Paint booth supply air filtration

Paint Booth Supply Air Filtration by Len Duello, David Reaman, Hollinee LLC, Ingleside,

and Hugh McPhail

Ill.; E-mail, [email protected]

aint booth supply air is filtered to remove particulate contaminants and in an effort to provide uniform airflow across the item to be painted. The negative impact of excessive particulates is inclusions in the paint, which disrupt the paint surface and appearance. Many of these paint disruptions are caused by particles greater than 10 microns in size. Uniform airflow is important for complete paint coverage, minimal overspray, and for optimum paint removal by the exhaust filters. According to the NAFA Guide to Air Filtration there are two kinds of air supplies, which require filtration (see Table I>. These are the down-draft and cross-draft air supplies. The down-draft air supply usually requires two-step processing. The first step employs a 90 to 95% atmospheric dust spot efficiency filter. Atmospheric dust spot efficiency is a test result available from an ANWASHRAE 52.1-1992 test that describes the percentage removal efficiency of ambient atmospheric dirt by an air filter. In poorer air quality environments, a prefilter is used to lengthen the lifetime of the 90 to 95% atmospheric dust spot efficiency filter. The prefilter is usually a 25 to 30% atmospheric dust spot efficiency filter. The second step in processing down-draft supply air is its passage through a diffusion media whose purpose is three-fold: (1) capture any particulates that shed off the 90 to 95% atmospheric dust spot efficiency filter; (2) filter any particles greater than 10 micron in size; and (3) provide uniform airflow to the paint application site. Down-draft diffusion media are usually composed of fire-retardant-treated, synthetic fibers that are highly resistant to fiber shedding and sometimes tackified. The other intentional supply of air to the paint booth is also filtered. Cross-draft intake filters are usually 20 to 30% atmospheric dust spot efficiency filters. The cross-draft air supply is used to direct airflow in the paint booth. When properly installed, used, and maintained all the air filtration devices fulfill their purpose of reducing and minimizing particles that cause paint and coating blemishes and defects. Airflow uniformity control is also an important aspect for proper operation of a paint booth. Mechanical means are available to improve or control long distance airflow uniformity both before and after the

P

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Table 1. Air-Supply

Filter Requirements

Air Supply

Filters

Purpose

Down draft

2530% DSE 90-95% DSE Diffusion media

Cross draft

2030%

Prefilter air Remove most particulates Capture shedded particles Remove all particles greater than 10 microns Airflow uniformity Filter make-up air

DSE

DSE = atmospheric dust spot efficiency-ANSIASHRAE

52.1-

1992.

supply air filters. Proper baffling, plenum size, and design can be used before and after the air supply filters to reduce turbulence. Proper booth design and blower control and operation can also help reduce turbulence and improve exhaust filter performance. Differential air velocities in the booth can cause airflow turbulence, especially in an improperly sized paint booth. Before the intake-air filters is the first opportunity to improve air velocity uniformity in the paint booth. An example of a mechanical means to control airflow uniformity before the supply air intake filters is presented by Kunec in U.S. Patent 5,779,534. In this instance the mechanical control is a plurality of nested perforated plates that distribute the airflow by means of spreading the flow within the plenum. One approach is to force the air entering the intake plenum through a multitude of ducts and attach filters to the bottom of each duct (see Fig. 1). Fluid Dynamics Solutions Inc. appears to have computed the airflow distribution for a similar scenario on their website, http://www.cfdpcs.comlCase2.htm. By varying the design parameters of the duct/filter combinations it is possible to change the airflow distribution to suit a particular requirement. The filters themselves can also be used to control air velocities within parts of or the whole paint booth. Gore et al. presents in U.S. Patent 5,512,017 the airflow resistance of various banks of bag filters is used to control air velocity into a region of a large paint booth. This patent also allows for the control of air velocities by using various numbers of filters of the same airflow resistance. Gore et al. indicate that slower air velocity can be created in the middle of a paint booth through the use of four air diffusing filters fed by only three bag Metal Finishing

Air IntakePlenum 0-l

Figure 1. Uniformity in airflow is obtained by proper design and use of the plenum baffle.

filters. The two outermost air-diffusing filters are fed by one bag filter each. The two innermost airdiffusing filters are fed by only one bag filter. The larger opening provided by the two diffusing filters slows the velocity of the air coming through the middle bag filter. This arrangement provides for a central region of airflow at approximately one-half the velocity of the outer region of airflow. For further details see Gore et al. U.S. Patent 5,512,017. In this case, sufficient space is allowed to keep the zones of differential air velocities where turbulence occurs away from the object to be painted. Mechanical approaches to airflow uniformity after the supply intake filters can also be employed. An example is presented by Parks et al. in U.S. Patent 5,743,793. In this instance diffuser plates positioned beneath the filters provide the mechani-

Air Intake Plenum

High airflow Drawmg baseUo"Gor~

Low airflow etalUSP 5,512,017

Figure 2. Sufficient space is allowed to keep zones turbulence occurs away from the object to be painted.

