Development of rectifying circuit with mixed refrigerants

Abstract

The use of mixed refrigerants for heat pump cycles has attracted the attention of the International Energy Agency as an important aspect of heat pump technology, and research and development activities in this area are being promoted all over the world. Mixed refrigerants have been applied to air conditioners and a unique technology has been developed for efficiently separating and varying the composition of refrigerants inside the refrigerating circuit. Computer programs for evaluating the thermodynamic properties of mixed refrigerants from those of the pure components have been developed and thermodynamic diagrams such as VLE diagrams, pressure-enthalpy diagrams, etc., were available for this work. The mixed fluorinated refrigerants R22 and R13B1 were proven experimentally to be promising for heat pump applications in cold districts. Using mixed refrigerants rather than pure R22 as the working fluids enhances the advantages of an inverter driven compressor. Techniques for estimating the composition of mized refrigerants operating in the rectifying circuit were developed and calculated values correlated well with experimental values. A small rectifier with coiled packings was also developed. The effects of packing shapes and column diameter were examined using a stand alone rectifier mock-up. The packed rectifier (about 15 mm in diameter and 210 mm in height, cooled at the top by 70 W) had a separation efficiency about seven times as much as an equilibrium separation of mixed components. In the case of R22 and R13B1, this corresponds to 70% versus 5% separation of R13B1. In the original air-to-air heat pump, the bottom of the rectifier, which was at an intermediate pressure, was connected with high pressure (condenser) and low pressure (evaporator) heat exchangers by capillary tubes in parallel with the expansion device. When the expansion device was open, the system operated with mixed refrigerants at the composition at which they were charged and resulted in a larger heating capacity. When the expansion device was shut off, mixed refrigerants flowed into the rectifier and rectification started by vapour formation in the interconnected capillary tubing without auxiliary heating devices. The vapourized lower boiling point refrigerant was cooled, liquified, and stored in a reservoir connected to the top of the rectifier. The heating capacity, input wattage, and fluid pressure of the air-to-air heat pump (operating with the separated refrigerants) decreased and its energy efficiency improved. Separation characteristics of the rectifier operating as a component in the air-to-air heat pump agreed well with results from preliminary tests of the rectifier only. The refrigerating circuit with the rectifier has been termed the ‘Auto Mixed Fron’ method. The newly developed rectifier for commercial models is so small that it is easily installed in the conventional outdoor unit. ‘Auto Mixed Fron’ air conditioner CS-223GR was initially marketed in Japan in November 1986.