Paint Booth Airflow and Filtration Fundamentals

Spray atomisation creates a fast-moving cloud of coating droplets and vapour. The booth must contain that cloud, carry overspray away from the operator and workpiece, remove captured particulate before discharge, and maintain a clean, stable environment around the wet coating. These functions depend on the combined performance of the supply opening, intake filters, plenum, booth enclosure, exhaust filters, ductwork, fan and discharge point. [S171; S172; S174; S175]

This chapter explains the fundamentals that apply across dry-filter liquid-coating booths. Water-wash booths, powder systems, recirculation systems and specialised solvent controls require their own engineered arrangements and operating procedures.

Chapter objectives

After this chapter, the reader should be able to:

• trace the complete airflow path through a paint booth;
• distinguish crossdraft, semi-downdraft, side-downdraft and downdraft arrangements;
• explain why average velocity, distribution and direction must be considered together;
• interpret differential pressure without mistaking it for a direct airflow reading;
• separate the functions of intake filters and exhaust overspray arrestors;
• recognise how filter loading changes airflow, fan duty and coating quality;
• identify pressure-balance symptoms at doors, gaps and filter frames;
• apply practical checks for worker exposure and post-spray clearance;
• separate particulate capture from vapour and VOC control; and
• troubleshoot airflow and filtration as one connected system.

The booth enclosure is the capture zone, but the enclosure alone does not create control. Air must enter through the intended clean path, travel through the work zone without harmful recirculation, pass through the correct exhaust treatment and discharge to a safe location. DOSH Malaysia describes an enclosure hood such as a spray-painting booth as a local exhaust ventilation control whose effectiveness depends on adequate, uniformly distributed flow and the performance of the complete system. [S171; S172]

Airflow arrangement describes the main direction between the clean-air supply and the exhaust. Crossdraft booths move air broadly from one end to the other. Semi-downdraft systems introduce air overhead and move it diagonally to a low exhaust. Side-downdraft systems supply from above and extract low at the sides. Full downdraft systems move air from a ceiling plenum towards a floor or low-level exhaust. These are established families, but the best choice depends on work geometry, operator movement, process, building layout, contamination sensitivity and the engineered ventilation duty. [S184]

The intended arrangement is only the starting geometry. A large product, fixture, platform, open door or operator can split the stream and create wake zones. Evaluate airflow with the normal part, support and work position in place, while preserving a safe test method.

Direction determines where overspray and vapour travel. The spray gun ejects coating at much higher local speed than the general booth airflow, so extraction cannot simply pull every droplet backwards at the gun. The control objective is to contain and sweep the dispersed cloud after atomisation, keeping the operator out of the downstream plume where the designed process permits. [S172; S177]

Arrange the part and spraying sequence so the operator does not repeatedly spray into their own breathing zone or block the path to the exhaust. Use turntables, fixtures or planned access where appropriate so the work can be presented to the airflow rather than forcing the painter to stand downstream. Check that large surfaces do not form a wall across the booth or create a protected pocket behind the component.

Crosscurrents from doors, fans, compressed-air blow-off, nearby extraction, vehicle movement or building pressure can overcome the intended stream at an opening. Increasing fan speed may not correct a badly directed crosscurrent and can create more turbulence. First remove the disturbance, restore the intended openings and check distribution.

Air velocity is the local speed of air at a stated plane and under stated operating conditions. Volume flow is related to average velocity and open area:

Volume flow = average velocity x effective open area

This relationship is useful only when the measurement plane and effective area are defined. A single centre reading cannot represent a large booth face, ceiling or filter bank. Use a planned traverse or the booth's specified verification method, compare readings across the intended plane, and record the operating condition of doors, fans, dampers, filters and production equipment.

Uniformity matters because two booths can have the same calculated average while one has strong jets and weak pockets. High local velocity can disturb the spray pattern, increase turbulence, dry the coating unevenly or draw contamination from gaps. Low local velocity can allow overspray to linger, escape or settle on the work. DOSH guidance therefore links effective enclosure control with both sufficient velocity and uniform distribution across the capture face. [S171]

Use smoke visualisation or another approved qualitative method only under safe non-spraying conditions and with materials suitable for the booth. Smoke can reveal reverse flow, dead zones, leakage and slow clearance, but it does not replace quantitative measurement or exposure assessment.

Differential pressure is the pressure difference across a filter bank or another system section. It indicates resistance to airflow. Clean media has an initial resistance at a stated flow. As dust or overspray accumulates, resistance normally rises. The fan and controls then determine whether airflow falls, fan speed rises, power changes or the system reaches an alarm or control limit. [S174; S179; S180]

Pressure drop is not a direct measure of capture efficiency, remaining filter life or booth velocity. A low reading can mean a clean filter, low airflow, a missing panel, a split medium, an open bypass or incorrectly located pressure taps. A high reading can indicate loading, wet or collapsed media, excessive flow, a blocked downstream path or an unsuitable replacement. Always interpret pressure with airflow, physical condition and system configuration.

