Inputs
Enter hood opening dimensions and exhaust flow. Advanced options help model sash position and effective free area.
Example Data
| Scenario | Width | Height | Sash (%) | Free-area | Flow | Velocity |
|---|---|---|---|---|---|---|
| Typical commissioning (Imperial) | 4.0 ft | 2.5 ft | 80 | 0.95 | 760 CFM | 100 fpm |
| Energy-optimized (Imperial) | 3.0 ft | 2.0 ft | 70 | 0.95 | 399 CFM | 100 fpm |
| Typical commissioning (Metric) | 1.2 m | 0.8 m | 80 | 0.95 | 0.365 m³/s | 0.50 m/s |
Formula Used
Face Velocity is the average air speed through the hood opening:
- V = Q / A
- A_effective = (Width × Height) × (Sash%/100) × Free-area factor
Use consistent units: CFM with ft² yields fpm, and m³/s with m² yields m/s.
How to Use This Calculator
- Select the unit system that matches your measurements.
- Enter the hood opening width and height.
- Enter exhaust flow from design or measured balancing data.
- Set sash open percent to reflect the operating position.
- Adjust the free-area factor if the opening is partially blocked.
- Confirm the target velocity range required by your project.
- Optional: enter measured point readings to review uniformity.
- Press Calculate to view results above.
- Download CSV or PDF to attach to commissioning records.
Technical Guidance
1) Why face velocity matters
Face velocity is the average air speed entering a fume hood through the sash opening. It is commonly evaluated during commissioning to confirm that the hood can capture and transport contaminants without excessive turbulence. Many projects set an operational band of 80–120 fpm (approximately 0.40–0.60 m/s) at a defined sash height, with field verification recorded for turnover and safety files.
2) Opening area drives performance
Airflow alone does not determine velocity. The effective opening area changes with sash position and with restrictions such as frames, inserts, and equipment. This calculator models that using sash percentage and a free-area factor. For example, a 4.0 ft × 2.5 ft opening has 10.0 ft² gross area. At 80% sash and 0.95 free area, the effective area becomes 7.6 ft². With 760 CFM, the calculated velocity is 100 fpm.
3) Interpreting low and high results
Results below target can indicate insufficient exhaust, an overly open sash, or poor distribution across the opening. Values above target can increase energy use and may raise noise or cause turbulence at the sash. Use the “required flow” outputs to understand how much exhaust is needed to reach the selected target range at the current effective area.
4) Using measured point readings
Field testing often includes multiple traverse points across the sash plane. Entering at least three readings allows the calculator to report min/avg/max and a simple uniformity span. As a practical guideline, large spreads relative to the average can suggest drafts, poor baffle settings, obstructions, or unstable sash use.
5) Documentation and practical notes
Record the unit system, sash position, room conditions, and test method with each result. Small changes in sash opening can meaningfully shift velocity: if effective area increases by 10%, velocity decreases by roughly 9% at the same flow. Use the CSV/PDF exports to keep consistent, auditable commissioning records for laboratories and process spaces.
FAQs
1) What is face velocity in a fume hood?
It is the average air speed entering through the sash opening, calculated as exhaust flow divided by effective opening area. It supports commissioning checks and helps compare performance against project targets.
2) Why does sash height change the result so much?
Sash height changes the opening area. With the same exhaust flow, a larger effective area reduces face velocity. Managing sash position is often the fastest way to correct low velocity.
3) What is the free-area factor used for?
It accounts for reductions in open area caused by frames, inserts, or partial blockages. A value below 1.00 models less-than-full free area and increases calculated velocity for the same flow.
4) Should I rely on calculated velocity or measured readings?
Use both when possible. The calculation estimates average velocity from flow and area, while measured points reflect real airflow distribution and room effects such as drafts and turbulence.
5) What does the LOW or HIGH status mean?
LOW means the calculated velocity is below your target band, and HIGH means it is above. It is a screening indicator; containment performance still depends on hood design and test method.
6) How do I use the “required flow” outputs?
They show the exhaust flow needed to hit the selected targets at the current effective area. If you change sash percent or free-area factor, the required flow updates accordingly.
7) What inputs should I record for commissioning files?
Record unit system, hood dimensions, sash position, measured exhaust flow, target band, and room conditions. Add any traverse readings and notes about drafts, obstructions, or baffle settings.