In geothermal power generation, ammonia-water (NH3-H2O) mixtures are considered as working fluids in order to improve efficiency from low enthalpy heat sources. During the evaporation process, the NH3-H2O working pair as zeotropic working fluid has the advantage to evaporate not isothermal temperature but over a temperature glide to adapt better to the heat source temperature profile. By today, there is a lack of performance data from operational generation units available in literature that can be used for scientific works based on computational calculations to compare with. In this paper the data logs from the evaporation process of the geothermal power plant are analyzed in order to review the evaporator's performance. Data had been gathered for a period of 5 years and three key performance parameters were defined: pressure drop, vapor fraction and heat transmission capacity. The main parameters that influence the performance are the temperatures, pressures and flow rates. It turned out, that the NH3 mass fraction plays only a minor role within the evaporation process. (C) 2017 Elsevier Ltd. All rights reserved.