side channel blowers

94 Pumps, Compressors and process Components 2011

Compressors

Side channel blowers

Introduction

The idea for a “ring vortex com-pressor” goes back to Prof. Eugen Oesterlen, who taught at the Tech-nische Hochschule in Hanover. In 1937 he pro posed an air compressor based on the already known side channel pump principle. The initial test com-pressors were destroyed during air raids in 1945. In 1955 his ideas about “ring vortex compressors” gained new attention. Since the high pressure cha-racteristics that these devices were found to have made it possible to achieve substantial differential pres-sures at low rpm speeds, the compres-sors were intended for use in vacu-um cleaners that were to be powered by 2-pole asynchronous motors. Un-til then, high-speed radial blowers driven by a.c./d.c. motors had been the standard. These motors, however, required carbon brushes, which wore out quickly.

Following years of development work, the first ring compressor went

Optimization of side channel blowers from an environmental standpointDr.-Ing. habil. R. Dittmar, Dr.-Ing. T. Grohmann, Dipl.-Ing. M. Kempf

into standard production in 1964. With an impeller diameter of 256 mm, it achieved a pressure difference of 5 kPa at a volumetric flow rate of 90 m3/h. However, its sound pres sure level of 79 dB(A) was still high, and its efficiency of less than 40 % was still low. Despite these modest figures compared with other pressure-gene-rating devices, the “ring” or side chan-nel compressors, with their unique hydraulic characteristics, took over in important markets, such as the prin-ting, pneumatic conveying, sewage aeration, and packaging industries.

Design and operation

When represented in a Cordier dia-gram, which compares various pres-sure-generating devices in dimension-less form /1/, side channel blowers assume a position between radial machines and piston machines; see Figure 1. Thus, side channel blowers are the type of pump/compressor that offers the highest single-stage pres-

sure levels ψ. They can achieve values up to 30. Figure 2 shows a cross-sec-tion through a side channel blower. The gas is drawn in through the suc-tion ports and silencers. After the gas enters the side channel, the exchange of momentum with the blades of the rotating impeller compresses the gas and transports it to the pressure port. It then passes through the absorption silencer and leaves the machine. To-day, side channel blowers are built in single- and multiple-stage designs for multi-stage pressure ratios up to 3 and for volumetric suction flow rates up to 3000 m3/h. Demands of plant opera-tors for environmental improvements and pressure from new laws and regu-lations are leading to ongoing innova-tions in blower technology. The main focus is on:– efficiency,– controllability and power/weight ratio,– noise and – modifications to implement envi-

ronmentally friendly technologies.

Fig. 1: Cordier diagram /1/ Fig. 2: Design and operation of a side channel blower

Diameter number δ

Spee

d n

um

ber

σ

Axial machines

Diagonal machines

Radial machines

Side channel maschines

Rotary lobe machines

Recip. piston machines

Pumps, Compressors and process Components 2011 95

Compressors


Several examples are described below.

Efficiency

As regards polytropic efficiency

(1)

where

(2)

In general, hydraulic efficiency is cons-trained by losses due to fluid friction, volumetric efficiency is constrained by flow through gaps, and mechani-cal efficiency is constrained by friction effects between the impeller and the casing and contact friction in bearings and seals. While fluid friction is of little consequence, especially at high flow rates ϕ and low pressures Ψ, effi-ciency at high pressures is dependent on gap losses. This suggests that there are opportunities for optimization for individual performance curve ranges.

