A 1-MW geothermal wellhead power plant incorporating a Lysholm or helical screw expander (HSE) was field tested between 1980 and 1983 by Mexico, Italy, and New Zealand with technical assistance from the United States. The objectives were to provide data on the reliability and performance of the HSE and to assess the costs and benefits of its use. The range of conditions under which the HSE was tested included loads up to 933 kW, mass flowrates of 14,600 to 395, 000 lbs/hr, inlet pressures of 64 to 220 psia, inlet qualities of 0 to 100%, exhaust pressures of 3.1 to 40 psia, total dissolved solids up to 310,000 ppM, and noncondensible gases up to 38% of the vapor mass flow. Typical machine efficiencies of 40 to 50% were calculated. For most operations efficiency increased approximately logarithmically with shaft power, while inlet quality and rotor speed had only small effects. The HSE was designed with oversized internal clearances in the expectation that adherent scale would form during operation. Improvements in machine efficiency of 3.5 to 4 percentage points were observed over some test periods with some scale deposition. A comparison with a 1-MW back-pressure turbine showed that the HSE can compete …