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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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