The demand for compressorswith high efficiency and at

the same time wide operatingranges has grown extraordinarlyin recent years. With radial com-pressors, which cover a substantialpart of the medium volume flowdemand, the efficiency and operat-

ing range depend not only on therotor but also very much on the dif-fuser and its guide vanes. Owing tothe complex (unsteady, friction-affected and three-dimensional)flow between rotor and diffuser, ithas hitherto been almost impossi-ble to determine an optimized dif-

fuser for a specific rotor withoutexperimentation. Through exten-sive measurements and calcula-tions, Sulzer Turbo has achievedthe key to more effective diffuserdesigning and hence the exploita-tion of this capability for enhanc-ing at the same time the efficiencyand the operating range of a radialcompressor.

THE DIFFUSER AS KEYCOMPONENT

The two key components determin-ing the behaviour of a radial com-pressor stage are the rotor and thediffuser (Fig. 1 ). The rotor servesto transmit energy from the rotat-ing shaft into the medium (fluid)being compressed. Rotor design hasreached a level from which no deci-sive improvements may be expect-ed. On the other hand, diffuser opti-mization offers big prospects. Dif-fuser design is subject to far fewermechanical constraints than rotordesign. It presents an interestingapproach for potential improve-ments to a radial compressor.

4 S U L Z E R T E C H N I C A L R E V I E W 3 / 9 8 3788

I N N O V A T I O N S

Small Steps,TOMAZ KMECL

PETER DALBERTSULZER TURBO

Gaseous media are compressed and delivered byradial compressors in various areas of technology.A new designing system evolved by Sulzer Turboenables the efficiency of radial compressors to beimproved and their optimal operating rangeswidened. Energy costs can be cut significantly inthis way.

1 The rotor and diffuser in a single-stage compressor.

Volute

Diffuser

Rotor

Gas

THE PURPOSE OF THE DIFFUSER

The diffuser has to convert asmuch of the kinetic energy as pos-sible into pressure energy. Withradial compressors, about half ofthe energy imparted to the fluid inthe rotor resides in the outflowvelocity. Various diffuser types areemployed, depending on the appli-cation and the design. A basic dis-tinction is made between vanelessand vaned diffusers (Fig. 2).

The vaneless diffuserThe vaneless diffuser is a simple,annular component surroundingthe rotor. In it, the radial compo-nent of the diffuser flow velocity isreduced by the increasing cross-section. Despite the simple designof the vaneless diffuser, its flowconditions are complex. Vanelessdiffusers are used on a large scalein radial compressor stages, be-cause they assure a wide operatingrange. Moreover, they are relative-ly cheap to design and fabricate. Solong as the demand for the widestpossible operating range has firstpriority, the choice falls to vanelessdiffusers.

The vaned diffuserVaned diffusers are used whereimportance is attached to the high-est possible efficiency. By means ofdeflecting vanes in the diffuserguiding the flow in the radial direc-tion, the velocity of the fluid can be

reduced more than with a vanelessdiffuser. In addition, the flow paththrough the forced deflection isshortened (Fig. 3 ). Higher effi-ciencies and more compact geome-tries are obtained. Nevertheless,vaned diffusers also have disad-vantages compared with vanelessones: If the flow rate for a givenvaned diffuser is much above thedesign flow rate, the profile lossesincrease considerably due to thehigh incidence angle. The efficien-cy then falls so steeply that opera-tion is no longer purposeful. Dur-ing recent years, the need for com-pressors with high efficiency com-bined with a wide operating rangehas increased exceptionally,because they enable energy coststo be cut. Accordingly, the advan-tages of both kinds of diffusersought to be united in a new diffusertype.

THE ANSWER: “LOW-SOLIDITY”

Numerous investigations haverevealed that high efficiencies andwide operating ranges can beattained with diffusers having fewvanes of short length. These arecalled “low solidity diffusers”(LSD). The term “solidity” (σ)denotes the closeness of the vanecascade and is the ratio of chordlength (s) to pitch (t) (Fig. 4).Low solidity diffusers have noactual channel in the diffuser. Onthe one hand, they tend to widen

the stable operating range at lowflow rates, while, on the otherhand, they widen it compared withconventionally vaned diffusers athigh flow rates in addition(Fig. 4).The decisive factor in optimizingan LSD is the magnitude of thesolidity σ. In general, σ should beso big that the flow is still deflect-ed sufficiently in the radial direc-tion at low flow rates, though atthe same time σ must be so smallthat the smallest possible profilelosses occur at high flow rates. Inview of this, determining of opti-mum σ is very important.

