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This article provides a stepwise, expert method to diagnose and correct low fume hood face velocity for safe laboratory operation and regulatory alignment.
1. Know the Target and the Variables.
Most research hoods target 80–120 fpm face velocity at a standard sash opening, often 18 inches, with 100 fpm used for setpoint calculations unless a performance test justifies a different value.
Face velocity equals exhaust flow divided by sash opening area.
# Core formulas. FaceVelocity_fpm = ExhaustFlow_cfm / OpeningArea_ft2. OpeningArea_ft2 = (SashWidth_in * SashHeight_in) / 144. RequiredExhaust_cfm = TargetFaceVelocity_fpm * OpeningArea_ft2. 2. Verify Measurement Before Adjustments.
Confirm instruments are calibrated and suited to low velocity measurement ranges.
Use a traverse method across the sash plane with at least 6–10 evenly spaced readings, then compute the average and standard deviation.
Perform qualitative smoke visualization to check for reverse flows, eddies, and bypass performance.
Caution: Do not introduce smoke or test gases with active work inside the hood. Test on an empty, clean hood.
3. Rapid Diagnostic Decision Tree.
| Observation | Likely Cause | High-leverage Fix |
|---|---|---|
| Velocity low at all sash heights. | Insufficient exhaust cfm or closed damper. | Open or recalibrate damper, increase fan setpoint, verify VAV command, confirm filter and duct losses. |
| Velocity acceptable at low sash but drops when raised. | Exhaust control not tracking sash area on VAV hood. | Recommission sash sensor, recalibrate VAV controller, enable area tracking curve. |
| Velocity uneven across opening. | Blocked baffles or internal loading near slots. | Reposition equipment, clear rear and bottom baffles, set minimum 2 inches clearance behind objects. |
| Smoke escapes at corners or floor level. | Incorrect baffle setting or missing lower slot capture. | Set baffles to balanced configuration, verify bypass function, adjust lower slot opening per OEM. |
| Velocity unstable over minutes. | Room pressure swings or supply diffuser drafts. | Stabilize room pressurization, redirect supply diffusers away from the sash plane, add air curtain only if needed. |
| Velocity low only on this hood. | Local duct obstruction, failed damper actuator, clogged prefilter or HEPA if equipped. | Inspect duct, repair actuator, replace filters, verify stack discharge is not throttled. |
| All hoods show low velocity. | Exhaust fan speed or control fault, shared filter loading, stack icing or wind effects. | Increase VFD speed within design limits, clear shared filters, inspect stack cap and wind band. |
4. Calculate the Required Exhaust Flow.
Use the sash width and working height to compute the opening area and the cfm needed to meet setpoint.
# Example. SashWidth_in = 48. SashHeight_in = 18. Target_fpm = 100. Opening_ft2 = (48 * 18) / 144 # 6.0 ft². Required_cfm = 100 * 6.0 # 600 cfm. # If measured average is 72 fpm, current cfm ≈ 72 * 6.0 = 432 cfm. # Increase exhaust by ~168 cfm to reach 600 cfm, then reverify uniformity and stability. 5. Restore Capture by Eliminating Internal Flow Blockages.
Remove large instruments that sit flush against the rear wall since they short-circuit flow away from the slots.
Raise items on 1 inch standoffs so air can sweep under and behind the load.
Keep chemicals off the work surface except during active work to free the baffle path.
6. Set Baffles and Bypass Correctly.
Three-zone baffle settings typically split capture among the upper, middle, and lower slots to balance thermal, mid-plane, and heavy vapor capture.
Verify the bypass opens as the sash closes to prevent extreme high velocities and flow starvation at large openings.
7. Fix Supply and Room Factors First.
Confirm the room is slightly negative to the corridor for chemical labs unless a specific hazard requires neutral or positive pressure in a specialized space.
Redirect supply diffusers so they do not blow toward the sash plane or user position.
