A comparison between superheat control and semi-flooded control in a CO2 refrigeration cycle.

To limit the impact on the environment all products related to energy must be designed for less energy use. The evaporator is a major component in a refrigeration/heat-pump system. An efficient evaporator plays a key role in saving energy and for a safe system operation. In this paper an experimental study is performed with a refrigeration cycle having two modes of evaporator control: superheat and semi-flooded. The superheat mode is controlled by means of controlling the mass flow rate with the temperature at the outlet of the evaporator (Direct Expansion method) where the superheat levels are typically higher than 5 K. The semi-flooded evaporator control mode allows to reduce the superheat level down to 0.5 – 1 K, by means of a vapor quality sensor that is used for regulating the mass flow rate. The sensor is of capacitive type and produced by HB Products in Hasselager, Denmark. CO2 is considered as a promising refrigerant due to its rapid reaction time to small changes in pressure and temperature, low Global Warming Potential (GWP) and Ozone Depletion Potential (ODP). Measurements are performed on a standard transcritical CO2 refrigeration unit for the two different control modes. A comparison between the two systems is made. The maximum capacity of the evaporator is 3.2 kW. The stability of the two systems is investigated, which is done by looking at the difference in superheat over time.