Improving the energy efficiency of low-pressure blowers

Twin-screw blowers hold some significant advantages over rotary-lobe blowers, particularly in terms of energy efficiency.

To improve the efficiency of equipment that uses air blowers for low-volume flows (300 to 5,000 m³/hr), engineers need to look beyond lobe-style blowers. These blowers are widely used in applications such as wastewater aeration, pneumatic conveying, and mixing, and designs have evolved from two to three-lobe designs, mainly to reduce discharge pulsations. But in regards to energy efficiency, lobe blowers have not seen significant improvements over the past 50 years.

Screw compressors, on the other hand, rely on internal compression — as opposed to external compression in lobed blowers — and this dramatically improves efficiency and can produce sizable energy savings. They also run quieter, vibrate less, and are more reliable. Here’s a look at the theoretical and practical details.

Design basics
Positive-displacement, rotary-lobe blowers consist of a pair of two or three-lobed rotors that turn inside an oval-shaped casing. External power drives one rotor while synchronous gears drive the other.
As the rotors turn, air is drawn through the inlet side and forced out the outlet port against system pressure. There is no change in the volume of the air within the machine. The blower only displaces air from the suction end to the discharge end against discharge-system resistance.

Oil-free screw blowers, in contrast, are positive-displacement rotary devices with screw-shaped male and female rotors. These rotate towards each other while the volume of air trapped between them and the housing decreases. The rotors do not contact each other and are synchronized by timing gears.

Each screw blower has a fixed, built-in internal pressure ratio. This means that the outlet port is designed with a certain, fixed geometric profile. To obtain the best efficiency, the internal pressure ratio should match the required working pressure.

Theoretical performance
To better understand a screw blower’s advantages, let’s first look at a p-V (pressure-volume) diagram for a lobe-type blower. Air in the piping outside the blower exhaust port is at a higher pressure. As the rotor lobes uncover the exit port, air from the pipeline flows back into the flute space between the rotor and casing. This back flow of air equalizes pressure and compresses the entrapped air externally at constant volume (seen as line 1-2 in the accompanying diagram.) As the rotors continue to turn, air is pushed into the discharge line against full system pressure (represented by line 2-3.)

Screw blowers operate differently.  At the start of the compression cycle, gas at suction pressure fills the flute spaces formed by the unmeshed rotors just under the suction flange. Gas continues to fill the flute spaces until the trailing lobe crosses the inlet port. At that point, gas is trapped inside the flute space (and equals displaced volume Vs.)

As the lobes mesh into the flute space, flute volume decreases and pressure increases as long as the gas is trapped. Gas leaves the flute space when the leading lobe crosses the discharge port. This discharge volume =
Vs/vi, where vi is the ratio of initial to compressed volumes inside the screw blower. Further rotation and meshing of the rotors forces gas into the discharge line.

The bottom line is that internal compression reduces energy consumption, as represented by the green area in the p-V diagram.

For optimal compression efficiency, the volume ratio, vi, should be sized so that the internal compression ratio matches the system compression ratio: pi = pe. If the ratios do not match, gas will be over or undercompressed.

With overcompression, as the name implies, gas is compressed more than the system requires, which takes more work. Gas compressed internally to a higher pressure must then expand down to external working pressure.

In the case of undercompression, the internal discharge pressure is less than system discharge pressure, and gas from the discharge line flows back into the flute space and equalizes pressure at constant volume, resulting in extra work from ideal compression.