www.worldpumps.com
WORLD PUMPS October 2013
10 Product Focus
Screw blowers overcome traditional lobe limitaions
A continuous reliable regu-
lated supply of quality air is
a critical requirement for
the aeration diff user systems
within municipal and industrial
wastewater treatment plants in
order to guarantee continuity of
process operations and clean,
uncontaminated discharge into
the environment. Yet despite the
vital role that air supply represents
in the treatment process, the
majority of these plants rely on
basic lobe blower technology that
has experienced little develop-
ment since its introduction in the
late 19th century.
It was to improve this scenario
and provide waste water treat-
ment plant operators with a reli-
able low pressure, oil-free, positive
displacement blower system that
combines optimum performance
with protection of the process, the
environment and operational
energy costs, that Atlas Copco
developed its range of innovative
Atlas Copco developed its range of sustainable screw technology ZS blowers, which
combine optimum performance with protection of the process, the environment and
operational energy costs. The operating principles, proven components and feature
benefi ts of these screw blowers were specifi cally developed to overcome the limitations
of lobe type blowers.
and sustainable screw technology
ZS blowers. The operating princi-
ples, proven components and
feature benefi ts of these screw
blowers were specifi cally devel-
oped to overcome the limitations
of lobe type blowers.
Wire-to-air measurement
In making a true wire-to-air
performance comparison between
the old and new technologies it is
necessary take into account not
just comparable shaft power
ratings but all of the potential
power losses that can occur in a
blower package − from motor and
electrical system to transmission,
fi lters, and other design features
that contribute to pressure drops
which are costly in energy terms.
For instance, a 50 mbar/0.7 psi
pressure drop may appear to be
small but when a blower is only
providing a 500 mbar/7 psi
discharge pressure this adds up to
10% of the power consumption.
Lobe blower data is commonly
off ered by giving the air intake fl ow
volume and the shaft power of the
bare element whereas low pres-
sure screw compressors
are quoted by listing
the FAD (free air
delivered) at the
unit outlet
and the power consumption at the
terminals of the power supply.
The air fl ow path before and after
the blower element includes air-
inlet fi lter, air-inlet silencer, air-outlet
silencer and the check valve. The
pressure drop over these compo-
nents has to be added to the
performance data of the lobe
blower element. Such losses, as
well as those incurred by electrical
and mechanical transmission, are
not always considered in the data
of lobe blowers but they are always
taken into account in the low-pres-
sure screw compressor data.
The transmission losses, measured
from the terminals of the power
supply to the shaft power of the
blower element, consist of the
losses of the electric motor and
the belt drive transmission losses
from the motor shaft to the
element. These fi gures vary gener-
ally as a function of the blower
size and the operating point but
typical values, for a 110 kW small
lobe type blower with an output
of 1,000 m³/h operated at 0.7 bar
compared to a 75 kW screw
blower, are as follows:
Screw blowers also have some
degree of air fl ow path and trans-
mission losses, but they are
already taken into account when
listing the system data, measured
at the power supplies input and
the compressed air outlet
As an example, the ZS screw
blower features an integrated
gearbox that allows better trans-
mission of the power from the
motor to the element and gains
more than 3% saving compared to
a belt/pulley system. Moreover, the
screw blower’s inlet fi lter and
smooth discharge system are
specifi cally designed to minimise
pressure drops.
To deliver a fl ow of 1600 m³/hr
(942 cfm) at a pressure of 0.8 bar
(11.6 psig), a tri-lobe blower would
consume 61 kW (82 hp) on
average whereas a comparable
screw blower demands only 43kW
(58hp) for the same volume. In
fact, the SER (Specifi c Energy
Requirement) is on average 28%
lower and at a 600 millibar 50%
load this can increase to as much
as 34%.
It is clear that waste water treat-
ment plant operations and many
other industry low- pressure
blower applications can benefi t
substantially from improved reli-
able performance and signifi cant
energy savings through the
replacement of conventional lobe
equipment with innovative and
sustainable screw technology.
Atlas Copco’s latest ZS range of
oil-free, energy-saving positive
displacement screw blowers is
designed to meet today’s low
carbon economy needs. Its proven
screw blower technology is on
average 30 percent more energy
effi cient in comparison to the
majority of conventional lobe
low-pressure air solutions.
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