Confined Space Ventilation Calculator: Purge Time, Air Changes and Atmosphere Check

Plan the ventilation of a tank, pit, silo or pipe before entry: air changes, purge time, and your oxygen and flammable gas readings checked against the limits you enter.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the size of the space (as a volume, a rectangular box or a cylinder) and the fresh air flow your fan really delivers at its duty point. The calculator gives the air changes per hour and the time for the fan to move one volume of air through the space.

Enter the contaminant level at the start and the level you want to reach, in the same unit, and a mixing factor. A space is never mixed perfectly, so a factor of 3 to 10 is normal and a higher number gives a longer, safer purge time. The calculator gives the number of volumes of air and the purge time.

If something is still leaking or evaporating inside, enter the release rate to see the steady level the fan can hold it to. Then type in your oxygen and flammable gas readings and the limits from the standard or code of practice, and the calculator tells you if the readings are inside them.

Formulas used

  • C(t) = C_0\,e^{-Qt/(KV)}
  • t = K\,\frac{V}{Q}\,\ln\frac{C_0}{C_t}
  • \mathrm{ACH} = \frac{Q}{V}
  • C_{ss} = \frac{G}{Q}

Worked example

With the default numbers in the calculator above:

  1. Volume of the space: 30.00 m3
  2. Air changes per hour from the fan: 20.00 per hour
  3. Time for the fan to move one volume of air: 3.00 min
  4. Number of space volumes of air needed: 23.03 volumes
  5. Purge time to reach the target: 69.08 min
  6. Oxygen reading against the limits: 20.90 percent: inside the limits of 19.50 to 23.50 percent
  7. Flammable gas reading against the limit: 0.00 percent of LEL: within the limit of 5.00 percent

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: How do you know the size of the space? Rectangular tank, pit or room (length, width, height).

  1. Volume of the space: 15.00 m3
  2. Air changes per hour from the fan: 40.00 per hour
  3. Time for the fan to move one volume of air: 1.50 min
  4. Number of space volumes of air needed: 23.03 volumes
  5. Purge time to reach the target: 34.54 min
  6. Oxygen reading against the limits: 20.90 percent: inside the limits of 19.50 to 23.50 percent
  7. Flammable gas reading against the limit: 0.00 percent of LEL: within the limit of 5.00 percent

Common questions

What are the rules for confined spaces in Queensland?

In Queensland a confined space is covered by the confined spaces part of the Work Health and Safety Regulation 2011 and the Confined spaces code of practice, with AS 2865 for the detailed methods. In general the business must identify the confined spaces, assess the risks, control them (isolate the space, ventilate it, test the air), use an entry permit, have a person outside who can communicate, and have rescue and first aid ready. Check the current versions with WorkSafe Queensland.

How long should I ventilate a confined space before entry?

It depends on the volume, the fan airflow and how much of the contaminant is there. This calculator gives a planning estimate from the dilution equation. The air must still be tested before entry and kept under watch while people are inside.

What oxygen level is safe in a confined space?

Normal air has about 20.9 percent oxygen. The usual limits in Australian guidance are about 19.5 percent as the lowest and 23.5 percent as the highest. Take the limits from the current AS 2865 and the code of practice, and enter them in the calculator.

Why does the calculator ask for a mixing factor?

Air in a real space does not mix perfectly. Pockets and dead ends keep contaminant longer, so the mixing factor lengthens the purge time to allow for that.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Work Health and Safety Regulation 2011 (Qld), confined spaces; Confined spaces Code of Practice (Queensland); AS 2865 Confined spaces; ventilation dilution equation from any industrial ventilation text.

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