Experimental investigation of a R744 two-stage compressor cycle for transport refrigeration.

State of the art in transport refrigeration is the single-stage vapour compression cycle with the synthetic re-frigerants R404A and R410A, which have both a high Global Warming Potential (GWP). Current research activities focus amongst others on new energy efficient vapour compression cycles and the application of low GWP-refrigerants. Two-stage compressors enable a broader variety of inter-stage configurations. A two-stage vapour compression cycle with an internal heat exchanger at medium pressure is a widely used inter-stage system at low and medium evaporating temperatures in commercial refrigeration. This cycle leads to a considerable increase of the system’s energy efficiency compared to a single-stage unit, especially for R744 in combination with low temperature applications. Due to its thermodynamic properties, the natural refriger-ant R744 with GWP=1 might be an eco-friendly and competitive alternative to the commonly used synthetic high-GWP-refrigerants R404A and R410A. This paper shows the experimental results of an R744 two-stage compression refrigeration test unit with an internal heat exchanger at medium pressure for transport refrigeration. The test unit provides simultaneous cooling and freezing for multi-compartment vehicles at two different pressure levels: at medium pressure for chilled food and at low pressure for frozen food. The design of the system is based on a commercial transport refrigeration system whereas the refrigeration components are specially built for R744. First measurements at an ambient temperature of 30°C are presented and discussed.