A direct evaporator for a high temperature organic Rankine cycle (ORC) plant with toluene as a working fluid is designed and tested. The exhaust gas from a 800 kWe combined heat and power plant is cooled on the shell side of the present heat exchanger, while the working fluid is heated and evaporated within eight helically coiled tubes, constituting a tube bundle. A method to obtain optimal design parameters for this type of heat exchanger is presented, considering the heat source, the ORC and the available space at the test site. After manufacturing, the apparatus is tested to validate the design procedure, focusing on the employed heat transfer and pressure loss correlations on the shell side. It is shown that the predicted values of the overall heat transfer coefficient and the shell side Nusselt number are in good agreement with experimental data, showing a maximum deviation of 5.5%. The measured shell side pressure loss is slightly higher than the predicted value, indicating that the correlation underestimates the pressure loss coefficient by up to 7% at low Reynolds numbers, but has a good accuracy at higher Reynolds numbers. It is observed that it is essential to adjust the mass flow rate of the working fluid in each coil to obtain a homogenous vapor quality. A reliable operation of the direct evaporator with a maximum heat flow of 225 kW is shown.