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where

cal control. This approach is similar to that found used in the Fluid Dynamics Solutions Inc. example but is on the paint booth side of the filters. Another aspect in airflow uniformity control is short-term velocity inconsistencies in the filtration field. These velocity inconsistencies can be caused by inconsistencies within an individual air filter. Air velocity inconsistencies caused by an individual filter can be caused by variations within the media of that filter. Inconsistencies in basis weight, thickness, density, fiber laydown, and binder amount and placement can cause inconsistencies in short-term velocities. Variability in basis weight of the air-intake filter media can affect air velocity consistency. Weight of the media is the mass per unit area of the media. Basis weight is reported in a variety of units-common units are ounces per square yard (OSY), grams per square meter (GSM), and pounds per ream. The best basis weight results are obtained when the media is weighed under controlled conditions of humidity and temperature. Target basis weights for air supply filtration media vary by application and supplier. Basis weight is measured by a variety of means; usually the media are cut to a certain size and weighed. The mass per unit area is either calculated directly or by use of a conversion table. The most important factor when comparing variations of different media is that they are measured using the same sample size and sampling plan. Variability in thickness of the air intake filter media can affect air velocity consistency. Thickness of the media is the distance between the air entrance and air exit side of the media in the filter. Thickness is reported in a variety of units, common units are inches and thousandths of an inch. Target thickness for air-supply filtration media vary by application and supplier. Thickness is measured by a variety of means. The media thickness is usually measured before being made into a filter. Thickness can be measured as compressed or under varying degrees of compression using a variety of platen shapes and sizes. The most important factor when comparing variations of different media is that they are measured using the same method. Thickness is a function of weight, fiber type, fiber denier, and compression/compressibility. Variability in density of the air intake filter media can affect air velocity consistency. Density is the weight per unit volume of the media. Density is reported in a variety of units. Target densities for air-supply filtration products vary by application and supplier but usually are controlled to below certain amounts as per a specification so that the media meet flow requirements at a specified static Metal Finishing

pressure. Density is calculated by dividing the basis weight by the thickness. The most important factor when comparing variations of different media is that they are measured using the same method. Density is a function of weight, fiber type, fiber denier, fiber laydown, and compression/compressibility. Variability in fiber laydown of the air-intake filter media can affect air velocity consistency. Uniform fiber laydown of the media is the consistency of fiber-to-fiber distance and consistency in orientation of the fiber to both the airflow and neighboring fibers. Uniformity of fiber laydown can be measured by image analyzers and ranked or determined by calibrated visual-inspection panels. Target fiber laydown uniformity for airsupply filtration products is presented by consistency of measurement results. The most important factor when comparing variations of different media is that they are measured using the same method. Uniform fiber laydown is a function of the equipment of manufacture, controls of the process of manufacture, basis weight, fiber type, and fiber denier. For those media containing binders, variability in binder add-on of the air-intake filter media can affect air velocity consistency. Binder is any adhesive agent whose purpose it is to provide strength and stability to the media. Binder add-on is the percentage of dried binder applied to the media that provides for its strength. Target binder add-on for air-supply filtration media vary by application and supplier. Binder add-on can be determined by weight add-on in the process or by an analytical method that removes known fractions of the media. The most important factor when comparing variations of different media is that they are measured using the same method. Binder add-on uniformity is a function of weight, fiber type, fiber denier, equipment of application, and proper controls of the process of manufacture. For those media containing binders, variability in binder placement in the air-intake filter media can affect air-velocity consistency. Binder placement can be characterized by its evenness of amount through the thickness of the media and across the width and length of it. Binder placement is characterized by either visual and tactile observation or by analytical technique. Target binder placement for air-supply filtration media varies by application and supplier. The most important factor when comparing variations of different media is that they are observed using the same method. Binder placement uniformity is a function of weight, fiber amount, equipment of application, and proper controls of the process of manufacture. Glenn Schuyler of Rowan Williams Davies and Irwin Inc. in his Technotes, Issue No. 9, article

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entitled “Rules of Thumb for Providing Uniform Airflow” indicates that “filtered paint booth supplies require a uniform supply of air to function correctly.” He further indicates that when airflow impinges on a filter that the flow is turned, slowing the airflow. This results in an increase in static pressure with resultant increased flow in that small portion of the media. This produces short-term air-velocity inconsistencies in the filtration field. In order to reduce the impact of short-term airvelocity inconsistencies across the filtration field on long-term airflow turbulence, inconsistencies within the air-supply filtration media need to be minimized. Higher regions of basis weight, density, and fiber clumping can result in localized lower air velocities through the media. Lower regions of basis weight, density, thin spots, and lack of binder can result in localized higher air velocities through the media. These regions of short-term air velocity inconsistencies across the filtration field may have an impact on long-term airflow turbulence. On the positive side, there are benefits to improved airflow uniformity within a paint booth. This can be seen in recent work at the University of Kentucky (UK). The UK Department of Mechanical Engineering has been working with Toyota Motor Mfg. Co. to further reduce wastes from the painting

of automobiles. Kozo Saito of the Center for Robotics and Manufacturing Systems and Abraham Salazar of the Department of Mechanical Engineering reports that through proper airflow design the overspray lost in the process can be reduced by nearly one-third (see http://www.ccs.uky.edu/research/ abraham.html). Supply air filtration products have a critical impact on paint booth operation. They remove particulate contaminants that may result in inclusions in the paint, which disrupt the paint surface and appearance. Supply air filtration products can also play a part in providing uniform airflow, which is important for complete paint coverage and for optimum paint removal by the exhaust filters. BIOGRAPHY

Len Duello has a B.S. in Chemistry from Georgia State University. The National Air Filtration Association certifies him as a NAFA Certified Air Filtration Specialist. He has 22 years of experience in nonwovens, including process engineering and product development work. For the last 3 years Duello has been a Product Development Engineer for Hollinee LLC, primarily working in air-filtration media development. He is listed as either an inventor or co-inventor on three U.S. patents. MF

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