Calculator Form
Ventilation Performance Graph
The graph compares purge time against different fan capacities using your entered duct efficiency and purge cycle settings.
Formula Used
These equations estimate ventilation performance for planning purposes. Site conditions, hose friction, bends, obstructions, contaminant source strength, and monitoring requirements can change actual performance.
How to Use This Calculator
- Enter a space name to identify the calculation.
- Select imperial or metric units.
- Choose rectangular or cylindrical geometry.
- Enter the space dimensions.
- Enter fan capacity, target ACH, duct efficiency, safety factor, and purge cycles.
- Submit the form to show results above the form.
- Review actual ACH, required airflow, purge time, and adequacy.
- Use the export buttons to save CSV or PDF records.
Example Data Table
| Example | Shape | Volume | Target ACH | Fan | Efficiency | Effective Airflow | Purge Time |
|---|---|---|---|---|---|---|---|
| Rectangular utility vault | Rectangular | 1,920.00 ft³ | 20 | 1,200 CFM | 85% | 1,020.00 CFM | 11.29 min |
| Cylindrical access shaft | Cylindrical | 1,570.80 ft³ | 15 | 900 CFM | 80% | 720.00 CFM | 10.91 min |
| Large process chamber | Rectangular | 4,500.00 ft³ | 12 | 1,800 CFM | 75% | 1,350.00 CFM | 13.33 min |
Frequently Asked Questions
1. What does this calculator estimate?
It estimates confined space volume, effective airflow, required airflow, actual air changes per hour, and purge time from the values you enter.
2. Why is duct efficiency included?
Real airflow drops because of hose length, bends, friction, and leakage. Duct efficiency reduces rated fan flow to a more realistic planning value.
3. What is purge time?
Purge time is the estimated time needed to replace the chosen number of space volumes using the effective airflow, not the fan nameplate rating.
4. What does target ACH mean?
ACH means air changes per hour. It shows how many times the full space air volume is replaced in one hour.
5. Is adequate status enough for entry approval?
No. Entry approval depends on permit requirements, atmospheric testing, hazard classification, rescue readiness, and competent supervision, not only this estimate.
6. Can I use metric units?
Yes. Choose metric to enter dimensions in meters and fan capacity in cubic meters per minute. The page also shows converted values.
7. Why use a safety factor above 1.00?
A safety factor adds margin for uncertainty, system losses, and changing field conditions. Higher values demand more airflow before the result is marked adequate.
8. What should I export?
Export the result when you want a quick calculation record for planning discussions, toolbox talks, equipment selection notes, or site documentation.