Engineering Notes
Zephyr Fans
PENNBARRY
19
Effect Of Changes In Speed
Increases or decreases in fan speed will alter the volume
flow rate through a system. Figure 2 illustrates the increase
in flow rate when the fan speed increases 10% to Point 2.
The 10% increase in flow rate, however, extracts a severe
power penalty. According to the fan laws (see below), the
power increase is 33%. This fact is often startling to the sys-
tem designer who finds a flow deficiency. Only 10% more air
is needed but the connected motor horsepower is not capa-
ble of a 33% increase in load. (Note that the increased power
requirements are the result of increased work done).
The greater air volume flow rate moved by the fan against the
resulting higher system resistance to the flow is a measure of
the increased work done. In the same system, the power
increases as the cube of the speed ratio; the fan efficiency
remains the same at all points on the same system curve.
Effect Of Change In Speed
(Fan Size and Gas Density Remaining Constant)
For the same size fan, D
c
= D and, therefore, (D
c
/ D) = 1.
When the density does not vary, p
c
= p and the density
(p
c
/ p) = 1.
Q
c
= Q (RPM
c
/ RPM) P
vc
= P
v
(RPM
c
/ RPM)
2
P
tc
= Pt (RPM
c
/ RPM)
2
HP
c
= HP
c
(RPM
c
/ RPM)
3
P
sc
= P
s
(RPM
c
/ RPM)
2
D = diameter c = change
HP = horsepower t = total
Q = CFM s = static
P = static pressure v = velocity
p = density
NOTE: Direct drive fans can change speed only through the use of a speed
controller and cannot increase speed beyond catalog maximum. Fans must
operate on single phase in order to change speed.
Effect Of Density On System Resistance
The resistance of a duct system is dependent upon the den-
sity of the gas flowing through the system. A gas density of
0.075 lb/ft
3
is standard in the fan industry. Figure 3 illustrates
the effect on the fan performance of a density variation from
the standard value.
The pressure and horsepower vary directly as the ratio of the
gas density at the fan inlet to standard density. This density
ratio must always be considered when selecting fans from
manufacturers’ catalogs or curves.
Effect Of Change on Density
(Fan Size and Speed Remaining Constant)
When the speed of the fan does not change, RPM
c
= RPM
and, therefore (RPM
c
/ RPM) = 1. The fan size is also fixed,
D
c
= D and therefore (D
c
/ D) = 1.
Q
c
= Q
P
tc
= Pt (p
c
/ p)
HP = HP (p
c
/ p)
P
sc
= P
s
(p
c
/ p)
P
vc
= P
v
(p
c
/ p)
D = diameter
c
= change
HP = horsepower
t
= total
Q = CFM
s
= static
P = static pressure
v
= velocity
p = density
See Table 1, which provides air density adjustment factors,
so fans can be selected to account for non-standard density.
NOTE: PennBarry’s Fansizer software can make density corrections for you.
Table 1. Unity Basis = Standard Air Density of .075 lb/ft
3
At sea level (29.92 in. Hg barometric pressure) this is equivalent to dry air at 70° F.
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