What is "free air" cfm?
Posted by Peter Haugen on Fri, Apr 29, 2011 @ 12:45 PM
“Free Air” cfm is a measure of airflow. CFM means cubic feet per minute. Many manufacturers of electric portable vacuum collection systems refer to “free air” cfm in their marketing materials when they are talking about the performance (or suction) of their vacuum collection systems.
A “free air” cfm number is not the actual cfm of a vacuum collection system on the jobsite. Let’s examine why this is true.
Let’s start by looking at the fans themselves. These backwards inclined airfoil fans are typically referred to by their “free air” cfm rating number which is typically the highest cfm number on the fans fan curve, In our industry we typically use the following three fan sizes:
- The 11” diameter fan is called a 2,500 “free air” cfm fan
- The 12.3” diameter fan is called a 3,500 “free air” cfm fan
- The 14” diameter fan is called a 5,000 “free air” cfm fan
The fan manufacturers test these fans in a lab setting by spinning them at 3,500 rpm’s and then plotting the performance in the form of a fan curve. This fan curve shows the “free air” cfm and static pressure (the two elements of fan performance). As cfm increases static pressure decreases and as the cfm decrease the static pressure increases (see chart below).
The equipment manufactures then take a fan and mounts it on the motor shaft; puts that assembly in a cabinet and then restrict the air flow by adding filters (pre-filters & HEPA’s). This added resistance lowers your motor rpm from 3,500 rpm (in a lab setting) to a range of 2,500 –3,000 rpm (in an actual unit setting). At this slower rpm you will never achieve the fan manufactures “free air” cfm numbers in the field. The table below.
12.3" Diameter Fan Performance Table
Performance Performance at 3,500 motor rpm at 3,000 motor rpm
3,500 cfm @ 0" s.p. 3,100 cfm @ 0" s.p.
3,300 cfm @ 1" s.p. 2,700 cfm @ 1" s.p.
3,150 cfm @ 2" s.p. 2,500 cfm @ 2" s.p.
2,800 cfm @ 3" s.p. 2,200 cfm @ 3" s.p.
2,600 cfm @ 4" s.p. 1,800 cfm @ 4" s.p.
2,250 cfm @ 5" s.p. 1,400 cfm @ 5" s.p.
(this data derived from fan curve chart created by Continental Fan company)
Even though “free air” cfm is not the actual “in the unit/on the job” cfm, is it a good idea to compare vacuum collection systems by comparing the “free air” cfm numbers? Yes and no. Yes, if you compare “free air” cfm along with other features like: portability (weight & size), first cost, operating cost (filter replacement), electrical requirements, warranties, etc. No, if you compare units using just “free air cfm numbers.
If you want a full discussion of this topic I recommend our "How to select an electric portable vacuum collection system guide." It’s an in depth discussion of what features to compare and why. Plus, it includes a list/chart of most of the portable vacuum collection units on the market.