1、SSPC-Guide 16August 1, 2003SSPC: The Society for Protective CoatingsGuide 16Guide to Specifying and Selecting Dust Collectors11. Scope1.1 This guide will assist end-users in the selection ofdust collectors to be used in conjunction with containmentsystems to control emissions to the environment, enh
2、ancevisibility, and reduce worker exposures within the contain-ment. It will discuss the purpose of dust collector use inpainting projects; define dust collector terminology; de-scribe the types of dust collectors and the mechanisms ofcollection; and discuss efficiency, operation, and mainte-nance o
3、f dust collectors.2. DefinitionsThe definitions below do not appear in the SSPCProtective Coatings Glossary.Aerosol: A system of small liquid or solid particlessuspended in a gas, in this case air. The aerosol particlecan be a single particle or an aggregate of connectedsmaller particles. The size o
4、f these particles can be in therange of 0.002 to 100 micrometers in diameter. Particlesthat are of the most significant health concern are between0.2 and 5.0 micrometers in aerodynamic equivalent diameter.Air to cloth ratio: A size or rating of the fabric filtermedia that is expressed in terms of ai
5、r flow capacity versusfabric media area, in units of cubic feet per minute persquare feet of fabric. The ratio represents the averagevelocity of the gas stream through the filter media and issometimes referred to as the filtration velocity in feet perminute (ft/min).Can Velocity: Also known as “Appr
6、oach Velocity.”The velocity of the dust-laden air as it passes upwardbetween the filter media. The can velocity is calculated bydividing the volume of air of the system (cfm) by theeffective cross-sectional area of the filter chamber. Thiseffective area is calculated by subtracting the area occupied
7、by the filters (sum of the areas of the number of the bags)from the total cross sectional area of the filter chamber.Dust Collectors: A subset of a larger group of gascleaning devices. Dust collectors are used to remove largeamounts of particulate matter (typically greater than 1 grainper cfm) from
8、gas streams and come in a wide range ofdesigns to meet various industrial applications. For thepurpose of this document, the gas stream is assumed to beair at ambient temperatures.Dust Load: The quantity of dust in the air stream.Usually expressed as grains of dust per cubic foot of air(grains/cfm).
9、ft/min: feet per minute, units of speed (velocity).Grains: A measure of the mass of particles in the air.One pound contains 7,000 grains.Inches of Water: Units of pressure equal to the pres-sure exerted by a column of liquid water an inch high atstandard temperature. Typically expressed in inchesw.c
10、.Inlet Loading: The amount of material going into thecollector. Extremes of high or low loading might allow slightlyhigher can velocities.Static pressure (SP): The flow potential force within aduct or device that acts in all directions and is measuredrelative to the surrounding atmospheric pressure.
11、 Staticpressure can be positive or negative with respect to theexternal atmosphere.Velocity pressure (VP): The pressure required to ac-celerate air from zero velocity to a given velocity, propor-tional to the kinetic energy of the air stream. Velocity pres-SSPC-Guide 16August 1, 20032sure can only b
12、e exerted in the direction of airflow and isalways positive.Total pressure (TP): The algebraic sum of the staticpressure (SP) and the velocity pressure (VP); typically ininches of water. TP can be positive or negative.Velocity: The time rate of movement including thedirection of movement; feet per m
13、inute (ft/min).3. The Purpose of the Dust CollectorThe purpose of a dust collector is to remove entrainedparticulate matter from air streams to maintain compliancewith particulate emission laws and to reduce damage causedby dust to people, equipment, the finished product, oradjacent property. The co
14、ncept is very simple; however, theprocess of separating particulate from air can be complex.The complexity arises from the extremely heterogeneousnature of the size and physical characteristics of particlesand from the quality and quantity of the air stream that is tobe cleaned. A thorough understan
15、ding of the purpose andintended use of the dust collector is needed before selec-tion of a dust collector can be made.The dust collector is a critical component of the con-tainment system for the maintenance painting process. Theexhaust fan attached to the dust collector draws contami-nated air into
16、 the collector. This creates negative pressureinside the containment that draws clean air into the contain-ment from the outside. The airflow through the containmenthelps to remove contaminants from the breathing zone ofthe workers. Airflow can also improve visibility inside thecontainment. The abil
17、ity of the dust collector to provide bothnegative pressure and airflow within the containment isdirectly related to the design, construction, and mainte-nance of the containment system, dust collector, and sup-porting parts (e.g. duct work). To prevent contaminantsfrom being released into the enviro
