Cleanroom Air Changes Per Hour and FFU Coverage Calculator
Air changes per hour (ACH) and ceiling coverage are the first numbers checked when a cleanroom is laid out with fan filter units. This calculator turns room size and FFU data into ACH, coverage and downflow velocities.
Your numbers
Count only units that are running at the design speed.
Use the fan curve value at your installed pressure drop, not the catalog maximum.
Your results
This is a sizing aid. Required air change rates and velocities depend on the ISO 14644 class, occupancy, heat load and your validated design. Confirm with your cleanroom designer and qualification testing.
Air changes per hour vs FFU count
Sweeps Number of FFUs from 10 to 120, holding your other inputs fixed.
Cleanroom records in one searchable place
Optional: ask how to connect particle logs and filter changes to your maintenance system, privately.
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How to use the calculator
Enter the room length, width and height, the count of FFUs that are operating, the airflow each delivers at its real operating point and the filter face area. The tool returns the total supply airflow, ACH, the percentage of the ceiling covered by filters, the mean velocity across the floor and the face velocity at each filter. Use the chart to see how ACH scales with FFU count and to compare a few layout options before talking to your designer.
- Use the fan curve value at the installed pressure drop.
- Count standby FFUs separately and keep them out of the input.
- Run it again after any filter change that raises resistance.
The method and the numbers it relates to
ACH is supply airflow in volume per hour divided by room volume. Coverage is FFU count times filter face area divided by floor area. Mean velocity is supply airflow divided by floor area. These are the screening metrics used when comparing designs for ISO 14644-1 classes. Higher classes (lower particle limits) typically need more ACH and higher coverage; unidirectional ISO 5 areas are usually designed around ceiling coverage that is high and a mean velocity in the neighborhood of a few tenths of a metre per second.
Worked example
A 12 m by 8 m room, 3 m high, has a volume of 288 cubic metres. Sixty FFUs at 1000 cubic metres per hour supply 60,000 cubic metres per hour, which is about 208 air changes per hour. At 0.72 square metres each, the filter face covers 43.2 square metres of the 96 square metre floor, which is 45% coverage. Mean downflow velocity is about 0.17 metres per second, and face velocity at each filter is about 0.39 metres per second.
Common mistakes
The most common error is using catalog maximum airflow, which overstates ACH once the filter loads. Another is mixing up total room volume and the volume of the clean zone in a bay-and-chase fab. Remember that ACH alone does not prove cleanliness: the class is proven by particle counting, recovery tests and airflow visualization under ISO 14644. Do not size from this tool alone for a validated area.
Frequently Asked Questions
What ACH do I need for my ISO class?
There is no single value in the standard. ISO 14644 defines particle limits, not air change rates. Designers choose ACH from experience, heat load and recovery needs, and then prove the class by testing. Ask your cleanroom designer for the design basis.
What is the difference between ACH and ceiling coverage?
ACH measures the volume of air supplied relative to room size. Coverage measures how much of the ceiling is filter face. Two rooms can have the same ACH but very different coverage, which affects flow pattern and particle removal.
Should I use rated or measured FFU airflow?
Use measured or fan curve values at the installed resistance. Filters load over time, so the flow drops unless the controller compensates. Rated maximum flow tends to overstate real ACH.
Does this include return air or makeup air?
No. It calculates recirculation through the FFUs only. Makeup air, exhaust and pressure cascade are separate calculations with their own limits.
If you also track cleanroom monitoring and maintenance in an ERP or CMMS, Netray can help connect those records so filter changes and particle trends are visible in one place on your own servers.
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