Abstract
M.Ing.
Since the promulgation of the Montreal Protocol many refrigerants needed to be phased out. R-22, which is a widely used refrigerant in refrigeration systems, was one of these. Many replacements have been found throughout the years but very few have the same refrigeration capacity without being penalised by an increase in pressure drop. R-407C is one of the refrigerants having the potential to replace R-22 as it has the same theoretical coefficient of performance and has a lower global warming potential. However, due to its zeotropic characteristics there is a degradation in heat transfer during evaporation and condensation attributed to mass transfer resistance. Thus, augmentation techniques are needed not only to increase the heat capacity, but also to achieve an increase without incurring an excessive pressure drop. One approach to cope with this problem is to make use of the recently developed herringbone micro-fin tubes. Unfortunately very little data exists for refrigerants undergoing condensation inside herringbone micro-fin tubes. There is also little pressure drop information available for this type of tube. An experimental set-up was designed to determine the characteristics of this type of tube due to the scarcity of information. With the aid of current literature, various techniques were used to determine the pressure drops inside the herringbone micro-fin tube. One of these techniques was
the use of the Kattan-Thome-Favrat flow regime map which helped to identify the flow patterns inside the tube. Knowledge of the type of flow occurring inside the tube helped to clarify the behaviour of the pressure drop relationships. The type of refrigerant being used also affected the behaviour of the pressure drop curves. A low-pressure refrigerant had a higher pressure drop due to the high vapour velocities achieved. Another cause for excessive pressure drop is the friction created by the high velocity vapour and condensate inside the tube. Many relationships for the friction factor exist and these are used to analyse the experimental data.The experimental facility comprised of a vapour compression loop and a water loop. The vapour compression loop consisted of a hermetically sealed compressor with a cooling capacity of 9.6 kW, a manually operated expansion valve and an evaporator. Three condensers were tested, namely a smooth tube, a helical micro-fin tube, and a herringbone micro-fin tube. The condensers were of
the tube-in-tube type with the refrigerant flowing in the inner tube and the water in counter flow in the annulus. The hot water loop was used as a source for the evaporator and a cold loop as a heat sink for the condenser. Three refrigerants were tested, namely R-22, R-134a, and R-407C, all operating at a nominal saturation temperature of 40°C and at mass fluxes between 300 and 800 kg/m 2s. Accurate sensors and transducers were used to measure the temperatures, pressures, and mass flows at predefined points. Video cameras were attached to sight glasses to aid in the identification of the type of flow regime. Data were captured using a computerised data acquisition programme designed specifically for use with the experimental study. The experimental results showed that transition between the annular and intermittent flow regimes occurred at around 25% vapour quality for the herringbone micro-fin tube, as opposed to 30% for the helical micro-fin tube and 50% for the smooth tube. Pressure drops for the herringbone micro-fin tube were higher than those for the smooth tube but slightly lower than those for the helical micro-fin tube when using refrigerants R-22 and R-134a. The correlation of Liebenberg was modified for the pressure drops inside the herringbone micro-fin tube and gave a mean deviation of 12%. The efficiency ratio for the herringbone tube using R-22 was 1.85 and 1.69 when compared with the helical micro-fin and smooth tube respectively. For R-134 the efficiency ratio was 2.02 and 2.13 when compared with the helical micro-fin and smooth tube respectively, while for R-407C it was 1.58 and 1.26 for the two respectively. It was also concluded that R-407C could be used as a replacement refrigerant for R-22when used with a herringbone micro-fin tube.