18、nment, all air leaving thecontainment should pass through the dust collector. Thecontainment system is only as good as the weakest elementthat makes up the containment system.The primary purpose of the containment system is tocontain debris and protect the environment. The primarypurpose of the dust
19、 collector is to provide airflow throughthe containment to reduce worker exposure to airbornedust, improve visibility, and maintain an inward airflow(negative pressure).Containment systems and methods of monitoring re-leases are discussed in detail in SSPC-Guide 6, “Guide forContaining Debris Genera
20、ted During Paint Removal Opera-tions.” Knowledge of the containment structure is critical inselecting the dust collector and will be discussed as needed toaid in the understanding of dust collector functions.4. Types of Dust CollectorsThere are four major types of dust collectors: centrifu-gal, wet,
21、 electrostatic, and fabric collectors. Each of thesewill be briefly discussed.4.1 Centrifugal Collectors: These collectors sepa-rate particulates from air streams by gravitational, inertial,or centrifugal force. Collection efficiency is affected byparticle size, particle velocity, collector design,
22、and dustconcentration. Centrifugal collectors can be designed toremove large particles effectively and economically; how-ever, they are not generally suitable for the collection of fineparticles. Most centrifugal collectors (e.g. cyclones) areused in stationary applications; however, some vacuumsyst
23、ems incorporate centrifugal collectors.4.2 Wet Collectors: Wet collectors separate particu-lates from air streams by impacting the particles with waterspray. They are commonly referred to as scrubbers. Wetcollectors are typically used for high temperature and mois-ture-filled gas streams. Neither of
24、 these conditions typicallyexists in maintenance painting applications.4.3 Electrostatic Precipitator Collectors (EPCs):EPCs separate particulates from the air by introducing acharge into the atmosphere and using a negatively chargedplate to precipitate (attract) the charged particles. EPCs areprima
25、rily used for heavy-duty dust applications such asutility boilers and cement kilns. While they have the abilityto effectively filter the concentration of interest to theindustrial painting industry, they generally cannot handlethe size range of dust typically generated by industrialpainting operatio
26、ns.4.4 Fabric or Cartridge Filter Collectors: This type ofcollector separates particulate from air streams by strain-ing, impaction, interception, diffusion, and electrostaticcharge. Air must pass through a specially designed fabricthat retains the dust on the fabric but allows the cleaned airto pas
27、s through. Fabric dust collectors are the type ofcollector used most frequently in the industrial paintingindustry.Fabric dust collectors (hereafter called dust collectors)are further broken down into two primary types based on thefilter configuration (bag and cartridge dust collectors), bothof whic
28、h are used heavily in the painting industry. Both bagand cartridge systems can attain capture efficiencies ofSSPC-Guide 16August 1, 2003399+% of particles over the entire size range of 0.1 to 100micrometers. Fabric filtration systems are normally de-signed to collect dry particulate; therefore, thei
29、r use withwet removal processes requires careful consideration.4.4.1 Baghouses: Baghouses or bag collectors ac-count for more than 80% of the fabric dust collectionsystems used in all dust collection applications today (pri-marily fixed plant facilities, although they are also used inabrasive recycl
30、ing equipment). Baghouse dust collectorsuse vertically mounted tubes or bags of fabric media rang-ing from five to twelve feet in length to remove contami-nants. They can filter a wide variety of materials (e.g.fibrous, particulate, moisture laden, and hydroscopic mate-rials). They are used at tempe
31、ratures up to 500 F and canhandle extremely heavy dust loads. Many abrasive recy-cling systems utilize baghouse collection systems due tothe heavy to extremely heavy particulate loading (2 togreater than 10 grains per cubic feet of air per minute (gr/cfm), and varying particulate size (0.1 to 100 mi
32、crometers).4.4.2 Cartridge Collectors: Cartridge dust collectorsare used on dry dust applications or when high concentra-tions of small (less than 1 micrometer) particulate are likely,but are capable of filtering a broad range of particulate sizes(0.1 to 30 micrometers). Most mobile dust collectors
33、utilizecartridge collector systems. While the abrasive blasting pro-cess generates particulate ranging from 3 to 100 micrometersin size, the dust collection system is primarily responsible forremoving fine particulate suspended in the air (i.e., smallparticulate).Cartridges are composed of rigidly p
