The power production from the conversion of heat at low and medium temperature is a viable way to recover energy from several industrial and utility processes. In this context, small-medium expanders for Organic Rankine Cycles and two-phase expanders for energy recovery in heat pumps are gathering an increasing attention. The design and setup of flexible test benches to carry out experimental investigations on these devices has a primary importance to their performance optimization. In this study, a test rig based on a hot gas bypass cycle to test expanders for superheated and two-phase organic fluids is proposed. This configuration leads to numerous advantages like compact design, high flexibility and a fast transition to stable steady conditions. After an experimental campaign to assess the correct response of the test bench in design condition, a numerical model was developed to study the behavior of the cycle. The management of the test rig, both with and without the expanders, for performing the tests at different operating conditions was analyzed, with particular focus on the required setting of valves and compressor. The analysis allowed the determination of the operating limits of the rig, and showed the impact of using different fluids in the same test rig at several operating conditions. The working range of the test bench is related to the condenser power (designed for 25 kW and 30 bar maximum pressure) and the ambient temperature. With R404A, in the 17-30 bars condenser pressure range, superheated and saturated vapor expanders may be tested at different values of cycle minimum pressure (between 3.7-7.2 bar and 3.6-6.4 bar respectively). With R410A the working range is narrower, with the possibility of testing 2 phase expanders between 6.4 and 8.7 bars of cycle minimum pressure in the 21-23 bars interval of condenser pressure and superheated vapor expanders between 5.7 and 7.2 bars of cycle minimum pressure in the 23-30 bars interval of condenser pressure.