R723 (NH₃/DME) as an alternative to NH₃

The previously described experiences with the use of NH₃ in commercial refrigeration plants with direct evaporation caused further experiments on the basis of NH₃ by adding an oil soluble refrigerant component. Main goals were improved oil transport and heat transmission with conventional lubricants, along with a reduced discharge gas temperature for the extended application range with single stage compressors.

The result of this research project is a refrigerant blend of NH₃ (60%) and dimethyl ether "DME" (40%). It was developed by the Institute of Air Handling and Refrigeration (ILK) in Dresden, Germany, and has been applied in a series of actual systems. As a largely inorganic refrigerant it received the designation R723 due to it its average molecular weight of 23 kg/kmol in accordance to the standard refrigerant nomenclature.

DME was selected as an additional component for its good solubility and high individual stability. Its boiling point is -26°C, the adiabatic exponent is relatively low, it is not toxic and available in high purity. In the above-mentioned concentration NH₃ and DME form an azeotropic blend characterised by a slightly higher pressure level than pure NH₃. The boiling point lies at -36.5°C (NH₃ -33.4°C), 26 bar (abs.) of condensing pressure corresponds to 58.2°C (NH₃ 59.7°C).

The discharge gas temperature in air conditioning and medium temperature ranges decreases by about 10 to 25 K (Comparison of discharge gas temperatures) and allows for an extended application range to higher pressure ratios. Thermodynamic calculations conclude a single-digit percent rise in refrigerating capacity compared to NH₃. The coefficient of performance is similar and even more favourable at high pressure ratios, confirmed by experiments. Due to the lower temperature level during compression, an improved volumetric and isentropic efficiency can be expected, at least with reciprocating compressors in case of an increasing pressure ratio.

Due to the higher molecular weight of DME, mass flow and vapour density increase by nearly 50% compared to NH₃, although this is of little importance to commercial plants, especially in short circuits. In conventional industrial refrigeration plants, however, this is a substantial criterion with regard to pressure drops and refrigerant circulation. These considerations again show that the preferred application area of R723 is in commercial applications and especially in liquid chillers.

Material compatibility is comparable to that of NH₃. Although non-ferrous metals (e.g. CuNi alloys, bronze, hard solders) are potentially suitable, provided minimum water content in the system (< 1000 ppm), a system design similar to typical ammonia practice is recommended.

Mineral oils or (preferred) polyalpha olefin are suitable lubricants. As mentioned before, the proportion of DME leads to improved oil solubility and partial miscibility. Furthermore, the relatively low liquid density and an increased DME concentration in the oil enhances oil circulation. PAG oils would be fully or partly miscible with R723 for typical applications, but are not recommended because of the chemical stability and high solubility in the compressor crankcase (strong vapour development in the bearings).

Tests have shown that the heat transfer coefficient at evaporation and high heat flux is significantly higher in systems with R723 and mineral oil compared to NH₃ with mineral oil.

Further characteristics are toxicity and flammability. The DME content lowers the ignition point in air from 15 to 6%. However, the azeotrope is ranked in safety group B2, but may receive a different classification in case of a revised assessment.