Using Rotary Lobe Compressors for Wastewater Treatment

AERZEN’s Delta Hybrid range of blowers uses unparalleled technical knowledge in the manufacture of screw-type compressors and positive displacement blowers to yield pioneering technology in the form of the world’s first rotary lobe compressor range.

Image Credit: Aerzen

The Delta Hybrid range combines compressor and blower technology in a blend of unique capabilities for creating both positive and negative pressures for air and other neutral gases, synchronizing the advantages of both in one rigorously designed system.

The Delta Hybrid is at the forefront of compressor development, as demonstrated by its seven patents already granted or pending. When it comes to lower pressure requirements, the Roots principle, based on isochoric compression, is the technology of choice and when the situation needs higher pressures, the screw-type compressor which creates internal compression comes to the fore.

Advantages

Based on AERZEN’s earlier Delta Screw and Delta Blower series, both hugely successful and justifiably famous, the Delta Hybrid range is a rational and novel extension of these devices.

Versatility in Numbers

• The Delta Hybrid provides suction volume flow at rates from 110 m³/hour to 9.000 m³/hour
• It can manage overpressures up to 1,500 mbar
• Its control range is between 25% and 100%
• It has nominal sizes from DN 100 to DN 300

Applications

The Delta Hybrid has found application in water treatment for potable use, wastewater treatment, and ventilation of rivers and lakes, among various other uses.

Benefits

The Delta Hybrid provides many benefits to the customer, such as:

• Excellent energy optimization
• Low lifespan costs
• Tremendous reliability and a very long service life
• Distinctly higher range of uses and applicable pressures
• Processed air is oil-free and absorbent-free
• Low maintenance needs

Most importantly, the Delta Hybrid comes with the AERZEN guarantee.

Technology

Two Profiles, One Packaged Unit

The Delta Hybrid incorporates the advantages of novel screw-type compressors and positive displacement blowers in one synthesized unit. In fact, the latest rotary lobe compressor includes two separate rotor profiles in a single machine. The first is a twisted 3+3 blower system, which works for lower differential pressures up to 800 mbar, and the second is a 3+4 compressor system, which can manage much higher pressures of up to 1,500 mbar.

Specially designed 3+4 compressor profile with interior compression for lower pressure applications. Image Credit: Aerzen

Compression Principle of Twisted 3+3 Rotary Piston Profile (Delta Hybrid L)

Just like the positive displacement blowers with straight profiles, the twisted profiles of the rotors function on principles of isochoric compression. However, the twist combines with the concept of twin stages to optimize the use of energy by leveraging the outcomes of this combination on the machine’s physical properties.

The fact that the medium flows in through a diagonal inlet port, and into the flow opening of the conveying chamber, means that the recoil is reduced, leading to a more efficient filling of the blower stage. Gas-dynamic shock is another effect that causes an initial compression of the medium within the conveying chamber. This results in operation at higher volume flow efficiency.

The result of this design is compressed air available at unmatched cost-efficiency owing to the diagonal movement of air through the stage, different from the flow in a positive displacement blower. The rotor movement itself creates the conveying chambers in combination with the housing walls. With various rotor profiles, internal compression takes place which increases the volumetric efficiency. This is the greatest variance between the Delta Hybrid design and the positive displacement blower.

3+3 Blower profile with twisted shafts and patented pulse charging as well as lower crushing losses. Image Credit: Aerzen

After the blower stage fills with gas more efficiently, the usable volume flow will go up. However, the conveying chamber retains its size, resulting in increased volumetric efficiency.

Gas-Dynamic Shock

Within a compressible medium which is moving freely through a space such as a pipeline, if a sudden closure of flow is imposed such as a sudden valve closure, an impulse will be observed that travels in the retrograde direction from the stop position to the inlet, at the speed of sound. As the impulse passes, it will cause the medium behind it to compress as well. The medium relaxes to the original state at the inlet, provided it is open.

However, if this is closed before the inlet approaches it, internal compression takes place, the degree varying with the extent to which the impulse moved within this area prior to the closure of the inlet. If the twisted rotary piston creates isochoric compression, the following formula will determine the effect of the coupling performance:

The coupling performance P_k is directly affected by the pressure difference Δp. This Δp can be determined by the equation

It is known that p₁ continues to be constant through the gas-dynamic shock, which results in a minor pressure difference and thus minimal coupling performance to maintain the constant volume flow.

When investigating a screw-type configuration, the effect of using internal compression with respect to power rudiments becomes obvious.

Image Credit: Aerzen

Compression Principle 3+4 Screw (Delta Hybrid S/H)

Screw compression of a medium causes less isochoric compression, with internal compression contributing to most of the compression. This saves energy due to the use of a pre-set pressure ratio within the stage. If the internal pressure ratio matches the downstream system pressure, wasteful isochoric compression can potentially be avoided. Similarly, in this case, the process air passes diagonally through the stage, with the conveying chamber consisting of the stage housing and the rotor blade.

Since the rotors mesh, creating a barrier to the backwash of air which directs the air in the right path. It is the screw rotation that causes the medium to be carried from the intake port, located on the upper part, to the discharge side which is on the opposite wall and at a lower point, also being of a specified size which causes pre-compression of the medium until it achieves the set pressure.

The benefit of using internal compression is that it carries the medium into the system at a pre-set intake pressure, which is already created before the medium reaches the system. Hence, it uses up the energy which would normally have been needed to counteract the system pressure when the medium is in the compressor stage, either in part or under ideal conditions, in full, saving the amount of power consumed.

Image Credit: Aerzen

In order to understand this better, a p-V diagram is shown wherein the plane may be assumed to denote the power requirement. In this case, isochoric compression when the system pressure is 1 bar (ü) in a positive displacement blower would require power that corresponds to the plane illustrated as the dotted line (------). In case the medium has already been precompressed to 0.8 bar (ü) within the stage, the consumption of power is less as denoted by the solid line indicating the curve. It is therefore carried into the system at 0.8 bar admission pressure (ü), meaning the only power additionally used is for an isochoric compression of 0.2 bar (ü), indicated as a violet rectangle (1). The distance between the planes shows the difference in the power consumed when using isochoric and internal compression to realize the same level of compression for the same volume of medium.

Image Credit: Aerzen

Conclusion

Energy is used by the positive displacement blower to create compression, which is responsible for approximately 90% of the costs incurred over its lifecycle. Therefore, this design is a critical factor in realizing a better ecological balance and becoming a more competitive business, which is dependent on the appropriate utilization of resources. Delta Hybrid units from Aerzen achieves energy efficiency with up to 15% savings on energy consumption, which represents a considerable return on investment. Considering the volume and compression rate for each application, this can indeed make it possible to recover the full investment in just two years. Therefore, the Delta Hybrid unit represents excellent power economy and outstanding competitiveness for global businesses.

This information has been sourced, reviewed and adapted from materials provided by Aerzener Maschinenfabrik GmbH.

For more information on this source, please visit Aerzener Maschinenfabrik GmbH.