مقالات تکنولوژی و تجهیزات ابکاری

finishing equipment & plant engineering

SPRAY BOOTHS

GLOBAL FINISHING SOLUTIONS, DALLAS, TEXAS

Before learning the features, benefits, and uses for spray booths, it is important to know the basics that apply to all spray booths: the reasons for using a spraybooth, what a spray booth can and cannot do, the various federal, state, andlocal agencies that give approval to a new spray booth installation, NationalFire Protection Association Bulletin 33 (NFPA-33) as it relates to spray boothdesign and booth classifications, the difference between code compliance andenvironmental compliance, how to determine booth efficiency, and the mostcommon types of spray booths and how they are used.The various codes and agencies that govern spray booth classification, installationand operation can be very confusing. Understanding the codes and howthey apply to spray booths allows for identifying the most appropriate booth.The purpose of a spray booth is to confine the application of a hazardous material to a restricted controlled environment. Spray booths prevent hazardous overspray and volatiles from escaping confinement and causing fire or explosionhazard to nearby operations. They control the air-fuel mixture so that a combustible combination cannot occur. In addition, spray booths provide a clean environment in which to paint.

 REGULATION OF SPRAY BOOTHS

The primary function of a paint spray booth is to reduce the likelihood of firesand explosions. A secondary consideration is protecting the operator from toxic materials. This protection is best done with respirators, protective clothing, and hoods. Spray booths cannot be designed to adequately protect the operator from overspray contamination. It is not unusual for part geometry to require the spray gun to be directed near the operator.A spray booth is not an emission control device even though some end usersassume that a spray booth is an emission control device that must complywith Environmental Protection Agency (EPA) standards. EPA standards placelimitations only on the amount of toxic material in the form of solvent vapor,known as volatile organic compounds (VOCs), entering the environmentthrough the booth exhaust stack.A spray booth is designed to collect solid particulates only, not solvent vapors.To comply with EPA requirements, exhaust air may need to be treated withequipment installed outside the spray booth. A carbon adsorption system or anincineration system, for example, are acceptable methods for collecting VOCs.Traditional code inspections deal with the design of the spray booth.Inspectors evaluate hardware and installation methods for compliance withthe Occupational Safety and Health Administration (OSHA) standards,National Fire Protection Association (NFPA) Bulletins 33 (Spray Applications)and 70 (National Electrical Code or NEC), and any local ordinances. A separateenvironmental quality review is conducted to determine the amount of pollutantsthe installation will emit.A new spray booth installation is approved or denied by the authorityhaving jurisdiction. For example, in areas dealing with public and employeesafety, the authority may be an official of a federal, state, or local agency. Or the authority having jurisdictionmay be a regional official,such as a fire chief or marshall,building or electrical inspector,fire prevention bureau inspector,or labor or health departmentinspector. For insurancepurposes, the authority may bean insurance inspector or representativeof a rating bureau.Greater environmental concernhas also led to increasinginvolvement by new agencieshaving jurisdiction. There aremany agencies that dictatecompliance. Depending onstate and region, one or moreof the following federal stateand local agencies may play apart in approving a new boothinstallation. OSHA is concerned with employee health and safety. Familiarization with thefollowing OSHA codes and the booth design and safety requirements that eachgoverns is important. The relevant OSHA codes are OSHA 1910.107 SprayFinishing, OSHA 1910.94 Ventilation and OSHA 1910.95 Noise Exposure.OSHA relies on the current National Fire Prevention Association (NFPA)Bulletin 33 to formulate guidelines on fire prevention.In addition to NFPA-33, OSHA also bases compliance decisions on theelectrical guidelines outlined in the current NFPA-70 (National ElectricalCode). For guidelines on the acceptability of certain spray booth components,OSHA refers to Underwriters Laboratory (UL), ETL Testing Laboratories(ETL), Factory Mutual (FM), and Industrial Risk Insurers (IRI). These organizationsevaluate equipment according to fire and safety standards.The Environmental Protection Agency regulates the allowable amount oftoxic material in exhaust stack emissions, liquid, and solid waste streams. TheEPA has no jurisdiction over booth design, which is designated by NFPA-33.

State Agencies

Federal agencies, such as OSHA, often maintain state offices to enforce theirown federal regulations and to administer any state mandated variationsin those regulations. Also, each state has an environmental agency (such asGeorgia Environmental Department) that conducts a review of all installations.The purpose of the review is to obtain a disclosure or prediction regardingthe level of pollutants the booth will emit.If the level is acceptable, the state agency issues a permit to operate an air contaminantsource. If the pollutant level is unacceptable, the agencymay deny thepermit, require the use of exhaust air treatment equipment, or require the use ofa different coating material.Filing an application for a permit to operate an air contaminant source cancause delays in installing and operating the equipment. The permit to operateis needed before the equipment can be used, and often before installation andassembly can begin. The application forms are usually complicated, and whencompleted the application is subject to administrative review before approval.

