Comparing screw blowers and turbo blowers for wastewater treatment

Positive displacement blowers are no longer preferred for the aeration
process in wastewater treatment facilities due to their low efficiency and high
energy demands. More modern technologies including turbo blowers, rotary lobe
compressors, and positive displacement screw blowers are available today, all
of which promise energy efficiency and energy savings.

Hurll Nu-Way presents a comparative analysis of positive displacement
screw blowers and turbo blowers to help wastewater treatment facilities make
the right choice to suit their aeration requirements.

Blower type

Rotary screw blowers are positive displacement blowers with precision
timing gears that maintain minute clearances between two intermeshing oil-free
screw elements that are never in contact. However, contact parts such as gear and
bearings in the blower package are exposed to wear and tear resulting in the
gap between the screws getting larger over time, leading to air leaks. The
Teflon coating on the rotary screw air ends also deteriorates after a couple of
years of operation, leading to loss of air flow delivery.

In turbo blowers or centrifugal blowers, the energy is transferred from a rotating shaft to air
or gas. A pressure rise is achieved by adding kinetic energy to a continuous
flow of air through the rotor or impeller. The turbo blower differs from the turbo
compressor by the lower pressure ratio, with compressor being above 2.5. Centrifugal
compressors increase pressure up to several hundred kPa, while the centrifugal
blower performs in the middle range up to 200kpa.

Bearings

Rotary screw blowers use cylinder roller bearings that have to operate
in hot environments with high load, leading to wear and tear over time and requiring
replacement after 3-5 years of operation, which results in major repair costs.

A turbo blower’s air bearing design utilises the speed of the air to
avoid contact of the surfaces, and is therefore free from friction and wear. It
can only be used for high speed applications. Bump type air foil bearings,
developed by NASA and used in some of the turbo blower models have an advantage
over leaf type air bearings with their higher load capacity. Their durability
is measured not by the hours of operation but in the number of start-ups and
turn-offs. Tested for 20000 start/stops, air foil bearings offer the assurance
of many years of operation without any decrease in blower efficiency.

Motor

The rotary screw blowers are equipped with standard induction motors,
which lose about half their efficiency at lower operating speeds.

Turbo blowers use high efficiency high speed motors called Permanent
Magnet Synchronous Motors or PMSM, which utilise permanent magnets to provide higher
power output to a given frame size. Thanks to direct connection with the impeller,
no power is lost during transmission. PMSM motors comply with the super premium
efficiency standard, IE4, which was only published by International
Electrotechnical Commission in 2014. The PMSM motor is about 5-8% more
efficient than the induction motor.

Controller

Screw blowers feature proprietary microprocessor controls, which
restrict the customers in choice and flexibility.

Turbo blower manufacturers use only major brands of controllers such as Allen-Bradley
or Siemens for reliability and ease of servicing/replacement as well as for the
flexibility to modify running parameters.

Range

A screw blower has a limited operating range, with power up to 355 kW,
pressure up to 120 kPa and flow capacity up to 154m³/min.

Turbo blowers deliver a flow capacity up to 700m³/min, pressure up to
200 kPa and power up to 600 kW.

Total efficiency

A blower package comprises of a blower (compressor), motor, inverter (variable
frequency drive), blow-off valve, controller, inlet filter and cooling system,
assembled and operating inside the enclosure.

‘Wire to Air’ (Total) Power Consumption is the total energy used to
produce the required flow and pressure for any particular application. It
includes all power losses in the motor, variable frequency drive, inlet filter,
guide vane, valves, cooling system, loss by compressor suction, temperature
rise and pressure drop. The complete blower system must be tested for
efficiency, as it’s the ‘wire to air’ power consumption that is reflected in
the electricity bill.

High speed turbo blowers are far superior to rotary screw blowers in
terms of overall ‘wire to air’ efficiency and ‘total ownership cost’. This is
because centrifugal compression efficiency is higher than rotary screw
compression; turbo blowers do not lose capacity; package efficiency of turbo
blowers is superior to screw blowers due to PMSM motor, direct transmission
without any gear/bearing losses, and no loss of capacity due to coating wear;
and turbo blowers provide major savings in ongoing maintenance costs.

Conclusion

Based on technical characteristics, turbo blowers outperform rotary
screw blowers on every parameter; however, the final choice will depend on
multiple factors, including the size of the plant. Hurll Nu-Way advises
wastewater treatment facilities to consult with experts before making their
selection.

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