.
  
  Section::::See also.
  
  BULLET::::- Asian dust
  
  BULLET::::- Bull dust
  
  BULLET::::- Coal dust
  
  BULLET::::- Occupational dust exposure
  
  Section::::Further reading.
  
  BULLET::::- PM 10 and Fugitive Dust in the Southwest Ambient Impact, Sources and Remedies, US Environmental Protection Agency
   BULLET::::- Methods of disposal – Methods of dust removal and disposal vary with the material, plant process, volume, and type of collector used. Collectors can unload continuously or in batches. Dry materials can create secondary dust problems during unloading and disposal that do not occur with wet collectors. Disposal of wet slurry or sludge can be an additional material-handling problem; sewer or water pollution problems can result if wastewater is not treated properly.
  
  Section::::Fan and motor.
  
  The fan and motor system supplies mechanical energy to move contaminated air from the dust-producing source to a dust collector.
   Packed-bed scrubbers consist of beds of packing elements, such as coke, broken rock, rings, saddles, or other manufactured elements. The packing breaks down the liquid flow into a high-surface-area film so that the dusty gas streams passing through the bed achieve maximum contact with the liquid film and become deposited on the surfaces of the packing elements. These scrubbers have a good collection efficiency for respirable dust.
  
  Three types of packed-bed scrubbers are:
  
  BULLET::::- Cross-flow scrubbers
  
  BULLET::::- Co-current flow scrubbers
  
  BULLET::::- Counter-current flow scrubbers
   Tubular precipitators consist of cylindrical collection electrodes with discharge electrodes located on the axis of the cylinder. The contaminated gases flow around the discharge electrode and up through the inside of the cylinders. The charged particles are collected on the grounded walls of the cylinder. The collected dust is removed from the bottom of the cylinder.
  
  Tubular precipitators are often used for mist or fog collection or for adhesive, sticky, radioactive, or extremely toxic materials.
  
  Section::::Electrostatic precipitators (ESP).:Unit collectors.
   BULLET::::- Dust concentration and particle size – For minerals processing operations, the dust concentration can range from 0.1 to of dust per cubic foot of air (0.23 to 11.44 grams per cubic meter), and the particle size can vary from 0.5 to 100 micrometres (µm) in diameter.
   Air flow gives the bag structure. Dirty air flows through the bag from the inside, allowing dust to collect on the interior surface. During cleaning, gas flow is restricted from a specific compartment. Without the flowing air, the bags relax. The cylindrical bag contains rings that prevent it from completely collapsing under the pressure of the air. A fan blows clean air in the reverse direction. The relaxation and reverse air flow cause the dust cake to crumble and release into the hopper. Upon the completion of the cleaning process, dirty air flow continues and the bag regains its shape.
   BULLET::::- A means of removing the collected particulates
  
  BULLET::::- A housing to enclose the precipitator zone
  
  The following factors affect the efficiency of electrostatic precipitators:
  
  BULLET::::- Larger collection-surface areas and lower gas-flow rates increase efficiency because of the increased time available for electrical activity to treat the dust particles.
  
  BULLET::::- An increase in the dust-particle migration velocity to the collecting electrodes increases efficiency. The migration velocity can be increased by:
  
  BULLET::::- Decreasing the gas viscosity
  
  BULLET::::- Increasing the gas temperature
  
  BULLET::::- Increasing the voltage field
  
  Section::::Electrostatic precipitators (ESP).:Types of precipitators.
  
  There are two main types of precipitators:
   Dust-Off was developed and introduced in 1970 by an employee at Falcon Safety Products who discovered that the pressurized blasts used to sound the alarm in the company’s signal horns could also remove dust from photography equipment and film without having to touch the surface.
   If the voltage drop across the dust layer becomes too high, several adverse effects can occur. First, the high voltage drop reduces the voltage difference between the discharge electrode and collection electrode, and thereby reduces the electrostatic field strength used to drive the gas ion-charged particles over to the collected dust layer. As the dust layer builds up, and the electrical charges accumulate on the surface of the dust layer, the voltage difference between the discharge and collection electrodes decreases. The migration velocities of small particles are especially affected by the reduced electric field strength.
   In high-resistance dust layers, the dust is not sufficiently conductive, so electrical charges have difficulty moving through the dust layer. Consequently, electrical charges accumulate on and beneath the dust layer surface, creating a strong electric field.
  
  Voltages can be greater than 10,000 volts. Dust particles with high resistance are held too strongly to the plate, making them difficult to remove and causing rapping problems.
   These rely on air pulled with a suction force through piping systems from the point of dust formation to a waste disposal unit. LEV systems consist of four elements: dust hoods at the point of dust formation, ventilation ducts, an air cleaning device (waste separator or dust collector) and an air moving device (a fan, otherwise known as an impeller). The air, containing dust and chips from the woodworking operation, is sucked by an impeller. The impeller is usually built into, or placed close to, the waste disposal unit, or dust collector.
   The third, and generally most common problem with high resistivity dust is increased electrical sparking. When the sparking rate exceeds the "set spark rate limit," the automatic controllers limit the operating voltage of the field. This causes reduced particle charging and reduced migration velocities toward the collection electrode. High resistivity can generally be reduced by doing the following:
  
  BULLET::::- Adjusting the temperature;
  
  BULLET::::- Increasing moisture content;
  
  BULLET::::- Adding conditioning agents to the gas stream;
  
  BULLET::::- Increasing the collection surface area; and
  
  BULLET::::- Using hot-side precipitators (occasionally and with foreknowledge of sodium depletion).
   In low resistance dust layers, the corona current is readily passed to the grounded collection electrode. Therefore, a relatively weak electric field, of several thousand volts, is maintained across the dust layer. Collected dust particles with low resistance do not adhere strongly enough to the collection plate. They are easily dislodged and become retained in the gas stream.
   Efficiency can be greatly increased by minimizing target size, i.e., using 0.003 in. (0.076 mm) diameter stainless steel wire and increasing gas velocity to more than 1,800 ft/min (9.14 m/s).
  