Local Agencies

City and county authorities conduct code inspections to evaluate hardware andinstallation methods for compliance with OSHA, NFPA-33 (Spray Applications),NFPA-70 (National Electrical Code), and any local ordinances. Some municipalitiesare now writing EPA compliance into their local ordinances as well.The burden of compliance falls on the end user. Ignorance of the regulationsand procedures is not a defense against prosecution, and penaltiesfor noncompliance are becoming more severe. Become familiar with all theagencies having jurisdiction, including the environmental agency review andapplication requirements.

Spray Booth Classifications

Spray booth classifications are outlined in NFPA-33. NFPA classifies boothareas according to the types of electrical equipment and other possible ignitionsources that can safely be used within those areas.Class I covers flammable gases and vapors and Class II covers combustibledusts. Divisions 1 and 2 cover locations in the classified area in which theseflammable gases, vapors, and dusts are handled. Most industrial booths areClass I. Class I, Division 1 areas are the inside of the spray booth and the insideof the ductwork. Class I, Division 2 is any area within a 10-ft radius of the openface of a spray booth when the spray gun is not interlocked with the exhaustfan to prevent spraying unless the fan is operating. When the spray gun and fanare interlocked, the Class I, Division 2 area extends five feet back from the openface. This area also extends three feet from a conveyor opening and includes thearea above the ceiling of the booth (see Figs.1 to 3).Equipment located in the Class I, Division 1 atmosphere must be classifiedas explosion proof. In practice there should be no electrical itemsinside a spraybooth. Electrical equipment in the Class I, Division 2 atmosphere must be thirdpartylisted (such as UL, ETL, ER) and must not produce sparks under normaloperating conditions.

MEASURING BOOTH

EFFICIENCY

By design, a spray booth collectssolids known as particulateemissions. Efficiency factors,specifically grain count,measure how effectively a spraybooth and filter system will bein trapping these particulateemissions.The following formula isused to determine the relativeefficiency of a specific system.The grain count, or relativeefficiency, can be altered bymaking changes in equipment(transfer efficiency),coatingmaterial(percent of solids inpaint), and the air flow (cfm),rather than changes only inbooth design. For example, ifa painter switched from conventionalair spray equipmentto HVLP equipment, the highertransfer efficiency possible with HVLP would lower the grain count. Because ofits ability to trap particulate matter, a spray booth can help the end user meetEPA requirements. Unfortunately efficiency factors have at times been misrepresentedas providing an assurance that a spray booth will meet EPA requirements Although some spray booth designsare more efficient than others at preventingmaterial from entering theenvironment, high-efficiency factorratings do not automatically ensureEPA compliance.

TYPES OF SPRAY BOOTHS

A spray booth consists of a work compartmentwhere spraying takes place,an exhaust chamber for collectingparticulate, an exhaust fan and motor, and an exhaust duct to the exterior of thebuilding. Paint booths are categorized by the method of collecting the oversprayand the direction of air flow in the booth. There are subcategories in each case.

Dry-Filter Booths

There are several types of dry filters available for use in spray booths. The rectangularpad type is available in many grades and types. The roll media type is alsoavailable in many sizes, grades, and types. This designation is a slight misnomeras the media is rolled for ease of shipment but is unrolled and applied as a largerectangular block of filter media. Roll media filters should not be confused withcontinuous roll. Continuous roll media come on spools in large, long rolledcoils. As the filter becomes contaminated, the clean section is advanced. Thiscan be hand or motor operated.Cardboard baffle and light density Styrofoam filters are also available; however,dry baffle exhaust systems havealmost entirely disappeared, exceptwhen used with paint filters as prebaffles.A single or double row of bafflesis placed vertically in front of a normalpaint arrestor bank. This provides theprimary collection surface for oversprayand effectively protects the filtersfrom rapid loading; however, they nowbecome part of the collection systemand must be cleaned and maintained.This application originated with thecollection of high solids paints thatcaused heavy loading and forced rapidchange of filter media. The physicalcharacteristics of the high solids materialsallow collection through a troughat the base of the baffles. In some cases,this reclaimed material is reusable or itcan be returned to the manufacturer.