Volumetric efficiency– Reducing the gap losses between

the impeller, cover and casing,– Geometric optimizations at the in-

terrupter, depending on the tip speed u, the pressure ratio p2

/p1, and

the number of blades,– Taking into account the optimal

blocking ratio for the design range (blade cell volume/total volume of blade ring);

Hydraulic efficiency– Reduction of pressure losses through

the use of more favorable port posi-tion and design,

– Modified side channel cross-sec-tions, variable in the circumferential direction,

– Reduction of losses due to impact at the base of the blade,

– Design of low-separation flow chan-nels in the blade cells;

The potential for improving ηmech is li-

mited and will not be discussed fur-ther here. Fig. 3: Environmentally relevant improvements in side channel blowers in the past 20 years

1997 2006 2010

Improvedgap sealing

Reduction oflaminar sep. in blade cell

Optimalport design/position

Modified path ofside channel flow

Interrupteroptimization

Dyn. gap sealing

Year

Figure 3 shows, among other things, the relative efficiency improvements due to individual optimization mea-sures over the past 20 years.

As a result, a 50 % increase in effi-ciency was achieved. In other words, today only 2/3 of the shaft output of 1990 is needed to achieve the same hydraulic performance. The efficien-cies of modern side channel blowers are approaching those of positive displacement machines and radial blowers.

By focusing design work on the performance characteristics of side channel blowers, engineers have been


Compressors


able to achieve favorable efficiencies at specific target operating points; see Figure 4.

At this operating point the re-quired shaft output can be minimized by dimensioning the blower to have carefully matched parameters, such as side channel cross-sectional area AK

, impeller diameter D, and blade area A

S. In the example shown in Figure 4,

the required power was reduced by 50 % at the given operating point.

The statutory demands on the ef-ficiency of electric drive motors are also increasing continuously. For ex-ample, Directive DIN EN 60034-30 must be implemented by mid-2011. It requires an increase in the efficiency of IEC standard motors of 2 to 4 % de-pending on the shaft output /2/.

Power/weight ratio, controllability

Frequency converter technology for electrical motors, which has been im-proving steadily since the early 1990s, is allowing the range of applications in which side channel blowers are used to be expanded considerably. The de-gree of variation can be reduced, sin-ce a single controlled blower can be used to achieve rpm ratios of up to 10. Furthermore, frequency conver-

ter technology also makes it possible to dynamically set any desired opera-ting points in the performance curve. For example, it is possible to continuous ly adapt pressures, tempe-ratures, and volumetric flows to the current process parameters /3, 4/ by adding electronic modules. In this way, losses caused by conventio nal thrott-ling devices, bypasses, or switching operations can be avoided, operating costs can be cut, and environmental impact can be reduced. Figure 5 shows

the performance map of a single-stage high-performance blower with local flow velocities in the transsonic range. It can be seen that the control range extends across suction volume-tric flows of up to 1100 m3/h and suc-tion-side differential pressures of up to 60 kPa. The higher efficiency of over 50 % in the center of the map is note-worthy. High tip speeds u permit size reductions while maintaining hydrau-lic performance; this manifests itself in a higher power/weight ratio:

Fig. 4: Performance curves for an operating point OP with a) non-modified blower geometry andb) geometry modified for optimal efficiency at the given OP

Fig. 5: Suction-side performance map of a single-stage high-performance side channel blower

(3)

As can be seen in Figure 3, the power-weight ratio of side channel blowers has increased during the past 20 years by a factor of nearly 10. In other words, only 10 % of the former design envelope is required to achieve the same hydraulic performance. The weight and the amount of material used in these devices have decreased significantly. This too represents a contribution to environmentally con-scious innovation.

Noise

Increasingly strict noise emission standards are forcing manufacturers and operators of industrial facilities to reduce the noise generated by machi-nery. In /5/ the permissible noise le-vels were reduced by 5 dB(A). The new action value (daily exposure value) is now 80 dB(A).


Compressors


In order to meet the new require-ments, primary machine design mea-sures make the most sense since they avoid the possibility of affecting other facility components. Figure 6 shows the dominant noise sources in side channel blowers.

