The Large Hadron Collider (LHC), presently under construction at CERN, requires large refrigeration capacity at 1.8 K. Compression of gaseous helium at cryogenic temperatures is therefore inevitable. Together with subcontractors, Linde Kryotechnik has developed a prototype machine. This unit is based on a cryogenic axial-centrifugal compressor, running on ceramic ball bearings and driven by a variable-frequency electrical motor operating at ambient temperature. Integrated in a test facility for superconducting magnets, the machine has been commissioned without major problems and successfully gone through the acceptance test in autumn 1995. Subsequent steps were initiated to improve efficiency of this prototype. This paper describes operating experience gained so far and reports on measured performance prior to and after constructional modifications.

A new numerical procedure has been developed to solve the meridional equations of motion in an axial flow turbomachine. It is based on the so-called streamline-curvature method. The primary aim of this project was to reduce the computing-time of existing programs. The procedure has been tested. The new program is coupled with a program for the calculation of end-wall-boundary layers on axial flow compressors. This combination makes the simulation of real flow conditions possible. The pitch wise deviation angles and blade-row efficiencies are generally given as input. For compressor blades of the NACA-65-family they can be called from stored empirical data as function of geometry and the upstream and downstream flow conditions. The paper presents an exact description of the numerical procedure and a computed example.

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