Water-Wash Booths

These booths may use pumps or be pumpless. Low static pressure-pump-typebooths with recirculating headers and piping are the most common types ofwater-wash booths. In contrast, high static pressure-pump-type booths are. usually foundin automotiveplants and aredescribed asgrain-count booths,meaning thatthey are consideredto have ahigher collectionefficiency thanstandard waterwashboothsbecause of higherinternal static pressure and scrubbing action. For a booth to be considered agrain-count booth, it should not release more than 3 grains (weight) to theatmosphere per 1,000 cfm of exhaust air. Test procedures are necessary to measurewasher efficiency.Pumpless booths also come in two forms, those requiring high pressure andthose with low pressure. In pumpless types that require high internal static pressureas a means to circulate and scrub water, high velocity air moves water upthrough the exhaust chamber. It is then released at a high point and returns tothe water tank through an exposed water curtain. Pumpless types with low staticpressure usually are fitted with a water holding pan and little or no water movementthrough the exhaust plenum. Collection depends on an abrupt change ofair direction to impinge overspray onto the water surface.

Draft Classifications

Booths are also classified by the method of draft. Cross-draft booths are characterizedas having air flow designed in a horizontal movement (Fig. 4). Air travelsparallel to the floor, from the face of the booth to the rear of the exhaust chamber.The majority of booths are designed as cross-draft. The booth can have theface open to the atmosphere, closed with input plenum, or closed with filterdoors.In the downdraft booth, the air flow is from overhead and moves down towardthe building floor (Fig. 5).The building floor normally has a sunken pit toaccept either dry-filter or water-wash exhaust. A bar-type grating is laid over thepit opening. The booth can also be placed on an elevated platform to avoid pitconstruction. The top of the booth may be open or enclosed with a filter inputplenum. Most downdraft booths have overhead, filtered input plenums. A boothwith a filter plenum is normally used in conjunction with a heated air make-upunit. This is considered a must for a clean paint job.A semidowndraft booth combines features of the cross-draft and downdraftbooths. The method of inputting the air to the booth makes it a semidowndraft.Air is introduced to the booth through the ceiling in the first 25% to 30% of boothlength (Fig. 6). This input air may be introduced by relying on the suction of theexhaust fan or it can be pressurized. For the best results, air make-up should beused and the booth should be positively pressurized. The exhaust is placed at thebooth rear as would be the case in a normal cross-draft booth.A second style of semidowndraft places a floor level filtered exhaust plenumdown each side of the booth. A full air input plenum is located in the booth ceil-ing as would be the case of a normal downdraft booth. The air flow is from theceiling of the booth down and out through each side plenum. No pit or elevatedplatform is required for this booth.

SPRAY BOOTH DESIGN AND SIZING

Selecting the booth and sizing it for an application requires review of severalareas. Knowledge about the facility and production process are important inchoosing the right equipment. Take the time to understand the application, anddo not forget future plans that may influence the choice of spray booth design.The following are some general guidelines for selection and sizing.

1. Maintenance: All booths require regular maintenance for optimum performance.As a first step, determine the capability of the maintenance departmentor maintenance contractor. This will determine the sophistication levelof the equipment required.

2.Budget: Always take the budget into consideration when choosing the spraybooth. Balancing the application requirements and available funds will helpidentify the most effective exhaust chamber, air flow, and booth optionsfor the job.

3.Selecting the Booth Design: The first step in selecting an appropriate boothdesign for an application begins with an investigation of the finish qualitylevel and the production requirements. This step will help determine thedirection of air flow through the booth, as well as the appropriate filtrationmethod, either dry filter or water wash.

Production Requirements

Part Size and Configuration

The size and style of the part, the carrier that conveys it through the booth, andthe relationship of the spray gun to the part, all play a role in determining thedirection of air flow as well as the velocity or speed of air through the booth.Air flow and velocity are needed to transport paint overspray into the filters.There are three types of air flow through a booth as discussed above: cross-draft,semidowndraft, and downdraft.Production Rate and Transfer EfficiencyProduction rate is a measure of the number of parts that can be finished withina certain time frame, usually per hour or per shift. Transfer efficiency is thepercentage of material beingsprayed that adheres to thepart; the remainder is overspray.The type of applicationequipment—conventional,electrostatic, or HVLP (highvolume, low pressure)—determineshow efficiently paintis transferred from the gunto the part. Together, productionrate and transfer efficiencyinfluence the choiceof air flow Material Being SprayedThe type of material being sprayed affects the choice of filtration or exhaustmethod, either dry filter or waterwash, to remove overspray from the booth.A dry-filter or paint-arrestor booth traps airborne paint particles (overspray)in disposable filters. A dry-filter is used in the majority of applications.Depending on the material being sprayed, removal efficiency ranges from95% to 99%. If more than one type of material is being sprayed, be sure thatthe materials are compatible. The combination of incompatible materialsin the dry filter can be a cause of spontaneous combustion. In a water-washbooth, air washing action traps the paint solids from overspray. Water-washsystems should be used for very heavy spray rates (over 20 gal/8-hr shift/10ft of exhaust chamber width). Removal efficiency for a water-wash booth canbe as high as 98% to 99%, depending on the type of material being sprayed.

