Filter Selection

Filter selection is not a search for one highest percentage. Every dry arrestor represents a balance among droplet capture, paint-holding capacity, airflow resistance, physical integrity, service life and cost. A media that performs well in a low-rate intermittent booth may block prematurely in continuous high-solids production. A coarse high-capacity stage may control bulk wet overspray but require a downstream stage where fine mist or sensitive equipment must be protected. [S178; S192; S193]

This chapter develops the selection route introduced in Chapters 032 and 033. It is an educational and application-selection guide. The installed booth design, coating Safety Data Sheet, applicable Malaysian requirements and current product documentation remain controlling. Where an exact product value is required, Refer to the product label, Technical Data Sheet, or MKTECH representative.

Chapter objectives

After this chapter, the reader should be able to:

• define the selection duty from paint type, solids content, spray rate and overspray volume;
• convert booth airflow and effective filter area into a meaningful face-velocity condition;
• distinguish overall arrestance, size-dependent capture, holding capacity, resistance and service life;
• establish the available resistance allowance without destabilising the booth;
• decide when a single-stage, backed or multi-stage arrangement should be evaluated;
• screen coating compatibility, fire behaviour, handling and Malaysian disposal requirements;
• compare operating cost on an accepted-production basis rather than purchase price alone;
• use a complete decision tree to produce a technically defensible shortlist; and
• recognise field symptoms that indicate the selected system is unsuitable or incorrectly applied.

Begin with the normal production condition, not an empty booth with clean filters. The filter must receive the actual coating cloud generated by the spray equipment, part geometry, painter or robot path, transfer efficiency, output and booth airflow.

The selection record should describe the process in observable terms that another engineer can verify.

Paint type is the first technical branch because the clean filter never operates alone; it becomes a paint-loaded composite.

Check the mixed coating, hardener, thinner, cleaning materials and any alternation between products. The complete media construction includes paper or fibre, binder, adhesive, backing, frame and surface treatment.

Solids content is not a filter-efficiency value. It is one contributor to the mass that may reach the exhaust. Spray rate is also not overspray volume: a high-rate efficient application can place less material on the bank than a lower-rate application with poor transfer or difficult part geometry. Treat the three quantities separately.

The most useful loading description combines deposited mass, time and distribution. Exact paint capture may require a controlled material balance, but a sound first selection can be formed from normal consumption, transfer condition, production duration and filter inspection.

Use the relationship conceptually:

Overspray load reaching the bank = coating used - coating deposited on acceptable work - other controlled losses

Do not substitute a generic transfer-efficiency percentage. Part size, recesses, gun distance, fan pattern, robot path, painter technique, colour change and reclaim practices can materially change the result.

Classify the duty for selection:

• Light/intermittent: long non-spray periods, modest daily mass and limited local loading.
• Moderate/repeating: regular batches with predictable full-bank loading.
• Heavy/continuous: sustained application or high mass requiring substantial internal capacity.
• Concentrated/local: one zone receives a disproportionate load because of work position or spray direction.
• Mixed droplet: bulk wet overspray and fine mist reach the bank together.

The category is a communication aid, not a performance rating. Define it with actual production information and revise it when the coating, equipment, part family or work method changes.

The same filter can behave differently at different airflow conditions. Air must carry overspray from the work zone to the exhaust face, pass through the installed media and remain within the fan/system capability. Filter-face velocity depends on the flow through the bank and the effective open filter area.

Filter-face velocity = airflow through the bank / effective filter area

Use consistent units and exclude blocked frames, sealed-off zones and non-flow area from the denominator. Average velocity alone is not enough: a partially bypassed or uneven bank can contain high local velocity beside stagnant loading zones. [S171; S172; S194]

Do not correct an unsuitable filter by increasing fan speed beyond the booth's approved operating method. This may change containment, turbulence, motor load, noise, discharge behaviour and fire protection. Selection must fit the complete system.

Required efficiency should state what must be captured, under which test or service condition and for what downstream purpose. EPA research shows that overall mass arrestance and penetration by droplet size describe different aspects of overspray-arrestor behaviour. A high mass percentage can be dominated by large droplets while a small mass of finer droplets still penetrates. [S192; S193]

Ask five questions whenever a performance percentage appears:

1. What test method or verified service method produced it?
2. What challenge aerosol, coating or particle-size distribution was used?
3. At what airflow or face-velocity condition was the filter tested?
4. Was the result measured clean, loaded or across a defined loading sequence?
5. Does the figure apply to one layer, a complete composite or the installed multi-stage system?

General-ventilation ratings are not automatically overspray-arrestor ratings. Compare only declared paint-arrestor data that match the intended coating and airflow condition. Refer to the product label, Technical Data Sheet, or MKTECH representative.

The filter receives only part of the fan's total pressure capability. Intake filters, diffusers, enclosure openings, exhaust filters, ducts, dampers, discharge fittings and dirty surfaces all contribute resistance. The available allowance must be established from the actual system pressure profile and acceptable airflow range. [S171; S179; S180]

Use this sequence:

1. Verify the normal production airflow and containment condition.
2. Confirm the current fan, drive, control mode, damper and motor state.
3. Measure or obtain the clean-system pressure profile at defined points.
4. Separate intake, booth, exhaust-filter and downstream-system losses where practical.
5. Determine the earliest allowed endpoint from airflow, containment, fan/motor, filter, physical and fire controls.
6. Reserve margin for realistic loading and measurement variation.
7. Evaluate the proposed first, intermediate and final stages as one combined system.

Adding pressure losses arithmetically is valid only when the values refer to the same airflow and compatible test conditions. Media resistance normally changes with airflow, loading and installation. Do not add a clean value at one velocity to a loaded value at another and present the result as a system prediction.

