Improving Fan System Performance
12
drivetrain without allowing slip. These sudden
load changes can be problematic for both motors
and fans. Another problem with synchronous belts
is the limited availability of pulley sizes. Because
the pulleys have a mesh pattern, machining them
alters the pitch diameter, which interferes with
engagement. Consequently, pulleys are available
in discrete sizes, which precludes an important
advantage of belt drives: the ability to alter operating
rotational speeds by adjusting sheave diameters.
Because of these factors, synchronous belts are not
as widely used as V-belts in fan applications.
In contrast, V-belts are widely used because of
their efficiency, flexibility, and robust operation.
V-belts have a long history in industrial applications,
which means there is a lot of industry knowledge
about them. An important advantage to V-belts is
their protection of the drivetrain during sudden
load changes. Service conditions that experience
sudden drivetrain accelerations cause accelerated
wear or sudden failure. While synchronous belts
tend to transfer these shock loads directly to the
shafts and motors, V-belts can slip, affording some
protection. Although they are less efficient than
synchronous belts, V-belts offer many advantages
such as low cost, reliable operation, and operating
flexibility. In applications that use standard belts,
upgrades to V-belts should be considered.
Although they are not commonly used, gear systems
offer some advantages to belt systems. Gear systems
tend to be much more expensive than belt drive
alternatives; however, gears tend to require less
frequent inspection and maintenance than belts
and are preferable in applications with severely
limited access. Gears also offer several motor/fan
configurations, including in-line drives, parallel-
offset drives, and 90-degree drives, each of which
may provide an attractive advantage in some
applications. Gear-system efficiency depends largely
on speed ratio. In general, gear efficiencies range
from 70 to 98 percent. In large horsepower (hp)
applications (greater than 100 hp), gear systems
tend to be designed for greater efficiency because
of the costs, heat, and noise problems that result
from efficiency losses. Because gears require lubri-
cation, gearbox lubricant must be periodically
inspected and changed. Also, because gears—like
synchronous belts—do not allow slip, shock loads
are transferred directly across the drivetrain.
Ductwork or Piping. For most fan systems, air is
directed through ducts or pipes. In general, ducts
are made of sheet metal and used in low-pressure
systems, while pipes are sturdier and used in
higher-pressure applications. Because ducts are
used for most air-moving applications, “duct” will
be the common reference for this sourcebook; how-
ever, most of the same principles can be applied to
pipes.
In ventilation applications in which a fan pulls
directly from a ventilated space on one side and
discharges directly to an external space (like a
wall-mounted propeller fan), duct losses are not a
significant factor. However, in most applications,
ducts are used on one or both sides of a fan and
have a critical impact on fan performance. Friction
between the airstream and the duct surface is usu-
ally a significant portion of the overall load on a fan.
As a rule, larger ducts create lower airflow
resistance than smaller ducts. Although larger ducts
have higher initial costs in terms of material and
installation, the reduced cost of energy because of
lower friction offsets some of these costs and should
be included during the initial design process and
during system modification efforts. For more
information, refer to the fact sheet titled Fan
System Economics on page 55. Other considera-
tions with ducts are their shape and leakage class.
Round ducts have less surface area per unit cross
sectional area than rectangular ducts and, as a
result, have less leakage. In hot or cool airstreams,
this surface area also influences the amount of
heat transferred to the environment.
Duct leakage class, typically identified by the
factor C
L
(which has units of cfm/linear foot) is an
indicator of duct integrity. Variables that determine
C
L
include the type of joints used in construction,
the number of joints per unit length of duct, and
the shape of the duct. Depending on the length
of the duct system, leakage can account for a
significant portion of a fan’s capacity. This is
especially applicable to systems with rectangular
ducts that have unsealed joints. In many cases, the
system designer can improve the performance of
the ventilation system by specifying ducts that
have low C
L
s. For more information see the fact
sheet titled System Leaks on page 37.
Introduction to Fan Systems