Eclipse Engineering Guide

CHAPTER 2 – FAN LAWS & BLOWER
Combustion air blowers are normally rated in terms of standard
cubic feet (scf) of air; that is, 70°F air at Sea Level
(29.92" Hg) barometric pressure. Density of this air is 0.075
lb/cu ft, and its specific gravity is 1.0.
Although fuel/air ratios are usually stated in cubic feet of air
per cubic foot or gallon of fuel, it’s the weight of air per
weight of fuel that’s important. As long as air temperature and
pressure are close to standard conditions, blower and burner
sizing charts can be used without correction. However, if air
temperature, gauge pressure or altitude change the density of
air by any significant amount, blower ratings have to be corrected
from actual cubic feet (acf) to standard cubic feet to
insure the proper weight flow of air reaches the burner.
Centrifugal fans are basically constant volume devices; at a
given rotational speed, they will deliver the same volume of
air regardless of its density.
APPLICATION ENGINEERING
18
RPM
"R"
Volume
"V"
For blower wheel with eight segments, Theoretical Flow = 8 x V x R
If, for example, a blower has a wheel made up of eight segments,
each with a volume V, and the wheel is rotating at R
rpm, the theoretical flow rating of the blower will be 8 x V x
R, because each fan wheel segment fills with air and empties
itself once each revolution.
The actual volume delivered is strictly a function of the carrying
capacity of the wheel and its speed. Cfm, whether it is
standard (scfm) or actual (acfm) is the same. Consequently, if
the density of air is reduced by temperature, pressure, or both,
the blower will deliver a lower weight flow of air, even
though the measured volume hasn’t changed.
Air density also affects the pressure developed by the blower
and its power consumption. Because air density is related to
temperature, pressure, and altitude (barometric pressure) – see
pages 20 and 21 – it is possible to relate blower performance to
these factors with a set of realtionships known as fan laws.