Design and test results of a refrigeration compressed air dryer using a natural refrigerant (R744).

A compressed air dryer is a common application for a vapor compression cycle. Typical working conditions for this circuit are evaporating temperature between 0°C and 5°C and condensing temperature variable between 35°C and 65°C. Generally, a HFC refrigerant fluid is used in this kind of application, in particular R134a , R407C and R404a. In any case all these refrigerant HFC fluids have a high GWP value. As consequence it is interesting to look for alternative refrigerant fluids with lower GWP value but with equal or comparable energy efficiency. Compressed air dryer with cooling capacity above 50kW are usually designed using flooded evaporator with special control to guarantee stable air dew point temperature. In this study results from design and test on a compressed air dryer unit using R744, in transcritical condition as refrigerant fluids, are presented. A special dryer unit has been designed using R744, built and tested. All the refrigerant and air circuit parameters working at nominal condition and at no air load condition have been recorded. Tests have been planned also at extreme condition in terms of high and low water temperature at the gas cooler. The results demonstrate how, from an air side point of you, performances are changing. The considered parameters are the performance at the evaporator heat exchanger and the dew point temperature reached by the air. In addition also the economic impact of this design has been considered. The conclusion is that, for this application the performances of the R744 system is comparable with a standard HFC dryer, while a big difference is obtained in terms of energy consumption. Finally, a short overview on the economic impact of the new design is presented and possible future improvement on the efficiency in particular related to the control.