It is characteristic of these units that the turbulent intermixing zones in the ports and in the side channel – along with the motor fan – produce broad-spectrum noise upon which the tonal noise components, which them-selves are highly objectionable in sub-jective terms, is superimposed. They are produced when the blades pass by the interrupter between the suction and pressure ports. As the blade ap-

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proaches the interrupter on the pres-sure side, pressure variations, which radiate outward in the form of noise, are produced. On the interrupter suc-tion side, the processes that occur when the pressure is relieved as the blade cells open are responsible for the noise that is emitted. Under certain operating conditions periodic turbu-lence separations occurring along the faces of the blades are a further source of tonal noise. The noise calculation is based on the Lighthill equation, which is fundamental in aeroacous tics; (4) found in /9/.

The left side characterizes the acous tic field; the right side with its three ad-ditive terms characterizes the acous-tic sources. The first term enclosed in the braces describes broad-spectrum turbulent flow noises; the second des-cribes fluid interactive forces at fixed boundary surfaces, and the third re-presents mass flows that vary over time. The turbulent flow areas in the side channel, the changes of direction, the silencers, and the motor fan may be assigned to the first term. Fluid in-teractive forces are generated at the pressure-side interrupter and by tur-bulent separations that occur on the faces of the blades; see second term in (4). The nonstationary, periodic pres-sure relaxation processes that occur on the suction side of the interrupter can be represented by the third term

Fig. 6: Noise sources of side channel blowers

turbulentflow

magnetic noises

motor fan noise Pressure release processes,Blade entry

Pressure side Suction side

periodic turbulent separations

(4)

.

After the source terms have been ex-pressed relative to location and time, their acoustic effects may be calcula-ted /6, 7, 8/. For a single-stage blower fitted with hoses on both sides, the A-weighted overall sound pressure level is

(5)


Compressors


where K1 stands for a design series-

specific constant and L´p stands for the motor fan sound level at rotatio-nal speed n´. For a side channel blower under high loads, in other words for high differential pressures Δp across the blower, the following applies to the narrow-band blade tonal sound level at frequency

acoustics it is a common practice to define acoustic efficiency as the ratio of acoustic to hydraulic output.

In water treatment and sewage treat-ment facilities using ozone or oxy-gen, the specifications that apply to– seal integrity,– the oxygen resistance of all materials used, and– noise emissionsare strict. Likewise, in order to transfer gas in fuel cells– high operating reliability and a long

life cycles,– low drive power,– low noise levels, and– high control accuracyare required.

Technical specifications for the blower and its accessories must also be ob served when blowers are used in sewage aeration systems and in flue gas analysis and filtration sys-tems. The large number of modifica-tions made on machines used for en-vironmental applications presents a major challenge for manufacturers in terms of design variation manage-ment, quality assurance, and cost control.

Summary

Side channel blower technology has advanced noticeably in recent years. Efficiency levels, which now ap-proach those of volumetric pressu-re-generating machines, high power/weight ratios, the ability to control devices over a broad range of rota-tional speeds, and low noise levels have improved the image of this type of blower. 20 years ago it was consi-dered by many to be a dying breed. The high single-stage pressure levels, which cannot be achieved by other compressors, pumps and similar ma-chines, permit high pressure ratios at low speeds. This offers design ad-vantages for high-load components and for shaft bearing systems. Appli-cation-specific modifications of the blower now permit it to be used in sensitive areas like medical and en-vironmental technology as well as in markets of the future like “residu-al energy utilization” and “renewable energies.”

Fig. 7: Narrow-band spectrum of a side channel blower

(6)

(7)

Here, the level increases substantial-ly with tip speed u; it decreases as the number of blades z increases. The last additive term characterizes the “rapi-dity” of the entry of the blade into the pressure-side interrupter – in other words, what angle ϕmax

is available until all of the edges of the blade s

max

are completely enclosed by the inter-rupter. Accordingly, in a side channel blower a large angle ϕ

max results in

small blade tonal sound levels. A narrow-band spectrum typical

of a side channel blower is shown in Figure 7.

