Fume Hoods

Laboratory fume hoods provide critical containment for chemical vapors, gases, and particulates. HVAC system design must ensure proper exhaust rates, face velocities, and room air balance while optimizing energy consumption.

Fume Hood Types

Conventional (Constant Air Volume)

Fixed exhaust rate regardless of sash position:

• Simple operation
• Higher energy consumption
• Face velocity varies with sash position
• Suitable for teaching labs, limited budgets

Variable Air Volume (VAV)

Exhaust modulates with sash position:

• Maintains constant face velocity
• Significant energy savings (40-60%)
• More complex controls
• Standard for research laboratories

Auxiliary Air (Add-Air)

Supplemental air supply at hood face:

• Reduces conditioned air consumption
• Not recommended (containment concerns)
• Prohibited by many institutions
• Legacy installations only

Ductless (Filtered)

Carbon filter recirculation to room:

• No duct connection required
• Limited chemical compatibility
• Filter saturation monitoring critical
• Not appropriate for all applications

Face Velocity Requirements

Design Face Velocity

ASHRAE and AIHA recommendations:

Sash Position Considerations

Face velocity calculated at working opening:

$$V_{face} = \frac{Q_{exhaust}}{A_{opening}}$$

Full Open Sash: Maximum exhaust, design face velocity Partially Closed: Reduced exhaust (VAV), maintained face velocity Fully Closed: Minimum exhaust (bypass air or slot)

Exhaust System Design

Individual Exhaust

One fan per hood:

• No cross-contamination
• Independent operation
• Higher capital cost
• More roof penetrations

Manifold Exhaust

Multiple hoods share exhaust system:

• Lower cost
• Fewer penetrations
• Dilution of concentrated vapors
• Requires careful design

System Configurations

Constant Volume Manifold:

• Bypass dampers at each hood
• Central fan runs constant
• Individual hood VAV possible

Variable Volume Manifold:

• Total exhaust varies with demand
• Variable speed fan(s)
• Significant energy savings
• Complex controls

Exhaust Fan Selection

Stack Design

Discharge above roof:

• 10 ft minimum above roof
• 2 ft above adjacent air intakes
• Adequate exit velocity (3,000+ fpm)
• Rain protection

Room Air Balance

Makeup Air

Supply air must balance exhaust:

$$\dot{V}{supply} = \dot{V}{fume\ hoods} + \dot{V}{general\ exhaust} - \dot{V}{infiltration}$$

Maintain slight negative pressure relative to corridor.

Air Change Rates

Laboratory air changes (total supply):

Supply Air Distribution

Position supply to avoid disrupting hoods:

• Maintain <50 fpm at hood face
• No supply directly opposing hood
• Perforated ceiling or high sidewall

Control Systems

VAV Hood Controls

Monitor and modulate:

• Sash position sensor
• Face velocity measurement
• Exhaust damper/valve
• Alarm on low face velocity

Room Pressure Control

Track supply with exhaust changes:

• Direct pressure control
• Offset tracking
• Cascade control
• Response time matching

Building Automation

BAS integration requirements:

• Hood status monitoring
• Alarm management
• Energy monitoring
• Occupancy scheduling

Performance Testing

ASHRAE 110 Method

Standard test protocol:

• Face Velocity Test: Measure velocity uniformity
• Flow Visualization: Observe smoke patterns
• Tracer Gas Test: Quantify containment (as manufactured: <0.05 ppm; as installed: <0.10 ppm)

Periodic Verification

Annual testing minimum:

• Face velocity measurement
• Visual inspection
• Alarm verification
• Sash sensor calibration

Continuous Monitoring

Real-time parameters:

• Face velocity or exhaust flow
• Sash position
• Room pressure differential
• Alarm status

Energy Efficiency

VAV Hood Benefits

Energy savings from VAV:

$$Savings = \sum hours \times (Q_{CV} - Q_{VAV}) \times \Delta h \times Cost$$

Typical 40-60% reduction in hood-related energy.

Occupancy-Based Control

Further reduction when labs unoccupied:

• Setback face velocity (50-60 fpm)
• Maintain minimum air changes
• Rapid return to normal on entry

Sash Management

Operator behavior programs:

• Close sashes when not in use
• Alarm on extended full-open
• Training and awareness
• Significant savings potential

Heat Recovery

For high exhaust volumes:

• Runaround coils (no cross-contamination)
• Heat pipe systems
• Energy wheels (careful with contaminants)
• Return on investment analysis

Special Hood Types

Perchloric Acid Hoods

Dedicated system with:

• Stainless steel construction
• Wash-down system
• No manifold connection
• Direct exhaust

Radioisotope Hoods

HEPA filtration with:

• Bag-in/bag-out filter
• Higher face velocity
• Glove box option
• Special waste handling

Walk-In Hoods

Large equipment accommodation:

• Higher exhaust volumes
• Multiple sash options
• Floor-level exhaust

Effective fume hood HVAC integration ensures occupant safety while managing the substantial energy requirements of these critical containment devices through appropriate system design and control strategies.

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