Enter Ventilation and Vapor Data
Concentrations can be entered as volume percent or as a percentage of the selected LEL.
Example Data
| Input | Example value | Why it matters |
|---|---|---|
| Space volume | 100 m³ | Larger spaces require more clean-air exchange. |
| Nominal airflow | 500 m³/h | Sets the starting removal capacity. |
| Mixing efficiency | 75% | Accounts for dead zones and uneven circulation. |
| Initial concentration | 50% LEL | Defines the starting condition. |
| Target concentration | 10% LEL | Defines the planning endpoint. |
| Safety factor | 1.25 | Extends model time for cautious planning. |
Formula Used
The calculator uses a first-order, well-mixed dilution model with optional continuous vapor generation.
C(t) is concentration at time t. Css is steady concentration. C0 is initial concentration. Ct is target concentration. Qeff is effective clean-air flow. V is space volume. G is vapor source flow. Cin is incoming vapor concentration.
How to Use This Calculator
- Choose a gas preset or enter a verified LEL value.
- Enter the volume of the enclosed space.
- Enter the nominal outdoor ventilation flow.
- Set realistic mixing and fresh-air percentages.
- Enter current and target vapor concentrations.
- Add an ongoing vapor source when one exists.
- Choose a planning safety factor, then calculate.
- Verify the real atmosphere with approved gas monitoring.
Vapor Dilution Planning
Understanding Vapor Dilution
A vapor cloud becomes less hazardous when clean air replaces contaminated air. This calculator estimates how long dilution may take in a defined space. It models a well-mixed room, enclosure, pit, process area, or ventilation zone. It does not confirm that an atmosphere is safe. Use calibrated gas detection and site procedures before entry or restart.
Lower Explosive Limit Targets
The lower explosive limit, or LEL, marks the lowest vapor concentration that can ignite under specified conditions. That target creates a margin before reaching the combustible range. Your target must match local rules, permits, and equipment requirements. Never treat one calculated value as a clearance authorization. Leaks, pockets, and uneven airflow can change real conditions quickly.
Effects of Airflow and Sources
This page converts volume and airflow into metric units internally. Mixing efficiency reduces the nominal airflow when dead zones exist. The fresh-air fraction reduces removal when air recirculates. A continuous vapor source raises the expected steady concentration. When that steady value equals or exceeds your target, ventilation alone cannot achieve the target. Stop the source, increase removal, or revise the work plan.
Model Assumptions
The primary dilution model is C(t) = Css + [C0 - Css] e^(-Qeff t / V). C0 is the starting vapor concentration. Css is the steady concentration produced by incoming vapor and ventilation. Qeff is effective clean-air flow. V is volume. The calculator rearranges this relationship to solve for time. It then applies your selected safety factor to create a planning duration.
Entering Reliable Values
Enter the room volume and select its unit. Enter the fan or exhaust flow and select its unit. Choose a gas preset or enter its LEL value. Add the initial and target concentration values. Each value may use volume percent or percent LEL. Then enter mixing efficiency, fresh-air fraction, source generation, and safety factor. Submit the form to view the result above the inputs. Review the warnings before relying on any number.
Limits of Nameplate Flow
The effective airflow is lower than fan nameplate flow. Long ducts, filters, bends, pressure losses, and poor make-up air reduce performance. Air may also bypass the contaminated zone. Gas may collect near floors, ceilings, trenches, or equipment. A single well-mixed model cannot show those patterns. Use smoke testing, ventilation verification, and multi-point monitoring where conditions demand them.
Continuous Release Conditions
Continuous releases need special attention. The calculation treats the source as a constant vapor flow. Actual releases may pulse, evaporate, or vary with temperature. A high source rate may create a steady concentration above your target. In that case, waiting longer will not solve the problem. Isolation, repair, capture ventilation, or additional exhaust may be necessary. Protect workers from ignition sources throughout the response.
Using the Estimate Safely
Use the result as a conservative planning estimate. Confirm fan operation and exhaust discharge locations. Verify that make-up air does not spread vapor elsewhere. Test the atmosphere at relevant heights and locations. Keep records of assumptions, readings, and corrective actions. Qualified safety professionals should carefully review high-risk spaces, confined areas, and emergency events.
Frequently Asked Questions
1. What does this calculator estimate?
It estimates the time required for a well-mixed space to fall from a starting vapor concentration to a chosen target. It adjusts nominal airflow for mixing efficiency and fresh-air fraction. It can also include a continuous vapor source.
2. Does the result prove the atmosphere is safe?
No. The result is a planning estimate only. Atmospheric testing, ignition control, ventilation verification, permits, and site procedures remain necessary. Use competent safety review for hazardous, confined, or poorly ventilated spaces.
3. Why does the calculator show that the target is unreachable?
The continuous source or incoming vapor may create a steady concentration equal to or above the target. Waiting longer cannot fix that condition. Stop or reduce the source, increase effective exhaust, or improve the fresh-air path.
4. What is mixing efficiency?
Mixing efficiency represents how much nominal airflow actually reaches and dilutes the contaminated zone. Low values can reflect dead zones, poor fan placement, obstructions, short-circuiting air, or stratification.
5. What does fresh-air fraction mean?
Fresh-air fraction is the portion of airflow that removes vapor rather than recirculating it. A lower percentage reduces the effective dilution flow. Confirm whether any return air is safe and permitted.
6. Can I enter concentrations in percent LEL?
Yes. Choose Percent of LEL for the initial or target basis. The calculator converts that entry using the LEL value. You may instead enter direct volume percent values.
7. What source rate should I use?
Use an estimated vapor generation rate in litres per minute when a release continues. Enter zero after isolation or when no continuing source exists. Consider evaporation and temperature effects before relying on a source estimate.
8. Why is effective airflow lower than fan airflow?
The calculation multiplies nominal airflow by mixing efficiency and fresh-air fraction. This creates an adjusted value that better reflects imperfect distribution and recirculation. Actual system performance may still differ.
9. What safety factor should I select?
Choose a factor based on site rules and uncertainty. A factor above one increases the planning duration. Do not use a safety factor to replace monitoring, source control, or required engineering safeguards.
10. Can this be used for pits or confined spaces?
It can support preliminary comparisons, but pits and confined spaces often have poor mixing and vapor layering. Treat them as higher-risk cases. Use formal entry controls, trained personnel, and atmospheric monitoring.
11. Which gas value should I enter?
Use an LEL value from a reliable safety data source for the actual vapor and operating conditions. Presets are convenience values. Verify the correct material, temperature range, and local safety requirements.