34、leated elementsproviding a large amount of filter area in a comparativelysmall housing. Cartridges are typically mounted eitherhorizontally or vertically within the filter housing. Cartridgesystems should not be used when the loading is expectedto exceed 3 gr/cfm.5. Types of Cleaning MechanismsDust
35、collectors are typically classified by theircleaning mechanism. There are three common types:shaker, reverse pulse, and pulse jet. Baghouses utilizeeither shaker, reverse pulse, or pulse jet systems. Car-tridge collectors utilize either reverse pulse or pulse jetsystems. In the industrial painting i
36、ndustry, almost all cur-rent equipment uses pulse jet systems.5.1 Shaker Systems: In shaker systems, dirty airenters at the center of the bag area and the dust cakeaccumulates on the interior surface. Shaker systems relyon a vibrating mechanism to shake the filters. This createsa sine wave along the
37、 top of the bag that ripples down thelength causing a portion of the dust cake to change shapeand fall into the hopper. Pressure drops across the baghousedetermine the shake interval. Typical shake intervals rangefrom 10 to 30 seconds. Shaking speed and frequency aredependent upon the diameter and l
38、ength of the bag. Mul-tiple baghouse filters are packaged in separate compart-ments in the dust collector. During cleaning, one compart-ment at a time is shut down for the shaker process, while theremaining compartments remain in operation. This is calledoff-line cleaning. Shaker systems have a low
39、air-to-clothratio (A/C). This means that the units are typically oversized(i.e., requiring high air volumes) and many bags are re-quired in order to maintain filtration efficiency during thecleaning cycle. Additionally, because shaker baghouseshave multiple bags per compartment, the time necessaryfo
40、r filter replacement is increased.5.2 Reverse Pulse Systems: Reverse pulse systemshave largely supplanted shaker systems and are currentlybeing replaced by pulse jet systems. The design of the dustcollector for reverse pulse systems allows the dust-ladenair to enter at the top of the unit. The dust
41、collects on theinterior of the bags or cartridges. Heavy particles fall intothe hopper, helping to extend bag life. The filtered air is thenvented to the atmosphere. Reverse pulse systems aremulti-compartment systems. Cleaning is accomplished byisolating one compartment at a time (while the othersco
42、ntinue to operate). Pulses of compressed air enter througha venturi nozzle, which creates a shock wave. The wavecauses the dust cake on the inside of the bags to fractureand fall into the hopper. Typical pulse durations range from30 seconds to several minutes. Reverse pulse systems areconsidered the
43、 least aggressive cleaning method, but onewhich extends bag life.5.3 Pulse Jet Systems: Pulse jet systems are consideredto be the most aggressive and effective filter cleaning approach.An FHWA publication, Lead-Containing Paint Removal, Con-tainment and Disposal (FHWA-RD-94-100)1, published inFebrua
44、ry 1995, indicated that only those dust collectors usingpulse jet cleaning mechanisms were effective over the durationof the study. Pulse jet systems are the only systems wherecleaning occurs while contaminants are entering the com-partment.In pulse jet systems, the inlet air stream enters the unita
45、nd passes vertically between the bags or cartridges. Dustaccumulates on the outside surfaces of the filters. Pulse jet1Lloyd M Smith and Gary L. Tinklenberg, Lead-Containing Paint Removal, Containment and Disposal, FHWA RD 94-100, Federal HighwayAdministration, McLean, VA , February 1995.SSPC-Guide
46、16August 1, 20034systems continuously remove some dust from the bag orcartridge through an adjustable cleaning frequency dictatedby a timer or differential pressure across the filters. Whenthe differential pressure point is reached, a high-pressurejet of compressed air is directed inside the filter
47、for 0.1 to0.15 seconds at intervals ranging from 1 to 30 seconds. Thepulse creates a shock wave similar to that used in thereverse pulse system, causing the dust cake to fracture andfall into the hopper. Proper adjustment of the cleaningfrequency in these systems is necessary for optimal par-ticulat
48、e removal efficiency. If the duration of the pulse is tooshort, cleaning is inefficient. If the duration of pulsing is toolong, the bag life is reduced. For optimal operation, thefrequency of cleaning should be adjusted so that differentialpressure across the collector ranges from 3 to 6 incheswater
49、 column (w.c.). Some manufactures employ timersrather than differential pressure drops to determine thecleaning sequence.The pressure of the compressed air at the delivery tubecan severely affect bag life. Too high a pressure or amisdirected jet of air due to misalignment of the deliverytube can damage the top several feet of the bag or car-tridge. Average compressed air pressures range from 40 to90 pounds per square inch (psi).One of the popular features of pulse jet systems is theon-line cleaning feature, which allows work to continuewithout interruption of project operations. However, if
