Air Blower Sizing and Selection
The air blower size is specified in terms of its output capacity in cubic foot of air per
minute (cfm) and the back pressure against which it pumps. A good rule of thumb for initial
calculations is to plan on providing 1 to 2 cfm for each foot of sparger pipe.
Back pressure is sometimes expressed in inches H20. Because two things cannot occupy
the same space at the same time, to propel air to the bottom of a tank requires, in turn, lifting
the column of liquid that stands above it up and out of the way. Thus, pumping air down to
the bottom of a 48-in. deep water tank requires a static pressure of 48 in. H20 (equivalent to
27.7 oz/in2); but, because many of the liquids to be agitated weigh more than water, and
because there are dynamic pressure losses associated with the pumping, for a first-cut
approximation the back pressure can be estimated at an ounce per inch (48 oz/in 2 for a 48-in.
deep tank).
The use of a conventional air compressor to supply the air is discouraged because of the
high probability of introducing oil into the agitated tanks. Further, the cfm versus back-
pressure ratings of air compressors align poorly with the usual requirements for agitation air.
For large installations, with a resulting need for copious volumes of air, a multistage
turbine blower may be a good match. For most small to average size installations, the cfm
versus back pressure needed is usually in a range not particularly well suited to traditional
fans, turbine blowers, or compressors; in recent years, however, the development of the
regenerative blower has filled this niche quite well.
In installing a blower, the need for filtration of the air cannot be overemphasized; read
instructions carefully regarding proper filter types. Also double check the vendor's suggested
method for adjusting air flow rates (for some blowers it is imperative that the outlet not be
throttled but that, instead, excess air be dumped).