Figure 6-47. Various configurations of centrifugal compressors.
of its low pressure ratio stages. The low tip stresses in this application make it a feasible design. Figure 6-48 shows several fabrication techniques. The most common type of construction is seen in A and B where the blades are fillet-welded to the hub and shroud. In B the welds are full penetration. The disadvantage in this type of construction is the obstruction of the aerody-namic passage. In C， the blades are partially machined with the covers andthen butt-welded down the middle. For backward lean-angled blades， this technique has not been verysuccessful， and there has been difficulty in achieving a smooth contour around the leading edge.
Dillustrates a slot-welding technique and is used where blade-passage height is too small (or the backward lean-angle too high) to permit conven-tional fillet welding. InE， an electron-beam technique is still in its infancy， and work needs to be done to perfect it. Its major disadvantage is thatelectron-beam welds should preferably be stressed in tensionbut， for theconfiguration ofE， they are in shear. The configurations of G through Juse rivets. Where the rivet heads protrude into the passage， aerodynamic performance is reduced.
Materials for fabricating these impellers are usually low-alloysteels， such as AISI 4140 or AISI 4340. AISI 4140 is satisfactory for most applications; AISI 4340 is used for larger impellers requiring higher strengths. For cor-rosivegases， AISI 410 stainless steel (about 12% chromium) is used. Monel K-500 is employed in halogen gas atmospheres and oxygen compressors because of its resistance to sparking. Titanium impellers have been applied to chlorine service. Aluminum-alloy impellers have been used in greatnumbers， especially at lower temperatures (below 300 0F). With new developments in aluminumalloys， this range is increasing. Aluminum and titanium are sometimes selected because of their low density. This lowdensity can cause a shift in the critical speed of therotor， which may be advantageous.
Bibliograph'
Anderson， R.J.， Ritter， W.K.， and Dildine， D.M.， ..An Investigation of the Effect ofBlade Curvature on Centrifugal Impeller Performance，'' NACA TN-1313， 1947.Balje，O.E.， ..Loss and Flow-Path Studies on CentrifugalCompressors， Parts IandII，'' ASME Paper Nos. 70-GT-1 2-A and 70-GT-12-B， June 1970.Balje，O.I.， ..A Study of Reynolds Number Effects inTurbomachinery，'' Journal of EnAineerinA for .ower， ASMETrans.， Vol.86， SeriesA，1964， p. 227.Bammert，K.， andRautenberg，M.， ..On the Energy Transfer in CentrifugalCompressors，'' ASME Paper No. 74-GT-121.
Boyce，M.P.， ..A Practical Three-Dimensional Flow Visualization Approach tothe Complex Flow Characteristics in a CentrifugalImpeller，'' ASME PaperNo.66-GT-83， June 1983.
Boyce， M.P.， and Bale， Y.S.， ..A New Method for the Calculation of Blade Load-ings in Radial-Flow Compressors，'' ASME Paper No. 71-GT-60， June 1971.Boyce，M.P.， ..New Developments in CompressorAerodynamics，'' Proceedingsof the 1st TurbomachinerySymposium， TexasA&M， October 1972.
Boyce，M.P.， andBale，Y.S.， ..Diffusion Loss in a Mixed-FlowCompressor，''Intersociety Energy conversion Engineeringconference， SanDiego， Paper No.729061， September 1972.
Boyce，M.P.， andDesai，A.R.， ..Clearance Loss in aCentrifugalImpeller，'' Proceedings Of the 8th Intersociety Energy Conversion Engineering Confer-ence， August1973， Paper No. 739126， p. 638.
Boyce，M.P.， andNishida，A.， ..Investigation of Flow in Centrifugal Impellerwith TandemInducer，'' JSMEPaper，Tokyo，Japan， May 1977.Boyce，M.P.， ..How to Achieve On-Line Availability of Centrifugal Compres-sors，''Che凡ical Weekly， June1978， pp. 115.127.Boyce，M.P.， ..Rerating of Centrifugal Compressors-PartI，'' Diesel and Gas Turbine Worldwide， October1988， pp. 46.50.
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