Booth Airflow Drop and Dust Nibs Mid-Job
A downdraft booth is in the middle of a multi-panel repair. The booth has a floor exhaust filter and ceiling intake filter. The painter notices the booth feels stuffy, overspray hangs longer than usual, and the freshly cleared hood shows a scatter of dust nibs and a few fine lint strands. The booth's differential pressure gauge reads higher than normal across the intake filter. The exhaust filter was changed two jobs ago, but the intake filter has not been changed in several months. The booth temperature is steady at 75°F. The painter wants to keep spraying to finish the job and clean up the nibs later.
1. Interpret the gauge reading and the symptoms, and explain the relationship between the intake filter, airflow, and the dust nibs.
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The elevated differential pressure across the intake filter is the key reading: a rising pressure drop means the intake filter is loading/restricting. With restricted intake, the booth cannot pull the designed volume of air, so airflow falls - matching the 'stuffy' feel and overspray that hangs instead of being carried down and out.
Dust nibs follow from that:
- Reduced downdraft velocity means airborne particles (lint, dust, overspray solids) stay in the work zone longer and settle onto the wet film instead of being captured by the floor exhaust.
- A loaded intake filter can also shed or fail to capture incoming particles, and a saturated filter no longer performs its job of delivering clean air.
- Overspray hanging increases the chance of dry overspray landing as nibs/lint on the hood.
So the nibs are a symptom of the airflow problem, not an independent contamination event. The steady 75°F rules out temperature as the cause.
2. Evaluate the painter's plan to 'finish and clean up later,' and state the correct course of action.
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Finishing the job is the wrong call. Continuing to spray in a booth with degraded airflow means:
- Every panel cleared from now on carries the same nib/lint and overspray risk - you multiply the rework.
- Poor exhaust means solvent and overspray accumulate, raising health/fire risk and degrading film quality.
- 'Cleaning up later' means sanding and buffing nibs out of fresh clear, which risks cutting through and requires rework anyway.
Correct course:
- Stop spraying.
- Change the loaded intake (ceiling) filter and verify the exhaust filter is still serviceable.
- Confirm airflow - differential pressure back to the normal range and a visual/feel check that overspray is being carried down and out.
- Clean the booth (walls, floor, ceiling) to remove accumulated overspray and dust before respraying.
- Tack and prep the affected hood, then re-clear the panels that were sprayed under degraded conditions.
Preventive point: intake and exhaust filters both have service intervals; tracking pressure drop and changing on condition (not just the exhaust on a schedule) prevents this.