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Halogen free (natural) refrigerants R290 (Propane) as DME was selected as an additional compo- nent for its good solubility and high individ- ual stability. Its boiling point is -26°C, the adiabatic exponent is relatively low, it is not toxic and available in high purity. In the abovementioned concentration NH3 and DME form an azeotropic blend character- ised 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 condi- tioning and medium temperature ranges decreases by about 10 to 25 K (Fig. 24) and allows 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 simi- lar and is 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 pres- sure 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 practise is recommended. Mineral oils or (preferred) polyalpha olefin are suitable lubricants. As mentioned be- fore, the proportion of DME leads to im- proved oil solubility and a 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 typi- cal applications, but are not recommended because of the chemical stability and high solubility in the compressor crankcase (strong vapour development in the bear- ings). Tests have shown that the heat transfer coefficient at evaporation and high heat flux is improved in systems with R723 and min- eral oil compared to NH3 with mineral oil. Further characteristics are toxicity and flam- mability. The DME content lowers the igni- tion 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. Resulting layout criteria Experiences with the NH3 compact systems described above can be used in plant tech- nology. However, the component layout has to be adjusted considering the higher mass flow. By appropriate selection of the evapo- rator and the expansion valve, a very stable superheat control must be ensured. Due to the improved oil solubility, "wet operation” can have considerable negative results compared to NH3 systems with non-soluble oil. With regard to safety regulations, the same criteria apply to installation and operation as for NH3 plants. Suitable compressors are special NH3 ver- sions which possibly have to be adapted to the mass flow and the continuous oil circu- lation. An oil separator is usually not neces- sary with reciprocating compressors. BITZER NH3 reciprocating compressors are suitable for R723 in principle. An individual selection of specifically adapt- ed compressors is possible on demand. alternative refrigerant R290 (propane) can also be used as a sub- stitute refrigerant. Being an organic com- pound (hydrocarbon), it has no ozone depletion potential and a negligible direct global warming effect. To take into consid- eration however, is a certain contribution to summer smog. Pressure levels and refrigerating capacity are similar to R22, and its temperature behaviour is as favourable as with R134a. There are no particular problems with mate- rial. In contrast to NH3, copper materials are also suitable, so that semi-hermetic and hermetic compressors are possible. Com- mon mineral oils of HCFC systems can be used here as a lubricant over a wide appli- cation range. Polyol esters (POE) and poly- alpha-olefins (PAO) offer even more favor- able properties. Refrigeration plants with R290 have been in operation world-wide for many years, mainly in the industrial area – it is a "proven" refriger- ant. Meanwhile R290 is also used in smaller com- pact systems with low refrigerant charges like residential air-conditioning units and heat pumps. Furthermore, a rising trend can be observed in its use with commercial re- frigeration systems and chillers. Propane is offered also as a mixture with Isobutane (R600a) or Ethan (R170), in order to provide a similar performance to halocarbon refrigerants. Pure Isobutane is mostly intended as a substitute for R12 in small systems (preferably domestic refriger- ators and freezers). The disadvantage of hydrocarbons is their high flammability, therefore they are classi- fied as refrigerants of "Safety Group A3". Based on the refrigerant charge quantities commonly used in commercial systems, the system design and risk analysis must be in accordance with explosion protection regu- lations. 30PDF Image | REFRIGERANT REPORT 21
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