Environmental Engineering Reference
In-Depth Information
Besides providing speed control, VSDs have “soft start” capabilities, which is a progressive
increase of speed during startup that avoids mechanical stress of the motor and equipment and
eliminates voltage fluctuations during the startup of large motors (WSU, 2003b).
Fans and pumps
Although many different types of fans and pumps are available, centrifugal pumps and fans
are the ones most frequently found in industrial applications.
One of the most common problems that reduces the efficiency of centrifugal fans and
pumps is mismatched size for the application. During design, engineers tend to oversize
pumps and fans to compensate for future losses of performance or potential future demands.
However, running oversized pumps and fans wastes energy, money, and increase emissions.
Also, oversized pumps and fans create flows larger than required, which forces the use of
valves and dampers to reduce flows to requirement.
As seen in the previous section, energy can be saved by reducing the revolutions per
minute (rpm) of the motor that powers the pump or fan using VSDs. Other solutions include
using multiple pumps, assigning small auxiliary pumps, replacing slower motors, and trim-
ming impellers (DOE, 2005a).
The affinity laws for pumps and fans relate power (
P
i
), rotational speed of the shaft (
N
i
),
and flow (
V
i
) according to the following equations:
3
PN
PN
⎛⎞
=
⎜
⎝⎠
[11.1]
1
1
2
2
VN
VN
[11.2]
1
=
1
2
2
According to these equations, a reduction of rotational speed of 10 percent produces a
reduction in flow of 10 percent but a reduction in power consumption of 27 percent!
Whenever possible, the replacement of old pumps and fans for more efficient newer
models is a good investment that reduces energy consumption and emissions. Currently, pumps
have efficiencies that range from 75 to 80 percent, which is a good improvement from 45 per-
cent averages in the 1970s (Pehanich, 2007). Centrifugal fans are less efficient than pumps
with typical efficiencies of 50 to 60 percent. When applications allow, centrifugal fans can
be replaced with vane-axial fans that have efficiencies up to 80 percent (Naughton, 1990).
Compressed air
Compressed air is a versatile energy carrier with multiple uses in food-processing plants.
Typically, compressed air is expensive because of the low efficiency in the compression
system, air leaks in distribution lines, and inappropriate use.
Air compression is an inefficient process. Only 15 percent of the energy consumed by a
compressor is converted into pneumatic compressed air energy and the rest, 85 percent, is lost
to waste heat (DOE, 2003). Moreover, in existing systems the wire-to-work efficiency is
around 10 percent, and when the distribution lines are included in the analysis, just 5 percent
of the energy spent by the motor reaches the point of use (Dalzell, 2000). Because of the
inherent low efficiency of compressed air systems, the best practice to reduce energy con-
sumption is to avoid them whenever possible. When alternatives to pneumatic systems exist,
then it is better to use the alternatives. For instance, if an electric tool or a hydraulic actuator
can do the same job compressed aircan, then they are better options in the long run. Obviously,