Finish Quality

The quality of the finish on the completed part has become more critical ascustomer’s expectation levels have increased. The total process mustnow beconsidered in order to achieve first-time-through quality levels.The spray booth design is one key aspect. Air flow, direction, filtration, airvelocity, and balance are critical to accomplishing the various desired qualitylevels. Unpressurized cross flow designs would be at the low end and pressurizeddowndrafts at the high end of quality potential.One key thing to consider is that a spray booth is only one part of theprocess. Many other phases of the process must be designed and controlledto achieve the desired qualitylevel. That would includethe preparation and cleanlinessof the object going intothe booth, the maintenanceof the booth and surroundingprocesses, the quality ofcompressed air to the tools(including spray gun), thequality of clothing andequipment the painter uses,and the quality of the paintpreparation activities. Thefinish quality can only be asgood as the design and controlof the process

Determining the Booth Size

Determining booth size is the second step in selecting the application. It is dependenton booth location and the type of operation (manual or automatic). Reviewof the facility layout and proposed booth location is recommended to determinewhether the allotted space is adequate for the size and style booth. The type offinishing operation, either manual or automatic, also determines the size of thebooth (see Figs. 7 and 8). A properly sized booth for manual spray operations willgive the operator and the finishing equipment adequate room in which to work.Adequate means enough space for the operator to move around, stoop down,bend over, and allowan even, fluid armmotion. For an automatedapplication,the correct booth sizewill provide enoughspace for automaticequipment to operateeffectively. Thisincludesallowing forthe operation of sideto-side and overheadreciprocators, andproviding the necessaryclearances forelectrostatic equipment.During finishing, there should be sufficient velocity through the boothand past the equipment to keep it in clean operating condition. When conveyorsare transporting parts through the booth, the booth size is directly related toconveyor speed.Minimum and maximumpart dimensions determine thebooth width, height, and depth.Acceptable booth width will allowat least 3 ft on either side of thepart, at least 6 ft of work space foreach operator in multiple-operatorapplications, and a minimum of 2ft from all conveyor openings. Todetermine the width, measure thediagonal dimension of the largestpart, including the fixture orpallet it is on, and add a 2- to 4-ftclearance on each end. This spacepermits the part to be turned if necessaryand enables the operator towork comfortably.Adequate booth height willallow at least 2 ft above the largestpart and allow for conveyorheight or include a housing forthe conveyor rail. Booth height isdetermined by the overall height ofthe largest part, plus 2 to 3 ft clearance.Add the height of the holdingfixture if the part is moved by aconveyor. This measurement givesthe operator sufficient room to coatthe top of the part without coatingthe booth ceiling. The part should. also be high enough above the floor to allow the operator room to spray the loweredges and the underside easily.Sufficient working depth will allow at least 3 ft between the rear of the part andthe water-wash tank or filter pads, at least 3 ft between the front of the part andthe booth face or intake filters, and allow for automatic machines, such as reciprocators,in conveyorized applications. Working depth should be sufficient forthe part, including the fixture or pallet, to be entirely within the booth enclosureduring finishing, plus allow for clearance at the rear. There should be a minimumof 3 ft between the part and the tank in a water-wash booth or the filters in a dryfilterbooth.Conveyor openings are required when a conveyor moves parts through thespray booth. Conveyor openings should allow 6-in. minimum clearance aroundthe part. A vestibule is a protected entry into the booth (see Fig. 9). It provides betterair flow control through the booth by effectively blocking the tunnel leadinginto and out of the booth with the product. The vestibule length should be a minimumof the gap between parts so the vestibule always contains a part.

