Confined Space Entrant · Practice Tool
Ventilation Calculations
Three calculations from the course. How many air changes an hour a blower gives you, what size blower a space needs, and how long to purge before anyone enters.
Always plan for a minimum of 7 air changes prior to entry.
Read this first. A practice tool, not an authority. Your entry permit, your gas monitor and your written procedure decide whether a space is safe to enter. Purge time is an estimate and must be extended for ducting length, bends in the ducting, and a flammable or toxic atmosphere.
Calculation 1
Air Changes per Hour
How many air changes an hour you get from a blower of a known capacity in a space of a known volume. Some entry procedures specify an ACH, typically 15 to 30.
Air Changes per Hour
CFM × 60 ÷ VOL = ACH
Air Changes per Hour
CFM × 60 ÷ VOL
—
Air changes per hour
Worked examples from the course material
| CFM | VOL | ACH | Load |
|---|---|---|---|
| 800 | 1100 | 43.64 (manual: 44) | |
| 1200 | 2900 | 24.83 (manual: 24.82) |
10 CSE - Student Manual - V2.2 250519, section 3.5.3. Both rows are that section's Practice #1 and #2.
Calculation 2
Blower Capacity Needed
The other way round. Given the volume of the space and the air changes per hour your procedure requires, what size blower do you need?
Blower Capacity
VOL × ACH ÷ 60 = CFM
A low hazard confined space requires at least 50 cfm of clean respirable air per worker, on top of whatever this gives you.
Blower Capacity
VOL × ACH ÷ 60
—
Blower capacity needed
Worked examples from the course material
| VOL | ACH | CFM | Load |
|---|---|---|---|
| 2300 | 20 | 767 (manual: 766, use 800) | |
| 3900 | 25 | 1,625 |
10 CSE - Student Manual - V2.2 250519, section 3.5.3. Both rows are that section's Practice #1 and #2.
Calculation 3
Purge Time
How long to run the blower before entry. Two steps: air changes per minute first, then the time to reach the air changes you need.
Step 1 — Air Changes per Minute
CFM ÷ VOL = ACM
Step 2 — Purge Time
AC ÷ ACM = PTM
An estimate. Add time for ducting length, bends in the ducting, and the presence of a flammable or toxic atmosphere.
Always plan for a minimum of 7 prior to entry.
Step 1 — Air Changes per Minute
CFM ÷ VOL
Step 2 — Purge Time
AC ÷ ACM
—
Purge time
Worked examples from the course material
| CFM | VOL | AC | PTM | Load |
|---|---|---|---|---|
| 570 | 1500 | 7 | 18.4 min |
10 CSE - Student Manual - V2.2 250519, section 3.5.3. The row is that section's Practice #1.