For the initial machine one sees an in-tense tonal increase in the sound le-vel at 4.5 kHz. By optimizing the in-terrupter and blades, it was possible to reduce this level by 13 dB, a level that is subjectively experienced by hu-man beings as roughly “half as loud.” For specific noise comparisons in aero-

(8)

Figure 3, which shows acoustic effici-ency for the past 20 years, illustrates the progress that has been made in combating noise. The reduction of the acoustic output to 1/3 of the initial value corresponds, with the same hy-draulic performance, to a noise level reduction of about 5 dB. Currently, a research consortium consisting of manu facturers and universities is wor-king hard to reduce further the noise produced by compressors, pumps and similar machines and also, in particu-lar, to take the psychoacoustic aspects into account.

Specific blowers for environmental technology

In recent years, specific side channel blowers have been developed to meet the high demands of the environmen-tal industry.

Here are some examples:For conveying biogas, only compres-sors having– low leak rates,– non-corrosive surfaces exposed to gas, and – in some cases ATEX conformity are suitable.


Compressors


Subscripts

ak AcousticDS Pressure sideG Blowerhyd HydraulicK Side channelmech Mechanicalpol PolytropicR Resting variableS Bladevol VolumetricW Shaft1 Entry2 Exit

References

/1/ Grabow, G.: Das erweiterteCORDIER-Diagramm für Fluidenergie- maschinen und Verbrennungsmo-toren, Deutscher Verlag für Grund-stoffindustrie, Leipzig/Stuttgart, 1993

/2/ DIN EN 60034-30, Beuth-Verlag, 2009

/3/ Schreiber,C.: Investigation –Control, GDD-Report 92.08.09, 2009, unpublished

/4/ GDD, Druckerzeuger für strömen-de Medien, DE-Gebrauchsmuster[German utility model] 2002 20 578.9

/5/ EC Noise Directive 2003/10/EC of Feb. 15, 2003

/6/ Dittmar, R.: Side Channel Noise, Overview and Summary, GDD-Report 96.05.07, unpublished

/7/ Grohmann, T.: Lokalisierung und Klassifizierung tonaler Schallquellen in Seitenkanalgebläsen, Diss. Univ. Erlangen-Nuremberg, 2009

/8/ Dittmar, R.: Geräusch von Seiten-kanalverdichtern, GDD-Report 96.01.02, 2002, unpublished

/9/ Lighthill, M.J.: On sound genera-ted aerodynamically, Part I: General theory; Part II: Turbulence as a Source of Sound; Proc.Roy.Soc. London (A)211 and 222 (1952, p. 564–587); (1954, p. 1–31)

Dr.-Ing. habil. Rudi Dittmar,Manager Development Engineering Gardner Denver Deutschland GmbH, Bad NeustadtDr.-Ing. Thomas Grohmann,Development EngineerGardner Denver Deutschland GmbH, Bad NeustadtDipl.-Ing. (TU) Mario Kempf,Development EngineerGardner Denver Deutschland GmbH, Bad Neustadt

Symbols and abbreviations:

A m2 Area

a m/s Speed of sound

c´ m/s Fluctuation velocity

D m Impeller diameter

N/m3 Fluctuation force relative to volume

fST

Hz Blade frequency

K1, K

2dB Constants

Lp

dB Sound pressure level at 1 m distance

M Nm Torque

kg/s Mass flow

kg/(sm3) Mass flow relative to volume

n – Polytropic exponent

n, n´ min–1 rpm

P W Power

p´ N/m2 Sound pressure

p N/m2 Pressure

q kg/(ms4) Acoustic source term

R m2/(s2K) Gas constant

smax

m Length of wetted blade outside edges

T K Temperature

t s Time

u m/s Tip speed

V m3 Volume

m3/h Volumetric flow

z – Number of blades

δ – Diameter number

η – Efficiency

ρ kg/m3 Density

σ – Speed number

ϕ – Flow number

ϕmax

degrees Interrupter enclosure angle on the pressure side

ψ – Pressure number

ω 1/s Angular frequency

side channel blowers

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