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Understanding CFM and True Air Flow Rate

Posted on 15 October 2020

It is important to make the distinction between fan speed CFM without a filter installed (sometimes referred to as the “free flow” air rate) and true air flow rate CFM with a high efficiency filter installed. In order to properly filter the air and scrub the air of contaminants, a multi-stage high-efficiency filter, like the one found in your Austin Air unit, must slow the rate of the air moving through the filter to remove contaminants from the air. If air can pass through a filter too quickly without slowing down then this is a red flag that this filter is not effective and doesn’t have a high efficiency rating. This is the same with any air purifier, however, most manufactures do not make this distinction and only publish the fan speed CFM without ever publishing the true air flow rate with a high efficiency filter installed (the part that really matters). Austin Air publishes both the fan speed CFM and the True Air Flow raing for its models. It’s other manufactures not being forward as we are that causes customers to become very confused when comparing models.

The air change calculations for dental operatories that we’ve provided have always been calculated using the true air flow rate CFM of 250 and not the fan speed, so our ACH ratings and your fallow time between patients remains the same. Below is our ACH reference table and it uses the data that we have always provided. This information also includes the formula to calculate ACH based on specific operatory sizes.

Our dental landing page originally included a badge that read “1 ACH per 5 minutes in 100 sqft”. This value actually only equates to 12 air changes per hour in a 10′ x 10′ operatory and under-rates the machine’s capabilities. Some dentists mentioned that they found this badge a bit confusing so based on their feedback we decided to update it with the “250CFM True Air Flow Rate” badge.

Air that is filtered at a MERV-17 level or higher (the same as what Austin’s products provide) is far more important than simply providing air changes using the HVAC system and spreading aerosols throughout your clinic… This is the key point that many HVAC companies are missing. They are skilled when it comes to heating and cooling applications, but they have very little to no experience when it comes to medical-grade air filtration as this is a new field for many of them. It’s vitally important that they understand the products they’re working with, especially if they’re providing paid services and reports to their clients.

We believe there is misinformation being spread which could stem from our competitors and is understandable given the circumstance that most of them cannot supply product for months due to off-shore manufacturing delays, but we’re happy to properly inform where education may be needed. We hope that information is helpful.

Please reach out to us at info@catija.com if we can be of any assistance.

Hourly Air Change Performance (ACH) in a Dental Operatory – Download PDF

The above data is provided to align Austin’s ACH performance with the RCDSO guidelines: “COVID-19: Managing Infection Risks During In-Person Dental Care (May 25th, 2020)”

How to calculate ACH for individual operatories of any size:

Calculate cubic volume of operatory using: v=l*w*h
Deduct volume of cabinetry using c=l*w*h adding the volume of multiple cabinets together if necessary
v-c=a where a=air volume in operatory
a/TAR=x where x equals the number of air changes per minute using the TAR (True Air Flow Rate, 250CFM for 400-series and 125CFM for 200-series models)
ACH=60/x

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