Booth Air Requirements

The final step in selecting the booth is establishing the minimum air velocityand volume requirements. The spray booth should be located to allow for properair entry and flow through the booth. An open-faced booth should be locatedwith the face at least booth height dimension from any wall (see Fig. 10). Whenthis placement is not possible, air input plenums will provide adequate air flow.A spray booth requires a minimum air draft or velocity, measured in lineal feetper minute (fpm), to carry overspray through the booth, past the operator or theautomatic equipment, and deposit it into either the water curtain or filter pads.As a rule, OSHA inspectors rely on the guidelines specified in NFPA-33 requirementsin the booth during spraying operations. Although the NFPA-33 guidelinecovers most spray operations, greater air flow may be required when specific typesof finishing equipment are used. The high-pressure atomization equipment usedto break up higher solids materials, for example, produces high atomization pressuresand consequently high fluid stream velocity at the tip of the spray gun. This can cause overspray tore bound and may expose the operator to toxic materialspresent in the paint. Velocity should always be sufficient to carry the oversprayaway from the operator and into the exhaust chamber.The velocity possible in a booth depends on the fan size. Most standard boothsoffered in the market come equipped with fan and motor packages sized to deliverthe necessary draft. Draft requirements take into account real-world static pressuresincluding resistance to air flow from entry losses, stack filters, and duct work.Static pressure is the amount of resistance air must overcome while movingfrom point A to point B. Static pressure in a spray booth is encountered intwo areas: intake and exhaust filtersand intake and exhaust duct work.The static pressure of any filter isdetermined by how much air will passthrough that filter. Air-intake filtersfor downdraft spray booths are denserand pass less air than air-intake filtersfor either cross-draft or semi downdraftbooths. Consequently, air-intakefilters for downdraft spray booths havea higher static pressure rating than theair-intake filters for other booths.When intake or exhaust filtersbecome clogged with dirt or materialoverspray, the amount of air thatcan pass through the filter decreases.When air flow is restricted, the filter’sstatic pressure or resistance to air flowincreases. Air intake and exhaust ductsalso influence static pressure.Air volume and velocity aredecreased when elbows, reducers, transitions,and long runs are added to ducts. Elbows introduce angles and increaseresistance to air flow. Reducers and transitionsalso increase the static pressure in duct work.The ideal situation is to keep duct work to aminimum.Static pressure is also a factor when choosingan air replacement unit. Because of the similaritiesto an exhaust booth, pressure drops in andout of the unit must be considered.Tables I and II give recommended spraybooth velocities covering average conditions.The figures are all based on empty booths andinclude the face opening plus any conveyoropenings. These are recommendations only, andare not meant to replace local or state regulationson minimum air velocity.In NFPA-33 (section 5–2) air velocity requirementsare defined. According to the guidelines,a booth needs to “provide adequate ventilation to maintain the concentration offlammable vapors or combustible vapors or mists in the exhaust stream below 25%of the lower flammable limit (lfl) of the paint.” Lower flammable limit is defined asthe concentration level at which a particular atomizedsolvent will ignite.The volume of air needed to move through the booth and into the exhaustchamber is measured in cubic feet per minute (cfm). Use the following formulato determine the volume of exhaust air:where area is the cross-sectional area in square feet of all openings in the spraybooth. When air input plenums are used the conveyor openings may be ignored.When connecting vestibules are used, the opening between adjacent booths maybe ignored. Velocity is the speed or velocity of air required by code. Speed of airmovement is measured in feet per minute (fpm). Cubic feet per minute (cfm) isthe volume of air moving through the booth. This relationship between boothsize, the velocity of the air movement, and the volume of air being moved isshown in Fig.11.

INDUSTRIAL-TYPE SPRAY BOOTHS

Dry-Filter Booths

As with any type of paint-arrestor spray booth (see Fig. 12), the booth’s mainfunction is to remove the airborne particles from the spray booth exhaust air bymeans of disposable filters. The standard booth is typically designed to operateat 125 fpm air velocity. The booth normally provides an enclosure to accommodatea spraying operation. It limits the escape of spray and residue and safelydirects them to a filter and exhaust system.Dry-filter spray booths are ideallysuited for low- to high-productionoperations; lighter sprayrates; materials that stay wet, suchas enamels, high solids, and waterbasecoatings; materials that do notreact chemically with each other;and limited budgets.Some of the styles of dry-filterbooths include the floor-type,bench-top, and bench models.While the floor-type booth is availablein a wide variety of sizes, thisbooth isdesigned for the workplace where space is limited. Thebench-top booth is perfectly suitedto sit on top of an existing workbench. Depending on availablespace, this booth may fit therequirements perfectly. Somebooths come with a leg kit for freestandingapplications. The bench booth provides greater paint arrestor fron- tal area for increased capacity in comparison to the bench-top booth. It isdesigned with a table height shelf. Both the bench-top and bench booths areperfect for spraying small objects and decorating and stenciling.Use of the dry-filter spray booths requires a regular schedule of filter replacement.Codes require that filters be inspected after each period of use and thatclogged filters be discarded and replaced immediately. Used filters must beremoved to a safe, well-detached location or placed in a water-filled metal drumand disposed of at the close of the day’s operation.A draft gauge is typical standard equipment with dry-filter spray booths. Thegauge is designed to indicate when paint filters have become sufficiently loadedand replacement is required.Keep in mind that high-transfer-efficiency spray systems, when used in combinationwith high-holding-capacity dry filters, result in lower operating costsand higher production rates. There are two filtration principles that apply to dryfilters, baffle and strainer, each havingadvantages and disadvantages.