  Section::::Wet scrubbers.:Types of scrubbers.:High-energy scrubbers.
   Dust explosions may be classified as being either "primary" or "secondary" in nature. Primary dust explosions may occur inside process equipment or similar enclosures, and are generally controlled by pressure relief through purpose-built ducting to the external atmosphere. Secondary dust explosions are the result of dust accumulation inside a building being disturbed and ignited by the primary explosion, resulting in a much more dangerous uncontrolled explosion that can affect the entire structure. Historically, fatalities from dust explosions have largely been the result of secondary dust explosions.
  
  Section::::Conditions required.
  
  There are five necessary conditions for a dust explosion:
   BULLET::::- Similar Design Reference, out of print, Gaylord & Gaylord, Design of Bins.
  
  Cement, lime and lead industry accepted dust loads (for structural loading) are:
  
  Process ductwork is intended to convey large volumes of dust. some of this dust will settle to the bottom of the duct during power outages and normal operation.
  
  The percentage of duct cross section filled with dust is often assumed to be as follows:
  
  BULLET::::- Duct slopes level to 30 degrees, 25% of cross section.
  
  BULLET::::- Duct slopes, 30 degrees to 45 degrees, 15% of cross section
   A study from 2008 finds that the initial saltation of sand particles induces a static electric field by friction. Saltating sand acquires a negative charge relative to the ground which in turn loosens more sand particles which then begin saltating. This process has been found to double the number of particles predicted by previous theories.
   A number of designs are available, with capacities ranging from 200 to 2,000 ft³/min (90 to 900 L/s). There are two main types of unit collectors:
  
  BULLET::::- Fabric collectors, with manual shaking or pulse-jet cleaning - normally used for fine dust
  
  BULLET::::- Cyclone collectors - normally used for coarse dust
  
  Fabric collectors are frequently used in minerals processing operations because they provide high collection efficiency and uninterrupted exhaust airflow between cleaning cycles. Cyclone collectors are used when coarser dust is generated, as in woodworking, metal grinding, or machining.
  
  The following points should be considered when selecting a unit collector:
   Dust-laden gas or air enters the baghouse through hoppers and is directed into the baghouse compartment. The gas is drawn through the bags, either on the inside or the outside depending on cleaning method, and a layer of dust accumulates on the filter media surface until air can no longer move through it. When a sufficient pressure drop (ΔP) occurs, the cleaning process begins. Cleaning can take place while the baghouse is online (filtering) or is offline (in isolation). When the compartment is clean, normal filtering resumes.
   Particle collection in wet scrubbers
  
  Wet scrubbers capture relatively small dust particles with large liquid droplets. In most wet scrubbing systems, droplets produced are generally larger than 50 micrometres (in the 150 to 500 micrometres range). As a point of reference, human hair ranges in diameter from 50 to 100 micrometres. The size distribution of particles to be collected is source specific.br
   There are three main types of centrifugal fans:
  
  BULLET::::- Radial-blade fans - Radial-blade fans are used for heavy dust loads. Their straight, radial blades do not get clogged with material, and they withstand considerable abrasion. These fans have medium tip speeds and medium noise factors.
  
  BULLET::::- Backward-blade fans - Backward-blade fans operate at higher tip speeds and thus are more efficient. Since material may build up on the blades, these fans should be used after a dust collector. Although they are noisier than radial-blade fans, backward-blade fans are commonly used for large-volume dust collection systems because of their higher efficiency.
   When laying a radiant barrier over the insulation on the attic floor, it is possible for dust to accumulate on the top side. Many factors like dust particle size, dust composition and the amount of ventilation in the attic affect how dust accumulates and thus the ultimate performance of radiant barrier in the attic. A study by the Tennessee Valley Authority mechanically applied a small amount of dust over a radiant barrier and found no significant effect when testing for performance. However, TVA referenced a previous study which stated that it was possible for the radiant barrier to collect so much dust that its reflectivity could be decreased by nearly half.
   In the simple, gravity-spray-tower scrubber, liquid droplets formed by liquid atomized in spray nozzles fall through rising exhaust gases. Dirty water is drained at the bottom.
  
  These scrubbers operated at pressure drops of 1 to 2 in. water gauge (¼ to ½ kPa) and are approximately 70% efficient on 10 µm particles. Their efficiency is poor below 10 µm. However, they are capable of treating relatively high dust concentrations without becoming plugged.
  
  Section::::Wet scrubbers.:Types of scrubbers.:Low- to medium-energy scrubbers.
  
presuppositions: Moving objects should be able to prevent dust from sticking to it.
corrections: Humidity in the air allows dust to stick to surfaces, even if it is moving.
raw_presuppositions: Moving objects should be able to prevent dust from sticking to it. Moving objects should be able to prevent dust from sticking to it.
raw_corrections: Humidity in the air allows dust to stick to surfaces, even if it is moving. Humidity in the air allows dust to stick to surfaces, even if it is moving.