Slide 003: Ventilation Systems for Blast Operations

Slide Visual

Slide Overview

This slide covers the design, components, and performance verification of local exhaust ventilation (LEV) systems used with blast cabinets, including dust collector types, ductwork design, and system monitoring.

Instruction Notes

Local Exhaust Ventilation Principles

LEV systems capture airborne contaminants at or near the source of generation before they reach the operator's breathing zone. A blast cabinet LEV system consists of four elements:

1. Hood (the cabinet itself): The enclosure acts as a capture hood. Openings (glove ports, door gaps) are the critical points where containment can fail.
2. Ductwork: Transports captured dust to the air cleaning device. Must maintain minimum transport velocity to prevent settling.
3. Air cleaning device (dust collector): Separates particulates from the air stream before exhaust.
4. Fan/blower: Provides the energy to move air through the system. Typically located downstream of the dust collector to protect the fan from abrasive particles.

Dust Collector Types

Type	Filtration	Efficiency	Capacity	Maintenance	Best For
Cartridge collector	Pleated polyester/cellulose	99.9% at 0.5 μm	400–2000 CFM	Pulse-jet self-cleaning	Most blast cabinets
Bag collector	Woven/felt bags	99% at 1 μm	500–5000 CFM	Manual shaking or pulse	Heavy-duty / multiple cabinets
Cyclone separator	Centrifugal	90% at >10 μm	200–2000 CFM	Minimal — empty collection bin	Pre-separator before filter
HEPA afterfilter	Glass fiber media	99.97% at 0.3 μm	Secondary unit	Replace cartridge	Toxic dusts (lead, Cr VI)

Ductwork Design Parameters

• Minimum transport velocity: 3500–4500 fpm for abrasive dust (ACGIH Industrial Ventilation)
• Duct diameter: Size for required airflow at transport velocity. Q = V × A (CFM = fpm × ft²)
• Horizontal runs: Keep minimal — each foot of horizontal duct adds friction and settling risk
• Elbows: Use long-radius (R/D ≥ 2) elbows to minimize pressure loss
• Clean-out ports: Install at every direction change and every 10 feet of straight run

System Performance Verification

Monitor these parameters to ensure LEV is functioning properly:

• Differential pressure gauge: Mounted across dust collector. Normal range varies by manufacturer — document baseline reading when filters are new.
• Capture velocity at openings: Measure with a velometer or smoke tube at glove ports and door seals. Minimum 200 fpm inward.
• Static pressure in duct: Measure at test ports with manometer. Compare to design values — decrease indicates leaks or fan degradation.
• Visual smoke test: Release smoke tube at cabinet openings while system runs. All smoke should be drawn inward. Any outward escape indicates containment failure.

Common Ventilation Problems

• Clogged filters (high ΔP) — reduce airflow and capture velocity
• Duct leaks (low ΔP, low capture) — seal with foil tape and mastic
• Undersized ductwork — restricts airflow, causes noise and vibration
• Fan belt slippage — reduces RPM and airflow

Key Talking Points

• The dust collector is only as good as its weakest component — the filter, the ductwork, and the fan must all be working
• Horizontal ductwork is the enemy — keep runs short and vertical where possible
• A differential pressure gauge is the single most important monitoring tool
• Smoke tube testing is the easiest way to verify containment — do it regularly

Learning Objectives (Concept Check)

• Identify the four components of a local exhaust ventilation system
• Compare dust collector types and their appropriate applications
• Perform a basic ventilation system performance check using ΔP readings and smoke testing

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Last Updated: 2026-03-19