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Circulating lubricant and its effects on the energetic efficiency of refrigeration circuits: state of the art and perspectives. Claudio Zilio Dipartimento di Fisica Tecnica – Università degli Studi di Padova Via Venezia, 1 – 35131, Padova claudio.zilio@unipd.it Introduction The main purpose of the lubricant in a refrigerating circuit is to promote a safe hydrodynamic lubrication inside the compressor by creating a thin film within the moving parts (pistons, thrust or shaft bearings, etc.) thus reducing the wear generated by the friction. The oil contributes to electric motor cooling inside hermetic and semi-hermetic compressors. Furthermore the action of the lubricant is fundamental for the tightness of the stuffing box in open-type compressors and for ensuring a good volumetric efficiency in screw or scroll compressors. It is important to point out that the lubricant fluid inside the compressor carter, or in the oil separator or in any other volume in the refrigerating circuit, has to be considered as a mixture of oil and refrigerant. So the thermophysical properties must be evaluated according to the actual composition of the mixture: small amount of refrigerant mixed with the oil can reduce markedly the viscosity of the lubricant. Furthermore, constraints may arise for the lubricant return to the compressor. Accordingly, the circulating refrigerant has not to be considered as a pure fluid but as a mixture of pure refrigerant and some amount of oil. This aspect can affect strongly the efficiency of the heat exchangers (evaporator mainly). Suitable correlations must be adopted for the evaluation of the thermophysical properties of the circulating mixture that, from a thermodynamic point of view, should be treated as a binary zeotropic mixture (Thome, 1995) at least in the temperature and pressure range where the two components, the oil and the refrigerant, are completely miscible. Qualitatively, refrigeration technicians are used to speak about refrigerant complete miscibility in oil when the liquid phase has a unique composition in thermodynamic equilibrium conditions, at set pressure and temperature. When two separate liquid phases with different composition arise at defined pressure and temperature, we speak about “partial miscibility”. If separated liquid phases are present at any temperature, pressure and composition, the two fluids are considered as “un-miscible”. The “solubility” concept is suitable to define the refrigerant amount (on mass or molar basis) mixed in the oil. The consequence of refrigerant solubility in oil is that some lubricant is discharged by the compressor. The ratio between the lubricant mass flow rate discharged by the compressor and the refrigerant mass flow rate is defined as the Oil Circulation Ratio (OCR). Depending on the circuit lay-out, the oil and the refrigerant type and the operating conditions, the discharged lubricant will distribute with different hold-up in each single component or line. The lubricant hold-up is considered to affect the efficiency of the refrigerating machine.PDF Image | Circulating lubricant and effects of refrigeration circuits
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