Intake filtration reduces dust, fibres, insects and other particles entering with make-up air. Its main production purpose is a cleaner wet-film environment and more uniform air distribution. A staged intake may include coarse protection followed by a finer final supply filter, but the correct arrangement depends on the air source, finish requirement, booth design and fan capacity.

Filter efficiency cannot be selected independently of resistance. A finer or deeper filter may improve particle control but can also change pressure drop and fan duty. General-ventilation filters may be classified and tested under the ISO 16890 series, including airflow resistance and dust-loading behaviour, but laboratory classification does not by itself predict service life in a particular paint booth. [S181; S182; S183]

Install media in the stated airflow direction and provide full support. Seal the perimeter, panel joints and access doors so air cannot choose a lower-resistance route around the filter. Keep the clean side protected during replacement; contamination released downstream of the final intake stage can deposit directly on wet work.

Do not assume that a visually clean filter has low resistance or that a dark filter has failed. Use the specified differential-pressure and airflow checks with inspection. For exact media class, dimensions, orientation, initial and final resistance, rated airflow, compatibility and replacement limit, Refer to the product label, Technical Data Sheet, or MKTECH representative.

The exhaust filter or overspray arrestor captures coating droplets and solids carried from the work zone. It protects the downstream plenum, duct, fan and discharge path from excessive deposits and reduces particulate emissions. It is not the same product function as an intake filter, and the two should not be interchanged merely because they fit the same frame.

Overspray loading depends on coating solids, spray rate, transfer efficiency, part geometry, gun technique, production time and airflow distribution. A filter that performs well with one coating process may load rapidly, drain, soften, shed, react or permit breakthrough with another. Chapter 33 explains construction families; Chapter 34 develops the selection route.

Inspect the complete bank for uniform loading. A clean corner beside a heavily loaded centre can indicate poor distribution, a blocked part position or air bypass. Darkening or deposit alone is not a reliable end-of-life rule unless the filter instructions use it. Track differential pressure, airflow, loading pattern, downstream cleanliness and production condition together.

Never cut, compress, stretch, reverse or stack media unless the product and booth instructions permit that arrangement. Uncontrolled modification changes flow passages, support, resistance, holding behaviour and fire characteristics.

Booth balance is the relationship between supplied and exhausted air, including every intentional opening and leakage path. The designed pressure condition may be negative, neutral or positive relative to the surrounding room depending on the booth type and process. The correct target is the engineered operating condition, not a universal pressure sign.

Too much exhaust relative to supply can pull dirty shop air through door gaps, panel joints and cable openings. This may increase dust defects, make doors difficult to operate and disturb temperature control. Too much supply relative to exhaust can drive vapour, odour or overspray out through openings. A nominal pressure indication can still hide local leakage if seals or distribution are poor.

A fan delivers airflow at the pressure required by the system. The operating or duty point is where the fan characteristic and system resistance meet. Filters, plenums, diffusers, dampers, ducts, bends, stacks and deposits all contribute resistance. AMCA and US Department of Energy guidance treat fan performance as a system interaction rather than an isolated nameplate value. [S179; S180]

With a fixed-speed fan, rising resistance commonly reduces delivered airflow. With a variable-speed or pressure-controlled fan, the controller may increase speed and power to maintain a set condition. The booth can therefore appear to hold airflow while the motor approaches its limit or energy use rises. A control signal is not proof that the physical system is healthy.

Trend the indicators that matter to the installed design: filter-bank pressure drop, booth pressure, representative airflow, fan speed, motor current or power, damper position and alarms. Investigate a change in relationship, such as more fan effort for the same airflow, before the system loses control.

Do not compensate for a restricted filter by bypassing media, opening uncontrolled gaps or exceeding equipment limits. Restore the designed air path and use a compatible replacement.

Coating quality depends on material preparation, atomisation, application technique, temperature, humidity, flash-off and curing as well as booth airflow. The airflow system cannot correct an unsuitable coating process, but it can introduce or amplify visible defects.

Release coating appearance only after the air system is stable under the normal production arrangement. An empty-booth test may not show wake zones created by the actual part, fixture or operator.