Maintain 3 feet of clearance in front of the hood and restrict cross-traffic that creates cross drafts.
Caution: Avoid ceiling fans and personal fans in front of hoods because they disrupt containment and invalidate velocity measurements.
8. Recommission VAV Control.
Check sash position sensor alignment and scaling so area tracking is accurate over the full travel.
Verify minimum and maximum flow setpoints, damper authority, and controller gain to prevent oscillation.
Trend controller inputs and outputs for at least 15 minutes and remove limit alarms before acceptance.
9. Inspect the Exhaust Path End-to-End.
Confirm damper orientation, blade stop settings, and that fail-safe direction matches the safety basis.
Check duct static pressure at the hood collar and several upstream points to locate a sudden loss increase caused by debris or a crushed section.
Inspect filters or scrubbers for differential pressure above design values and replace or service as required.
10. Acceptance Testing Before Return to Service.
Document face velocity grid results at the defined sash height and record the measured average and range.
Perform qualitative smoke tests for containment at corners and around the operator position.
Complete a tracer gas or standardized performance test when required by policy to validate containment at the restored flow.
11. Minimum Documentation Package.
| Record | Content | Who | Date |
|---|---|---|---|
| Field sheet. | Instrument ID, calibration date, grid points, sash height, average fpm, cfm. | Tester. | YYYY-MM-DD. |
| Controls trend. | VAV setpoint, actual cfm, damper position, duct static. | Controls tech. | YYYY-MM-DD. |
| Mechanical inspection. | Baffle positions, bypass function, duct inspection notes, filter DP. | HVAC tech. | YYYY-MM-DD. |
| Safety verification. | Smoke visualization, alarm function, signage, user briefing. | EHS. | YYYY-MM-DD. |
12. Worked Case: Low Velocity With VAV Fault.
A 60 inch hood at 18 inches measured 70 fpm average with high variation at the lower left grid points.
The VAV controller was capped at 500 cfm while the required flow was 60 * 18 / 144 * 100 equals 750 cfm.
The sash sensor scale was off by 20 percent, and a microscope blocked the lower baffle slot.
After recalibrating the sash sensor, lifting the microscope on a 1 inch riser, and increasing the max flow to 800 cfm, the average stabilized at 102 fpm with uniformity within plus or minus 10 fpm.
13. Operating Rules That Prevent Recurrence.
Keep the sash as low as practical during work to reduce required cfm and stabilize flow.
Position work items at least 6 inches inside the sash plane.
Close the sash when unattended to maintain energy performance on VAV systems.
Schedule semiannual hood inspections and annual performance tests in line with your lab policy.
14. Quick Field Checklist.
# Fume hood low velocity triage. 1. Confirm instruments and grid method. 2. Record sash width and working height. 3. Compute opening area and target cfm. 4. Verify room pressure and supply diffuser aim. 5. Clear baffles and raise equipment on risers. 6. Recheck velocity uniformity. 7. Inspect damper actuation and controller scaling. 8. Measure duct static and filter differential pressure. 9. Adjust VFD or damper to reach target cfm. 10. Smoke check and document results. Caution: Specialized hoods, such as perchloric acid or radionuclide hoods, have unique baffle and washdown requirements. Do not apply generic settings to these units.
FAQ
Is face velocity alone sufficient to judge safety.
No. Use qualitative smoke and standardized performance testing where required to verify containment.
What grid should I use across the sash plane.
Use a uniform grid that covers corners and midlines. Capture at least 6–10 points for small hoods and more for wider hoods.
How much variation across points is acceptable.
Keep point readings within a narrow band around the average. Large corner deficits indicate baffle or blockage issues.
How do cross drafts affect readings.
Air from supply diffusers or traffic can reduce capture and create false low or unstable readings. Redirect diffusers and control traffic.
Should I upgrade to VAV to fix low velocity.
VAV improves stability and energy use but still requires correct sizing, tuning, and maintenance.