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pseudocritical region (with a reduction up to 75% of HTC for the lower diameter tubes). For the higher diameter’s tubes, with 1% OCR, the HTC penalization was found to be negligible. Perspectives for the future Previous considerations clearly indicate the need for refrigerating compressors with low as possible OCR: according to the data available in the open literature, present author believes that OCR lower than 0.5% is advisable. Nevertheless, two interesting alternatives have been recently proposed “offbeat”: the use of suitable additives or the use of the so called “nano-lubricants”. Use of additives Oil additives are commonly used in refrigeration technology. The main purposes are usually anti-corrosion, anti-oxidation, anti-wear, anti-seize. In 1990 a U.S. Patent (n. 4,936,280, 1990) claimed that the use of additives based on sufficiently polar molecules can improve the distribution of the lubricant film over the metallic surface. The consequent reduction of the film thickness promotes a reduction of the mass transfer resistance and of the pressure drop linked to the lubricant film itself characterized, as already mentioned, by viscosity much higher than the pure refrigerant. Nowadays several polar additives are available. Some manufactures of chillers equipped with flooded evaporators claimed an improvement of more than 30% of the machine efficiency. In the open literature only a few data were obtained systematically. Kedzierski (2007) studied the “pool boiling” of the mixtures R134a/POE and R123/MO with an additive. With R134a/POE the HTC was measured to increase up to 73% with 5 kW·m-2 heat flux and even 95% with 22 kW·m-2. With R123/MO no improvement was found in pool boiling. On the basis of the mentioned results Kedzierski put forward some hypothesis about the characteristics of the additive for an heat transfer improvement: − Oil and additive must have “chemically different” molecules, i.e. the structure of the basic molecule or of the molecular chain don’t have to be the same in oil and additive: e.g. if the MO is naftenic it is not advisable to use a naftenic based additive molecule; − The liquid-vapour surface tension of the additive must be higher than lubricant one: − The viscosity (liquid phase) of the additive must be higher than the lubricant one. For further details please see the cited paper. Use of “nano-lubrificants” In the recent year the lubricant industry is dealing with an intense research activity in the field of nanotechnologies with the aim of improving the tribological performance of lubricants (see for example Liu et al. 2004). The results achieved in the industrial applications are extremely encouraging showing very good performances when adding carbon nanotubes or nano particles of copper oxides. Nevertheless several aspects have still to be investigated before making this technology ready for large industrial application.PDF Image | Circulating lubricant and effects of refrigeration circuits
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