Laboratory exhaust workflow
Fume Hood Airflow Verification Workflow
A face-velocity number is one recorded condition, not a substitute for containment testing or a coordinated laboratory ventilation design.
Direct answer
Document the hood identity, sash position, room condition, controls, and instrument first. Measure a required equal-area traverse, calculate the average and distribution, compare them only with owner-approved criteria, and complete the applicable flow-visualization or containment test. For system planning, separately total active and standby exhaust, coordinate makeup air and room offset, and calculate conditioned-air load from matched psychrometric states.
Four separate decisions
| Decision | Required evidence | Site tool |
|---|---|---|
| What average velocity is implied by measured flow? | Sash opening and volume airflow | Face Velocity Calculator |
| How are point readings distributed? | Equal-area traverse readings | Traverse Calculator |
| What exhaust scenario follows from active/standby states? | Hood counts, documented flows, controls basis | Multi-Hood Planner |
| What thermal load follows from that exhaust? | Flow, density, two air states, recovery | Exhaust Load Calculator |
Step 1: preserve the test condition
Record the hood and room as tested: hood identifier and type, sash height, equipment in the hood, door and window positions, supply-air operation, exhaust-control status, and instrument/calibration information. ASHRAE notes that hood performance depends on opening geometry, room air movement, researcher movement, internal heat, baffles, exhaust arrangement, and replacement-air delivery.
Step 2: measure a traverse, not one convenient point
OSHA describes taking multiple readings at different points in a plane perpendicular to airflow and averaging them. A NIOSH field evaluation used equal-area grid patterns and recorded average velocity, standard deviation, opening dimensions, and total exhaust flow. Follow the selected test method for grid layout and point placement; the Traverse Calculator summarizes readings but does not define the test.
Step 3: keep velocity and containment separate
Face velocity is relevant, but it is not the complete containment result. ASHRAE describes face-velocity, flow-visualization, tracer-gas, and sash-movement checks. The owner establishes the certification program, acceptance criteria, responsible agency, and retest process. Do not turn a generic web result into a certification label.
Step 4: connect the hood to the air system
A laboratory can contain constant-volume and variable-volume hoods with different minimum and full-open flows. Build explicit scenarios rather than hiding simultaneity in one factor. Then reconcile hood exhaust with other exhaust, supply, return, transfer, door leakage, and the pressure-control sequence. The Multi-Hood Planner exposes its counts and airflow assumptions.
Step 5: calculate the conditioned-air consequence
Exhaust usually requires replacement air. Use the Psychrometric Calculator for indoor and outdoor moist-air states, take the absolute enthalpy difference, and apply it with documented airflow and density in the Exhaust Load Calculator. This isolates thermal load; fan energy, plant efficiency, reheat, humidification, and controls remain separate.
Worked example
A 48-by-18-inch face at 600 CFM implies 100 ft/min average velocity. Nine equal-area readings average the same 100 ft/min but span 96 to 104 ft/min. Four hoods with two active at 600 CFM and two in 150 CFM standby produce a 1,500 CFM scenario, versus 2,400 CFM fully connected. At 0.075 lb/ft³ and a 10 Btu/lb enthalpy difference, that scenario represents 67,500 BTU/h of thermal load before recovery. Each result answers a different question.
Common mistakes
- Using outside cabinet dimensions instead of the clear opening at the recorded sash position.
- Mixing readings from different sash positions or room-control states.
- Using average velocity as proof of containment.
- Applying an undocumented diversity or makeup-air percentage.
- Calling thermal kWh electrical kWh without plant efficiency.
- Using a fume-hood workflow for a biosafety cabinet, laminar-flow bench, or another containment device.
Frequently asked questions
What face velocity should a hood have?
This guide does not prescribe a universal value. Use the hood design, owner program, applicable requirements, and qualified test procedure.
Can these tools certify a hood?
No. They calculate and organize inputs; qualified testing must follow the applicable procedure and owner acceptance program.
Why calculate exhaust energy?
It makes the conditioning consequence visible when evaluating operating schedules, sash management, controls, or recovery—without weakening required containment.
Related reference
Use the Fume Hood Airflow Formulas for equations, unit conversions, a field record checklist, and scope limits.