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Oil Circulation Ratio (OCR) and components hold-up Since the Eightees several investigations have been carried out for the development of in-line OCR measurement techniques. Nowadays, the only technique complying to a Standard is that according to ASHRAE Standard 41.4 – 1984 “Standard method for measurement of proportion of oil in liquid refrigerant”. As mentioned in the title, the Standard deals with liquid phase only. When applying this technique, one has to vent at least three samples of liquid withdrawn from the refrigerating circuit. Hence the method is not suitable for “in-line” dynamic measurements. The sensors proposed in the open literature are based on different transduction principles: e.g. refractive index measurement, viscosity measurement, density measurement, speed of sound measurement, ultraviolet radiation absorption, etc.. As already mentioned, when the oil is discharged by the compressor, a different lubricant hold-up occurs in the circuit components and lines. Prof. Radermacher’s group (Cremaschi et al., 2005) measured oil hold-up inside the condenser, the evaporator, the liquid and the suction lines in air conditioning systems with OCR of about 1% and 5% respectively, for different refrigerant mass flow rates. The investigated fluids were R410A/POE, R410A/MO, R22/MO, R134a/POE and R134a/PAG. The reported measurements indicate that the oil hold-up inside the circuit components can be considerable at some OCR values for given refrigerant mass fluxes in vapour-phase lines. The hold-up is linked to oil/refrigerant mixture viscosity and pipes orientation. The same group proposed the same study on a transcritical CO2 system. The paper is in press ((Hwang et al., 2007). Wujek and Hrnjak (2006) have considered the effects of a PAG oil in a prototype of refrigerating unit operating with transcritical R744 cycle. Their observations show that increasing the OCR, in absence of separator of oil, the COP of the system tends to decrease: in particular, the Authors observed a decrease in the gas cooler efficiency and higher pressure drop at the evaporator. However the installation of an oil separator caused additional pressure drop in the discharge line. As a consequence the no improvement of the COP was measured even though the OCR was reduced. It’s evident that the mentioned analyses are strictly linked to the investigated system: a different circuit arrangement, different refrigerant, different oil or different mass flow rate of the refrigerant itself may lead to completely different lubricant hold- up in each single component. It is a common belief that the compressor and circuit design should be carried out having the OCR reduction as a target. Effects of the lubricant on heat transfer The lubricant type and the related OCR are fundamental for compressor efficiency and strongly affects also the two-phase heat transfer process inside circuit heat exchangers. Shen and Groll (2005a e 2005b) critically reviewed some of the most outstanding papers available in the open literature dealing with the effects of the lubricant mixed with the refrigerant on the heat transfer coefficient (HTC) and on pressure drop during the boiling process (2005a) and the condensation (2005b). Both the mentioned works evidenced the high scattering of the experimental data available. The measurement presented by several Authors indicate every possiblePDF Image | Circulating lubricant and effects of refrigeration circuits
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