Baffle Filters

The baffle principle creates a highturbulence in the air flow as theair moves through the filter. Theheavier overspray particulates areforcefully deposited at variousdepths in the filter. This process,called depth loading, is optimizedwith the slit and expanded kraft filter.Baffle filters areavailable in metalpanels, corrugated filters, pleated and expanded kraft, and Styrofoam pads.Metal panels have excellent holdingcapacity,but their ability to trap a highpercentage of solids from the spray booth is limited and the exhaust air is poor.Also their efficiency is low. The metal panels are most efficient when intermittentproduction exists, or when used as a precollector to reduce the replacementfrequency of more efficient filters. Corrugated filters also have excellentholding capacity and poor efficiency/performance. Pleated kraft filters haveexcellent holding capacity with fairefficiency. Generally, pleated kraftis used in light production situationsand with slow-drying coatings.Expanded kraft filters exhibitgood efficiency but only fair holdingcapacity. And lastly, Styrofoampads have excellent holding capacitywith fair efficiency.Strainer Filters

The second primary filtrationprinciple is the strainer filter.This filter simply screens overspray from the air stream. Particles finer thanthe screen work through the screen, where as larger particles become trapped.Strainer filters come in two types. Nonwoven cloth filters have excellent efficiencybut poor holding capacity. Another disadvantage is that they are faceloaded. Fiberglass filters are a little less efficient, showing good efficiency anda somewhat better,but still only fair, holding capacity. The front surface loadsquickly, which is also disadvantageous.

Combination Baffle/Strainers

Any time you combine the best technologiesfrom two different sources,the end product is one that’s betterthan each component. So it onlymakes sense that the combinationof the superior properties of bafflefilters and strainer filters producesa filter with the highest effectivenesspossible. These high-capacityfilters can range as high as 99.5%efficiency, depending on paint formulation.

Water-Wash Booths

Water-wash spray booths (see Fig. 13) use a type of air washing action to trappaint particles. They are designed to continually break up paint accumulatingon the surface of the tanks into minute, easier to handle solid particles ofpaint or a skimming system.Overspray laden air is first drawn into the exhaust chamber. The heavierpaint particles are separated from the air and forced into a water curtain atthe chamber front. The air then enters a washer where it passes in front ofa manifold containingnumerouswater-spraynozzles where it iswashed a secondtime. In additionto passing thesewater nozzles, theair is forced tomake numerousturns throughoutits journey.Centrifugal forcediscards waterand solid particlesat each turn. Thedeposited waterand solid particlesto this pointfall back into thewater tank.Water-wash booths are ideally suited for heavier spray rates (over 20 gal/8-hrshift/10 ft of chamber width); all types of paints including primers, topcoats,enamels, epoxies, urethanes, and water reducibles; finishing operations thatare conveyorized and where automatic coating equipment is used or largeamounts of coating material are sprayed; and high-production applications.Features of water-wash booths include up to 99.6% collection efficiency,depending on paint formulation; continuous ventilation rate (constant staticpressure); and agitation systems for more effective paint-killing action.The water-wash action removes the liquid from most paints and reducesit to extremely small particles. It is a nonflammable, nonsticky waste, whichmay be nonhazardous. The sludge formed is skimmed from the tank top, orscooped from the tank bottom, and placed in drums.There are several potential challengesassociated with water-washsystems such as maintenancedowntime, operating costs, andsludge disposal costs. The additionof a sludge removal systemcan greatly minimize these problems.The benefits of a propersludge removal system are numerousand include reduction in theoverall volume of disposed materialbecause the end product is adrier sludge; the final water content,with some systems, may below enough to permit the driedsludge to be classified as nonhazardous;the result of cleanerbooth water can eliminate nozzleclogging in the air-wash section of the booth; and higher production due toincreased up time.

AUTOMOTIVE, TRUCK, AND TRAILER BOOTHS, PREP STATIONS,

AND PAINT MIX ROOMS

There are a variety of vehicular spray booths available, including cross-draft,downdraft, and semidowndraft, which were discussed above. Figures 14, 15, and16 show models of these three types of vehicular spray booths.The prep work station (see Fig. 17) is a filter exhaust system that traps sandingdust at the source, returning a clean, even flow of air to the work areasaround the part. They are also used to exhaust paint overspray on some lightpainting applications. They come in semidowndraft and downdraft designs.During sanding or prep work, the overhead plenum recirculates clean, filteredair to the work station. During priming, the inside/outside damper vents solventvapors to the outside. The advantages of utilizing a prep work station include aquick return on investment; a cleaner work area because a prep work station canbe equipped to control both dust and vapors; increased productivity due to lowermaintenance and easy cleanup; and lower energy costs (shop air is recirculatedafter filtering, so heating and air conditioning bills are lower).A paint mix room (see Fig. 18) is designed to provide a bright, clean, wellventilatedarea for mixing paints and related materials. These “clean-air” roomshelp provide a contaminant-free mixing operation and a safe work environment.The paint mix room downdraft ventilation system pulls in air from aroundthe mixing room and through a first-stagefilter to collect large dirt and dust particles.The prefiltered air then moves through theceiling fan for continuous air exchanges.Air then moves through the ceiling plenumfilter to further purify room air of contaminantsfor a clean air mixing environment.