Use a single stage when one declared construction can provide the required bulk capacity and downstream protection within the available resistance and frame arrangement. Use a backed or multi-stage system when coarse wet load and fine-mist capture impose conflicting duties, or when downstream sensitivity justifies a separate final stage.

A downstream stage is not permission to leave a failed first stage in service. If bulk paint reaches the fine stage, resistance may rise quickly and the complete bank can lose airflow.

No option proceeds to cost comparison until it passes the compatibility and safety screen.

Reject or escalate a candidate when:

• the supplier does not declare it suitable for the coating or moisture condition;
• the complete media construction can soften, shed, collapse or react in service;
• the coating residue is susceptible to self-heating and the arrangement is not specifically suitable;
• incompatible coatings would accumulate in the same bank or container;
• the booth's fire-protection, interlock or access arrangement cannot be maintained;
• the media cannot be installed in the intended direction with continuous support and sealing;
• used filters cannot be safely handled, classified, contained or sent to an authorised destination; or
• changing the filter system would compromise an applicable permit, booth approval or engineering control.

OSHA's dry-filter provisions are used here as transferable safety principles: airflow indication, filter inspection, control of discarded media, protection around the filter area and prevention of incompatible or self-heating residues. Malaysian requirements and the authority having jurisdiction govern the installation. [S175; S185]

In Malaysia, the waste producer classifies the used filter from the generating process and captured materials. Disposal cost and handling effort therefore depend on the residue, segregation plan, container and authorised route—not only the clean media mass. [S188; S189]

Unit price is visible; process cost is distributed. Compare candidates over a common production basis such as accepted parts, coated area, production hours or coating throughput. Use the measure that best represents value at the site.

Total filter-system cost = media + change labour + disposal + energy + cleaning + filtration-related quality loss

Keep the comparison transparent. If a cost cannot be isolated reliably, record how it was estimated and avoid presenting it as a guaranteed saving.

The following decision tree converts the ten required inputs into a shortlist and release path.

Apply the tree in this order:

1. Identify the paint system. Confirm mixed coating, carrier, cure chemistry, tack and residue hazards.
2. Define solids and spray rate. Use the normal condition and credible peak, not only nominal equipment capacity.
3. Estimate overspray volume and pattern. Distinguish total mass from local concentration and fine mist.
4. Confirm booth type and effective filter area. Establish frame depth, support, access and loading distribution.
5. Verify airflow and fan response. Use the occupied normal production arrangement.
6. Define capture duty. State downstream protection and the comparable test basis needed.
7. Set the resistance allowance. Evaluate clean and loaded stages within the complete fan/system condition.
8. Choose architecture. Shortlist single-stage, backed or staged media families.
9. Pass safety and disposal gates. Confirm compatibility, fire controls, segregation and Malaysian waste route.
10. Compare operating cost. Use accepted production and observed change-out evidence.
11. Verify installation and production behaviour. Release only after the sealed bank provides stable airflow, containment and downstream cleanliness.

Use a structured shortlist so one attractive property cannot hide a system failure.

Do not score a mandatory safety, compatibility or legal requirement. It is a pass/reject gate. Weighted scoring is suitable only for remaining commercial and operational differences after all mandatory conditions pass.

A controlled evaluation confirms system behaviour without claiming a universal product ranking. Keep the coating, booth, part family, application method and production basis comparable.

Record:

• exact filter identity, construction, stages, orientation and installed area;
• coating and mix condition, spray equipment and representative production output;
• clean pressure, airflow/containment condition and fan/control state;
• loading distribution, run-off, sag, damage, bypass and downstream deposits;
• change-out trigger and production achieved before that trigger;
• labour, waste container and disposal route;
• finish defects or cleaning work reasonably attributable to filtration; and
• deviations, stoppages and process changes that invalidate comparison.

The evaluation must not exceed the booth, filter, fan, fire-control or coating operating limits. It does not replace current product instructions. Refer to the product label, Technical Data Sheet, or MKTECH representative.

Correct the verified cause. Changing to a coarser filter to reduce pressure may increase downstream penetration; changing to a finer filter to improve capture may shorten life or reduce airflow. Re-run the complete selection tree after a significant change.

A purchase request should describe the required duty without inventing a product result. Include:

• filter function and booth location;
• paint and coating-family compatibility;
• media family, stage role and physical format;
• installed dimensions, effective area, orientation, backing and support;
• declared test method and airflow condition for required capture data;
• clean resistance and declared change endpoint under comparable conditions;
• holding-capacity or service information with its test/service basis;
• fire, handling, storage and installation information;
• required TDS, SDS or disposal information;
• packaging, traceability and revision identification; and
• contact route for application confirmation.

Avoid descriptions such as “high efficiency”, “high capacity”, “low pressure drop” or “fire retardant” without the declared basis. Refer to the product label, Technical Data Sheet, or MKTECH representative.

• Do not select an exhaust filter by price, colour, thickness or appearance alone.
• Do not compare percentages from different test methods as equivalent.
• Never add a stage without confirming fan duty, frame support, sealing and airflow.
• Do not use a loaded-filter fire classification inferred from the clean media.
• Keep coatings capable of reacting or self-heating in separate controlled filter and waste streams.
• Never bypass media or increase fan limits to hide excessive resistance.
• Stop spraying for loss of containment, abnormal pressure, collapse, run-off, breakthrough, heat, smoke or smouldering odour.
• Treat used filters according to the captured residue and Malaysian waste requirements.
• Maintain ventilation, interlocks, grounding and fire protection in their designed state.

The Filter Inspection and Replacement chapter begins on the following page of the printed handbook (page 287), outside this chapter extract.