Choose your space size and desired air changes. Add contaminant limits and safety factor. Get required flow, purge minutes, and a matched fan recommendation.
Use dimensions for quick volume, or enter total volume directly. Add advanced contaminant fields when you need dilution ventilation or purge time estimates.
Sample inputs and resulting ventilation targets for planning discussions.
| Scenario | Volume | ACH | Safety | Duct loss | Recommended delivered flow |
|---|---|---|---|---|---|
| Small valve vault | 18 m³ | 20 | 25% | 15% | 450 m³/h |
| Manhole with longer duct | 9 m³ | 30 | 30% | 25% | 585 m³/h |
| Tank entry staging | 60 m³ | 12 | 20% | 10% | 864 m³/h |
Example values are illustrative and must be validated for your job hazards.
The calculator selects the larger airflow between ACH and dilution methods, then applies a safety factor. If duct loss is entered, it estimates the fan rating needed to deliver the target flow.
Always confirm with atmospheric testing, permit requirements, and manufacturer fan curves. Do not rely on a single calculation for entry authorization.
Confined spaces can accumulate toxic vapors and displace oxygen quickly. Ventilation is used to dilute contaminants, control heat, and maintain acceptable oxygen levels before and during entry. It also improves mixing so instruments reflect conditions more consistently.
Many sites start planning with 10–20 air changes per hour (ACH) for routine entries and increase to 20–30 ACH when odors, welding, or solvent work is expected. Higher targets are common when monitoring shows slow clearance. For unknown conditions, start higher and adjust after readings stabilize.
The calculator converts volume into required airflow using Q = V × ACH ÷ 60. For a 18 m³ vault at 20 ACH, the base flow is 6.0 m³/min (360 m³/h) before adding contingency. If you enter feet and CFM, the tool converts units internally for consistent sizing.
Field conditions rarely match perfect assumptions. A 20–30% safety factor is often applied to cover imperfect mixing, changing work rates, door openings, and instrument lag. The tool multiplies the base requirement by (1 + safety%). Consider higher safety when hot work or intermittent emissions are planned.
Long ducts, bends, reducers, and dirty filters reduce delivered flow. Planning values of 10–25% loss are common for moderate runs, while complex setups may exceed 30%. If the duct run is long or has many bends, use a higher loss value.
If a contaminant is produced at a steady rate, the dilution method uses Q = G ÷ (Callow − Cbg). Example: 1200 mg/min with an allowable 50 mg/m³ and background 5 mg/m³ requires about 26.7 m³/min (1600 m³/h).
For pre-entry purge, the well-mixed model estimates time to drop from an initial concentration to a target using t = (V ÷ Q) × ln(Cinitial/Ctarget). With 18 m³ and 10 m³/min delivered, reducing 120 to 20 mg/m³ is about 3.2 minutes.
Place the duct outlet near the hazard zone, avoid short-circuiting to the opening, and verify flow direction. Re-check with atmospheric testing, especially after work changes. Keep the fan intake away from vehicle exhaust, and document assumptions using the CSV/PDF outputs.
Use m³/h when Metric is selected and CFM when Imperial is selected. The calculator converts internally and applies duct loss to estimate delivered flow.
Use ACH for general air turnover planning. If you have a known contaminant generation rate and an allowable concentration, the dilution method is more defensible. The calculator uses the larger airflow of the two.
Common planning values are 20–30%. Increase it when mixing is poor, the task is variable, duct routing changes, or monitoring shows slow clearance. Do not use safety factor to replace required controls.
It represents the percentage of rated fan flow you expect to lose due to duct length, bends, restrictions, filters, and setup inefficiencies. Higher losses require a larger fan rating to deliver the same target flow.
The purge model assumes the space is well mixed and the airflow is steady. Dead zones, stratification, and short-circuiting can lengthen real clearance times. Always verify with atmospheric testing.
It helps you size ventilation, but oxygen hazards require continuous monitoring, a permit, and a rescue plan. If oxygen is low or unstable, treat it as an immediately dangerous condition and follow your procedure.
Record the space volume, chosen ACH, safety factor, duct loss assumption, recommended delivered flow, and the fan model rating. Export the CSV/PDF and attach monitoring logs for a complete entry record.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.