AIR MAKEUP

An air makeupunit canlower heatingand coolingcosts. Whenair make-up is added, the building exhaust systemworks more efficiently. The information inthis section will help to determine when an airmake-up system is needed.Air make-up is the air required to maintain safeand effective building operation by replacingexhausted air. When an exhaust fan is installed ina building, exhausted air must be replaced fromoutside. This is done either through the cracks andopenings in a building or with an air make-up, orair replacement, unit, which introduces outsideair into the building. This air is usually filtered,cooled, or heated.Installing an exhaust systemwithout an air make-up unit is agood example of heating ventilationair by accident rather thanby design. Air always flows from ahigher pressure area to a lower pressurearea. Installing an exhaust fanin a building creates negative pressurewithin the interior space. Airwill flow from the higher pressureoutside the building to the lower pressure inside. Because most buildings ar

closed in, the flow is restricted, but not completely. Cracks around doors andwindows and in the masonry and vent stacks allow air to flow into the building.This air creates drafts and cold spots until itcanmix sufficiently with space airto reach room temperature. The normal heatingsystem must work longer andat higher temperature to heat the air seeping from the outside. In addition to the increased heating cost, the negative pressure keeps the exhaust fan from doinits job—exhausting contaminants from the space.Exhaust fans are rated for a certain air delivery measured in cubic feet perminute (cfm). Thisrating is based on a specific static pressure. Static pressure isthe friction the fan must overcome to exhaust air. The more cracks and openingsin the building (and the larger they are), the easier it is for air to move into thebuilding. As the static pressure rises, the exhaust air decreases.

When to Install an Air Make-Up Unit

Use the following checklist to determine if abuilding needs an air make-up unit.

1. Gravity systems, such as vent stacksfrom a gas-fired furnace or water heaterthat normally draw air out of the building,are pulling outside air in.

2. Exhaust systems are not operating efficiently,resulting in a build-up of contaminatedair within the facility.

3. The inside perimeter of the buildingis cold because the outside air is beingpulled into the building.

4. Exterior doors are hard to open or closebecause of the pressure exerted by outsideair entering the building throughthem.

5. It is difficult to maintain an even temperaturethroughout the interior space.OSHA requires the work compartmentof a spray booth to be maintained at aminimum temperature of 65°F. To meetthis regulation, it is mandatory that heatedair make-up be used during the wintermonths.Installing an air make-up unit sized to the building will improve exhaustsystem efficiency and provide greater control over the interior temperature. Withthe correct balance of air, it is easier to control air pressures to alleviate problemsin opening or closing doors. Balance also prevents contaminants or odors fromtravelling to different areas of the building. The air make-up unit reduces fuelbills by eliminating drafts.

 Sizing

The air make-up system should be sized according to the spray booth exhaustvolume plus 10%. If the air make-up duct will be physically connected to thespray booth, then the 10% extra capacity can be disregarded; however, somemeans of volume adjustment must be allowed so that a proper input/exhaustvolume balance can be obtained. This can be in the form of an adjustable driveon the air make-up and/or exhaust fan or volume dampers in the system. Ifthe installation is new, then the manufacturer will know the needs of boththe exhaust fan and the air make-up system. If the booth is older, the exhaustvolume can be determined from the manufacturer’s literature, computingfrom known booth velocity or from fan curves.Air make-up is most easily sized during initial booth purchase and installation.To determine if you require an air replacement unit, multiply yourspray booth’s exhaust fan rated capacity (cfm) by 20 (based on three changesper hour: 60 minutes/3 = 20). Using a 10 ft wide x 8 ft high spray booth ratedat 125 fpm (with a total of 10,000 cfm exhausted) would be 20 times 10,000,or 200,000 ft3 of air.If your shop’s cubic foot area is less than 200,000 ft3 of air, you shouldinstall an air replacement system.