Ventilation is an engineering control, but it does not automatically reduce every hazardous substance below the applicable exposure requirement. Coating droplets, solvents, isocyanates, metals and other ingredients require assessment from the actual Safety Data Sheet, process and workplace exposure conditions. Fire-safe ventilation and flammability control do not by themselves prove toxic-exposure control. [S153; S160; S175; S177]

Keep the operator on the clean-air side of the spray cloud where the booth design and task allow. Avoid directing the gun towards another person, an exit or an uncontrolled opening. Maintain mechanical ventilation during spraying and for the required period afterwards. Respiratory, skin, eye and body protection must be selected through the current chemical-risk assessment and site programme.

Post-spray clearance is the time needed for airborne contamination to be removed after spraying stops. HSE guidance recommends establishing clearance under worst-case service conditions, including near the filter-change condition, and clearly communicating when entry or removal of respiratory protection is permitted. [S176; S177]

Do not use odour or visible mist alone as the clearance indicator. Some hazardous vapours are poorly detected by smell, and fine aerosol may remain after the obvious cloud disappears.

Dry overspray arrestors primarily capture coating droplets and solids. Ordinary particulate filters generally do not remove solvent vapours or VOCs. EPA guidance makes this distinction explicit: a booth can capture overspray particulate while vapour remains in the exhaust stream. [S178]

VOC, hazardous-air-pollutant and odour control may require coating reformulation, improved transfer efficiency, enclosure, dedicated adsorption, thermal or catalytic treatment, wet collection, or another engineered process. Selection depends on the actual coating chemistry, emission rate, regulatory requirement and control-system design.

Discharge treated air to the approved location away from doors, occupied zones and air intakes so contaminants are not drawn back into the building. Malaysia's Environmental Quality (Clean Air) Regulations 2014 establish the national environmental framework for air-pollution-control systems and emissions. Site design, notification, monitoring and limits must follow the current requirements applicable to the installation. [S173]

Captured filters and residues remain process waste. Store and dispose of them according to coating chemistry, fire risk, Safety Data Sheet information and Malaysian waste requirements. Do not treat a dry filter as harmless because the carrier frame is paper or synthetic fibre.

Use a repeatable release check at start-up, after maintenance, after filter change and whenever airflow, coating quality, odour, fan load or booth pressure changes.

Record the booth identity, coating process, filter type, installed condition, pressure readings, airflow result, fan state, inspector and action taken. Trends are often more useful than isolated readings because they show how resistance and fan response change across filter life.

• Use the booth only for the coating processes, work sizes and operating modes covered by its current instructions and risk assessment.
• Confirm mechanical ventilation, airflow indication, interlocks and alarms before spraying.
• Keep doors and intentional openings in their stated operating position; do not defeat interlocks or pressure controls.
• Keep ignition sources, unsuitable electrical equipment, hot work and incompatible materials out of the spray area.
• Maintain ventilation during spraying and throughout the established post-spray clearance period. [S175; S176]
• Select respiratory, eye, face, skin and body protection from the coating Safety Data Sheet and current chemical-risk assessment.
• Isolate energy and follow the filter-change procedure before opening plenums, reaching into fans or servicing the air path.
• Treat used filters and deposits as coating-contaminated material; control ignition, exposure, storage and disposal.
• Never operate with missing, damaged, bypassed or improvised filter media.
• Follow the current authority, fire code, environmental approval, booth manual and site procedures. NFPA 33 is a recognised fire-safety reference where adopted by the authority having jurisdiction. [S173; S185]

1. Confirm the booth, coating, part size and operating mode are authorised for the planned work.
2. Inspect the make-up-air intake and discharge location for blockage, contamination or re-entry risk.
3. Check intake and exhaust filters for correct identity, orientation, support, sealing and condition.
4. Inspect doors, panels, lights, floor grilles, plenums and intentional openings.
5. Start the ventilation system and confirm fan rotation, stable running, indicators, interlocks and alarms.
6. Record differential pressure across each controlled filter bank.
7. Verify airflow direction and distribution using the specified method and normal booth configuration.
8. Place the work and fixture so they do not block the designed path or force the operator downstream of the plume.
9. Confirm the booth pressure condition contains overspray without drawing dirty air through uncontrolled gaps.
10. Check the clean-air side, work zone and exhaust path for loose dust or residue.
11. Confirm protective equipment, safe entry, exit and post-spray clearance arrangements.
12. Spray only when the system is released and keep ventilation operating throughout the process.
13. Watch for changing door force, pressure, airflow, fan sound, visible escape, dust or finish quality during the run.
14. Stop if containment, ventilation, filter integrity, interlock or fire-safe operation is uncertain.
15. After spraying, maintain ventilation and access controls for the established clearance period.
16. Record readings, filter condition, abnormalities, corrective work and release decision.

The Paint-Stop and Overspray Filters chapter appears on the following pages of the printed handbook, outside this chapter extract.