S U L Z E R T E C H N I C A L R E V I E W 3 / 9 8 5

Big Results

3 The different flowpaths (red) through thevaneless (a) and thevaned (b) diffuser.

2 The differencebetween vaneless andvaned diffusers.

ba

4 Arrangement of the vanes in a conven-tional and a low solidity diffuser.

determined for any rotor. With thisnew design system for radial tur-bocompressors, a high diffuser(and therefore stage) efficiency aswell as a very wide operating rangecan be achieved. Figure 5 showsthe stage efficiency of three stagestested with different diffusers andplotted against a dimensionlessflow rate number φ. The blackcurve represents a stage withvaneless diffuser, the red curve onewith a conventionally vaned dif-fuser, and the blue curve one withan optimized LSD. It is clear thatthe efficiency of the stage withvaneless diffuser is far below thatof the two vaned diffusers. Thepeak efficiency of the stage withconventionally vaned diffuser isvery high locally, but it drops verysteeply at higher flow rates owingto disproportionally rising profilelosses and also high acceleration inthe narrowest cross-section h1

(Fig. 4 ) and the pressure dropassociated with this. For that rea-son, the operating range of thisstage is very narrow. With the opti-mized LSD, both high peak effi-ciency and wide operating rangeare secured. The efficiency gain with the LSD isabove all evident at high flowrates. Here, the stage efficiency atφ = 0.0625 is almost 15% higherthan with the conventionallyvaned diffuser.

6 S U L Z E R T E C H N I C A L R E V I E W 3 / 9 8

I N N O V A T I O N S

BIG DEVELOPMENT OUTLAY

The road from an idea to its suc-cessful translation into practiceis often long and costly. To con-firm the theoretical considera-tions, extensive measuring seriesare needed with different rotorsand diffusers. By drawing uponother core capabilities of theSulzer Corporation – in this case

the Sulzer Innotec specialists for measurements on turbomachinery –it was possible to carry out the developments within a reasonable timeand with an optimal costs/use ratio. On the Sulzer Innotec test stand(picture below), measuring series were run and evaluated with combi-

nations of 4 differentrotors and 25 diffusers(picture above).

All rotors anddiffusers were

calculated andproduced spe-

cially for thedevelopment.

The test stand forperforming themeasuring seriesof Sulzer Innotec.

OPTIMIZING THE DIFFUSERThe optimal value of σ depends onthe approach flow conditions andon the diffuser geometry. Thismeans the optimal σ depends onthe rotor and diffuser. Sulzer Tur-bo has developed a design systemthat enables an exactly matchingdiffuser with appropriate σ to be

Solidity σ = chord length of diffuser vane/pitch = s/t

Diffuser channel

sh1

h1

t

LSD

MODEST YET EFFECTIVE STEPSFor a medium-sized air compressorintended for air decompositionplants, where high reliability, min-imal space requirement and lowenergy consumption are of primeimportance, the use of a low solidi-ty diffuser offers considerable sav-ings. This potential may be illus-trated taking a four-stage 12.5-MWradial compressor as an example(Fig. 6).The big stage efficiency gainsecured with an LSD figures lessobviously in the overall efficiencyof the compressor. Nevertheless, itis clear that the contours of con-stant efficiency have become big-ger with the LSD (Fig. 6 , redcurves). At certain operatingpoints, the compressor efficiency isnearly 1% higher than with a con-ventionally vaned diffuser (bluecurves). With a compressor powerinput of 12.5 MW (at design point),

F O R M O R E D E T A I L SSulzer Turbo AGTomaz Kmecl, T733PostfachCH-8023 ZürichSwitzerlandTelephone +41 (0)1-278 31 19Fax +41 (0)1-278 32 05E-mail tomaz.kmecl@sulzer.ch

6 Efficiencies of two four-stagecompressor units with different

diffusers in the first stage.

S U L Z E R T E C H N I C A L R E V I E W 3 / 9 8 7

5 Efficiencies of three different kinds of diffuser.

1% more efficiency means 125 kWless drive input for the same com-pressor performance. That everyunnecessary kilowatt of drivingpower for such a compressor is val-ued between 3000 and 5000 Swissfrancs shows the significance ofthis enhanced efficiency. Accord-ingly, a diffuser optimization rais-es the competitiveness of a com-pressor substantially. Ω

Operating range withvaneless diffuserOperating range with vaned diffuser

∆η/η vaned (max.)

with vaned diffuser

with vaneless diffuser

with low solidity diffuser

η/η

vane

d (m

ax.)

Standardized stage efficiency

LSD

Flow rate number (φ)0.03 0.04 0.05 0.06 0.07 0.08

1.10

1.00

0.80

0.90

0.70

0.60

Out

flow

pre

ssur

e (b

ar)

V (m3/s)30 35

0.98 0.98 0.99 0.99 0.97 0.97

40 5045 55 60

8

6

4

2Design point

ηp/ηp (max.)