Fundamentals of Blast Room Ventilation
Proper ventilation design is critical for blast room operations. The ventilation system must balance three key requirements: effective dust capture, worker safety, and operational efficiency.
Core Ventilation Principles
Airflow Direction
Air must flow toward dust sources and collection points. For blast rooms:
- Downdraft: Air flows downward through floor
- Negative Pressure: Air flows toward walls/collection
- Minimum velocity: 3,500-4,000 FPM in breathing zone
Supply Air Balance
Supply air must match exhaust air volume to prevent negative pressure issues:
- Supply = Exhaust + 10% infiltration
- Filtered or clean supply air required
- Distribution through ceiling or walls
- Prevents exterior air leakage
Downdraft Blast Room Design
System Configuration
Downdraft systems pull air downward through the blasting area into a sub-floor plenum:
Key Components:
- Supply Air: Filtered replacement air from ceiling or walls
- Blasting Area: Open floor with proper ventilation
- Floor Grates: Perforated metal or slots distributed across floor
- Sub-floor Plenum: Collection chamber below floor (typically 18-24" height)
- Primary Separator: Cyclone removing 85-95% of heavy abrasive
- Main Collector: Baghouse or cartridge collector for final dust removal
- Fan: Appropriate horsepower for required CFM and pressure drop
Design Calculation: CFM = Booth area (sq ft) × 100-150 CFM/sq ft
Downdraft Advantages & Disadvantages
Advantages
- Superior dust capture
- Improved visibility
- High abrasive recovery
- Proven performance
- Minimal outside air
Disadvantages
- High initial cost
- Requires floor excavation
- Complex installation
- Longer implementation
- Permanent structure
Negative Pressure Blast Room Design
System Configuration
Negative pressure systems maintain room pressure slightly below surrounding areas (0.5-1.0" WC negative):
Design Principles:
- Room Sealing: Walls, ceiling, floor sealed to maintain pressure differential
- Controlled Entry: Doors/hatches with pressure relief
- Supply Air: Limited controlled supply through diffusers
- Exhaust Collection: Dust pulled from blasting area via exhaust ducts
- Pressure Control: Dampers and fans maintain -0.5 to -1.0" WC
- Dust Collection: Standard baghouse or cartridge collector
Negative Pressure Advantages & Disadvantages
Advantages
- Flexible construction
- Can retrofit existing
- Modular walls available
- Simpler installation
- Lower initial cost
Disadvantages
- Higher air volume
- More A/C needed
- Less precise control
- Door opening issues
- Operating complexity
Airflow Velocity & Distribution
Critical Velocity Requirements
| Location |
Velocity (FPM) |
Purpose |
| Breathing Zone |
3,500-4,000 |
Capture dust before inhalation |
| Main Ductwork |
3,500-4,000 |
Prevent dust settling in ducts |
| Branch Ducts |
2,000-3,000 |
Balance across multiple areas |
| Collector Inlet |
1,500-2,000 |
Prevent re-entrainment |
Velocity Calculation Formula
Velocity (FPM) = CFM ÷ Duct Area (sq ft)
Example: For 20,000 CFM in 12-inch diameter duct:
Area = π × (12/24)² = 0.785 sq ft
Velocity = 20,000 ÷ 0.785 = 25,478 FPM (too high!)
Solution: Use larger duct diameter to achieve proper velocity.
Supply Air Design
Make-Up Air Requirements
- Supply Volume: = Exhaust CFM + 10% infiltration allowance
- Air Quality: Must be filtered (minimum 45-65% efficiency)
- Temperature: Season-appropriate conditioning or heating/cooling
- Distribution: Uniform distribution across ceiling or upper walls
- Noise: Supply outlets designed for low velocity (<500 FPM)
Supply Air Calculation
If blast room requires 50,000 CFM exhaust:
Supply Air = 50,000 CFM + (50,000 × 0.10) = 55,000 CFM
Common Design Mistakes to Avoid
Mistake #1: Undersized supply air - Causes pressure imbalance and door opening difficulty
Mistake #2: Unbalanced CFM distribution - Some areas have inadequate capture
Mistake #3: Improper duct sizing - Excessive velocity or settling of dust
Mistake #4: Inadequate pressure drop planning - Fan too small for actual system resistance
Design Checklist
Complete Ventilation Design Review
- ☐ Room dimensions and blasting area defined
- ☐ Required CFM calculated based on booth size
- ☐ Breathing zone velocity verified (3,500+ FPM)
- ☐ Ductwork diameter sized for proper velocity
- ☐ Pressure drop analyzed for all components
- ☐ Fan horsepower determined
- ☐ Supply air volume calculated and balanced
- ☐ Dust collector sized appropriately
- ☐ Installation plan developed
- ☐ Budget and timeline established