Types of Heaters

An air make-up unit contains a heater to heat the air. The heater may be gasfired(direct or indirect), steam or hot water, or electric units. Direct gas-firedheaters are the most economical choice. Indirect gas-fired heaters are onlyused when there are restrictions against the use of direct units. Steam or hotwater heaters are the least efficient. They should only be used when there is anexisting boiler that has additional capacity to handle the air make-up system.Electric units should only be used when alternative fuels are not available.The cost of this fuel is quite expensive. The formula for calculating costs isas follows:where cfm is the actual cubic feet of air delivered by the air make-up perminute, T is the temperature of the air leaving the unit (same as the spacetemperature), To is the average outside air temperature during heating season,1.08 is the constant arrived by multiplying 0.075 (air density) by 0.24(specific heat) by 60 min/hr, H is the total hours of operation from Octoberthrough April inclusive, F is the BTU value of one unit of fuel (generally1,021for natural gas per cubic foot), E is the efficiency of the unit (0.92 for a directfired air make-up unit), and c is the cost of one unit of fuel (expressed in thesame units as those used for F). The following example illustrates how the fuelcost formula works.A 10,000 cfm air make-up unit in a building in St. Louis operates 60 hr perweek at 65°F space temperature. It is fueled by natural gas at $0.40/ft3. We findthe annual operating hours byRemember, this represents the greatest cost to operate the air make-up unit.Actual cost could be less.

Types of Air Make-Up Units

There are four basic air make-up styles available. They are defined by theirintake and discharge mechanisms and include horizontal intake/downdraftdischarge, horizontal intake/horizontal discharge, vertical intake/horizontaldischarge, and the floor-mounted vertical unit.The horizontal intake/downblast discharge unit is an air replacement unitfor inside or outside installation (see Fig. 19). The unit, when weather proofedmay go on the building roof, has a horizontal intake with a down blast discharge,and is curb mounted. The horizontal intake/horizontal discharge unitis an air replacement unit generally used indoors (see Fig. 20). The horizontalintake allows the unit to be mounted through the side wall of a building. Theunit has a horizontal discharge. The vertical intake/horizontal discharge unitis used indoors (see Fig. 21). The vertical intake allows for mounting throughthe roof of the building. It has a horizontal discharge. The floor-mountedvertical unit is an upblast furnace (see Fig. 22). All horizontal intake andfloor-mounted vertical units are available in either inside or outside models.

SUMMARY

This has been a basic overview of spray booths. Hopefully, an appreciationfor their complexity of application into a total finishing process has beenconveyed. Too often, the finishing process is not designed; it evolves, and thepurchase of any spray booth is considered as “all that is required.” Finishingand refinishing expertise should always be sought early in the process wheninitiating a new system or upgrade to an existing system.

finishing equipment & plant engineering

DESIGN AND OPERATION OF

CONVECTION DRYING AND CURING

OVENS

BY DAVID CARL

GEORGE KOCH SONS INC., EVANSVILLE, IND.

he three major processes at work in a finishing operation are the surface pretreatment,the coating application, and the drying and curing of the coating. There areseveral proven methods from which to choose. The processes are dependent uponeach other and are subject to design considerations, such as coating specifications,substrates, factory space availability, capital budget, environmental concerns, andmany others. Several options for the process are available. There are air-dry applications,low-temperature cures for woods, plastics, and even electrocoated parts, andthe more traditional higher temperatures for solids and powders.The equipment required to properly dry and/or cure the coating is just as varied.Infrared (gas and electric), radiant wall, conventional convection, and high-velocityconvection are but a few of the available options. Applications that combine methodsare becoming increasingly popular. From the point of view of an equipment supplier,by far the most often applied process is the direct gas-fired conventional convectionoven. Infrared or radiant wall designs are often incorporated for preheating; however,the completion of the cure still is accomplished by traditional means.The purpose of a drying and/or curing oven is to elevate the product and coatingto a particular temperature and hold this temperature for a set period of time. Thecombination of time and temperature serves to drive off solvents and set the coating.The desired outcome is for the combination of pretreatment, application, and cureto produce a coating with specific physical and chemical properties.Understanding the operation of a convection oven requires the examination of thesystems at work within the unit. There are five major components in an oven: the shell,the heater, the supply system, the recirculation system and the exhaust system. Each ofthese has an essential function, is comprised of several interlocking parts, and is subjectto problems from misadjustment and misapplication. When they work together properly,they produce the process necessary for the successful cure of a coating.

OVEN SHELL

The purpose of an oven shell is to contain the environment necessary for the curingprocess. The shell consists of the supporting structure, insulating and sealing materials,and openings. It must be of proper dimensions to house the product and processequipment while exposing the product to the required times and temperatures.A steel structure supports the enclosure and the product-conveying equipment.Most often the structure is built using wide flange or tubular steel on 10 foot centers.For ease of construction, the steel is located within the enclosure, exposing it to theelevated temperatures and cycling of the oven environment. Expansion becomes a problem.The beams in an oven that is 40 feet wide, operating at 450°F, will grow about 1in. as the oven temperature is elevated. Special slotted-hole connections must be usedto allow the structure to compensate for the expansion.To contain the heat, the process must be enclosed with proper insulating materials.Panels that are 30 in. wide are used with the necessary fiber insulation (1 in. of 4-ldensity insulation for every 100°F) sandwiched between aluminized metal skins. The